Techniques for Proper and Efficient Characterization, Validation, and Reliability Testing of Power Semiconductor Devices

Techniques for Proper and Efficient Characterization, Validation, and Reliability Testing of Power Semiconductor Devices
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Techniques for Proper and Efficient
Characterization, Validation, and Reliability Testing
of Power Semiconductor Devices
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Contents
Testing Power Semiconductor Devices with
Keithley High Power System SourceMeter® SMU
Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Breakdown and Leakage Current Measurements
on High Voltage Semiconductor Devices
Using Keithley Series 2290 High Voltage
Power Supplies and Series 2600B System
SourceMeter® Source Measure Unit (SMU)
Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Creating Multi-SMU Systems with High Power
System SourceMeter® Instruments . . . . . . . . . . . . . . 13
Simplifying FET Testing with Series 2600B
System SourceMeter® SMU Instruments . . . . . . . . . 27
Measuring Pulsed Waveforms with the High
Speed Analog-to-Digital Converter in the Model
2651A High Power System SourceMeter®
Instrument . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Methods to Achieve Higher Currents from I-V
Measurement Equipment . . . . . . . . . . . . . . . . . . . . . . 51
Combining Keithley Model 2651A High Power
SourceMeter® Instruments for 100A Operation . . . . 61
Optimizing Reliability Testing of Power
Semiconductor Devices and Modules with
Keithley SMU Instruments and Switch Systems . . . . 75
VDS Ramp and HTRB Reliability Testing of High
Power Semiconductor Devices with Automated
Characterization Suite (ACS) Software . . . . . . . . . . . 79
Testing High Brightness LEDs under Pulse Width
Modulation Using the Model 2651A High Power
System SourceMeter® Instrument . . . . . . . . . . . . . . . 85
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2
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Testing Power Semiconductor Devices
with Keithley High Power System
SourceMeter® SMU Instruments
Introduction
The proliferation of electronic control and electronic power
conversion into a variety of industries (e.g., energy generation,
industrial motor drives and control, transportation, and IT)
has spurred growth in power semiconductor device design and
test. To demonstrate technology improvements, new device
capabilities must be compared with those of existing devices.
The use of semiconductor materials other than silicon demands
the use of new processes. And, to be sustainable, these new
processes must be tuned to deliver consistent results and high
production yield. As new device designs are developed, reliability
measurements are performed on many devices over long periods.
Therefore, test engineers must identify test equipment that is not
only accurate but scalable and cost-effective.
Power module design engineers—the consumers of the
discrete power semiconductor components—work at the other
end of the semiconductor device testing spectrum. They integrate
the discrete components into designs for DC-DC converters,
inverters, LED controllers, battery management chips, and many
other devices. Driven by demands for higher energy efficiency,
these engineers need to qualify the devices they receive from
their vendors to ensure that they can withstand use in the
application, predict how the efficiency of the power modules may
be affected by the device, and finally validate the performance of
the end product.
Keithley’s SourceMeter SMU instruments give both device
test engineers and power module design engineers the tools
they need to make the measurements they require. Whether
they’re familiar with curve tracers, semiconductor parameter
analyzers, or oscilloscopes, they can obtain accurate results
simply and quickly. This application note highlights some of the
most commonly performed tests, the challenges associated with
them, and how Keithley SMU instruments can simplify the testing
process, especially when integrated into a Keithley Parametric
Curve Tracer (PCT) configuration.
Background on Power Device Characterization
The switching power supply is one common electrical circuit
element used in power management products. In its simplest
form (Figure 1), its main components include a semiconductor
such as a power MOSFET, a diode, and some passive
components, including an inductor and a capacitor. Many also
include a transformer for electrical isolation between the input
and output. The semiconductor switch and diode alternatively
VIN
VOUT
Figure 1. A simple schematic of a type of switching power supply.
switch on and off at a controlled duty cycle to produce the
desired output voltage.
When evaluating energy efficiency, it’s important to
understand the switching loss (energy loss that occurs during
the short periods when the device is changing states) and
conduction loss (energy losses that occur when the device is
either on or off). Keithley SMU instrument-based solutions can
help test engineers evaluate the device parameters that affect
conduction loss.
Semiconductor devices are often used to ensure circuit
protection. For example, some thyristor devices are used for
overvoltage protection. To achieve that objective, such devices
must trigger at the appropriate intended voltage and current,
must withstand the intended voltage, and must behave in circuit
with minimal current draw. High power instrumentation is
required to qualify these devices properly.
This note focusses on the characterization of static power
device parameters.1 These parameters can be divided into two
broad categories: those that determine the performance of the
device in its ON state and those that determine the performance
in its OFF state. Table 1 lists common ON-state and OFF-state
parameters for several power semiconductor devices that
Keithley SMU instruments support. Many tests involve the use of
multiple SMU instruments. Keithley’s ACS Basic Edition software
simplifies the test configuration by managing the configuration
and data collection of all SMU instruments in the test system.
Unlike general-purpose start-up software, ACS Basic Edition is
designed specifically for semiconductor device characterization
and includes a library of tests; users can focus on the test and
device parameters rather than the SMU instrument configuration.
The test results included in this note were acquired using ACS Basic
Edition software, which is included in our PCT configuration.
1 Tektronix solutions are available for transient characterization of power devices. For
more information, visit www.tek.com.
www.keithley.com3
Table 1. Common power semiconductor devices and tests.
Devices and Tests
ON-state
OFF-state
Diode
MOSFET
V F–IF
V DS –ID
V TH
VGS –ID
R DS(on)
IR
IGSS
IDSS
BV DSS
BV DG
BJT
IGBT
VCE –IC
VCE –IC
Gummel plot VGE –IC
ICEO
ICES
BVCES
BVCEO
BVCBO
Thyristor-Class Devices Keithley SMU
(e.g., SCR, Triac)
I-V Capability
Voltage:
VT
–40V to +40V
IH
Current: Up to
IL
100A (pulse)
ICEO
ICES
BVCES
BVCEO
V bo
V DRM
V RRM
ON-State Characterization
ON-state characterization is typically performed using a high
current instrument capable of sourcing and measuring lowlevel voltages. If the device has three terminals, then a second
SMU instrument is used at the device control terminal to place
the device in the ON state. For a typical configuration for
characterizing the ON-state parameters of a power MOSFET,
see Figure 2.
Keithley SMU instruments cover a wide range of currents
for power semiconductor devices. Series 2600A and 2600B
System SourceMeter SMU instruments include at least 1.5A DC
and 10A pulse capability for DC characterization. For very high
current devices, two Model 2651A High Power SourceMeter SMU
instruments can be used in parallel to generate current pulses
up to 100A.
Let’s examine the configuration details and measurement
challenges of a few ON-state parameters.
Output characteristics
One of the most readily recognizable set of test results for a
semiconductor device is a plot of its output characteristics.
Output characteristics are normally shown in graphical form
on the device’s data sheet and depict the relationship between
Force HI
Sense HI
Pulsed testing is common in power
semiconductor test because DC testing
can cause the device to self-heat, which
can alter the measured characteristics.
Implementing a pulsed test with multiple
SMU instruments requires precise control of the timing of
source adjustments and measurements and is often coordinated
by means of a computer program. For consistent results, it is
important to validate the system. This can be done by sourcing a
single pulse through the system and measuring the response at
the device. Capturing the complete pulse waveform as a function
of time allows selecting appropriate source and measure delays
so that the device turns on properly and measurements are made
after the system is settled. The high speed A/D converters in
Series 2650A High Power System SourceMeter SMU instruments
are useful for monitoring the voltages and currents at the device
as they relate to time. A diagnostic feature in ACS Basic Edition
2.0 software allows capturing the pulse shape of a single point
in the family of curves easily. Figure 4 depicts the results of a
pulse transient characterization of collector voltage and current
vs. time of an IGBT. For this particular example, delaying the
measurement 100μs after the start of the pulse ensures the
system has settled prior to measurement, allowing for more
consistent results between tests.
Power semiconductor devices are often high gain devices;
oscillation is common when characterizing such devices and
will result in erratic measurements. The high speed A/D
converters in the Series 2650A SMU instruments and the
pulse transient feature of ACS Basic Edition 2.0 software are
useful for verifying the presence of oscillation. Resolving this
VCE vs. ICE for IGBT (Family of Curves)
A
VDS
Force HI
High Current
SMU
Instrument
(10–100A
pulsed)
Sense LO
A
Voltage:
–3kV to +3kV
Current:
Down to 1fA
Force LO
VGS
40
VGE = 12.0V
VGE = 11.0V
VGE = 10.0V
VGE = 9.0V
VGE = 8.0V
VGE = 7.0V
30
IC (Amps)
Test
Category
the output voltage and current. For a
gated power semiconductor switch,
such as a MOSFET, IGBT, or BJT, output
characteristics are commonly referred to
as the “family of curves.” Figure 3 shows
the results for a power IGBT as generated
by ACS Basic Edition software.
20
10
SMU Instrument
Force LO
0
0
1
2
3
4
5
6
VCE (Volts)
Figure 2. Typical SMU configuration for ON-state characterization of
power devices.
4
Figure 3. Measured output characteristics for commercially available IGBT.
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6
25
5
20
4
15
3
10
2
5
Y1:V_Collector
Y2:I_Collector
1
ICE (A)
VCE (V)
Pulse Transient Data
0
0
0.0000
0.0001
0.0002
0.0003
0.0004
0.0005
0.0006
Time (seconds)
Figure 4. Pulse transient data of collector voltage and current vs. time
for an IGBT.
oscillation involves adding a resistor in series with the device
control or input terminal, for example, the gate of a MOSFET or
IGBT. The Keithley Model 8010 High Power Test Fixture easily
accommodates the addition of a discrete resistor.
ON-state voltage
The semiconductor device’s ON-state voltage directly impacts
the conduction loss. Examples of ON-state voltages include the
forward voltage of a power diode (V F), the ON-state saturation
voltage of a BJT or an IGBT (VCEsat), and the ON-state voltage of
a thyristor (V T ). Power consumed by or lost in the device is equal
to the product of the ON-state voltage and the load current.
This power is not delivered to the device. Typically, device
manufacturers want to characterize how the ON-state voltage
and, by extension, the conduction loss, varies with temperature
and load current. Keithley SMU instruments are commonly used
in these characterizations.
To measure the ON-state voltage, the high current SMU
instrument is configured to output current to the device and
measure voltage. For BJTs and IGBTs, a second, lower-power
SMU instrument is used at the base or gate terminal to place
the device in the ON state. Because power semiconductors are
typically high current devices, ON-state voltage is generally
measured using a pulsed stimulus to avoid any change in
device parameters as a result of device self-heating due to a
DC test current.
Two key elements help ensure a successful ON-state
voltage test: (1) accurate voltage measurement and (2) proper
cabling and connections. Accurate voltage measurements are
crucial because ON-state voltage varies with temperature. For
instance, a few millivolts of difference in the forward voltage of
a power diode can indicate a change of several degrees in the
temperature at the device. High speed A/D converters in the
Keithley Model 2651A High Power System SourceMeter SMU
Instrument let it make very accurate voltage measurements at
1μs intervals with pulse widths as short as 100μs.
Proper cabling and connections are equally key to
minimizing voltage errors. For power diodes, BJTs, and IGBTs,
typical test currents can range from 100mA to tens of amps
while ON-state voltages of 1–3V are very common. Thyristors
are ideal for use in ultra-high-power applications because they
have very low ON-state voltages (<2V) while conducting currents
that could be greater than 100A. During testing, such high
currents can generate voltage drops across test leads and test
lead connections between the instrument and the DUT. These
additional voltages create errors in the voltage measurement.
Four-wire or Kelvin connections eliminate most of these voltage
errors from the measurement by using separate test leads for the
voltmeter. Minimal current flows in these leads, creating virtually
no voltage drops between the instrument and DUT, so the
instrument measures the voltage seen at the DUT.
The use of low inductance cables helps ensure good pulse
fidelity (i.e., short rise and fall times) when testing high current
devices, which provides more time for measurement within
a given pulse width. The Model 2651A High Power System
SourceMeter SMU Instrument comes with a specialized low
resistance, low inductance cable assembly appropriate for
generating 100μs pulses at 50A.
Transfer characteristics
A device’s transfer characteristics allow evaluating its
transconductance and therefore its current carrying capability.
Transfer characteristics have an indirect relationship to
determining switching time and estimating switching losses. The
transfer characteristics are often monitored vs. temperature in
order to gauge temperature’s effect on the device’s maximum
current handling capability. Two SMU instruments are required
for measuring the transfer characteristics: one sweeps the input
voltage on the control terminal of the device and the second
biases the output terminal and measures the output current.
Typical transfer characteristic measurements include the gate
voltage vs. drain current plot for a MOSFET (V DS –ID), the gate
voltage vs. collector current plot for an IGBT (VGE –IC), and the
Gummel plot for a BJT (V BE vs. IC, IB).
In some cases, a wide range of output current is measured.
This is especially true for the Gummel plot of a BJT, where
several orders of magnitude of current are traversed. In these
cases, the Model 2651A is very useful because it can measure
currents from the nanoamp range up to 50A. Figure 5 depicts
a Gummel plot generated using a Model 2651A on the collector
and a Model 2636B on the base.
ON-resistance
The figure of merit for a power MOSFET is the product of
ON-resistance (R DS(on)) and gate charge (QG). The ON-resistance
is the key determinant of the conduction loss of the power
MOSFET. The conduction loss is equal to ID * R DS(on). Newer
devices have ON-resistances in the range of a few milliohms to
tens of milliohms at high current. This requires very sensitive
voltage measurement capability at the drain terminal. Measuring
ON-resistance requires the use of two SMU instruments: one
SMU instrument drives the gate into the ON state and a second
SMU instrument pulses a defined current at the drain and
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ON-resistance vs. Drain Current for a Power MOSFET
100
100
0.096
10
–1
10
–1
0.094
10
–2
10
–2
10–3
10–3
10–4
10–4
10–5
10–5
Y1: I_Collector
Y2: I_Base
10–6
10–6
10–7
10–7
10–8
10–8
0.4
0.6
0.8
1.0
1.2
V_Base (Volts)
Figure 5. Gummel plot for typical power BJT generated using Keithley Models
2651A and 2636B SMU instruments.
measures the resulting voltage. The ON-resistance is calculated
using Ohm’s Law and the programmed drain current and
measured drain voltage. Such a calculation can be automatically
configured in software.
ON-resistance is often characterized as a function of drain
current or gate voltage. Using software, both SMU instruments
can be triggered and swept so that this measurement is
performed within a single test. Figure 6 shows the calculated
R DS(on) vs. drain current results. This was all completed during
a single run of the R DS(on) test. For very high current devices,
two Model 2651As can be used in parallel to generate current
pulses up to 100A. ACS Basic Edition software manages the
configuration of both SMU instruments and the data collection.
ON-resistance increases with breakdown voltage, so any
process adjustments made to increase the breakdown voltage
will involve later testing of ON-resistance in order to assess the
overall impact of process changes. Obtaining more efficient
devices at higher voltages is one of the drivers for further
research on SiC and GaN devices, which offer ON-resistances
smaller than silicon devices’ at high withstand voltages.
OFF-State Characterization
To gain an adequate understanding of overall product efficiency,
the impact of the device on the overall circuit when the device
is turned off must be investigated. For high power devices, OFFstate characterization often involves the use of a high voltage
instrument capable of sourcing hundreds or thousands of volts
and measuring small currents. OFF-state characterization is often
performed between two device terminals (regardless of the total
number of device terminals), so a single SMU instrument is often
sufficient to perform the measurement. However, an additional
SMU instrument can be used to force the device into its OFF
state or to add certain stress to certain terminals.
Keithley SourceMeter SMU instruments cover a wide range
of voltages and currents for characterizing the OFF state of
power semiconductor devices. The Model 2635B and 2636B
SMU instruments offer 200V characterization with current
6
RDS (Ohms)
101
I_Base (Amps)
I_Collector (Amps)
Gummel Plot for BJT
101
Vgs = 10V
Vgs = 15V
0.092
0.090
0.088
0.086
0.084
10
20
30
40
50
I_Drain (Amps)
Figure 6. Results for ON-resistance of a power MOSFET as measured as a
function of drain current for two gate voltages.
measurement capability down to the femtoamp level. The Model
2657A SMU instrument extends high voltage characterization to
3kV while providing very low current measurement resolution
and accuracy.
Two primary DC tests are performed when the device is off:
breakdown voltages and leakage currents. Let’s consider these
individually.
Breakdown voltages
A device’s OFF-state breakdown voltage determines the
maximum voltage that can be applied to it. The primary
withstand voltage of interest to power management product
designers is the breakdown voltage between drain and source
of a MOSFET or between the collector and emitter of an IGBT
or BJT. For a MOSFET, the gate can be either shorted or forced
into a “hard” OFF state, such as by applying a negative voltage to
an n-type device or a positive voltage to a p-type device. This is
a very simple test that can be performed using one or two SMU
instruments. The lower power SMU instrument is connected to
the gate and forces the transistor off. It can force 0V for a gate
shorted test or force a user-specified bias voltage. A high voltage
SMU instrument, such as the Model 2657A, forces current into
the drain and measures the resulting drain voltage.
Most MOSFETs typically have breakdown voltages higher than
the value specified on the data sheet. Therefore, it is useful to
define the voltage limit of the drain SMU instrument to a value
higher than the specified breakdown voltage. Additionally, as
the drain terminal is driven closer and closer toward avalanche,
the behavior of the currents and voltages of the device under
test (DUT) may begin to change. These cases can take advantage
of the high speed A/D converters of the Series 2650A SMU
instruments. Without the need for any extra equipment, it’s
possible to catch a quick look at both the transient behavior
of current and voltage at the DUT. Figure 7 is an example of
transient characterization of a test of the breakdown voltage
between drain and source of a commercially available 600V
silicon power MOSFET. The Model 2657A is used to pulse a
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equipped with a safety interlock; when properly installed, this
interlock ensures that the hazardous voltages are disconnected
whenever a user opens the test fixture or accesses the wafer in a
probe station.
Transient Characterization of BVDSS Test
700
0.010
0.008
500
400
0.006
300
0.004
V_Drain
I_Drain
200
I_Drain (Amps)
V_Drain (Volts)
600
0.002
100
0
0.000
0.000
0.002
0.004
0.006
0.008
0.010
0.012
Time (seconds)
Figure 7. Transient capture of voltages and currents of BV DSS test using the
Model 2657A’s fast A/D converter. Sampling interval is 10μs.
current into the MOSFET and then measure the voltage and
current at 10μs intervals. The plot shows that the device actually
breaks down at approximately 680V.
Another way of characterizing breakdown voltages involves
forcing a voltage across the terminals of interest (e.g., drain
and source of a MOSFET) and measuring the resulting current.
The breakdown voltage is defined as the voltage at which the
current exceeds a specified threshold, for example, 1mA. Limit
the maximum current in order to prevent destruction of the
device during testing. Unlike traditional curve tracers and
power supplies, Keithley SourceMeter SMU instruments include
built-in programmable features to limit the maximum voltage
and current to the device precisely and quickly. As with any
protection device, the limit control of the SMU instrument has
a finite response time. Some devices may have extremely fast
and hard breakdown behavior in which the device impedance
changes by several orders of magnitude in a very short period.
When the device breaks down faster than the SMU instrument
can respond, use series resistors to limit the total maximum
current through the device.
Safety must be one of the first considerations for high
voltage characterization of power semiconductor devices. Keep
in mind the voltage ratings for all terminals, connectors, and
cables. For example, Keithley SourceMeter SMU instruments are
electrically floating, which means that measurement common is
not tied to protective earth (safety ground). Unless the user ties
measurement common to safety ground, then he must take high
voltage precautions at all terminals if the SMU instrument can
generate more than 42V.
When configuring a test system, it’s important to protect
the operator from electric shock. One primary way to do this
is to use a safe test enclosure that surrounds the DUT and any
exposed connections. The Keithley Model 8010 High Power
Test Fixture permits safe testing of packaged high power
semiconductor devices at up to 3kV. Pairing a safe enclosure with
a safety interlock also minimizes risk of electric shock when the
user changes test connections. Keithley SMU instruments are
In addition to protecting the operator, it is also important to
consider the interactions between all the instruments connected
to the device terminals. If a lower voltage SMU instrument is
connected to the device during a breakdown characterization,
a device fault can result in high voltage being present across
this lower voltage SMU instrument. Keithley’s Model 8010 High
Power Test Fixture includes built-in protection to protect the
lower voltage SMU instrument in such applications.
Leakage currents
Leakage current is the level of current that flows through two
terminals of a device even when the device is off. It factors
into the standby current of the end product. In most cases,
temperature and the voltage across the terminals of interest will
affect leakage current. Minimizing leakage current minimizes
power loss when the device is off. This power is consumed by
the device and is not output to the load and therefore contributes
to power inefficiency. When using a transistor or diode to switch
or rectify, it’s important to make a clear distinction between ON
and OFF states; therefore, a lower leakage current means having
a better switch.
While testing a device’s OFF state, it is generally desirable
to test the gate leakage current and drain or collector leakage
current. For power devices, these values are typically within
the nanoamp and microamp ranges, so they can be measured
using the sensitive current measurement capability of Keithley
SMU instruments. This capability can be greatly beneficial when
testing devices made of wide bandgap materials such as silicon
carbide, gallium nitride, and aluminum nitride, which typically
have higher breakdown voltages and lower leakage currents
than do silicon devices. Figure 8 is a plot of OFF-state drain
voltage vs. drain current results for a commercially available SiC
power MOSFET.
Triaxial cables are essential to achieving accurate low current
measurements in part because they permit carrying the guard
terminal. Guarding eliminates the effect of system leakage
currents by routing them away from the measurement terminal.
Additionally, guarding reduces settling time in high voltage
applications by virtually eliminating the need to charge the cable
capacitance. Using a guarded test fixture or prober allows the
benefits of guarding to be extended to the DUT.2 Keithley offers
triaxial cables and connections for all its SMU instruments that
are tailored for low current measurements, such as the Model
2636B and Model 2657A SMU instruments. The specialized high
voltage triaxial cables for the Model 2657A allow measurements
at 3kV with 1fA resolution. The Model 8010 test fixture includes
guarded connections to the device test board to permit current
measurements down to tens of picoamps.
2 For more details on guarding, review Keithley Application Note #3163,
Creating Multi-SMU Systems with High Power System SourceMeter SMU Instruments.
www.keithley.com7
Finally, system validation is important for low current
characterization to ensure that the measurement is settled.
Settling time increases as the expected current decreases.
Keithley products have auto delay settings that set reasonable
delays to achieve good measurements while taking the
instrument’s settling characteristics into account. However,
to account for any unguarded system capacitance, perform a
measurement validation by stepping voltage and measuring the
resulting current through the system. Use the results from the
validation to set additional source and measure delays in order
to achieve consistent measurements.
Drain Leakage Current
I_Drain (Amps)
10–7
10–8
Y1: I_Drain
10–9
10–10
0
200
400
600
800
1000
1200
1400
V_Drain (Volts)
Figure 8. A look at the drain leakage current as the drain voltage is swept
while the transistor is in the OFF state.
Electrostatic shielding is another important consideration
for low current measurements. An electrostatic shield is a
metal enclosure that surrounds the circuit and any exposed
connections. The shield is connected to measurement common
to shunt electrostatic charges away from the measurement node.
When testing devices within the Model 8010 High Power Test
Fixture, the test fixture chassis serves as an electrostatic shield.
8
Conclusion
Keithley SourceMeter SMU Instrument solutions can be used
with ACS Basic Edition software to provide a comprehensive
solution for testing high power semiconductor discrete devices.
ACS Basic Edition includes a library of tests for a variety of
power devices including FETs, BJTs, IGBTs, diodes, resistors,
and thyristors. Additionally, Keithley has the appropriate cabling
accessories and test fixtures that allow safe, accurate, and
reliable testing. Although these instruments and accessories
can be purchased separately, they are also available as part of
Keithley’s Parametric Curve Tracer Configurations. Refer to
Keithley’s website (www.keithley.com) or contact your local
Keithley representative for further information or system
configuration assistance.
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Breakdown and Leakage Current Measurements on High
Voltage Semiconductor Devices Using Keithley Series 2290
High Voltage Power Supplies and Series 2600B System
SourceMeter® Source Measure Unit (SMU) Instruments
Increased attention to energy efficiency has resulted in
electronics with higher power density. In grid-connected and
industrial applications, such as AC motor control, uninterruptible
power supplies (UPS,) and traction control (large hybrid and
electric transport vehicles,) the need to keep manageable cable
sizes pushes power conversion to higher voltages. For such
voltages, the semiconductor device of choice has historically
been the thyristor. Technological advances in device fabrication
and material processing is enabling the development of IGBTs
and MOSFETs with voltage ratings of thousands of volts. In
applications where possible, using IGBTs or even MOSFETs in
place of thyristors permits power conversion at high switching
frequencies. The migration to higher frequency reduces the size
of passive components used in the design and, thereby, improves
energy efficiency.
Keithley has long had a strong presence in high power
semiconductor device test with its high voltage source-measure
products, including the Models 237, 2410, and 2657A SMU
instruments. Most recently, Keithley released the Model 2290-5
5kV and Model 2290-10 10kV High Voltage Power Supplies. This
note considers the application of these power supplies to high
voltage semiconductor device testing.
High Voltage Device Tests
Basic characterization of high voltage semiconductor devices
typically involves a study of the breakdown voltage and
leakage current. These two parameters help the device
designer to quickly determine whether the device was correctly
manufactured and whether it can be effectively used in the target
application.
Breakdown Voltage Measurements
Measuring breakdown voltage is done by applying an increasing
reverse voltage to the device until a certain test current is
reached that indicates that the device is in breakdown. Figure 1
depicts a breakdown measurement on a high voltage diode
using a Series 2290 High Voltage Power Supply. Note that
the Series 2290 Power Supplies are unipolar supplies and
must be connected to the diode’s cathode in order to apply a
reverse voltage.
In qualifying breakdown voltage, measurements are typically
made well beyond the expected rating of the device to ensure
that the device is robust and reliable. The models 2290-5 and
2290-10 Power Supplies have a voltage range wide enough to test
many of the industry’s future devices.
Properly grounded
safe enclosure
Series 2290
High Voltage
Power Supply
A
Figure 1. Typical breakdown voltage measurement of a high voltage diode
using the Series 2290 High Voltage Power Supply.
Safety Considerations
When testing at high voltage, safety is of utmost concern. The
Series 2290 Power Supplies generate voltage up to 10kV, so
precautions must be taken to ensure that the operator is not
exposed to unsafe voltage:
• Enclose the device under test (DUT) and any exposed
connections in a properly grounded fixture.
• Use the safety interlock. The Series 2290 Power Supplies are
fully interlocked so that the high voltage output is turned off
if the interlock is not engaged (interlock switch closed.) The
interlock circuit of the power supply should be connected to
a normally-open switch that closes only when the user access
point in the system is closed to ensure that operators cannot
come in contact with a high voltage connection to the DUT.
For example, opening the lid of the test fixture should open
the switch/relay that disengages the interlock of the Series
2290 Power Supply.
• Use cables and connectors rated to the maximum voltage in
the system. Series 2290 Power Supplies provide a number of
appropriately-rated accessories that the test system designer
can use to interface to the device under test (DUT).
Leakage Current Measurements
In a typical power conversion application, the semiconductor
device is used as a switch. Leakage current measurements
indicate how closely the semiconductor performs to an ideal
switch. Also, when measuring the reliability of the device,
www.keithley.com9
leakage current measurements are used to indicate device
degradation and to make predictions of device lifetime.
Semiconductor researchers are finding materials to make
higher quality switches and produce devices with very small
leakage currents. Such currents may fall below the measurement
capability of the Series 2290 Power Supplies. In such cases,
couple the accurate sourcing ability of the Series 2290 Power
Supply with the precision low current measurement ability of
a Keithley SMU instrument. Using Keithley SMU instruments
improves the low current measurement resolution and accuracy
and also improves the accuracy of the current limit. As an
example, Keithley Models 2635B and 2636B SourceMeter® SMU
instruments have four current ranges at 1µA and below. The
current limit of the Keithley SMU instrument can be configured
as small as 10% of a range.1
Series (Pin 22) +5V
2600B
SMU (Pin 24) INTLK
(Pin 19) GND
Series (Pin 1) +5V
2290
Power (Pin 2) INTLK
Supply (Pin 3) GND
System Access Point
(use normally-open
switch to enable or
disable the interlock)
Figure 2. Correct wiring of interlocks from a Series 2600B SMU instrument
and a Series 2290 Power Supply to a single system access point, e.g. test
fixture lid.
To prevent unwanted measurement error when measuring
currents less than 1µA, use triaxial cables and electrostatic
shielding. Triaxial cables are essential in part because
they permit carrying the guard terminal from the current
measurement instrument. Guarding eliminates the effect
of system leakage currents by routing them away from the
measurement terminal. Use an electrostatic shield to shunt
electrostatic charges away from the measurement terminal.
An electrostatic shield is a metal enclosure that surrounds the
circuit and any exposed connections. The safe test enclosure
may serve as an electrostatic shield. For more tips on optimizing
low current measurements, refer to Keithley’s Low Level
Measurements Handbook, 7th Edition.
Safety Considerations
Review system safety whenever a new element, in this case the
SMU instrument, is added to the test circuit. In addition to the
safety issues considered under the topic of breakdown voltage
testing, the Series 2600B SMU Instrument is also capable of
generating voltages up to 200V. Like the Series 2290 power
supplies, Keithley Series 2600B SMU instruments have a safety
interlock to ensure operator safety during changes in the test
setup. For optimum system safety, the interlock of the Series
2600B SMU Instrument should be wired in parallel with the
Series 2290 Power Supplies. An example of this is shown
in Figure 2.
As a part of the system safety review, consider all potential
consequences of device failure. In a setup where both a Series
2290 Power Supply and a Series 2600B SMU Instrument are
employed, a device breakdown could result in high voltage
appearing at the input terminals of the SMU instrument. Because
the SMU instrument is not designed to handle these higher
voltages, it must be protected against possible damage by the
high voltage power supply. The Model 2290-PM-200 Protection
Module can be used for this purpose. The same module can be
used regardless of whether a Model 2290-5 5kV or Model 2290-10
10kV High Voltage Power Supply is used in the test circuit (see
1The current limit of an SMU instrument is an active current limit and has a finite
response time. To limit the maximum possible current in a circuit, use a series resistor.
10
Figure 3. The Model 2290-PM-200 Protection Module permits safe
connection of a single 200V SMU instrument into the test circuit. It is
designed to be used in a test circuit with either the Model 2290-5 5kV or the
Model 2290-10 10kV Power Supply.
Figure 3). Figure 4 illustrates the placement of the Model 2290PM-200 in the test circuit.
Using the test setup shown in Figure 4, the actual test
results when measuring leakage current of a high voltage
diode are displayed in Figure 5. The diode has a maximum
specified reverse current of 10µA when 3300V is applied at
room temperature. The results show that the diode meets
its specification. The reverse current grows at a faster rate
as the reverse voltage increases, indicating that the diode is
approaching breakdown.
Figure 6 depicts the actual test results when measuring the
collector-emitter cutoff current of a 4000V IGBT. In this test,
the gate and emitter terminals are shorted to ensure that the
device remains off (Figure 7a). An SMU instrument can also
be used to actively program the gate voltage. Using an SMU
instrument is useful if the leakage current measurements are
desired with the device in hard cutoff (with a bias less than 0V at
the gate terminal). Figure 7b depicts the setup using two SMU
instruments and a Series 2290 Power Supply.
This particular IGBT has a maximum specified cutoff current
of 100µA at 4000V. The performance of this IGBT is much better
www.keithley.com
Properly grounded
safe enclosure
Properly grounded
safe enclosure
Series 2290
Power Supply
Series 2290
Power Supply
Model
2290-PM-200
Protection
Module
Model 263xB
System
SourceMeter
Instrument
configured as
an ammeter
A
PM
Figure 4. Characterizing the leakage current of a high voltage diode using a
Series 2290 Power Supply with a Model 263xB SMU Instrument. Using the
SMU instrument enhances the resolution and accuracy of both the current
measurement and current limit.
Model 263xB
System
SourceMeter
Instrument
configured as
an ammeter
A
PM
Model
2290-PM-200
Protection
Module
Figure 7a. Test setup using a Series 2290 Power Supply and the Model 263xB
SourceMeter SMU Instrument to measure the cutoff current (ICES ) of an IGBT.
The short between the gate and emitter terminals keeps the device in the
off-state.
Leakage Current Measurements on 3300V Schottky Diode
Properly grounded
safe enclosure
0.00E+00
–5.00E–07
A
–1.00E–06
PM
Current (A)
–1.50E–06
–2.00E–06
–2.50E–06
Model 263xB
System
SourceMeter
Instrument
–3.00E–06
Model
2290-PM-200
Protection
Module
Model 263xB
System
SourceMeter
Instrument
configured as
an ammeter
Series 2290
Power Supply
A
PM
Model
2290-PM-200
Protection
Module
–3.50E–06
–4.00E–06
–4.50E–06
–4000
–3500
–3000
–2500
–2000
–1500
–1000
–500
0
Voltage (V)
Figure 5. Measurements of 3300V Silicon Carbide Schottky diode. Voltage is
applied with the Model 2290-5 5kV Power Supply and current is measured
with the Model 2636B System SourceMeter SMU Instrument.
ICES Measurement on 4000V IGBT
Collector-Emitter Leakage Current (ICES)
1.00E–05
Figure 7b. Test setup using a Series 2290 Power Supply and two Model 263xB
SourceMeter SMU instruments to measure the cutoff current (iCES ) of an
IGBT. The SMU instrument connected to the gate terminal can be used to
place a certain bias on the gate, e.g., to drive the device into hard cutoff.
than the specification. In fact, even at 4500V, the cutoff current
is not increasing rapidly, thereby indicating that the device is not
yet in breakdown.
The command sequence to generate the results shown in
both Figures 5 and 6 using either the Model 2290-5 or the Model
2290-10 and the Model 2636B SourceMeter SMU Instrument
is included in the Appendix. Note that open source language
Python™ 2 is used to send the information to the GPIB interface.
1.00E–06
Conclusion
1.00E–07
1.00E–08
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Collector-Emitter Voltage (VCES)
Figure 6. Ices measurements of 4000V IGBT. VCES is applied with the
Model 2290-5 Power Supply, and ICES is measured with the Model 263xB
SourceMeter SMU instrument. Gate and source terminals are shorted.
Testing high voltage semiconductor devices involves a
consideration of test system safety, wide voltage range, and
accurate current measurement. Coupling a Keithley Series 2290
Power Supply with a Keithley SourceMeter SMU instrument
and their associated accessories meets all these needs and
further facilitates research of high voltage materials and
semiconductor devices.
2Find out more about Python programming language at http://www.python.org
www.keithley.com11
Appendix
# Turn on the output of the 2290
# Import pyVisa and time modules into the Python
environment
import visa
import time
ki2290.write("HVON")
# Open a VISA session with the 2290 at GPIB address 14
# and 263xB at GPIB address 26
ki2290 = visa.instrument("GPIB::14")
ki263x = visa.instrument("GPIB::26")
# Reset and clear the status of the 263xB
ki263x.write("reset()")
ki263x.write("*CLS")
# Reset and clear any errors of the 2290
ki2290.write("*RST")
ki2290.write("*CLS")
ki2290.write("*RCL 0")
time.sleep(1)
print "Running Sweep . . . "
# Perform a sweep from 0 to 4500V and make current
measurements
# at each point of the sweep
for n in range(0,51):
ki2290.write("VSET " + str(voltage))
time.sleep(2) # Allow new voltage level to stabilize
currReading = ki263x.ask("print(smua.measure.i())")
time.sleep(1) # Allow measurement to be taken
# Configure the 263xB as an ammeter, set the current
limit
# and current measurement range
ki263x.write("smua.source.rangev = 0.2")
ki263x.write("smua.source.levelv = 0")
ki263x.write("smua.source.limiti = 1e-3")
ki263x.write("smua.source.autorangei = 1")
ki263x.write("smua.measure.lowrangei = 100e-9")
currReading = float(currReading)
# Configure the display of the 263xB and turn on the
output
ki263x.write("display.screen = display.SMUA")
ki263x.write("display.smua.measure.func = display.
MEASURE_DCAMPS")
ki263x.write("smua.source.output = smua.OUTPUT_ON")
ki2290.write("VSET 0")
# Define sweep variables for the programmed output
voltage
# and measured current readings
voltage = 0
currReading = ""
currRdgList = []
12
currRdgList.append(currReading)
voltage = voltage + 100
# Set the voltage of the 2290 to 0V and turn off its
output
ki2290.write("HVOF")
# Turn off the output of the Model 263xB
ki263x.write("smua.source.output = smua.OUTPUT_OFF")
# Print the current measurements
print "Sweep Complete. Current Measurements: ",
currRdgList
www.keithley.com
Creating Multi-SMU Systems with
High Power System SourceMeter®
Instruments
Introduction
The design and configuration of test systems for DC
characterization of power semiconductor devices using high
voltage and high current source measurement units (SMUs)
involves several steps:
• Selecting equipment to meet test demands
• Selecting cabling and fixturing to connect the instruments to
the device under test (DUT)
• Verifying system safety and instrument protection
• Optimizing the instrument setup to ensure
measurement integrity
• Controlling the instrumentation hardware
Power semiconductor discrete devices are designed so that
in the ON state, a device delivers a lot of power to the load
and consumes minimal power from the power source (high
efficiency); in its OFF state, the device delivers nearly zero power
to the load and consumes minimal power from the power source
(standby current is small). Therefore, characterization or DC
parametric test of power semiconductors can be broken into
two categories: ON-state and OFF-state characterization. This
application note considers the application of test to these two
categories. Specific examples of test systems built with a variety
of Keithley SourceMeter® Source Measurement Unit (SMU)
instruments will also be presented.
Select Equipment to Meet Test Demands
Power devices typically require high power instrumentation at
only one or two terminals. For example, characterizing the OFFstate of a high voltage n-channel MOSFET requires a high voltage
supply at the drain; all other terminals are driven with lower
voltage supplies. Conversely, when characterizing the ON-state
performance, high current flows from drain to source, thereby
requiring that only those two terminals be rated for maximum
power. Test researchers who are making the transition from
testing lower power devices to higher power devices can reuse
some of their existing test equipment at the gate and substrate
terminals. Being able to use the same test equipment for multiple
devices allows users to maximize their return on investment.
In order to select appropriate test equipment, it’s essential
to know the minimum and maximum current and voltages that
will be necessary to source and measure. If at all possible, select
equipment that has the capability to extend beyond these values
in order to accommodate the development of new devices.
Keithley’s Series 2650A and 2600B SMUs were designed
with evolving test systems in mind. The TSP-Link® inter-unit
communication bus supports creating mainframe-less systems
while still allowing sub-microsecond synchronization of multiple
SMU channels.
One of the most powerful features of the Series 2650A
and 2600B is the ability it offers to build systems that address
all of the application’s test requirements while maintaining
seamless system performance. The Series 2650A and 2600B
families includes 11 models that offer a variety of functions and
capabilities:
• Up to 50A pulse at 2000W (100A possible with two SMUs)
• Up to 3kV source at 60W, 1500V at 180W
• Sub-picoamp measurement capability
• Up to 1A or 3A DC on lower-power SMUs. This is ideal when
testing high power BJTs with large base currents.
This level of capability is generally unavailable in an offthe-shelf commercial test mainframe and would have once
required configuring a custom or semi-custom ATE. Moreover,
using stand-alone instruments allows the test engineer to add
new capabilities as new test needs evolve. Stand-alone high
power SMUs can extend the current and voltage capabilities of
semiconductor parametric analyzers and, therefore, the scope of
devices that can be tested.
Selecting Cabling and Fixturing to Connect
the Instruments to the Device
Determine the Interface to the Device
In the past, most power semiconductor manufacturers had to
package a device in order to test it because there was no widely
available technology that allowed delivering tens of amps or
thousands of volts to a device on wafer.
The availability of commercial prober solutions is allowing
many manufacturers to seize the opportunity to lower their cost
of test by testing devices on wafer.
Deciding whether to test packaged devices or devices on
wafer is a balance between the large capital costs of a prober
versus the smaller (but repeated) costs of packaging devices
prior to test. Keithley solutions apply to both packaged test and
wafer-level testing.
For testing packaged devices, system developers should
take advantage of commercial test fixtures, paying attention
to the supported device packages and any opportunity for
customization. Keithley offers the Model 8010 High Power
www.keithley.com13
Test Fixture, which is equipped to measure devices in TO-220
and TO-247 packages using supplied and optional device test
boards. Additionally, a device test board supplied with the Model
8010 allows customization for connecting to a variety of device
packages. System developers can create a custom circuit or can
use clips and an insulating block to connect to a device in an
unsupported package.
The Model 8010 supports the following Keithley SMUs:
• Up to two Model 2651A High Current High Power
SourceMeter Instruments (connected in parallel to allow up
to a 100A pulse)
• One Keithley Model 2657A High Voltage High Power
SourceMeter Instrument
• Up to two of the following SMUs: Models 2611/12B, Models
2635/6B, Model 4200-SMU, or Model 4210-SMU
Overvoltage protection circuitry for the lower voltage SMUs is
installed in the Model 8010 so that instruments are protected in
the event that a device failure results in an overvoltage from the
Model 2657A. With the Model 8010, SMUs may be connected to
up to three terminals on the device.
For some, a custom fixture may be necessary to handle
packages or device types/terminals that commercial fixtures
don’t support. In such cases, consider the following guidelines:
• Plan for the variety of device package types to be tested.
• Find or design a socket that is rated for the maximum
voltages and currents in the system.
• For high current testing, use a socket that has Kelvin
connections. This ensures that four-wire connections are
made all the way to the device pins and provides a true
measure of the device characteristics without voltage errors
induced by lead resistance in the socket.
• For high voltage testing, ensure that the socket is built with
high quality insulators to ensure that measurements can be
made to sufficiently low current levels.
• Use triaxial connections if possible for high voltage testing.
For the Model 2657A High Power System SourceMeter
Instrument, use the Keithley Model HV-CA-571-3 panel-mount
HV triax to unterminated cable assembly to create a test
fixture with high voltage triaxial connections. Ensure that
cables are properly terminated for high voltage creepage and
clearance specifications.
which results in devices with lower ON resistance. Knowledgeable
prober vendors have experience with a variety of chuck materials
and chuck designs in order to achieve low contact resistance during
ON-state characterization. In addition, they will have sufficient
expertise in obtaining good low current measurements through
the large chuck surface and will be able to provide guidance on
selecting the appropriate cabling to meet specific needs.
Connecting the Instrumentation
Connections from the instrumentation to the device under test
(DUT) are crucial to obtaining meaningful results. When using
the Keithley Model 8010 High Power Test Fixture, connecting
the instrument using Keithley supplied cables is straightforward.
Review Figure 22 in the “Example Systems” section of this
document for a connection diagram. If connections are
made to a prober or to a custom fixture instead, keep these
guidelines in mind:
1.Select the best cables for high current test.
Ensure that the cables used during test are rated for the
maximum current in the test system. Use cables designed
to achieve performance required for the high current,
low voltage measurements commonly encountered during
ON-state characterization.
In high current testing, pay attention to lead resistance
and lead inductance to avoid voltage source and
measurement errors.
Lead resistance
Some power devices have ON resistances in the range
of a few milliohms. Therefore, lead resistance can be on
the same order as the parameter under test. When high
current is applied, a small amount of lead resistance can
result in voltage errors. Small amounts of offset or noise
in the voltage measurement can lead to large errors in the
ON-resistance result.
Note the example of an RdsOn measurement of a
MOSFET setup in Figure 1, which depicts the instrument
configuration. Figure 2 illustrates how the lead resistance
is large relative to the device resistance and how the lead
resistance results in an 80% measurement error.
• Connect the enclosures of conductive fixtures to safety earth
ground. Ensure that non-conductive fixture enclosures are
rated to a voltage that’s twice the maximum voltage of the
test system. Follow all other safety precautions.
To eliminate errors, use separate cables for the voltage
measurement as depicted in Figure 3; use additional cables
to connect from the sense terminals of the instrument to
the DUT. The test current travels in one set of cables and
the voltage measurement is made through the sense lines in
which nearly zero current is flowing.
For testing devices on wafer, select a prober vendor with
experience in testing with high voltage and high current on wafer.
Power semiconductor devices are typically vertical devices with high
voltage or current connections terminals on the backside of the
wafer. This vertical orientation allows device designers to achieve
higher breakdown voltages. The wafers are typically ultra-thin,
In high current testing, four-wire connections, also called
Kelvin connections, are a must for accurate low voltage
and low resistance measurements. To maintain good
measurements, monitor force lead resistance to avoid
exceeding the SMU’s specification for the maximum voltage
drop between the force and the sense leads.
14
www.keithley.com
Lead inductance
Excessive inductance results in voltage overshoots when there
is a large change in current over a short period of time (V =
L[di/dt]). This is especially important for pulse testing, when
dt can be small. Excessive inductance requires the instrument
to force more voltage in order to achieve the desired voltage
at the DUT.
VM
Id
SMU
A
Vgs
Keithley Model 2651A-KIT-1 cables are designed to have
low resistance and inductance and are recommended
whenever the Model 2651A High Power System SourceMeter
Instrument is used.
2.Select the best cables for high voltage test.
Ensure that cables used during test are rated for the
maximum voltage in the test system. Use cables designed to
achieve the performance required for the high voltage, low
current measurements common in OFF-state characterization.
SMU
Figure 1. Typical instrument configuration for measuring RdsOn of a
power MOSFET.
In high voltage testing, ensure sufficient insulation and
minimize the effect of leakage currents and system
capacitance.
R HI ~ 6mΩ
HI
Vg > Vth
Device is ON
Proper insulation
Vm
Id =
40A
RdsON
= 15mΩ
SMU
LO
R LO ~ 6mΩ
Measured Resistance = RHI + RdsON + RLO = 27mΩ (80% error)
Use a cable rated for at least the maximum voltage of the
test system. Use high quality insulators in test fixtures to
achieve low current measurements. The insulation resistance
is in parallel with the resistance of the DUT and induces
measurement errors. (See Figure 4.) With the Model 2657A,
up to 3kV may appear across the test circuit, so the amount
of current generated through these insulators can be
significant relative to the current to be measured through the
DUT. For good results, ensure that the insulation resistance
is several orders of magnitude higher than the resistance
of the DUT.
Figure 2. Resistance of test leads is large relative to the DUT resistance.
Because the voltage measurement is made at the instrument’s output
terminals in a two-wire configuration, the measurement includes the sum of
the test lead resistance and the DUT resistance.
IDUT
IM
R DUT
HI
R HI ~ 6mΩ
Vg > Vth
Device is ON
SMU
Isense
~ 0A
RdsON
= 15mΩ
A
HI
SHI
RL
IL
Metal Mounting Plate
Insulating
Standoff
Vm
LO
Id =
40A
SLO
SMU
LO
R LO ~ 6mΩ
Measured Resistance = Vm/Id.
Since Isense ~ 0A, then Vm = Vds.
Measured Resistance = RdsON = 15mΩ
Figure 3. Use separate test leads to connect the device to the instrument’s
sense terminals. In this way, the voltage measured is only that across the
device. The resulting resistance measurement will be a true measurement of
the DUT resistance.
RL
IM = IDUT + I L
Figure 4. The current generated in the insulators affects the DUT current
measurement. To minimize measurement error, ensure that the resistance of
insulators (R L) is much higher than the resistance of the device (R DUT ).
Leakage currents and system capacitances
Use guarding to minimize the effect of insulators in the test
circuit. Guard is a low impedance terminal that is at the
same potential as the high impedance terminal. In Figure
4, the leakage from the insulators is still present, even with
www.keithley.com15
high quality insulators. This leakage may be problematic
when measuring currents in the nanoamp range. Note
how guarding improves the measurement in Figure 5. The
leakage currents are routed away from the high impedance
measurement terminal (HI), which eliminates them from
the measurement.
Note: Capacitance of second insulator is
irrelevant when circuit is floating.
IDUT
IM
To minimize settling times and leakage current, carry the
guard from the SMU all the way to the device pins. Doing
so eliminates the need to charge other system capacitances.
Because the guard voltage can be up to 3kV, make sure
that the guard is terminated a safe distance away from
other conductors.
R DUT
HI
RL
R
Metal Mounting Plate
x1
SMU
RL
V HI
A
C
C
Insulating
Standoff
Guard
Insulation
Inner
Shield
R
Outer
Shield
TRIAX CABLE
(outer jacket)
Insulation
A
VG
LO
x1
IM = IDUT
Figure 5. Guard is used to reduce leakage currents by reducing the voltage
across the insulators in the circuit to nearly 0V. Any remaining leakage is
routed away from the HI terminal where the measurement occurs.
Because the guard terminal is at the same potential as the
high impedance terminal, the guard voltage is a hazardous
voltage. Use triaxial cabling to carry guard and protect
operators from risk of an electric shock. In a triaxial cable,
the high impedance terminal is connected to the center
conductor, guard is carried on the inner shield, and the outer
shield is connected to ground. Figure 6 shows the crosssection of a triaxial cable.
Guarding can also minimize the effect of system capacitances.
System capacitances impact the settling of the voltage
source and the current measurement. The test setup must
allow for the charging of such capacitances and the settling
of currents at or below the expected noise floor of the
device measurement. The high impedance nature of these
setups results in long settling times. Figure 6 illustrates
how guarding can reduce the impact of cable capacitance.
A typical triaxial cable has a capacitance of about 40pF per
foot. This can quickly result in several hundred picofarads
of capacitance for a cable that is two or three meters long,
and tens of milliseconds of voltage settling time, depending
on the maximum current in the test setup. Placing guard on
the inner shield of the cable means there is no voltage drop
across the cable’s insulator. Therefore, the capacitance of
this insulator does not need to be charged. In steady-state
conditions, the Model 2657A is specified so that the guard
voltage is within 4mV of the high impedance terminal (HI).
The Keithley Model HV-CA-554 is a high voltage triaxial cable
that safely carries signal and guard voltages up to 3280V.
Keithley Model HV-CA-554 cables are designed to meet
the demands of a 3kV, low current measurement system.
16
Common
Figure 6. When V HI ≈ VG , the voltage drop across the capacitor and resistor
is 0V. Guarding virtually eliminates leakage current due to cable insulation
and minimizes response time by eliminating the need to charge cable
capacitance.
Converting to coaxial connection is a necessity in some
systems. For high voltage testing, SHV is the industry
standard coaxial connector. Keithley offers Model
SHV-CA-553 cable assemblies that allow adapting from
high voltage triaxial to SHV. These cable assemblies use a
triaxial cable so that the guard is carried as far as possible
before it must be terminated so that the connection can be
made to SHV. Using coaxial connections results in degraded
performance because the benefits of guarding are lost from
the point at which the guard is terminated. This means that
the remaining cable capacitance and the capacitance within
the test system must be charged.
When designing a test fixture, a user can take steps to
minimize the capacitances by shortening the length of the
cabling and device connections after the conversion to coax
from triax.
The effects of converting to coax become even more
significant on a probe station, where cabling and connections
are a function of the size of the wafer and device orientation
(vertical or lateral). When cable capacitances are taken into
account, capacitances in a probe station can easily be on the
order of nanofarads, resulting in significant capacitive charge
time and measurement settling time.
3.Establish a common reference for the SMUs.
Few test system issues are as misunderstood as grounding.
Here, a “ground” is defined as a connection to earth ground.
www.keithley.com
However, many people often use the word “ground” to
refer to the reference point for the SMUs in the test circuit.
In this note, this reference point will be referred to as
“circuit common.”
Earth ground
For safety, most systems have a connection to earth ground
to ensure that any faults within the instrument or test system
do not expose the user to the risk of electric shock. For
similar reasons, in high voltage systems, connect conductive
test fixtures and their associated accessories to earth ground.
Circuit common
Identifying circuit common is important in order to obtain
accurate source values and measurements. When connecting
multiple supplies to a DUT, it is important that the supplies
be referenced to the same point so that the desired voltages
appear at each DUT terminal. Consider the example shown
in Figure 7.
A
A
Vgs
Vds
Circuit
Common
Figure 7. When using stand-alone instruments, the outputs must have the
same reference so that the correct voltages and current appear at the device
under test (DUT). In this example, the source terminal of the FET must
be connected both to the LO of the gate supply and the LO of the voltage
supply so that VGS and V DS are accurate. Because the LO terminals of both
instruments are connected to the source terminal, this becomes circuit (or
measurement) common.
Performance of the device is specified based on the
relationship between V DS and VGS. We will consider
connections to circuit common in the light of the two test
configurations: ON-state characterization with a Model 2651A
SMU on the drain and OFF-state characterization with a
Model 2657A SMU on the drain.
Creating circuit common when performing ON-state
characterization using the Model 2651A high current SMU
The section titled “Selecting Cabling and Fixturing to
Connect the Instruments to the Device” addresses why
four-wire connections are required for the high current
instrument. Four-wire connections are also recommended for
the SMU connected to the base of a BJT or gate of a MOSFET
or an IGBT, even when there is little current flowing through
the gate. Let’s examine the reasons for this recommendation
as it relates to the connections to circuit common.
Note the measurement configuration in Figure 8 where
ON-state characterization is performed on a power MOSFET.
This configuration could apply for generating a family of
curves for the MOSFET. Circuit common is created when
the LO of the gate SMU (SMU 1) is tied to the LO of the
drain SMU (SMU 2). Because little to no current is flowing
in the gate-to-source loop, the gate SMU is measuring and
correcting the output voltage based on the measurement
at its force terminals, which is the difference between
the voltage at the gate terminal and the voltage at circuit
common, denoted as node S´ (prime) in Figure 8. Circuit
common is tied to the Source terminal of the FET (node S
in Figure 8) through a test lead that has a resistance R slead.
Because a high current (up to 50A pulse) is flowing in the
drain-to-source loop, we cannot ignore R slead. Even a 1mW
resistance here may cause a difference between VGS and
VGS´ (prime) of 50mV. Some devices are very sensitive to
changes in the gate-to-source voltage. A 50mV difference in
VGS may cause hundreds of milliamps or even an amp or
more difference in the drain current. To compensate for the
voltage drop between the connection in circuit common and
the actual device terminal, connect the sense terminals of
the gate SMU separately to the DUT as shown in Figure 9 1.
Because nearly zero current is flowing in the sense leads,
the gate SMU accurately measures the device voltage at the
FET’s Source terminal and can correct its output voltage to
maintain the desired VGS at the device.
In some cases, the response of the gate SMU must be slowed
to compensate for ringing or oscillation in the gate circuit.
This is done when high capacitance mode is enabled on the
gate SMU. However, this slower response time can slow the
feedback between the measurement of the sense voltage and
the correction of the output voltage. In such cases, connect
both the LO and Sense LO terminals of the gate SMU to the
Sense LO of the drain SMU. Because little to no current is
flowing in the gate-to-source loop, no voltage measurement
errors occur. However, this should not be done when testing
BJTs because significant current can flow between base
and emitter.
Creating circuit common when performing OFF-state
characterization using the Model 2657A high voltage SMU
For OFF-state characterization, connections between the gate
and drain SMUs and the DUT are made as shown in Figure 7.
If four-wire connections are desired, then simply connect the
Sense LO terminals of the gate and drain SMUs.
Device faults can result in the presence of high voltages
at lower voltage terminals. Therefore, the connection to
the gate, source, and substrate must be made with high
1 As nearly zero current flows in the sense circuit, the Sense LO of the gate SMU can
be tied to the Sense LO terminal of the drain SMU without any impact on the voltage
measurements.
www.keithley.com17
2651A’s Sense LO through the Model 2657A-LIM-3 so that it
is common with the other SMUs in the test setup. Connect
the Output LO terminal of the Model 2657A-LIM-3 to the LO
terminal of the Model 2651A as close as possible to the DUT.
These connections are illustrated in detail in Figure 21 in the
section of this note titled “Example Systems.”
HI
SHI
D
A
Vm
G
HI
Feedback
S
SLO
LO
A
SMU 2
Vm
Feedback
R slead
Id
SMU 1
LO
S’
Figure 8. Because high current flows through circuit common, the resistance
between circuit common and the FET’s source terminal (R slead ) will result
in voltage differences between measurements made at circuit common
and measurements made at the source terminal of the FET. Therefore VGS ≠
VGS ’ (prime) when two-wire connections are used to connect the gate SMU
(SMU 1) to the DUT.
HI
D
SHI
A
Vm
G
HI
Feedback
S
SHI
SLO
A
LO
SMU 2
Vm
Output LO
R slead
Feedback
SMU 1
Id
SLO
LO
S’
Figure 9. Using four-wire connections to the gate SMU negates the voltage
errors that arise because of R slead. In this way, the gate SMU can correct its
output voltage to maintain the desired VGS .
voltage connectors. To facilitate connections between the
LO and Sense LO of the two instruments, Keithley offers the
Model 2657A-LIM-3 LO Interconnect Module as an optional
accessory. The LO and Sense LO of three SMUs can be easily
routed through the Model 2657A-LIM-3. Additional SMUs can
be routed with slight modifications in connections. (Consult
Keithley’s online FAQs or Keithley field applications engineers
for more details.)
Creating circuit common for systems that will use both
Models 2651A and 2657A High Power SMUs
Given that complete testing of a power semiconductor device
involves both ON-state and OFF-state characterization, it is
likely that a test setup will involve both the Model 2651A and
the Model 2657A. To ensure the integrity of measurements
in both configurations, route the Model 2651A’s LO terminal
separately from the device under test. Route the Model
18
When performing ON-state characterization to devices
on wafer, the connections recommended in the previous
paragraph would result in three probe needles touching
down on the pad that connects to the Source terminal of the
FET. However, making three connections may be a problem
because there may be insufficient space on the pad for three
probe needles and because pad lifetime decreases as the
number of probe touchdowns increases. This problem can
be resolved by using the autosense resistor in the Model
2657A-LIM-3. The autosense resistor connects the output
Sense LO of the Model 2657A-LIM-3 to the output LO of the
Model 2657A-LIM-3 through a 100kW resistor. (See Figure
10.) Although true four-wire connections are not maintained
to the device under test, there is little impact on the voltage
delivered to the FET or IGBT gate terminal because the gate
current is small and the Model 2651A’s LO is routed through
a separate connection to circuit common2. Figure 21 in the
“Example Systems” section of this application note assumes
that the autosense resistor of the Model 2657A-LIM-3 is used
to connect to its Output LO and Sense LO terminals.
100kΩ
Output
Sense LO
Figure 10. In the Model 2657A-LIM-3, a 100kW resistor is used to connect
Output Sense LO to Output LO. This allows for quasi-Kelvin sense
connections in cases where there is insufficient space to place four
connections on the DUT. In cases where full Kelvin is desired, simply connect
separate cables to Output Sense LO and Output LO, and the 100kW resistor is
essentially ignored.
Reviewing System Safety and
Instrument Protection
When considering cabling and fixturing design, it is also
important to consider system safety. Think through a variety
of fault conditions, including those due to operator error and
those due to a change in the device state, in order to determine
what dangers are presented to the operator and to the
instrumentation.
One of the potential dangers of high current testing is the
risk of fire or burns. Enclose the device under test to protect
users from the potential for fire or flying projectiles if the device
fails during high current tests.
2If true four-wire connections are not made to the DUT, voltage errors may appear when
testing BJTs with high base currents.
www.keithley.com
The risk of electric shock is one potential danger for high
voltage testing. The risk of electric shock exists anytime an
instrument (or device) has the ability to output more than
42VDC. Proper test system configurations must include
mechanisms for protecting the operator and untrained users
from electric shock.
While power is applied, such users should be unable to access
systems in which high voltage is present. The safety interlock is
one mechanism for limiting access. All modern Keithley SMUs
include an interlock so that high power is only enabled when the
interlock line is engaged. Interlock is intended to be used with
a normally open switch at each system access point. When there
are multiple instruments that present a risk of electric shock,
wire the instruments so that the outputs of all instruments are
disabled if the system access point is opened. When there are
multiple access points, each point requires a separate switch and
all such switches should be wired in series. This is illustrated in
Figure 11.
The 5V supplies from all instruments are combined,
externally routed through the switch and used to enable the
interlock. When the 5V supplies from multiple SMUs are used,
use Schottky diodes to prevent the 5V supplies from each of
the SMUs from back-driving each other. Schottky diodes are
preferred because they have low forward voltage; therefore,
they are less likely to affect the voltage needed to enable the
interlock line.
lines of the other SMUs in the system. Review the instrument
specifications for the safety interlock pin to determine the
amount of current required to drive the interlock pin of each
SMU. Also review the specifications for the 5V power supply
pin to determine its current capability. Alternatively, an
external power supply may be used to drive the interlock lines
of the SMUs.
In addition to operator safety, it is important to protect
the investment in the system’s test instrumentation. Carefully
consider the effect of potential device failures. The test
configuration shown in Figure 12 is typical for measuring OFFstate leakage current of an n-channel FET near its specified
drain-to-source breakdown voltage. The lower voltage SMU at
the gate terminal ensures that the device is in the OFF state.
The lower voltage SMU at the source terminal provides a direct
measurement of the current at the source terminal. However, if
there is a sudden breakdown from drain to source, then there
is potential for a 200V SMU to be damaged by the 3kV SMU.
The same potential for damage exists between the gate and
drain terminals.
A
1000V
Model 2657A
SMU
(3000 V max)
Depending upon the number of SMUs in the test setup, the
5V supply from one SMU may be able to drive the interlock
0V
Model 2636B
SMU
(200 V max)
Schottky Diodes
A
A
0V
Model 2636B
SMU
(200 V max)
+5V
SMU 1
INTLK
Figure 12. In the event of a device breakdown or failure, there is potential for
the Model 2657A SMU to damage either Model 2636B SMU.
GND
+5V
SMU 2
INTLK
GND
+5V
SMU 3
INTLK
GND
+5V
SMU 4
INTLK
GND
System Access Point #1
(use normally-open
switch to enable or
disable the interlock)
System Access Point #2
(use normally-open
switch to enable or
disable the interlock)
Figure 11. Configuration for connecting multiple SMUs to a system with
multiple access points.
Use an overvoltage protection device to protect the lower
voltage SMU in the event of device breakdown or device failure.
The overvoltage protection device should have little effect on the
test circuit under normal conditions but will clamp the voltage
during an overvoltage condition. The Keithley Model 2657A-PM200 200V Protection Module is designed for use in test systems
that contain both the Model 2657A SMU and lower voltage
SMUs. In the event of an overvoltage, the protection module will
clamp the external high voltage source to approximately 2V in
microseconds. In an unclamped condition, picoamp-level current
source and measure capability is maintained on the lower
voltage SMU3.
Instrument damage can also occur when device failures
subject the instruments to high current. Ensure that the
3The overvoltage protection of the Model 2657A-PM-200 protection module is triggered at
~ 220V. Therefore, it is not recommended for use in SMUs with maximum output voltage
capability of less than 200V. Additionally, the Model 2657A-PM-200 is not recommended
for use in protecting the HI and LO terminals of the Model 2651A. The protection module
is not rated for the maximum output current of the Model 2651A.
www.keithley.com19
Make sure to select a resistor rated for the maximum current,
voltage, and power in the test system. Such resistors often serve
a dual purpose. During ON-state characterization of transistors,
resistors are often added in series with the gate to minimize the
oscillation and ringing in the gate voltage commonly seen in
high gain devices. In addition, during OFF-state characterization,
resistors are often used to limit the maximum current that can
flow during a breakdown, preventing premature device failure.
Optimizing the Instrument Setup to
Obtain Good Measurements
Once the system setup is complete, it‘s time to test its
functionality and optimize instrument setup to obtain the best
measurements.
For ON-state Characterization
In most ON-state testing performed today, the device is pulsed
on in order to minimize heating. In addition, the end application
for many power semiconductor devices involves operating
under pulsed conditions. Qualify the test system for pulsing
by outputting a pulse of the SMU through the test system and
capturing the response at the device terminals. The Model 2651A
High Power System SourceMeter instrument includes high speed
analog-to-digital converters (ADC) that allow the simultaneous
digitization of current and voltage waveforms. These ADCs are
useful in determining system pulse performance. If anomalies
in the pulse shape occur, review cabling to make sure that
lead inductance is minimized. Use the Keithley-supplied low
impedance coaxial cabling wherever possible and minimize the
inductive loop area of other leads.
For OFF-state Characterization
Understand the source and measure settling time of the system.
High voltage sources are generally used to perform OFF-state
measurements of the device, where currents are low and
the device is in a high impedance state. However, there may
be system capacitances that cannot be guarded out and the
device itself will have some capacitance. Power semiconductor
transistors typically have output capacitances on the order of
100pF or more. Device resistance in the OFF state can be 1GW or
more, resulting in a single RC time constant of 100 milliseconds
20
or longer. Note the plot of voltage vs. time for a charging
capacitor in Figure 13. In order to produce settled readings, it’s
important to wait at least four to six time constants (four time
constants = 99%), which may mean nearly a second of settling
time is required. Account for this settling time in estimating
production throughput or the time required for long tests, such
as device reliability testing.
Voltage vs. Time – Capacitive Charging, C = 100pF, R = 1GΩ
1000
900
800
700
Voltage (V)
instruments at all terminals are capable of handling currents
normally present at the specific device terminals. To limit
instrument damage when device failure results in higher
currents, use series resistors to limit the maximum current
that can flow into any instrument. During characterization of
breakdown voltage or leakage current, the user must be careful
to limit the amount of current through the device so that the
testing is not destructive. Although SMUs have programmable
current limits, these active limits take some finite time to
respond to changes in load (known as the “transient response
time.”) When the load impedance changes very quickly, then
currents above the programmable limit may flow. Adding a series
resistor enforces a maximum current at that terminal even under
transient conditions.
600
500
400
300
200
100
1T
0
0
2T
0.2
3T
4T
0.4
5T
6T
0.6
0.8
1.0
1.2
Time (s)
Figure 13. A simulated plot of voltage vs. time for a 100pF capacitor while it
is being charged. Readings are settled at four to six time constants. T = RC
time constant.
If coaxial connections are used instead of triaxial, be aware
that additional settling time will be required to obtain accurate
and repeatable low current measurements. In addition to the
RC of the device, the settling time must now include the RC
time constant of the cable and the probe station or fixture.
Characterize the total settling time by applying a voltage step
at the system input and measuring the current decay over time
at the output. The fast analog-to-digital converter (ADC) of the
Model 2657A or Model 2651A affords the user a quick and easy
way to capture settling time information. Select a measurement
delay by observing the time at which the current falls below the
expected noise floor for the device measurement.
Controlling the Instrumentation Hardware
Coordinating the source and measure sequence of the device
under test is not trivial when multiple instruments are in use.
Therefore, it is advantageous to use available software solutions
to simplify and/or eliminate extensive programming.
Use the supplied free start-up software to validate the test
system configuration and functionality. Series 2600B and Series
2650A SMUs are provided with TSP® Express software, free
start-up software that is served from the web interface of the
instrument. When the TSP-Link interface is used to create a
network of instruments, TSP Express software can be used to
control all of the instruments on the network. See Figure 14 for
an example diagram of the communication setup between three
SMUs. Using TSP Express software, the user can easily set up DC
www.keithley.com
and pulse sweeps on one or more SMUs and have other SMUs
assigned to output static bias voltage or program a step change in
bias voltages at each sweep. Built-in simplified graphing software
allows the user to plot results quickly and assess whether all
cabling, connections, and parameters have been correctly
configured. Figure 15 and Figure 16 illustrate the capabilities
TSP Express software.
Controller
LAN
Model 2636B SMU
(TSP-Link Node #1)
TSP-Link
Model 2651A High
Power SMU
(TSP-Link Node #2)
Model 2657A High
Power SMU
(TSP-Link Node #3)
Figure 14. TSP-Link technology provides mainframe-less system expansion
and enables communication between the instruments, precision timing, and
tight channel synchronization.
TSP Express software can also access the fast ADC capability
of the Model 2651A and Model 2657A high power SMUs. Use TSP
Express software to output a single pulse from the Model 2651A
through the test system and capture its response. These results
can be used to set the required source and measure delays to
ensure that data is taken during the settled portion of the pulse.
Simplify the testing of discrete semiconductor devices by
using software designed for parametric test. Such software
includes test libraries with pre-defined tests that simplify
gathering data for various devices. Keithley’s ACS Basic Edition
software is recommended for semiconductor device testing
with multiple-SMU systems in which the device is contacted
using a manual prober or test fixture. Keithley’s ACS Standard
software is recommended when interfacing with devices using a
semiautomatic or automatic prober.
Example Systems
This section provides detailed connections for several example
configurations. For more information on how to apply any of
these examples to a specific application, please contact your
local Keithley field applications engineer. Worldwide contact
information is available at the end of this application note or on
www.keithley.com.
Packaged Device Testing Using a Test Fixture
The Keithley Model 8010 High Power Device Test Fixture
completes the solution for testing power semiconductor
devices with Model 2651A and Model 2657A High Power System
SourceMeter instruments. Figure 22 depicts the connections
from the instrumentation to the Model 8010. Example device
testing configurations are considered in detail in the Model 8010
Interconnect Reference Guide (IRG), which can be downloaded
from www.keithley.com.
Custom test fixtures can be designed to incorporate
any number of SMUs for testing. For high current testing,
connections may be easily adapted from the Phoenix screwterminal connectors provided on the Model 2651A. For
highest integrity high voltage measurements using the Model
2657A, Keithley offers a bulkhead version of the custom HV
triax connector, already assembled with a triax cable that is
unterminated at one end. It is designed to be installed in a safe
enclosure. Connections can easily be soldered to the device or
adapted to another connector appropriate for the setup. This is
illustrated in Figure 17. The legend in Figure 18 applies to this
custom fixture connection example.
Wafer-Level Device Testing on a Probe Station
A few probe station vendors offer commercial solutions for
high power semiconductor device testing and other vendors
create custom solutions. Consult with a probe station vendor to
determine the types of probing solutions available for the current
voltage, and power required for a specific test application.
Prober manufacturers can contact Keithley Instruments for
information on how to use our custom high voltage triaxial cable
and connector.
As part of a recent review of probe station solutions, we
found that banana plugs and jacks are commonly used for high
current testing and that SHV is an industry standard for high
current connections. The high current Phoenix screw-terminal
connectors can be easily adapted to banana connectors. To adapt
to SHV solutions, Keithley offers the Model SHV-CA-553, a cable
assembly that has a high voltage triaxial connector on one end
and an SHV (coaxial) connector on the other end. The diagrams
that follow illustrate three example configuration connections to
a wafer-level device using banana and SHV connections.
The legend in Figure 18 applies to the wafer-level device
connection examples.
www.keithley.com21
Figure 15. TSP Express software allows quick setup of source and measurement parameters of all instruments connected over the TSP-Link network.
Figure 16. TSP Express software allows quick and easy plotting of measurements, such as the Vces vs. Ic plot shown here for an IGBT with a rated
BVces of 2500V.
22
www.keithley.com
Custom Test Fixture
Model
2657A
(HV Triax)
SHI
(HV Triax)
HI
(HV Triax)
SLO /
LO
Interlock
Relay
LO
SMU 1 Sense
LO and LO
Inputs
DIO
(HV Triax)
Earth
Ground
Model
2657A-PM-200
Protection Module
Model
2635B /
2636B
SMUA
(Ties together LO of
all input SMUs.
Ties together SLO
of all input SMUs)
Common
LO Output
(HV Triax )
(HV Triax)
---- SH----
SHI
(HV Triax)
(HV Triax)
Std. Triax
---- HI ----
HI
SMU 3 Sense
LO and LO
inputs
(HV Triax)
---- SLO / LO ----
Std. Triax
SLO /
LO
SMU 2 Sense
LO and LO
inputs
Model 2657A-LIM-3
Interlock
Out
Std. Triax
(HV Triax)
LO
Earth
Ground
DIO
Common
Sense LO
Output
(HV Triax )
Part # CA-558-2 (supplied with Model 2657A-LIM-3)
26xx
Interlock
Inputs
(includes 5V
from SMU
DIO port)
Part # CA-573-3 (supplied with Model 2657A-LIM-3)
Earth
Ground
4200
Interlock
Inputs
(includes 12V
from 4200)
Figure 17. Custom fixture connection example for high voltage testing.
Model 7078-TRX Standard 3-Lug Triax
Cable Assembly (Male to Male)
Model HV-CA-554 HV Triax Cable
Assembly (Male to Male)
Model SHV-CA-553 HV Triax to SHV
Cable Assembly (Male to Male)
Model HV-CA-571-3 HV Triax
Female Bulkhead Jack to
Unterminated Cable Assembly
Panelmount SHV connector
Banana Jack
Standard 3-lug triax feedthrough
Banana Plug
Model 237-TRX-BAN Triax to Banana Cable
Figure 18. Legend for wafer-level device connection example diagrams.
www.keithley.com23
Probe Station
Chuck
Connection Ports
Model
2657A
SHI
HI
SLO /
LO
LO
DIO
Earth
Ground
Model
2635B /
2636B
SMUA
Model
2657A-PM-200
Protection Module
SHI
(HV Triax)
---- SH---(HV Triax)
SMU 1 Sense
LO and LO
Inputs
---- HI ----
HI
(HV Triax)
---- SLO / LO ----
SLO /
LO
(HV Triax)
SMU 2 Sense
LO and LO
inputs
LO
Earth
Ground
DIO
Model
2635B /
2636B
SMUA
Model
2657A-PM-200
Protection Module
SHI
(Ties together LO of
all input SMUs.
Ties together SLO
of all input SMUs)
Common
LO Output
(HV Triax )
(HV Triax)
Interlock
Out
(HV Triax)
26xx
Interlock
Inputs
(includes 5V
from SMU
DIO port)
(HV Triax)
(HV Triax)
---- SLO / LO ----
LO
Part # CA-573-3 (supplied with Model 2657A-LIM-3)
(HV Triax)
---- HI ----
SLO /
LO
Interlock
Relay
SMU 3 Sense
LO and LO
inputs
---- SH---HI
Model 2657A-LIM-3
Common
Sense LO
Output
(HV Triax )
Earth
Ground
Earth
Ground
4200
Interlock
Inputs
(includes 12V
from 4200)
DIO
Part # CA-558-2 (supplied with Model 2657A-LIM-3)
Figure 19. Connections to wafer-level device for high voltage only testing using SHV connections.
SHI
SHI
G
G
G
N/C
G
SLO
N/C
Model
2600TRIAX
Probe Station
N/C
2651A-KIT-1
LO
Model
2635B /
2636B
SMUA
Chuck
Connection Ports
SLO
HI
Model
2651A
Model 237-TRX-BAN Triax to Banana Cable
HI
LO
DIO
SHI
HI
SLO /
LO
Interlock
Relay
LO
DIO
Figure 20. Connections to wafer-level device for high current only testing using banana connections.
24
www.keithley.com
Chuck
Connection Ports
Model
2657A
Probe Station
SHI
HI
Manual
Switch
SLO /
LO
LO
Model
2657A-PM-200
Protection Module
DIO
Earth
Ground
---- SH----
(HV Triax)
SHI
SHI
G
G
G
N/C
G
SLO
N/C
Model
2600TRIAX
---- HI ---N/C
---- SLO / LO ---SLO
CA-552 (supplied with 2651A-KIT)
LO
Model
2635B /
2636B
SMUA
(HV Triax)
HI
HI
Model
2651A
SMU 1 Sense
LO and LO
Inputs
(HV Triax)
Safety ground
connections not shown
SMU 2 Sense
LO and LO
inputs
LO
Model 2657A-LIM-3
Common
Sense LO
Output
(HV Triax )
(Ties together LO of
all input SMUs.
Ties together SLO
of all input SMUs)
Interlock
Relay
Common
LO Output
(HV Triax )
(HV Triax)
DIO
SHI
Model
2657A-PM-200
Protection Module
SMU 3 Sense
LO and LO
inputs
Interlock
Out
(HV Triax)
(HV Triax)
---- SH---HI
26xx
Interlock
Inputs
(includes 5V
from SMU
DIO port)
(HV Triax)
---- HI ---(HV Triax)
SLO /
LO
LO
---- SLO / LO ----
Earth
Ground
Part # CA-573-3 (supplied with Model 2657A-LIM-3)
Earth
Ground
4200
Interlock
Inputs
(includes 12V
from 4200)
DIO
Part # CA-558-2 (supplied with Model 2657A-LIM-3)
Figure 21. Connections to wafer-level device for high voltage and high current test.
Figure 22. Connections from SMU instruments to Model 8010 High Power Device Test Fixture.
www.keithley.com25
26
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Simplifying FET Testing with Series
2600B System SourceMeter® SMU
Instruments
Introduction
Field effect transistors (FETs) are important semiconductor
devices with many applications because they are fundamental
components of many devices and electronic instruments. Some
of the countless applications for FETs include their use as
amplifiers, memory devices, switches, logic devices, and sensors.
The most commonly used FET is the MOSFET, which is the basis
for digital integrated circuits.
Characterizing FETs’ current-voltage (I-V) parameters
is crucial to ensuring they work properly in their intended
applications and meet specifications. Some of these I-V tests
may include gate leakage, breakdown voltage, threshold voltage,
transfer characteristics, drain current, on-resistance, etc. FET
testing often involves the use of several instruments, including
a sensitive ammeter, multiple voltage sources, and a voltmeter.
Programming and synchronizing multiple instruments, however,
can be tedious and time consuming. The use of a turnkey
semiconductor characterization system is one alternative
approach that solves the integration problem and offers other
important benefits, but systems of this type typically cost tens
of thousands of dollars. A third approach involves using Source
Measurement Units (SMUs) to perform parameter testing on FETs
and other semiconductor devices. An SMU is an instrument that
can quickly and accurately source and measure both current
and voltage. The number of SMUs required in the test usually
depends on the number of FET terminals that must be biased
and/or measured.
This application note explains how to simplify I-V
measurements on FETs using a Series 2600B System SourceMeter
SMU Instrument with the TSP® Express embedded software tool.
The Series 2600B family of one- and two-channel SMUs offers a
range of instruments ideal for electrical characterization of FETs
that can source and measure over a wide range of voltage and
current. These SMUs have current resolution to 0.1fA and can
be current limited to prevent damage to the device. The TSP
Express software tool simplifies performing common I-V tests on
FETs and other semiconductor devices, without programming or
installing software. Figure 1 illustrates a typical test setup.
Field Effect Transistors
The field effect transistor is a majority charge-carrier device in
which the current-carrying capability is varied by an applied
electric field. The FET has three main terminals: the gate, the
drain, and the source. A voltage applied to the gate terminal (VG)
Figure 1. Model 2636B with the TSP Express software tool generating a drain
family of curves on a MOSFET
controls the current that flows from the source (IS) to the drain
(ID) terminals.
The many types of FETs include the MOSFET (metal-oxidesemiconductor), MESFET (metal-semiconductor), JFET (junction),
OFET (organic), GNRFET (graphene nano-ribbon), and CNTFET
(carbon nanotube). These FETs differ in the design of their
channels. Figure 2 illustrates the MOSFET, CNTFET, and JFET.
Gate
Source
n–
SiO2
p+
MOSFET
Drain
n–
Source
CNT
Gate
Drain
Source
p+
SiO2
Si Gate
n–
Carbon Nanotube FET
JFET
Drain
Figure 2. MOSFET (insulated gate), CNTFET (carbon nanotube channel), and
JFET (junction FET)
Using a Series 2600B SMU for FET Testing
A FET’s I-V characteristics can be used to extract many
parameters about the device, to study the effects of fabrication
techniques and process variations, and to determine the quality
of the contacts. Figure 3 illustrates a DC I-V test configuration
for a MOSFET using a two-channel Series 2600B SMU. In this
configuration, the Force HI terminal of Channel 1 (SMU CH1) is
connected to the gate of the MOSFET and the Force HI terminal
Channel 2 (SMU CH2) is connected to the drain. The source
terminal of the MOSFET is connected to the Force LO terminals
of both SMU channels or to a third SMU if it is necessary to
source and measure from all three terminals of the MOSFET.
www.keithley.com27
MOSFET
Drain
Gate
Force HI
Force HI
Source
A
A
SMU
CH1
Force LO
SMU
CH2
Force LO
Figure 3. MOSFET test configuration using two SMU channels
Once the device is set up and connected to the SMU,
the control software must be configured to automate the
measurements, such as with the TSP Express software tool
embedded into Series 2600B SMUs. As shown in Figure 4, it’s
simple to connect the instrument to any computer with the
supplied Ethernet cable. Entering the IP address of the SMU into
the URL line of any web browser will open the instrument’s
web page. From that page, the user can launch TSP Express and
configure the desired test using the project wizard. These tests,
or projects, can be saved and recalled for future use.
Series 2600B
Figure 5. Screen capture of MOSFET drain family of curves measured by the
two-channel Model 2636B using TSP Express software
Supplied
Ethernet cable
Figure 6. MOSFET I-V curve data displayed in a table in TSP Express
Any computer with
any web browser
Drain Current vs. Gate Voltage
1.00E+00
1.00E–01
Figure 4. Connecting a Series 2600B SMU to a computer
A common I-V test performed on a MOSFET is the drain
family of curves (V DS-ID). With this test, SMU CH1 steps the
gate voltage (VG) while SMU CH2 sweeps the drain voltage
(V D) and measures the resulting drain current (ID). Once the
two SMUs are configured to perform the test, the data can be
generated and plotted on the screen in real time. Figure 5 shows
a screen capture of a MOSFET drain family of curves created by
using the two-channel Model 2636B SMU with the TSP Express
software. This I-V data can be exported directly to a .csv file
with a single button click, then imported into a spreadsheet for
further analysis.
The data in the software can also be displayed in a table view
(Figure 6). For each SMU channel, the current, voltage, and time
appear in the table. This data can also be saved to a file.
Without changing connections to the device, the TSP
Express software allows users to perform other common I-V
FET tests such as the Drain Current (ID) as a function of Gate
28
Drain Current (A)
1.00E–02
1.00E–03
1.00E–04
1.00E–05
1.00E–06
1.00E–07
1.00E–08
1.00E–09
1.00E–10
1.00E–11
1.00E–12
0
1
2
3
4
5
6
Gate Voltage (V)
Figure 7. Drain current as a function of the gate voltage of a MOSFET
generated by the Model 2636B and the TSP Express software
Voltage (VG). For this test, the gate voltage is swept and the
resulting drain current is measured at a constant drain voltage.
Figure 7 shows the results of an ID-VG curve at a constant
drain voltage. However, in this case, the generated data was
exported to a file and plotted on a semi-log graph. This test
www.keithley.com
can be easily reconfigured to step the drain voltage as the gate
voltage is swept.
The ID-VG data shows the many decades of drain current that
were measured by the Model 2636B, from 1E-12 to 1E-2 amps.
The Model 2636B is a low current SMU with 0.1fA resolution,
so it’s suitable for other low current FET test applications
such as gate leakage current measurements. When measuring
low current, it is important to employ low level measurement
techniques, such as shielding and guarding, to prevent errors.
Further information on optimizing low current measurements is
available in Keithley’s Low Level Measurements Handbook.
Conclusion
Parameter testing of FETs is often performed using
multiple Source Measurement Units (SMUs). To simplify I-V
characterization of FETs, Series 2600B SMUs with embedded
TSP Express software allow generating I-V sweeps quickly and
easily. With their wide dynamic range of current and voltage
sourcing and measuring, Series 2600B SMUs are ideal tools for
semiconductor device characterization.
www.keithley.com29
30
www.keithley.com
Measuring Pulsed Waveforms with the High Speed
Analog-to-Digital Converter in the Model 2651A
High Power System SourceMeter® Instrument
For these inherently pulsed applications, it is important to
test the discrete components that make up the end product
under pulsed conditions. Test instruments with only DC
capability can deliver an amount of power to a device that causes
enough heat dissipation to alter its characteristics. The desire to
measure the true state of the device without the effects of selfheating is another motivation for pulsed characterization.
The use of a pulsed stimulus demands faster measurements.
The Model 2651A meets this need with its high speed ADCs
(analog-to-digital converters). Coupled with the ability to
measure asynchronously from the source, this feature makes
the Model 2651A suitable for many transient characterization
applications. The following demonstrates how to configure
the ADCs to perform measurements on pulsed waveforms and
considers techniques to obtain optimal results.
High Speed ADC vs. Integrating ADC
Traditional precision SMUs (source-measure units) use
integrating ADCs. An integrating ADC averages the signal over
a certain time interval known as the integration time. Figure 1
depicts a simplified dual-slope integrating ADC. This type of
ADC operates by charging a capacitor with the unknown signal
and then discharging the capacitor using a reference voltage. The
ratio of the charge and discharge times is proportional to the
ratio of the unknown signal to the reference signal.
While having the advantage of high accuracy and excellent
noise immunity, this ADC technology does not lend itself to
high speed measurements. The charge-discharge cycles on
the capacitor result in long inter-measurement intervals. For
example, though the smallest integration interval for the Model
2651A is 0.001PLC (16.67μs for 60Hz, 20μs for 50Hz), the smallest
inter-measurement interval is 50μs.
In addition to the two integrating ADCs for voltage and
current, the Model 2651A also includes two high speed ADCs
with the capability of sampling signals at burst rates of up to
C
R
Vin
–
Vref
To other timing,
control, and
comparison
circuitry
+
Figure 1. Simplified diagram of dual-slope integrating ADC.
1MHz.1 These ADCs use sampling technology similar to an
oscilloscope and take snapshots of the signal over time. A high
speed ADC of the Model 2651A has higher resolution (18 bits)
than an oscilloscope (typically 8 bits) resulting in more precise
transient characterization in comparable bandwidths.
Figure 2 illustrates the difference between the integrating
and high speed ADCs. While returning more readings,
the measurements performed by the high speed ADC are
less accurate and less repeatable than those performed by
the integrating ADC. For applications that demand higher
throughput, the lower accuracy can be tolerated, or if needed,
improved by averaging several readings. Typically, integrating
2.15
Sampled Data
Integrating ADC Data
2.10
2.05
Volts (V)
Green initiatives and energy efficiency standards worldwide
have motivated engineers to find ways to design more efficient
semiconductor devices and integrated circuits. In industrial
applications, engineers are trying to improve the efficiency of
switching power supplies and power inverters. In commercial
and residential applications, the push for LEDs (light emitting
diodes) drives the design of AC-DC converters to make these DC
devices operate on AC power and use pulse width modulation as
a light dimming technique.
2.00
1.95
1.90
1.85
0
200
400
600
800
1000
1200
Time (µs)
Figure 2. Comparison of possible results from integrating and sampling ADC technologies.
1Up to 5,000 readings can be acquired at the maximum acquisition rate.
www.keithley.com31
ADC measurements with integration rates of 0.01PLC or faster
can be made with similar accuracy using the high speed ADC.
Having two high speed ADCs ensures that voltage and
current measurement can be made simultaneously. The ability
to sample current is a unique feature of the Model 2651A and
may replace the need for a current probe and an oscilloscope in
some applications.
The combination of the Model 2651A’s high speed ADCs and
trigger model supports precisely timed measurements on pulsed
signals. Additionally, the Model 2651A introduces a feature that
allows the user to trigger measurements asynchronously from
source operations, such as before, during, or after a pulse. This
capability can also be used with the integrating ADCs. Figure 3
diagrams five examples of pulsed signals and measurements
that can be made with the Model 2651A. The sections below
discuss how to configure the Model 2651A to perform each of
these examples.
Figure 4 illustrates the relationship between a pulse, the
triggering conditions that create the pulse, and the definition of
high speed ADC measurement parameters.
General Information Regarding the Examples
In the sections that follow, each of the examples makes use of
a few common functions. The code for these functions is in
Appendix A.
Example #1: Digitizing the Top of a Pulse
a.
d.
Potential Uses
For some applications, such as thermal impedance of power
diodes and LEDS, characterizing the slope of the measured
voltage at the top of the pulse is important. This capability is
also useful for characterizing pulse amplitude flatness. The
high speed ADCs can digitize the top of the pulse when the
measurements are made synchronously with the source.
Averaging
Filter
b.
e.
How the Measurement is Made
The pulses are timed using trigger timers. Measurements are
triggered at the beginning of the pulse, but delayed to the settled
part of the pulse by programming a measure delay.
c.
The trigger model setup is depicted in Figure 5. The Test
Script Processor (TSP®) script for Example #1 is located in
Appendix B along with the commands for executing the test
and obtaining the results. The sample results taken using a 0.1W
resistor are shown in Figure 6.
Figure 3a.Example #1, measuring at the top of the pulse.
Figure 3b.Example #2, performing a spot mean measurement at the top of
the pulse.
Figure 3c. Example #3, digitizing the entire pulse.
Figure 3d.Example #4, triggering measurements to begin before the pulse.
Figure 3e.Example #5, triggering measurement to begin after the pulse.
Example #2: Performing a Spot Mean
Measurement at the Top of the Pulse
Potential Uses
Often, analysis software is used to average sampled data to
improve accuracy. The Model 2651A can automatically perform
averages on measurements.
Measurement
Interval
How the Measurement is Made
The averaging and median filters of the Model 2651A can be used
on the high speed ADC readings, making it possible to return
spot mean measurements. The same test performed in Example
#1 can be modified to return a spot mean measurement instead
of the raw sample data by changing a few lines of code.
Measure
Delay
Measurement
Trigger
Source Action
EndPulse
Trigger
Action Trigger
Time
Figure 4. Specifying a pulse using the Model 2651A.
32
The trigger model configuration is the same as used for
Example #1. Appendix C contains the TSP script for Example
#2 and the commands for executing the test and obtaining the
results. The test results are shown in Figure 6 next to the raw
sample data and are also listed in Table 1.
www.keithley.com
digio.trigger[1]
User asserts
line in script
smua.trigger
Idle
EVENT_ID
IDLE_EVENT_ID
mode = digio.TRIG_FALLING
stimulus
Arm
Layer
SWEEPING_EVENT_ID
No
Yes
trigger.timer[1]
stimulus
period
EVENT_ID
arm.stimulus
count =
passthrough = false pulseCount – 1
Arm
Count
>1?
Sweep
Armed
Trigger
Layer
ARMED_EVENT_ID
SWEEP_COMPLETE_EVENT_ID
trigger.timer[2]
stimulus
pulseWidth
passthrough = false
EVENT_ID
EndSweep
Action
count = 1
No
Yes
source.stimulus
Trigger
Count
>1?
Source
Action
SOURCE_COMPLETE_EVENT_ID
User-configured
connections
Static trigger model
connections
measure.stimulus
Measure
Action
MEASURE_COMPLETE_EVENT_ID
endpulse.stimulus
EndPulse
Action
PULSE_COMPLETE_EVENT_ID
Figure 5. Trigger model configuration for Examples #1 and #2.
Table 1. Spot mean measurement results for Example #2.
Current (A)
Voltage (V)
19.7816
1.97854
19.8421
1.98492
Measuring at the Top of the Pulse
3.0
25
2.5
20
How the Measurement is Made
Timers are again used to trigger the start and end of the pulse.
The trigger used to start the pulse is also used to start the
measurement process.
Current (A)
Potential Uses
At times, it is useful to characterize how a pulse is transmitted
through a device or system. These applications require that the
entire pulse be digitized, including the rising and falling edges.
This measurement is possible using the high speed ADCs to
measure asynchronously to the source operation.
2.0
15
1.5
Digitized Current (A)
Spot Mean of Current
Digitized Voltage (V)
Spot Mean of Voltage
10
Voltage (V)
Example #3: Digitizing the Entire Pulse
Including the Rising and Falling Edges
1.0
5
0.5
0
0
50
100
150
200
250
0.0
300
Time (µs)
Figure 6. Results of Examples #1 and #2. The load is a 0.1W resistor.
www.keithley.com33
digio.trigger[1]
User asserts
line in script
smua.trigger
Idle
EVENT_ID
IDLE_EVENT_ID
mode = digio.TRIG_FALLING
stimulus
Arm
Layer
SWEEPING_EVENT_ID
No
Yes
trigger.timer[1]
stimulus
period
EVENT_ID
arm.stimulus
count =
passthrough = false pulseCount – 1
Arm
Count
>1?
Sweep
Armed
Trigger
Layer
ARMED_EVENT_ID
SWEEP_COMPLETE_EVENT_ID
trigger.timer[2]
stimulus
pulseWidth
passthrough = false
EVENT_ID
EndSweep
Action
count = 1
No
Yes
source.stimulus
Trigger
Count
>1?
Source
Action
SOURCE_COMPLETE_EVENT_ID
endpulse.stimulus
EndPulse
Action
User-configured
connections
Static trigger model
connections
PULSE_COMPLETE_EVENT_ID
Asynchronous Measurement
measure.stimulus
Measure
Action
MEASURE_COMPLETE_EVENT_ID
Figure 7. Trigger model configuration for Example #3.
Digitize Entire Pulse
Figure 7 diagrams the trigger model configuration.
Appendix D contains the TSP script for Example #3 and the
commands for executing the test and obtaining the results. The
results for the example data are shown in Figure 8.
How the Measurement is Made
This example is arranged so that the user can specify how long
before the pulse the measurements should occur. Timers are
34
2.5
20
2.0
Current (A)
Potential Uses
Pulses are sometimes used to provide power stresses to the
device. It is useful to note the device state before the stress is
applied. This can be done by programming a pulse with a nonzero idle level and triggering the measurements before the pulse.
Current
Voltage
15
1.5
10
Voltage (V)
Example #4: Pre-Pulse Characterization:
Triggering Measurements before the Pulse
3.0
25
1.0
5
0.5
0
0
50
100
150
200
250
0.0
300
Time (µs)
Figure 8. Results from Example #3 showing the voltage and current
measurements as performed by the high speed ADCs. There is a 0.1W
resistive load.
www.keithley.com
digio.trigger[1]
smua.trigger
Idle
EVENT_ID
IDLE_EVENT_ID
mode = digio.TRIG_FALLING
stimulus
Arm
Layer
SWEEPING_EVENT_ID
No
stimulus
period
Arm
Count
>1?
Yes
trigger.timer[1]
EVENT_ID
count =
passthrough = false pulseCount – 1
arm.stimulus
Sweep
Armed
Trigger
Layer
ARMED_EVENT_ID
SWEEP_COMPLETE_EVENT_ID
trigger.timer[2]
stimulus
prePulseTrig
passthrough = false
EVENT_ID
EndSweep
Action
count = 1
No
Yes
trigger.timer[3]
stimulus
pulseWidth
passthrough = false
EVENT_ID
source.stimulus
Trigger
Count
>1?
Source
Action
count = 1
SOURCE_COMPLETE_EVENT_ID
endpulse.stimulus
EndPulse
Action
User-configured
connections
Static trigger model
connections
PULSE_COMPLETE_EVENT_ID
Asynchronous Measurement
measure.stimulus
Measure
Action
MEASURE_COMPLETE_EVENT_ID
Figure 9. Trigger model configuration for Example #4.
used to program the start of the measurement and the beginning
and end of the pulse.
Triggering Measurements Before the Pulse
20
The trigger model configuration for this example is shown in
Figure 9. Appendix E contains the TSP script of Example #4 and
the commands for executing the test and obtaining the results.
The results for the example code are shown in Figure 10.
Potential Uses
When using pulse testing to stress a device, the device must
also be characterized after the stress is applied. This is typically
done by sourcing a pre-defined test voltage or current after the
pulse. The test level is chosen so as not to cause any additional
thermal or electrical stress to the device. The measurement can
be made by sourcing a pulse with a non-zero idle level and using
the high speed ADCs to perform the measurement. The results
18
16
Current (A)
14
5
4
12
3
10
8
Voltage (V)
Example #5: Post-Pulse Characterization:
Triggering Measurements after the Pulse
6
Current
Voltage
Simulated Voltage Pulse
2
6
4
1
2
0
0
1000
2000
3000
4000
5000
6000
0
7000
Time (µs)
Figure 10. Results from Example #4. There is a 0.5W resistive load.
www.keithley.com35
digio.trigger[1]
User asserts
line in script
smua.trigger
Idle
EVENT_ID
IDLE_EVENT_ID
mode = digio.TRIG_FALLING
stimulus
Arm
Layer
SWEEPING_EVENT_ID
No
stimulus
period
Arm
Count
>1?
Yes
trigger.timer[1]
EVENT_ID
arm.stimulus
count =
passthrough = false pulseCount – 1
Sweep
Armed
Trigger
Layer
ARMED_EVENT_ID
SWEEP_COMPLETE_EVENT_ID
trigger.timer[2]
stimulus
pulseWidth
passthrough = false
EVENT_ID
EndSweep
Action
count = 1
No
Yes
source.stimulus
Trigger
Count
>1?
Source
Action
SOURCE_COMPLETE_EVENT_ID
endpulse.stimulus
EndPulse
Action
User-configured
connections
Static trigger model
connections
PULSE_COMPLETE_EVENT_ID
Asynchronous Measurement
measure.stimulus
Measure
Action
MEASURE_COMPLETE_EVENT_ID
Figure 11. Trigger model configuration for Example #5.
from the high speed ADCs indicate how the device recovers from
the stress.
Figure 11 diagrams the trigger model configuration for this
example. Appendix F contains the TSP script of Example #5 and
the commands for executing the test and obtaining the results.
The results for the example code are shown in Figure 12.
Getting Pulses and Measurements on Time
The ability to pulse in pulse-only current and voltage regions
coupled with the ability to make measurements with the high
36
6
Current
Voltage
Simulated Voltage Pulse
18
5
16
Current (A)
14
4
12
3
10
8
Voltage (V)
How the Measurement is Made
The pulses are timed using trigger timers. The measurement
is triggered by the EndPulse event of the trigger model,
which causes the pulse to return to the idle level. If desired,
a measurement delay can be used to postpone the start of the
measurements until after the falling edge occurs.
Triggering Measurements After the Pulse
20
2
6
4
1
2
0
–100
0
1000
2000
3000
4000
5000
6000
0
7000
Time (µs)
Figure 12. Results from Example #5. There is a 0.5W resistive load.
www.keithley.com
speed ADCs means that the user must carefully consider test
timing. In the Model 2651A, the taking of measurements is given
priority over source and display operations. Therefore, display
updates may not occur and source timing can be compromised if
the system is busy making or processing measurements. Consider
the following precautions in order to avoid erratic pulse timing.
These precautions can also help to prevent damaging devices
from the excessive power that can be generated from the
Model 2651A’s ability to output very high currents in pulse and
DC regions.
• For precise timing, always use fixed ranges for source and
measurement functions. (Note: Asynchronous measurements
mandate the use of fixed ranging for both the integrating and
high speed ADCs.)
• When operating in the extended pulse-only operation region,
obey all maximum duty cycle and pulse width limitations.
The instrument will turn off the output to prevent thermal
runaway. However, continuing to issue triggers for source
or EndPulse actions may result in source and/or EndPulse
action overruns.
• Sustained high speed data acquisition rates may result
in undesired pulse timing. This is more likely when the
measure interval for the high speed ADC is smaller than
10μs and when a pulse train has a high duty cycle. If a
particular configuration results in undesired pulse timing,
then abort the present test and perform one of the following
to obtain the desired timing: reduce the measurement trigger
frequency, reduce the measurement count, or increase the
measurement interval.
of the status model according to the trigger object in which
the action overrun is generated. (Each time a register is read,
all bits are cleared.) For the SMU trigger object, an action
overrun occurs when a new input trigger is detected before the
previously triggered action has been started2. Figure 13 shows
the SMU trigger overrun operation status register set. Note that
each action block (Arm, Source, Measure, and EndPulse) has
a corresponding overrun bit in the status model. Appendix G
contains the TSP script demonstrating how to monitor the Model
2651A’s operation overrun Event register for overruns to the
Source, Measure, or EndPulse action blocks.
Each of the SMU action blocks latches (or remembers) one
trigger, even if it cannot immediately act upon that trigger. An
action overrun is not generated by the latched trigger. However,
if multiple triggers are issued while an action is in process, then
overruns are generated. For instance, if the Model 2651A is busy
making a large number of high speed measurements, it may
not be able to respond to the EndPulse event. The first time the
EndPulse event is triggered, the SMU will hold off ending the
pulse and the result is a pulse with a longer width than expected.
If the SMU is still making or processing measurements when the
next EndPulse event is triggered, an EndPulse action overrun is
generated and the next pulse does not end.
Operation Status SMU A
Trigger Overrun Register
Arm Overrun (ARM)
Source Overrun (SRC)
Measure Overrun (MEAS)
End Pulse Overrun (ENDP)
• In asynchronous operation, all expected measurements must
be triggered before the End Sweep action occurs. Refer to
Example #5 for an example of how to use a measure delay to
postpone the start of measurements after the trigger.
• Any source polarity changes during a sweep incur a 100μs
delay before the source level is set. The number 0 is
considered a positive value.
–– For negative-going pulses that start from zero, use a
‘negative zero’, which is a negative number that is very near
zero, e.g., -1e-12. See the code for Examples #1, #2, and #3.
–– When the source polarity must be changed during a sweep,
the user must account for the polarity change delay in the
TSP script in order to obtain proper pulse timing.
A user can monitor the state of the instrument using its
status model. Action overruns set bits in different registers
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
To Trigger Overrun Bit
(TRGOVR) in
Operation Status SMU A
Summary Register
(status.operation.instrument.smua)
status.operation.instrument.smua.trigger_overrun
Figure 13. SMU trigger overrun operation status register set.
Conclusion
The high speed ADCs in Keithley’s Model 2651A provide a more
detailed look at measurements of pulsed waveforms. They can
be used for a variety of transient characterization applications
previously not possible with an SMU.
2For asynchronous measurement mode, a measurement action overrun is generated each
time a new measurement is triggered while the SMU is taking measurements. No triggers
are latched for asynchronous measurements.
www.keithley.com37
Appendix A. Common Functions, runPulse() and printData()
The examples in the following appendices use common functions. The TSP scripts for these functions are listed below. These lines of
code may be copied, saved, and run as a script so that the functions are available.
--Runs the pulse after it is configured
function runPulse()
smua.source.output = 1
delay(0.001)
smua.trigger.initiate()
digio.trigger[1].assert()
waitcomplete()
smua.source.output = 0
end
--Prints the results from the reading buffers.
function printData()
if smua.nvbuffer1.n == 0 then
print(“No readings in buffer”)
else
print(“Timestamps\tCurrent\tVoltage”)
for i = 1, smua.nvbuffer1.n do
print(string.format(“%g\t%g\t%g”, smua.nvbuffer1.timestamps[i], smua.nvbuffer1.readings[i], smua.nvbuffer2.
readings[i]))
end
end
end
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Appendix B. Code for Example #1, Digitizing the Top of a Pulse
This is the TSP script that is used to configure the Model 2651A SourceMeter instrument to digitize the top of the pulse when the
measurements are made synchronously with the source. The commands for executing the code and obtaining the results are listed
below the TSP script.
function digitizeTopSync(iAmpl, vLimit, pulseWidth, period, pulseCount, sampleInterval, measCount, measDelay)
reset()
errorqueue.clear()
status.reset()
local l_iZero --Variable for the idle level of the sweep
--[[
The ‘if’ statement below controls the idle level of the sweep. Set to 0 for positive pulses. Set to
‘negative zero’ for
negative pulses. Eliminates polarity change delay. See “Getting Pulses and Measurements On Time” for more
information.
]]
if iAmpl <0 then
local l_iZero = -1e-13
else
local l_iZero = 0
end
-- Prepare the reading buffers
smua.nvbuffer1.clear()
smua.nvbuffer1.appendmode = 0
smua.nvbuffer1.collecttimestamps = 1
smua.nvbuffer1.collectsourcevalues = 0
smua.nvbuffer2.clear()
smua.nvbuffer2.appendmode = 0
smua.nvbuffer2.collecttimestamps = 1
smua.nvbuffer2.collectsourcevalues = 0
--Set up the source
smua.source.func = smua.OUTPUT_DCAMPS
smua.sense = smua.SENSE_REMOTE --Enables remote sense (4W measurements)
smua.source.rangei = iAmpl
smua.source.leveli = l_iZero --Sets the pulse off value.
smua.source.limitv = vLimit -- Sets the DC voltage limit
smua.trigger.source.listi({iAmpl}) --Sets the values for the source sweep
smua.trigger.source.limitv = vLimit --Sets the voltage limit
smua.trigger.source.action = smua.ENABLE --Enables the source sweep
--Set up the measurements
smua.measure.rangev = vLimit -- Set the measure voltage range
smua.measure.adc = smua.ADC_FAST --Configures the SMU to use the high speed ADC
smua.measure.interval = sampleInterval --Sets the measurement interval
smua.measure.delay = measDelay --Sets the initial measurement delay
smua.measure.count = measCount --Sets the measurement count
smua.trigger.measure.iv(smua.nvbuffer1, smua.nvbuffer2) -- Configure the SMU to measure both voltage and
current
smua.trigger.measure.action = smua.ENABLE -- Enables measurements in sync with the source sweep
--Set up a digital I/O line to trigger the start of the period timer
digio.trigger[1].mode = digio.TRIG_FALLING
digio.trigger[1].clear()
-- Timer 1 controls the pulse period by triggering the pulse to begin
trigger.timer[1].delay = period
--The timer should always have a count of 1. The ‘if’ statement below configures the timer count.
if pulseCount > 1 then
trigger.timer[1].count = (pulseCount - 1)
else
trigger.timer[1].count = 1
end
trigger.timer[1].passthrough = true --Timer issues an event at the start of the first interval.
trigger.timer[1].stimulus = digio.trigger[1].EVENT_ID -- Period timer is triggered by a digital I/O line
trigger.timer[1].clear()
-- Timer 2 controls the pulse width
trigger.timer[2].delay = pulseWidth
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trigger.timer[2].count = 1
trigger.timer[2].passthrough = false --Pulse width timer only issues event when interval elapses
trigger.timer[2].stimulus = trigger.timer[1].EVENT_ID --Timer starts when period timer issues event
trigger.timer[2].clear()
-- Return to the bias level at the end of the pulse/sweep
smua.trigger.endpulse.action = smua.SOURCE_IDLE
smua.trigger.endsweep.action = smua.SOURCE_IDLE
smua.trigger.count = pulseCount --Sets the number of pulses to generate
smua.trigger.arm.count = 1 --Sets the number of times to iterate through the entire sweep
smua.trigger.arm.stimulus = 0
smua.trigger.source.stimulus = trigger.timer[1].EVENT_ID --Source starts when period timer generates event
smua.trigger.measure.stimulus = smua.trigger.SOURCE_COMPLETE_EVENT_ID -- Start measuring when the source
action is complete
smua.trigger.endpulse.stimulus = trigger.timer[2].EVENT_ID -- Start EndPulse action as soon as the pulse
width timer ends
end
Executing the Code
Send the following lines to configure and execute two 20A, 100μs pulses with a 200μs period and a voltage limit of 10V. Fifty
measurements are requested, taken at 1μs intervals with an initial measurement delay of 50μs.
digitizeTopSync(20, 10, 100e-6, 200e-6, 2, 1e-6, 50, 50e-6)
runPulse()
Results
The raw sample data is stored in the dedicated nonvolatile buffers, smua.nvbuffer1 and smua.nvbuffer2. Use the following command
to print the data through the remote command interface:
printData()
Sample results taken using a resistor are shown in Figure 6.
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Appendix C. Code for Example #2, Performing a Spot Mean Measurement at the Top of the Pulse
Below is the example code to configure the Model 2651A to perform a spot mean measurement at the top of the pulse. Highlighting
is used to distinguish the command lines that are different from those in Example #1 (Appendix B). The commands for executing the
code and obtaining the results are listed below the TSP script.
function spotMeanTopSync(iAmpl, vLimit, pulseWidth, period, pulseCount, sampleInterval, filtCount, measDelay)
reset()
errorqueue.clear()
status.reset()
local l_iZero --Variable for the idle level of the sweep
--[[
The ‘if’ statement below controls the idle level of the sweep. Set to 0 for positive pulses. Set to
‘negative zero’ for
negative pulses. Eliminates polarity change delay. See “Getting Pulses and Measurements On Time” for more
information.
]]
if iAmpl <0 then
local l_iZero = -1e-13
else
local l_iZero = 0
end
-- Prepare the reading buffers
smua.nvbuffer1.clear()
smua.nvbuffer1.appendmode = 0
smua.nvbuffer1.collecttimestamps = 1
smua.nvbuffer1.collectsourcevalues = 0
smua.nvbuffer2.clear()
smua.nvbuffer2.appendmode = 0
smua.nvbuffer2.collecttimestamps = 1
smua.nvbuffer2.collectsourcevalues = 0
--Set up the source
smua.source.func = smua.OUTPUT_DCAMPS
smua.sense = smua.SENSE_REMOTE --Enables remote sense (4W measurements)
smua.source.rangei = iAmpl
smua.source.leveli = l_iZero --Sets the pulse off value.
smua.source.limitv = vLimit -- Sets the DC voltage limit
smua.trigger.source.listi({iAmpl}) --Sets the values for the source sweep
smua.trigger.source.limitv = vLimit --Sets the voltage limit
smua.trigger.source.action = smua.ENABLE --Enables the source sweep
--Set up the measurements
smua.measure.rangev = vLimit -- Set the measure voltage range
smua.measure.adc = smua.ADC_FAST --Configures the SMU to use the high speed ADC
smua.measure.interval = sampleInterval --Sets the measurement interval
smua.measure.delay = measDelay --Sets the initial measurement delay
smua.measure.count = 1 --Sets the number of measurements to take each time the measure action block is
triggered
smua.measure.filter.type = smua.FILTER_REPEAT_AVG
smua.measure.filter.count = filtCount
smua.measure.filter.enable = smua.FILTER_ON
smua.trigger.measure.iv(smua.nvbuffer1, smua.nvbuffer2) -- Configure the SMU to measure both voltage and
current
smua.trigger.measure.action = smua.ENABLE --Enables measurements in sync with the source sweep
--Set up a digital I/O line to trigger the start of the period timer
digio.trigger[1].mode = digio.TRIG_FALLING
digio.trigger[1].clear()
-- Timer 1 controls the pulse period by triggering the pulse to begin
trigger.timer[1].delay = period
--The timer should always have a count of 1. The ‘if’ statement below configures the timer count.
if pulseCount > 1 then
trigger.timer[1].count = (pulseCount - 1)
else
trigger.timer[1].count = 1
end
trigger.timer[1].passthrough = true --Timer issues an event at the start of the first interval.
trigger.timer[1].stimulus = digio.trigger[1].EVENT_ID -- Period timer is triggered by a digital I/O line
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trigger.timer[1].clear()
-- Timer 2 controls the pulse width
trigger.timer[2].delay = pulseWidth
trigger.timer[2].count = 1
trigger.timer[2].passthrough = false --Pulse width timer only issues event when interval elapses
trigger.timer[2].stimulus = trigger.timer[1].EVENT_ID --Timer starts when period timer issues event
trigger.timer[2].clear()
-- Return to the bias level at the end of the pulse/sweep
smua.trigger.endpulse.action = smua.SOURCE_IDLE
smua.trigger.endsweep.action = smua.SOURCE_IDLE
smua.trigger.count = pulseCount --Sets the number of pulses to generate
smua.trigger.arm.count = 1 --Sets the number of times to iterate through the entire sweep
smua.trigger.arm.stimulus = 0
smua.trigger.source.stimulus = trigger.timer[1].EVENT_ID --Source starts when period timer generates event
smua.trigger.measure.stimulus = smua.trigger.SOURCE_COMPLETE_EVENT_ID -- Start measuring when the source
action is complete
smua.trigger.endpulse.stimulus = trigger.timer[2].EVENT_ID -- Start EndPulse action as soon as the pulse
width timer ends
end
Executing the Code
Send the following lines to configure and execute two 20A, 100μs pulses with a 200μs period and a voltage limit of 10. Two
measurements are requested, each with an average of 50 readings taken at 1μs intervals with an initial measurement delay of 50μs.
spotMeanTopSync(20, 10, 100e-6, 200e-6, 2, 1e-6, 50, 50e-6)
runPulse()
Results
The raw sample data is stored in the dedicated nonvolatile buffers, smua.nvbuffer1 and smua.nvbuffer2. Use the following command
to print the data through the remote command interface:
printData()
The results are shown in Figure 6 next to the raw sample data and are also listed in Table 1.
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Appendix D. Code for Example #3, Digitizing the Entire Pulse Including the Rising and Falling Edges
The example code below lists the commands necessary to digitize one to two pulses at the maximum sample rate. (Note: To increase
the pulse count, the sample interval or pulse period must be increased to allow sufficient time for data processing and to avoid
erratic source timing. For more details, see “Getting Pulses and Measurements on Time” in this application note.) The commands for
executing the code and obtaining the results are listed after the TSP script.
function digitizePulse(iAmpl, vLimit, pulseWidth, period, pulseCount, sampleInterval)
reset()
errorqueue.clear()
status.reset()
--[[
The ‘if’ statement below controls the idle level of the sweep. Set to 0 for positive pulses. Set to
‘negative zero’ for
negative pulses. Eliminates polarity change delay. See “Getting Pulses and Measurements On Time” for more
information.
]]
local l_iZero
if iAmpl <0 then
l_iZero = -1e-13
else
l_iZero = 0
end
-- Prepare the reading buffers
smua.nvbuffer1.clear()
smua.nvbuffer1.appendmode = 0
smua.nvbuffer1.collecttimestamps = 1
smua.nvbuffer1.collectsourcevalues = 0
smua.nvbuffer2.clear()
smua.nvbuffer2.appendmode = 0
smua.nvbuffer2.collecttimestamps = 1
smua.nvbuffer2.collectsourcevalues = 0
--Set up the source
smua.source.func = smua.OUTPUT_DCAMPS
smua.sense = smua.SENSE_REMOTE --Enable remote sense (4W measurements)
smua.source.rangei = iAmpl
smua.source.leveli = l_iZero --Sets the pulse off value.
smua.source.limitv = vLimit -- Sets the DC voltage limit
smua.trigger.source.listi({iAmpl}) -- Sets the amplitude values for the pulse sweep
smua.trigger.source.limitv = vLimit --Sets the voltage limit
smua.trigger.source.action = smua.ENABLE --Enables the source sweep
--Set up the measurements
smua.measure.rangev = vLimit -- Set the measure voltage range
smua.measure.adc = smua.ADC_FAST -- Configures SMU to use high speed aDC
smua.measure.interval = sampleInterval --Set measurement interval
smua.measure.delay = 0 --Configures no initial measurement delay
-- Set the measure count to be 50% greater than the width of the pulse to ensure we capture rising and
falling edges of pulse
smua.measure.count = pulseWidth / smua.measure.interval * 1.5
smua.trigger.measure.iv(smua.nvbuffer1, smua.nvbuffer2) -- Configure the SMU to measure both voltage and
current
smua.trigger.measure.action = smua.ASYNC -- Configure SMU measurements to occur asynchrnously with source
sweep
--Set up a digital I/O line to trigger the start of the period timer
digio.trigger[1].mode = digio.TRIG_FALLING
digio.trigger[1].clear()
-- Timer 1 controls the pulse period by triggering the pulse to begin
trigger.timer[1].delay = period
--The timer should always have a count of 1. The ‘if’ statement below configures the timer count.
if pulseCount > 1 then
trigger.timer[1].count = (pulseCount - 1)
else
trigger.timer[1].count = 1
end
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trigger.timer[1].passthrough = true --Timer issues an event at the start of the first interval.
trigger.timer[1].stimulus = digio.trigger[1].EVENT_ID -- Period timer is triggered by a digital I/O line
trigger.timer[1].clear()
-- Timer 2 controls the pulse width
trigger.timer[2].delay = pulseWidth
trigger.timer[2].count = 1
trigger.timer[2].passthrough = false --Pulse width timer only issues event when interval elapses
trigger.timer[2].stimulus = trigger.timer[1].EVENT_ID --Timer starts when period timer issue event
trigger.timer[2].clear()
-- Return to the bias level at the end of the pulse/sweep
smua.trigger.endpulse.action = smua.SOURCE_IDLE
smua.trigger.endsweep.action = smua.SOURCE_IDLE
smua.trigger.count = pulseCount -- Configures number of pulses to complete
smua.trigger.arm.count = 1
smua.trigger.arm.stimulus = 0
smua.trigger.source.stimulus = trigger.timer[1].EVENT_ID -- Trigger source sweep using events generated by
period timer
smua.trigger.measure.stimulus = trigger.timer[1].EVENT_ID -- Start measuring when the period timer starts
the source action
smua.trigger.endpulse.stimulus = trigger.timer[2].EVENT_ID -- Start EndPulse action as soon as the pulse
width timer ends
end
Executing the Code
Send the following lines to configure and execute a single 20A, 200μs pulse with a voltage limit of 10V and take high speed ADC
measurements at 1μs intervals.
digitizePulse(20, 10, 200e-6, 2e-3, 1, 1e-6)
runPulse()
Results
The raw sample data is stored in the dedicated nonvolatile buffers, smua.nvbuffer1 and smua.nvbuffer2. Use the following command
to print the data through the remote command interface:
printData()
The results for the example data are shown in Figure 8. The load is a 0.1W resistor.
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Appendix E. Code for Example #4, Pre-Pulse Characterization:
Triggering Measurements before the Pulse
Below is the TSP script for Example #4. The commands for executing the code and obtaining the results are listed after the
TSP script.
function measThenPulse(vPulse, vIdle, iLimit, pulseWidth, period, pulseCount, prePulseTrig, measCount,
sampleInterval)
reset()
errorqueue.clear()
status.reset()
--If the programmed pulse is negative and biased from zero, then program negative zero
if vIdle == 0 and vPulse <0 then
vIdle = -1e-12
else
vIdle = vIdle
end
-- Prepare the reading buffers
smua.nvbuffer1.clear()
smua.nvbuffer1.appendmode = 0
smua.nvbuffer1.collecttimestamps = 1
smua.nvbuffer1.collectsourcevalues = 0
smua.nvbuffer2.clear()
smua.nvbuffer2.appendmode = 0
smua.nvbuffer2.collecttimestamps = 1
smua.nvbuffer2.collectsourcevalues = 0
--Set up the source
smua.source.func = smua.OUTPUT_DCVOLTS --Set source function to DC Volts
smua.sense = smua.SENSE_REMOTE -- Enable remote sense (4W measurements)
smua.source.rangev = vPulse
smua.source.levelv = vIdle --Sets the pulse off value.
--The ‘if’ statement below sets the DC current limit
if iLimit > 20 then
smua.source.limiti = 20
else
smua.source.limiti = iLimit
end
smua.trigger.source.listv({vPulse}) -- Configures amplitude levels for source sweep
smua.trigger.source.limiti = iLimit --Sets current limit during pulsing
smua.trigger.source.action = smua.ENABLE --Enables source sweep
--Set up the measurements
smua.measure.rangei = iLimit -- Set the measure current range
smua.measure.adc = smua.ADC_FAST -- Configures SMU to use high speed ADC
smua.measure.interval = sampleInterval -- Sets measurement interval
smua.measure.delay = 0
smua.measure.count = measCount
smua.trigger.measure.iv(smua.nvbuffer1, smua.nvbuffer2) -- Configure the SMU to measure both voltage and
current
smua.trigger.measure.action = smua.ASYNC -- Configures measurements to occur asynchrnously from source sweep
--Set up a digital I/O line to trigger the start of the period timer
digio.trigger[1].mode = digio.TRIG_FALLING
digio.trigger[1].clear()
-- Timer 1 controls the start of the measurement and serves as system clock
trigger.timer[1].delay = period
--Timer must always have a count of 1 or more. The following ‘if’ statement sets the timer count
appropriately.
if pulseCount > 1 then
trigger.timer[1].count = (pulseCount - 1)
else
trigger.timer[1].count = 1
end
trigger.timer[1].passthrough = true --Timer issues an event at the start of the first interval.
trigger.timer[1].stimulus = digio.trigger[1].EVENT_ID -- Period timer is triggered by a digital I/O line
trigger.timer[1].clear()
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-- Timer 2 controls the delay from the start of measurement to the start of the pulse
trigger.timer[2].delay = prePulseTrig
trigger.timer[2].count = 1
trigger.timer[2].passthrough = false --Only issue event when timer interval has elapsed
trigger.timer[2].stimulus = trigger.timer[1].EVENT_ID
trigger.timer[2].clear()
--Timer 3 controls the pulse width
trigger.timer[3].delay = pulseWidth
trigger.timer[3].count = 1
trigger.timer[3].passthrough = false -- Only issue event when timer interval has elapsed
trigger.timer[3].stimulus = trigger.timer[2].EVENT_ID
trigger.timer[3].clear()
-- Return to the bias level at the end of the pulse/sweep
smua.trigger.endpulse.action = smua.SOURCE_IDLE
smua.trigger.endsweep.action = smua.SOURCE_IDLE
smua.trigger.count = pulseCount --Sets number of pulses to perform
smua.trigger.arm.count = 1
smua.trigger.arm.stimulus = 0
smua.trigger.source.stimulus = trigger.timer[2].EVENT_ID -- Start next pulse once prePulseTrigger time
interval has elapsed
smua.trigger.measure.stimulus = trigger.timer[1].EVENT_ID -- Start measuring when the period timer issues an
event
smua.trigger.endpulse.stimulus = trigger.timer[3].EVENT_ID -- Start EndPulse action as soon as the pulse
width timer ends
end
Executing the Code
Send the following lines to configure and execute three pulses which pulse from 1 to 5V with a width of 300μs and a period of 3ms
with a current limit of 20A. Ten measurements taken at 10μs intervals are taken 100μs before the pulse begins.
measThenPulse(5, 1, 20, 300e-6, 3e-3, 3, 100e-6, 10, 10e-6)
runPulse()
Results
The raw sample data is stored in the dedicated nonvolatile buffers, smua.nvbuffer1 and smua.nvbuffer2. Use the following command
to print the data through the remote command interface:
printData()
The results for the example code are shown in Figure 10. The load is a 0.5W resistor.
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Appendix F. Code for Example #5, Post-Pulse Characterization:
Triggering Measurements after the Pulse
Below is the TSP script that can be used to program the Model 2651A to perform Example #5. The commands for executing the code
and obtaining the results are listed after the TSP script.
function pulseThenMeas(vPulse, vIdle, iLimit, pulseWidth, period, pulseCount, measDelay, measCount, sampleInterval)
errorqueue.clear()
reset()
status.reset()
--If the programmed pulse is negative and biased at zero, then program negative zero
if vIdle == 0 and vPulse <0 then
vIdle = -1e-12
else
vIdle = vIdle
end
-- Prepare the reading buffers
smua.nvbuffer1.clear()
smua.nvbuffer1.appendmode = 1
smua.nvbuffer1.collecttimestamps = 1
smua.nvbuffer1.collectsourcevalues = 0
smua.nvbuffer2.clear()
smua.nvbuffer2.appendmode = 1
smua.nvbuffer2.collecttimestamps = 1
smua.nvbuffer2.collectsourcevalues = 0
--Set up the source
smua.source.func = smua.OUTPUT_DCVOLTS --Set source function to DC Volts
smua.sense = smua.SENSE_REMOTE -- Enable remote sense (4W measurements)
smua.source.rangev = vPulse
smua.source.levelv = vIdle --Sets the pulse off value.
--The ‘if’ statement below sets the DC current limit
if iLimit > 20 then
smua.source.limiti = 20
else
smua.source.limiti = iLimit
end
smua.trigger.source.listv({vPulse}) -- Configures amplitude levels for source sweep
smua.trigger.source.limiti = iLimit --Sets current limit during pulsing
smua.trigger.source.action = smua.ENABLE --Enables source sweep
--Set up the measurements
smua.measure.rangei = iLimit -- Set the measure current range
smua.measure.adc = smua.ADC_FAST --Configures SMU to use high speed ADC
smua.measure.interval = sampleInterval -- Sets measurement interval
smua.measure.delay = measDelay --Set the time from the end of the pulse to the start of measurements using
measure delay
smua.measure.count = measCount --Sets number of measurements
smua.trigger.measure.iv(smua.nvbuffer1, smua.nvbuffer2) -- Configure the SMU to measure both voltage and
current
smua.trigger.measure.action = smua.ASYNC -- Configures measurements to occur asynchronously with the source
sweep
--Set up a digital I/O line to trigger the start of the period timer
digio.trigger[1].mode = digio.TRIG_FALLING
digio.trigger[1].clear()
-- Timer 1 controls the start of the measurement and serves as system clock
trigger.timer[1].delay = period
--Timer must always have a count of 1 or more. The following ‘if’ statement sets the timer count
appropriately.
if pulseCount > 1 then
trigger.timer[1].count = (pulseCount - 1)
else
trigger.timer[1].count = 1
end
trigger.timer[1].passthrough = true --Timer issues an event at the start of the first interval.
trigger.timer[1].stimulus = digio.trigger[1].EVENT_ID -- Period timer is triggered by a digital I/O line
trigger.timer[1].clear()
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-- Timer 2 controls the pulse width
trigger.timer[2].delay = pulseWidth
trigger.timer[2].count = 1
trigger.timer[2].passthrough = false --Only issue events after timer interval elapses
trigger.timer[2].stimulus = trigger.timer[1].EVENT_ID -- Start timer interval when period timer issues event
trigger.timer[2].clear()
-- Configure SMU Trigger Model for Sweep
-- Return to the bias level at the end of the pulse/sweep
smua.trigger.endpulse.action = smua.SOURCE_IDLE
smua.trigger.endsweep.action = smua.SOURCE_IDLE
smua.trigger.count = pulseCount --Set number of pulses to complete
smua.trigger.arm.count = 1
smua.trigger.arm.stimulus = 0
smua.trigger.source.stimulus = trigger.timer[1].EVENT_ID --Trigger source action when period timer generates
event
smua.trigger.measure.stimulus = trigger.timer[2].EVENT_ID -- Trigger measurements when the end pulse action
is started
smua.trigger.endpulse.stimulus = trigger.timer[2].EVENT_ID -- Start EndPulse action as soon as the pulse
width timer ends
end
Executing the Code
Send the following lines to configure and execute three pulses that pulse from 1V to 5V with a width of 300μs and a period of 3ms
with a current limit of 20A. Twenty measurements taken at 10μs intervals are taken 100μs after the pulse ends.
pulseThenMeas(5, 1, 20, 300e-6, 3e-3, 3, 100e-6, 50, 10e-6)
runPulse()
Results
The raw sample data is stored in the dedicated nonvolatile buffers, smua.nvbuffer1 and smua.nvbuffer2. Use the following command
to print the data through the remote command interface:
printData()
The results for the example code are shown in Figure 12. The load is a 0.5W resistor.
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Appendix G. Monitoring the Operation Overrun Event Register
The following TSP script demonstrates how to monitor the Model 2651A’s operation overrun Event register for overruns to the
Source, Measure, or EndPulse action blocks.
function checkSwpStatus()
trigOvrEvent = status.operation.instrument.smua.trigger_overrun.event
print(“Overall smu trigger overrun event register value”, trigOvrEvent)
--For more detail look at bits 2, 3, 4
--bit 2 = source, bit 3 = measure, bit 4 = endpulse
--The bit.test function returns a boolean: true if set, false if not set
sourceOvr = bit.test(trigOvrEvent, 3)
print(“Source Overrun bit:”, sourceOvr)
measOvr = bit.test(trigOvrEvent, 4)
print(“Measure Overrun bit:”, measOvr)
endPulseOvr = bit.test(trigOvrEvent, 5)
print(“EndPulse Overrun bit:”, endPulseOvr)
--If any bit in
if trigOvrEvent
ovrBool
else
ovrBool
end
return ovrBool
end
the register is set, then return a Boolean set to true for overrun occurred.
> 0 then
= true
= false
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Methods to Achieve Higher Currents
from I-V Measurement Equipment
The most flexible test equipment for sourcing and measuring
current (I) and voltage (V) are source-measure units (SMUs) such
as Keithley’s Series 2600B System SourceMeter® instruments.
The Series 2600B also includes three new benchtop models that
offer best-in-class value and performance. These specialized
instruments are high performance I-V source-measure
instruments that are designed for use either as bench-top I-V
characterization tools or as building block components of
multi-channel I-V test systems. Each Series 2600B SourceMeter
instrument combines a precision power supply, a true
current source, a DMM, an arbitrary waveform generator with
measurement, an electronic load, and a trigger controller – all
in one instrument. In short, they can source I or V, and then
measure V or I, simultaneously. They also support both polarities
of I and V (sinking and sourcing power), referred to as “four
quadrant operation.”
By design, there is a limit to the maximum current or voltage
that a single SMU can source and measure. This paper will
present methods to achieve current levels during test sequencing
that are higher than the published DC (direct current)
specifications of a single SMU. Two techniques will be explored:
1.Pulse sweeps
2.Combining multiple SMU channels together
These techniques can be used to source and measure
currents up to 40A for high-power applications such as:
• Solar cells and other photovoltaics
• Power management devices such as power
MOSFETs and IGBTs
• High brightness light emitting diodes
• RF power transistors
Pulse sweeps
There is a limit to the DC maximum current or voltage that a
single SMU can source and measure. This limit is a function of
the inherent equipment design and is typically dependent on
design parameters such as the maximum output of the power
supply internal to the SMU itself, the safe operating area (SOA)
of the discrete components used in the SMU, the spacing of the
metal lines on the SMU’s internal printed circuit board, etc. Some
of these design parameters are constrained by maximum current
limits, some by maximum voltage limits, and some by maximum
power limits (I×V). A typical expression of the DC I-V limits of
a four quadrant SMU is shown in Figure 1. It shows a maximum
DC current of 3A (point A in the figure) and a maximum voltage
of 40V (point B). The maximum power the SMU can output
is 40W, which is achieved at point B (1A×40V). At point A the
power is lower at 18W. The difference can be explained, for
example, that the maximum at point B is constrained by the
maximum allowed power output of the on-board power supply,
whereas at point A the limit is based on the maximum current
(not power) that a key component can handle.
+3A
A 18W
B 40W
+1A
–40V
–6V
+6V
+40V
–1A
DC
–3A
Figure 1
Figure 1 shows the DC (or continuous wave, CW) I-V limits,
or performance envelope. Now consider if the SMU could
produce a time-varying waveform such as a pulse. If the pulse
waveform had 40V amplitude, 1ms pulse width, and a 50% duty
cycle, then the effective CW power averaged over several seconds
is 20W, not 40W. Depending on its design, it may be possible for
that SMU to source higher current in pulse mode than in DC
mode – the instantaneous maximum peak power in pulse mode
is higher than DC peak power, but the CW power dissipation
during pulse mode is less on average than in DC mode.
+10A
+5A
+3A
+1.5A
+1A
DC
0A
Pulse
–1A
–1.5A
–3A
–5A
–10A
–40V
–35V
–20V
–6V 0V +6V
+20V
+35V
+40V
Figure 2
www.keithley.com51
As an example, the pulsed I-V envelope is shown in
Figure 2 for the same SMU model shown in Figure 1. There
are constraints on the allowed pulse width and duty cycle, but
by pulsing the instantaneous power can be as high as 200W
(10A×20V). Although the instantaneous power may be high, the
CW power based on allowed pulse width and duty cycle is below
the DC power limit of 40W.
The higher instantaneous power when pulsing can be applied
to achieve higher power I-V sweeps. Consider a standard scenario
where a voltage bias is applied to a DUT (Device Under Test).
The voltage values are swept over time, from low voltage to
high voltage. Intermittently, the current is measured (schematic
shown in Figure 3). This generates I-V pairs that, when plotted,
give a typical I-V sweep such as that shown in Figure 4 for a P-N
junction diode (1N5400 component). P-N diodes are encountered
when measuring a solar cell or other photovoltaic (PV) device,
or high-brightness light emitting diodes (HB-LEDs). In this
sweep, voltage increments of 0.02V were used during the sweep.
As shown in Figure 2, when the applied voltage is less than 6V,
that particular SMU’s maximum allowed DC current is 3A, as
demonstrated in Figure 4.
The schematic for a pulsed voltage sweep is shown in Figure
5, which is equivalent to the DC sweep shown in Figure 3.
When current is not being measured, the sourced voltage values
simply return to zero to keep the averaged CW power within the
allowed limits. As seen in Figure 5, the V bias values at which
the I values are measured and the I sampling rates are identical
to those in the DC sweep shown in Figure 3. Performing pulse
sweeps in this manner allows identical I-V values as DC to be
achieved in the lower power regions (Figure 6), while allowing
I-V curves in the higher power region up to 10A to be achieved
(Figure 7).
− Sourced Voltage (V)
× Measured Current (A)
0
0.5
1
1.5
T ime (s ec)
2
2.5
3
Figure 5
Sourced Voltage (V)
Measured Current (A)
0
0.5
1.0
1.5
2.0
2.5
In Figure 6, the DC and pulse sweep I-V curves are so closely
overlaid that it is difficult to visually discern the differences
between the curves. Therefore to quantify the excellent
correlation, the relative percent difference between the curves
is also calculated and shown in Figure 6. At the higher current
values of most interest, ±2% correlation is achieved between
the DC and pulse sweeps (using the DC sweep values as the
reference). With excellent correlation established at the lower
current levels, we then use pulse sweeps to extend the I-V curves
to 3× higher currents of 10A maximum than could be achieved
with DC sweeps. This is shown in Figure 7.
3.0
Time (s)
Figure 3
3.0
3.0
2.5
4%
2.0
Measured current, I (A)
Measured current, I (A)
6%
DC Sweep
Pulse Sweep
Difference
2.5
1.5
1.0
2.0
2%
1.5
0%
1.0
-2%
0.5
-4%
0.5
0.0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
A p plied voltag e (V )
-6%
0.0
0.2
0.4
0.6
0.8
1.0
A pp lied voltag e (V)
Figure 4
Figure 6
52
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Combining multiple SMU channels
to achieve higher DC current
10
Pulse Sweep
DC Sweep
9
The most commonly-used method of combining SMU channels
to achieve higher DC currents is to put the current sources in
parallel across the DUT, as shown in Figure 8.
Measured current, I (A)
8
7
6
5
4
3
2
SMUB
FIMV
1
0
0.0
0.2
0.4
0.6
A pplied voltag e (V)
0.8
1.0
Force HI
Sense HI
Sense LO
Force LO
SMUA
FIMV
Force HI
Sense HI
Sense LO
Force LO
DUT
Figure 7
For many DUT types, a pulsed sweep can be substituted
for a DC sweep to achieve higher power I-V sweeps with little
impact on results. DUT types for which pulsed sweeps may not
adequately correlate to DC sweeps are those DUTs potentially
impacted by displacement current (second term in Maxwell’s
equation for current, Jtot = J + ∂D/∂t). Large displacement
currents can be generated at the sharp edges of the voltage
pulse, and DUTs such as capacitors can have their electrical
properties changed by large displacement currents.
However, there are many high power devices where pulsed
I-V testing must be performed to get optimal results. The
reason is that during high power CW testing, the semiconductor
material itself starts to dissipate the applied power via thermal
heating. As the material in the device heats up, the conduction
current decreases as the carriers have more collisions with the
vibrating lattice (phonon scattering). Therefore, the measured
current is erroneously too low, due to so-called self-heating
effects (http://www.keithley.com/data?asset=50742) caused by
Joule heating. Because devices such as these are typically run
in pulsed mode, intermittently, or AC and not run continuously
on, the erroneously-low DC-measured currents are not an
accurate characterization of their performance. In this case,
pulsed testing must be used, and pulse width and duty cycle
are explicitly stated in the device’s published datasheet (see
for example the inset in Figure 2 at http://www.fairchildsemi.
com/ds/1N%2F1N5400.pdf). Example devices that require pulse
testing are high-power RF power amplifiers and even low-power
nanoscale devices.
Figure 8
This test setup takes advantage of the well-known electrical
principal that two current sources connected to the same
circuit node in parallel will have their currents added together
(Kirchhoff’s current law). In Figure 8, both SMUs are sourcing
current and measuring voltage. The HI terminals refer to the
high impedance terminals of the SMU and the LO terminals
are the low impedance terminals. The “FORCE” terminals are
the ones forcing current, and the SENSE lines are used for the
four-wire voltage measurements. four-wire configuration is a
mandatory requirement when high values of current are involved
and is discussed later in this document. All of the LO terminal
(FORCE and SENSE) of both SMUs are tied to earth ground.
Characteristics of this particular configuration are:
• Source Current:
IDUT = ISMU A + ISMU B
• Load Voltage:
V DUT = VSMU A = VSMU B
• Maximum Source Current:
IMAX = IMAX SMU A + IMAX SMU B
• Maximum Voltage: Limited to the smaller of the two SMUs
maximum voltage capabilities. This is a result of very small
variations between units of the maximum voltage that can be
output when sourcing current. V MAX = Smaller of V MAX CMPL
SMU A and V MAX CMPL SMU B.
Other notes:
The primary tradeoffs when migrating from a DC sweep to a
pulse sweep are as follows:
• Set SMU A and SMU B output currents to the same polarities
to obtain maximum output.
• The pulse width must be wide enough to allow time for the
device transients, cabling and other interfacing circuitry to
settle so a stable, repeatable measurement can be made.
–– –While not absolutely required, the source polarity is
generally the same for the two SMUs in this configuration.
• The pulse width cannot be so wide so as to violate the test
instruments’ maximum pulse width and duty cycle limits,
which would exceed the allowed power duty cycle of
the instrument.
• When possible, have one SMU in a fixed source configuration
at a time and the other SMU performing the sweep. This
is preferable to having both sweeping simulataneously.
If both SMUs are sweeping, their output impedances are
naturally changing, for example, as the meter autoranges. In
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–– One ramification of having one SMU’s current fixed and the
others sweeping current is that for current levels well below
IMAX, one SMU will be sourcing or sinking much more
current than the other. The current levels are not balanced
between the two SMUs, but this does not cause any
accuracy or precision issues when done with high quality
SMUs. There is no particular reason to try to keep the SMUs
at approximately the same current throughout the sweep.
–– Therefore, to sweep from 0A to IMAX, set both SMUs
to source 0A and then sweep SMU B from 0A to
+IMAX SMU B. Next sweep SMU A from 0A to +IMAX SMU A.
Similar approaches can be used to sweep from IMAX to 0A,
or 0A to –IMAX, or –IMAX to 0A.
–– To sweep from –IMAX to +IMAX, first set SMU A to –
IMAX SMU A and SMU B to –IMAX SMU B. Sweep SMU B from
–IMAX SMU B to +IMAX SMU B and then sweep SMU A from
–IMAX SMU A to +IMAX SMU A. Again, there is no particular
reason to try to keep the SMUs at approximately the same
current. A similar approach can be used to sweep from
+IMAX to –IMAX.
• Both SMUs are sourcing current, but only let one SMU limit
the maximum output voltage via the compliance setting.
For example, set the I-source voltage compliance of SMU B
greater than the compliance of SMU A, i.e. V LIMIT SMU B >
V LIMIT SMU A.
–– If the DUT is an active source, the compliance setting of
SMU A must be greater than the maximum voltage the DUT
can source, to avoid putting the circuit and SMUs into an
unknown state. For example, if the DUT is a 9V battery
but SMU A’s compliance is set to 5V, the results will be
unpredictable and unstable.
–– Set the voltage readback measurement range of SMU
B equal to its compliance range. There is no special
requirement for SMU A’s measurement range, but be aware
of range compliance if the measurement range is less than
compliance range and the instrument allows differences
between real compliance and range compliance (such as
Keithley Model 24xx and 6430).
Now we apply this technique of combining SMUs in parallel
using the SMU models whose DC I-V power envelope are shown
in Figure 2. The same P-N diode DUT is used whose results are
shown in Figure 4 for a single SMU. By combining two SMUs
in parallel, we expect to be able to double the maximum DC
current measured from 3A to 6A.
54
This is confirmed by the results shown in Figure 9. Up to 3A,
the single-SMU and dual-SMU results are so closely correlated
that is difficult to visually discern any differences between the
results. As before, the relative percent differenence between the
results is calculated and plotted in the figure and, in most cases,
shows ±1% correlation is achieved between the single-SMU and
dual-SMU sweeps (using the single-SMU sweep values as the
reference). With excellent correlation established at the lower
current levels, we then use dual-SMU sweeps to extend the I-V
curves to 2× higher currents of 6A maximum than could be
achieved with single-SMU sweeps.
6
6%
2 S MUs in parallel DC Sweep
1 S MU DC Sweep
Difference
5
Measured current, I (A)
addition, the DUT’s output impedance may also be changing
significantly, for example, from high resistance off state to
low resistance on state. With so many of the impedance
elements in the circuit changing, this could increase overall
circuit settling time at each bias point. Although this is a
transient effect that damps out, nonetheless, fixing one SMU’s
source and sweeping the other usually results in more stable
and faster-settling transient measurements, therefore higher
test throughput.
4%
4
2%
3
0%
2
-2%
1
-4%
0
-6%
0.0
0.2
0.4
0.6
0.8
1.0
A p p lied vo lta g e (V )
Figure 9
Pulse sweeps while combining
multiple SMU channels
In this section we combine power-enhancement techniques of
the Pulse Sweep method with the method of combining multiple
SMU channels in parallel. Furthermore, we increase the number
of SMU channels from two to four by using two dual channel
SMUs such as the Keithley Model 2602B. As seen in Figure 2,
this SMU can achieve a maximum 10A pulse for DUT bias less
than 20V. Therefore the maximum current now achievable is 40A
(4×10A), which is more than 13× higher than the 3A that can be
achieved by using a single SMU with DC sweeps.
Not surprisingly, great care must be taken when
implementing this testing method. First, there is a personnel
safety aspect: When dealing with hazardous voltages, it is critical
to insulate or install barriers to prevent user contact with live
circuits. Failure to exercise these precautions could result in
electric shock or death.
There is also an aspect related to avoiding damage to the
measurement equipment or the DUT. The multiple pulses must
be tightly synchronized in time (on the nsec scale) so that one
piece of equipment is not applying power and damaging units
that are not turned on yet. Most SMUs on the market simply
do not have the capability to synchronize on sufficiently short
time scales and therefore are not suitable for implementing this
type of test methodology; however, the Keithley Series 2600B
SMUs have been intrinsically designed to do this. There are
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To set a baseline before combining SMUs, we perform a
10A pulse using a single SMU and observe the results on an
oscilloscope. A high power precision resistor (0.01W, ±025%, KRL
R-3274) was used as the test DUT and a pulse width of 300µs
was programmed. We expect the oscilloscope to show a nearly
square waveform of amplitude 0.1V (10A × 0.01W) and 300µs
width, and, in fact, those are the results we see in Figure 10.
As also shown in Figure 10, combining four SMUs in parallel
to pulse 40A across the same DUT results in the expected 0.4V
magnitude with excellent synchronization (low jitter) between
the channels.
With the pulse performance verified, we program a pulse
sweep combining four SMUs and repeat the I-V curve on the
P-N diode test DUT. The results are shown in Figure 12. We see
excellent correlation with the 1-SMU DC sweeps up to 3A, and
with the 1-SMU pulse sweep up to 10A. Then, we extend the
achievable I-V curve up to 40A.
40
4 SMU Pulse Sweep
1 SMU Pulse Sweep
1 SMU DC Sweep
35
Measured current, I (A)
other important considerations when using more than two SMUs
together, which will be discussed in the next section.
30
25
20
15
10
5
0
0.0
0.2
0.4
0.6
0.8
Applied voltage (V)
1.0
1.2
1.4
Figure 12
Figure 10
Using the pulse waveform shown in Figure 10 (40A
amplitude, 300µs width, generating 0.4V across a 0.01W
±0.25% resistor), repeatability testing was done to verify
pulse consistency. This is a particularly stringent test that
simultaneously checks both the high I sourcing performance
along with low V measurement performance. The results are
shown in Figure 11, with a 3σ standard deviation of 0.045%
observed across 25 repetitions done in quick succession.
Resistance (Ω)
A vera g e = 0.009962 Ω
3 σ = 0.045%
0.00996
0.00994
0.00993
0.00991
• Threshold voltage (gate to source), VGS(TH)
• Drain-to-source breakdown voltage, BV DSS
The ID –V DS curves for a variety of VGS values are shown
in Figure 13. The measurements and curves are the expected
results, with the effects of minor device self-heating observed at
around V DS=10V for the VGS=7V and VGS=8V curves. Self-heating
is expected for the pulse width of 1000µs that was used. Note
that nearly 800W peak power (20V × 40A) is achieved with this
tests setup.
0.00995
0.00992
• rDS(ON)–ID curves for different VGS values
For the three-terminal measurements, four SMUs are
connected in parallel across the drain-source nodes to enable
40A pulsed currents; a fifth SMU is connected across the gatesource nodes to provide the gate bias.
0.00999
0.00997
• ID –V DS curves for a variety of VGS values
• Continuous source-to-drain current and voltage, ISD and VSD
0.01000
0.00998
With the results of this technique (combining four
SMU channels and pulsing to achieve 40A) verified
on two-terminal devices (resistor and diode), the
technique is next applied to a three-terminal device, a
high-power MOSFET (IRFP240, datasheet available at
www.datasheetcatalog.org/datasheet/fairchild/IRFP240.pdf).
Such devices have high operating drain current ID(ON) (>20A at
V DS>20V, VGS>10V), and low drain-source on resistance rDS(ON)
(<0.2W at VGS=10V, ID=10A). Typical electrical parameters that
are measured on such a device include:
Puls e amplitude = 40A
Puls e duration = 200 µ s
Duty cycle = 0.1% max
DUT = 0.01Ω res is tor
0.00990
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
R ep etitio n n u mb er
Figure 11
www.keithley.com55
50
100
Puls e duration = 1000 µ s
Duty cycle = 0.1% max
Puls e duration = 1000 µs
Duty c yc le = 0.1% max
V
G S
= 8V
I SD , source to drain current (A)
I D, drain current (A)
40
30
V
G S
= 7V
V
G S
= 6V
V
G S
= 5V
V
G S
= 4V
20
From publis hed
datas heet
10
1
Meas ured
10
0
0
10
V D S , d rain to s o u rc e v o lta g e (V )
0.1
0
20
0. 4
0. 8
1. 2
1.6
2
V S D , s o u rc e to d ra in v o lta g e (V)
Figure 13
Figure 15
The rDS(ON)–ID curve was measured for VGS=10V up to
the maximum measurable drain current of ID=40A. The
results are shown in Figure 14, and, for comparison purposes,
the data from the device’s published datasheet (Figure 8 in
www.datasheetcatalog.org/datasheet/fairchild/IRFP240.pdf)
are also shown. The correlation is excellent when using this
multiple-SMU technique combined with pulse sweeping. With the
standard, single-SMU DC sweep, the curve would have ended at
3A, which is not sufficient to properly characterize the device.
2600B SMUs are mainframeless, the exact number of channels
can be expanded to the desired number without having to incur
the additional cost or power limitations of a mainframe.
Important test implementation details
This section describes key implementation details that
significantly improve the accuracy and precision of the results
obtained using this multi-SMU pulsed sweep approach.
Source readback
Figure 14
Finally, the ISD –VSD curves for VGS=0V are shown
in Figure 15. These are compared to the results
from the device’s published datasheet (Figure 13 in
www.datasheetcatalog.org/datasheet/fairchild/IRFP240.pdf), and,
again, correlation is very good.
With a modern smart SMU like the Keithley Series 2600B,
which has its own microprocessor, onboard memory, and math
and logic programming functions via an embedded open-source
scripting language, it is easy to run the SMU via a graphical userinterface for benchtop applications, or have the scripts resident
on the SMU with no need for a control PC for high-speed parallel
test production applications. Also, because the Keithley Series
56
Consider the case when a test applies a voltage to a DUT and
measures a current. Because an SMU has both source and
measure functions built into the same unit, it can also read back
the actual value of the applied voltage using its measurement
circuitry. This is a source-measure-measure sequence not just
source-measure: Source voltage, measure (readback) applied
voltage, measure resulting current across the device. A typical
reason why the programmed value for the source voltage is not
the same as the voltage applied to the DUT is that the DUT is
sinking a large current, which slightly loads the voltage source.
In that case, the actual measured voltage values that are read
back are typically slightly lower than the values that were
programmed. A comparison was done for the P-N diode DUT
used previously, and the results are shown in Figure 16. At the
maximum point of the I-V (current about 40A), the programmed
voltage is 1.3000V and the actual measured sourced value is
1.2917V, a small 0.64% error, which may or may not be impactful
depending on the actual application. The error manifests itself
primarily as a small offset on the voltage axis (left-right shift
on the X-axis in Figure 16) with little impact to the measured
current values (little shift on the Y-axis).
Four-wire measurements
Four-wire (Kelvin) measurements must be used when doing high
current testing. A four-wire measurement bypasses the voltage
drop in the test leads by bringing two very high impedance
voltage sense leads out to the DUT. With very little current
flowing into the SENSE leads, the voltage seen by the SENSE
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40
40
4 SMU Pulse Sweep, no source readback
4 SMU Pulse Sweep, with source readback
4 SMU Pulse Sweep, 4-wire
4 SMU Pulse Sweep, 2-wire
35
30
Measured current, I (A)
Measured current, I (A)
35
25
20
15
30
25
20
15
10
10
5
5
0
0
0.0
0.2
0.4
0.6
0.8
Applied voltag e (V)
1.0
1.2
1.4
0.0
0.2
0.4
0.6
0.8
Applied voltag e (V)
1.0
1.2
1.4
Figure 16
Figure 17
terminals is the virtually same as the voltage developed across
the unknown resistance.
Maximum of one voltage source at each DUT node
It is common in many test sequences to perform voltage sweeps
(force voltage) and measure current (FVMI). In the case of more
than one SMU connected in parallel to a single terminal of the
device, the obvious implementation would be to have all of the
SMUs in V-source mode and measure I. However, three factors
must be considered:
This is very important when high currents are being tested.
At 40A levels, even a small resistance such as 10mW in the
test cable can generate a voltage drop of 0.4V. So if the SMU
is forcing 1V at 40A current and the cable resistance is 10mW
and there are two test leads, the DUT might only receive a
voltage of 0.2V, with 0.8V dropped across the test cables. A more
detailed discussion of four-wire measurements can be found at
http://www.keithley.com/data?asset=10636.
Unlike source readback, which primarily impacted just the
source values, implementing four-wire measurements will result
in significantly more accuracy on both the sourced and measured
values. That is because most good-quality modern SMUs have an
analog feedback control loop; in other words, if it is programmed
to source, say, 1V, but the measured value at the DUT using
four-wire is only 0.2V, then the SMU will increase the current
it sources to compensate for the loads and voltage drops in the
circuit, until the four-wire voltage value reaches the programmed
source value (within the loop exit limits). The DUT’s bias will be
closer to the desired value; hence, the measured value will be
more accurate. Therefore, by enabling four-wire measurement
capability, both the sourced values and the measured values will
be more accurate (impacts both axes in an I-V curve).
The results of two-wire versus the more accurate four-wire
results are shown in Figure 17 for the P-N diode used previously.
As seen in that figure, due to the voltage drops in the test leads,
in two-wire mode the DUT sees only a small fraction of the
intended applied voltage, and, therefore, the forward current is
lower. At 1.3V bias in 2-wire mode (bearing in mind that the DUT
will get less voltage around 1.1V due to uncompensated voltage
drops from the resistance in the test leads), the measured current
is less than 20A, half of the real value (40A) when four-wire mode
is used. This is a significant error, justifying the benefit of fourwire full Kelvin (not quasi-Kelvin) testing.
Of course, to achieve best results, every effort must be made
to place the test leads for the four-wire Kelvin connection as
close to the DUT as is possible.
• SMUs when sourcing voltage are in a very lowimpedance state.
• DUTs can have impedances higher than an SMU that’s in
V-source mode. The DUT’s impedance can be static or
dynamic, changing during the test sequence.
• Even when all SMUs in parallel are programmed to output
the same voltage, small variations between SMUs related to
the instruments’ voltage source accuracy means that one
of the SMU channels will be at a slightly lower voltage (mV
order of magnitude) than the others.
So, if three SMUs are connected in parallel to one terminal
of a DUT, and each SMU is forcing voltage and outputting nearmaximum currents, and the DUT is in a high impedance state,
then all current will go to the SMU which is sourcing the slightly
lower voltage. This will most likely damage that SMU. Therefore,
when connecting SMUs in parallel to a single terminal of a DUT,
only one SMU should be sourcing V, as shown, for example, in
Figure 18 for the multiple SMUs connected to the drain of the
MOSFET whose results were shown in Figures 13–15.
Even if the configuration in Figure 18 is used, extreme care
must be taken so that throughout the sweep only one SMU
is in FVMI mode and that none of the SMUs in FIMV mode
automatically or inadvertently change to FVMI mode. An SMU
can change from FIMV to FVMI mode, for example, when an
SMU in FIMV mode reaches its programmed voltage level for
source compliance. When an SMU is sourcing current to a
programmed level in FIMV mode and the DUT sinks current, the
SMU will automatically increase its output voltage via an analog
feedback loop so as to maintain the programmed output current.
The maximum voltage it will use is set by the user via a source
compliance voltage level. When an SMU is in FIMV mode and
www.keithley.com57
entire sweep. The SMUs supplying current are supplying it at the
level measured by all SMUs at the previous bias point.
F
S
SMU1 FVMI S
F
F
HI
S
G
F
HI
S
F
HI
F
SMU5 FVMI
HI
S
S
F
F
HI
So in the case of four SMUs for the results shown in
Figures 12–17:
LO
SMU2 FIMV S
LO
• Put SMU1 in V-source mode; SMU2, SMU3, SMU4 in
I-source mode
F
SMU3 FIMV S
LO
F
SMU4 FIMV S
LO
• Determine Vstep
LO
• Initially SMU1 sources 0V (or a voltage level that will result
in a DUT current that is less than the maximum current that
SMU1 can handle on its own). SMU2–4 source 0A.
S
Figure 18
• Sweep loop:
source voltage compliance is reached, the SMU switches modes
and becomes a low-impedance voltage source and is at risk to
being damaged. To prevent that from happening:
• Voltage compliance levels should be set appropriately
(typically as high as possible)
• Control code can be written to monitor results during the
sweep and take corrective action to avoid compliance as the
instrument approaches compliance levels.
One practical implementation of a maximum of 1V-source at
each DUT node is to have no SMUs in V-source, all in I-source.
The sweep then would be entirely I-bias (not V-bias). While this
is easiest to implement, this method suffers from the fact that
I-V data will not be equally spaced on the voltage axis; they will
be equally spaced on the current axis. This might complicate or
confuse some standard analysis algorithms. A demonstration of
this is shown in Figure 19, where all SMUs are FIMV and none
are V-source. These results should be compared to Figure 13,
where one SMU is in V-source mode and, therefore, the data
points are equally spaced in voltage.
The method shown in Figure 18 works on the basis that
one SMU controls the output voltage while the rest of the SMUs
supplement the current. To do this, one SMU is configured as a
V source while the rest are configured as current sources for the
40
ID, drain current (A)
35
30
VGS = 7V
25
20
VGS = 6V
15
10
VGS = 5V
5
VGS = 4V
0
0
5
10
V DS , drain to s ource voltage (V)
Figure 19
58
15
20
–– All 4 SMUs measure I.
–– Calculate Itotal = ISMU1 + ISMU2 + ISMU3 + ISMU4
–– Ibias = Itotal/3
–– Set SMU1 to source voltage at the level of the previous
voltage plus Vstep
–– Set SMU2, SMU3, SMU4 to source current each at a
level of Ibias
–– Repeat loop until exit condition is reached
Mitigating excessive energy dissipation
due to device breakdown
When two SMUs of the same capability are connected in parallel
to a single node in the circuit, one SMU is always capable of
sinking all of the current being output by the other SMU. This
scenario can occur, for example, when a DUT breaks down,
becomes an open (near-infinite impedance), and is no longer a
continuity path where current can flow. There is a short time
during which 1 SMU has to sink all the current from the other.
However, when there are more than 2 SMUs connected in
parallel at a single circuit node, 1 SMU cannot sink all of the
current coming from the other SMUs. The SMU(s) that will be
forced to sink current if the DUT breaks down are the SMUs at
the lowest voltage or lowest impedance, most likely the ones
sourcing voltage. In order to protect the signal input of the
SMU forcing voltage, a diode such as the 1N5820 can be used.
This limits the amount of current that can go into the SMU. A
diode is preferable, because a fuse is too slow, and a resistor will
cause too large of a voltage drop a across it. A diode has a much
faster response than a fuse, and the diode has a much smaller
maximum voltage drop across it (typically around 1V) than
a resistor.
Adding diode protection to the test setup previously shown
in Figure 18 results in the circuit schematic shown in Figure
20. Although SMU5 is also in FVMI mode, it does not require
input protection because its Force-HI connection is connected to
the high-impedance gate node. Also, in the test setup shown in
Figure 20, extra test code has been implemented to ensure that
SMUs 2–4 will not reach compliance during the test sequence, to
ensure they don’t switch to FVMI mode. If this extra test code is
not used, hardware protection should be added to the inputs of
SMUs 2–4 in case any of them reach source voltage compliance.
www.keithley.com
lower end of that range. Keep cable lengths a short as possible
and in no case longer than one meter.
1N 5820
F
H I
S
G
F
H I
S
F
H I
S
F
SMU5 FVMI
H I
S
SMU1 FVMI
SMU2 FIMV
SMU3 FIMV
SMU4 FIMV
F
F
H I
L O
S
S
F
LO
S
F
LO
S
F
LO
S
F
LO
S
Figure 20
To demonstrate little impact on results of adding diode
protection to SMU1, we repeat the rDS(ON)–ID curve measured on
the IRFP240 part for VGS=10V up to the maximum measurable
drain current of ID=40A (results previously shown in Figure 14),
both with and without diode protection. The results are shown
in Figure 21. The results overlay so well they are almost
indistinguishable, and, upon calculating the relative percent
difference between the two curves and plotting, in most cases,
shows ±1% correlation is achieved between the no-diode
and diode cases. This confirms that diode protection can and
should be used.
It is also important to ground the LO terminals of both
SourceMeter units, as shown in Figures 8, 18, and 19. If the
DUT becomes grounded and the steps above are not followed
carefully, the SMUs could be damaged.
r DS (ON), drain to source on resistance (Ω)
1.5
No d io d e p ro te c tio n
W ith d io d e p ro te c tio n
Differe n c e
0.4%
0.6
-0. 4%
0.3
-1. 2%
0.0
-2. 0%
15
Four-wire Kelvin connections must be as close to the DUT as
possible (every mm matters). So if using banana test leads and
“piggybacking” the jacks, the Sense leads should be in front of
the Force leads, as shown in the photo below. Putting the Sense
leads behind the Force leads will degrade the results. While
“piggybacking” is acceptable for the SMUs forcing current, the
SMU forcing voltage must have its sense leads separated and right
at the DUT (for example using alligator clips) in order to have
proper 4-wire operation. Also, it should be noted that the voltage
readback should be done with the SMU forcing voltage, because
the current SMU’s voltage readings will all vary quite a bit due
to the connections, and will be different then what is actually
at the DUT.
1.2%
0.9
0
It is generally thought that guarding can minimize the effects
of cable charging, but this is typically more of a concern for high
voltage testing and not for high current testing. Guarding was
not used in the results shown in this paper.
2.0%
Puls e duration = 1000 µs
Duty c yc le = 0.1% max
1.2
Cables used in this document were Keithley Model 2600BBAN: 1m (3.3 ft) banana test leads/adapter cable, which provide
safety banana connections to Hi, Sense Hi, Lo, Sense Lo, and
guard. The test leads were connected to the DUTs using alligator
clips with boots (barrels accept standard banana plugs), such as
those in Keithley Model 5804 Test Lead Set.
30
45
I D , d ra in c u rren t (A )
60
75
The jacks used on the test fixture should be high quality.
In particular, some red jacks use high amounts of ferrous (Fe,
iron) content to produce the red coloring, and these jacks can
have unacceptably high levels of leakage due to conduction. The
resistance between the plugs to the case should be as high as
possible and in all cases >1010W.
In general, test cabling and test connections must all be designed
to minimize resistance (R), capacitance (C), and inductance (L),
between the DUT and SMU.
Many published test setups recommend to add a resistor
between the SMU and the device’s gate in the case of test a
FET or IGBT. For example in Figure 20, a 10kW resistor would
be added between SMU5 and the gate node. This resistor
can stabilize measurements, and, because the gate does not
draw much current, the resistor does not cause a significant
voltage drop.
To minimize resistance, use thick gauge wire (14 gauge is
acceptable, 12 gauge is better) wherever possible, and definitely
within the test fixture itself. Cable resistance can range from
30-300 mW/meter and higher, so obviously choose cabling at the
If voltages in excess of 40V will be used during the test
sequence, the test fixture and SMUs must have the proper
interlock installed and be operational according to normal
safety procedures.
Figure 21
Cabling and test fixture considerations
www.keithley.com59
Summary
Methods were shown how to increase from 3A to 40A the
maximum current level that can be measured:
1.Pulse sweeps
2.Combining multiple SMU channels in parallel to achieve
higher current
60
Example results using these techniques were given for
commercially-available devices, and the results show excellent
correlation with the published datasheets.
In addition, important test implementation factors were
discussed in detail, including source readback, four-wire
measurements, single V-source at each DUT node, and mitigating
excessive energy dissipation due to device breakdown.
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Combining Keithley Model 2651A
High Power SourceMeter® Instruments
for 100A Operation
Introduction
node. In Figure 2, two current sources representing SMUs and a
device under test (DUT) are connected in parallel.
Source-measure units (SMUs), such as the Keithley Model 2651A
High Power System SourceMeter instrument, are the most
flexible and most precise equipment for sourcing and measuring
current and voltage. Because of this, they are widely used to test
semiconductor devices such as MOSFETs, IGBTs, diodes, high
brightness LEDs, and more.
IDUT
Node A
I2
I1
With today’s focus on green technology, the amount of
research and development being done to create semiconductor
devices for power management has increased significantly. These
devices, with their high current/high power operating levels, as
well as their low On resistances, require a unique combination
of power and precision to be tested properly. A single Keithley
Model 2651A is capable of sourcing up to 50A pulsed and 20A
DC. For applications requiring even higher currents, Model
2651As can be combined to extend their operating range to
100A pulsed.
DUT
SMU #2
SMU #1
Node B
Figure 2: The sum of the currents entering the node equals the sum of the
currents leaving the node.
In Figure 2, we can see that two currents, I1 and I2, are
entering Node A and a single current, IDUT, is leaving Node A.
Based on Kirchhoff’s Current Law we know that:
This application note demonstrates how to collect Rds (on)
measurement data from a power MOSFET device by using a
pulsed current sweep to test up to 100A (see Figure 1); however,
it can be easily modified for use in other applications. The
document is divided into three sections: theory, implementation,
and example.
IDUT = I1 + I2
This means that the current delivered to the DUT is equal to
the sum of the currents flowing from each SMU. With two
SMUs connected in parallel, we can deliver to the DUT twice
the amount of current that can be delivered by a single SMU.
Using this method with two Model 2651As, we can deliver up to
100A pulsed.
Theory
Kirchhoff’s Current Law says that the sum of the currents
entering a node is equal to the sum of the currents leaving the
0.020
0.020
0.018
Id = 20A
Id = 40A
Id = 60A
Id = 80A
Id = 100A
Rds (ohms)
0.014
0.012
Vgs = 10V
Vgs = 6V
0.018
Rds (ohms)
0.016
0.010
0.008
0.016
0.014
0.006
0.004
0.012
0.002
0.000
2.0
2.5
3.0
3.5
4.0
4.5
Vgs (V)
5.0
5.5
6.0
0.010
6.5
0
10
20
30
40
50
60
70
80
90
100
Ids (Amps)
Figure 1: Example results after performing a pulsed Rds(on) current sweep (500μs pulse width and 0.01 NPLC) to test up to 100A on a power MOSFET device
using two Model 2651A SourceMeter instruments connected in parallel.
www.keithley.com61
Implementation
Region of Power Envelope
In order to create a current source capable of delivering more
current than a single SMU can provide, we put two SMUs, both
configured as current sources, in parallel. Below is a quick
overview of what needs to be done to successfully combine
two Model 2651As so that together they can source up to 100A
pulsed. The following sections explain each item in detail.
Both SMUs should be configured to operate in the same region
of the power envelope (see Figure 3). In order for one SMU to
sink all the current of the other SMU, the sinking SMU must be
operating in an equivalent region of the power envelope as the
sourcing SMU.
1.Use two Model 2651As, running the same version
of firmware.
2.Use the same current range for both SMUs.
3.Use the same regions of the power envelope (Figure 3) for
both SMUs.
4.Use 4-wire mode on both SMUs with Kelvin connections
placed as close to the DUT as possible.
5.Use the Keithley supplied cables. If this is not possible,
ensure your cabling matches the specifications of the
Keithley-supplied cable.
6.Set the voltage limit of both SMUs. (When the output of an
SMU reaches its voltage limit, it goes into compliance.) The
voltage limit of one SMU should be set 10% lower than the
other SMU.
7.Select the output off-mode of each SMU. This determines
whether an SMU will function as a voltage source set to
0V or as a current source set to 0A when the output is
turned off. When two SMUs are functioning in parallel as
current sources:
• The SMU with the lower voltage limit should have its
output off-mode set to NORMAL with the off function set
to voltage, and
• The SMU with the higher voltage limit should have its
output off-mode set to NORMAL with the off function set
to current.
When configured as a current source, the region of the
power envelope in which the SMU is operating is determined
by the source current range and the voltage limit value. When
combining SMUs in parallel, each SMU should be set to the same
source current range so the final determining factor for the
region is the voltage limit. As can be seen in Figure 3, the Model
2651A has three ranges of voltage limit values that determine
the operating region: >0V to ≤10V, >10V to ≤20V, and >20V to
≤40V. For example, if one SMU’s voltage limit is set to 20V, then
the other SMU’s voltage limit should be set to a value that is less
than 20V and greater than 10V in order to keep both SMUs in the
same operating region.
+50A
+30A
+20A
+10A
+5A
0A
–5A
–10A
DC
Pulse
–20A
–30A
–50A
–40V
–20V
–10V
0V
+10V
+20V
+40V
Figure 3: Power envelope for a single Model 2651A.
Identical Model
Connections
Both SMUs MUST be the same model, the Model 2651A. This
ensures that if the SMUs are forced into a condition in which
one SMU must sink all of the current from the other SMU, the
SMU that is sinking is capable of sinking all the current. For this
reason, combining different model SourceMeter instruments in
parallel is NOT recommended. In addition, both SMUs should be
running the same version of firmware to ensure that both SMUs
perform the same.
A simple connection diagram for combining two SMUs in parallel
for higher current can be seen in Figure 4.
Source Current Range
Cabling Considerations
Both SMUs should be set to the same source current range. How
an SMU responds to a change in current level can vary with
the current range on which it is being sourced. By configuring
both SMUs to source on the same current range, both SMUs will
respond similarly to changes in current levels. This reduces the
chances for overshoots, ringing, and other undesired SMU-toSMU interactions.
Cables capable of supporting the high levels of current that the
Model 2651A can produce should be used to obtain the desired
performance. The cable provided by Keithley with the Model
2651A is designed for both low resistance and low inductance.
We recommend using this cable from the Model 2651A to as
close to the DUT as possible. If the Keithley cable cannot be
used, use wiring with as low a resistance and inductance as
62
Because the Model 2651A can produce such high currents,
test leads (even those with very little resistance) can produce
significant voltage drops and create errors in the voltage
measurements. To eliminate these errors, use 4-wire mode on
both SMUs with Kelvin connections placed as close to the DUT
as possible.
www.keithley.com
HI
SHI
HI
SMU #1
LO
DUT
SHI
SMU #2
SLO
LO
SLO
A single Model 2651A is capable of sourcing up to 50A
pulsed and up to 20A DC. Using Ohm’s Law we can calculate the
maximum resistance allowed in our test leads so as not to exceed
the 3V limit under these maximum conditions. Ohm’s Law states:
V = I · R
where: V is voltage, I is current, and R is resistance. If we rewrite
this equation, solving for R we get:
R = V/I
To find the maximum resistance values allowed in our test
leads, we can substitute our limits for V and I.
Figure 4: Wiring diagram for connecting two SMUs in parallel using
4-wire mode.
R1
DUT
R1
R3
3V
_________
= 0.15W
20A DC
3V
____________
= 0.06W
50A Pulse
Based on these calculations, the resistance of each source
lead should not exceed 150mW when only DC testing is used and
should not exceed 60mW when pulse testing is used.
R2
R3
SMU #1
SMU #2
R2
Figure 5: Model for test lead resistances.
possible. We recommend that wire of 12 AWG or thicker be used
with a single Model 2651A. When combining SMUs for greater
current, 10 AWG or thicker wire should be used. Guidelines for
cabling should be taken seriously since wiring not rated for the
current being sourced can affect the performance of the SMU
and could also create a potential fire hazard.
The following sections discussing resistance and inductance
are provided to help you verify that the cables you are using will
allow your system to function properly.
Resistance
The Model 2651A has the ability to compensate for errors caused
by voltage drops due to resistance in the force leads when large
currents are flowing. This allows the Model 2651A to deliver or
measure the proper voltage at the DUT rather than at the output
of the instrument. This is done by using Kelvin connections.
The resistance of any cabling and connections between the
SMUs’ output and the DUT should be kept as low as possible
to avoid excessive voltage drops across the force leads. This is
because there is a limit to how large a voltage drop an SMU
is capable of compensating for without adversely affecting
performance. In the Keithley Model 2651A, this limit is 3V per
source lead, which is imposed by the Kelvin connections.
For example, in Figure 5 the length of the test lead
represented by R3 should be as short as possible in order to
minimize its resistance value (and thus the voltage drop across
R3). In this configuration, the current that flows through R 3 is
the sum of the current flowing through R1 and R 2. If we assume
R1 = R 2 = R3 and that both SMU #1 and SMU #2 are delivering
the same amount of current to the circuit, then the voltage drop
across R3 is twice as large as the voltage drop across R1 or R 2
because twice as much current is flowing through R 3 as there
is through R1 or R 2. The voltage drop that each SMU sees is the
sum of the voltage drop across R3 and the voltage drop across its
own lead resistance, R1 or R 2.
Inductance
The Model 2651A also has the ability to compensate for errors
caused by voltage drops due to inductance in the force leads.
As mentioned in the discussion about resistance, this allows the
Model 2651A to deliver or measure the proper voltage at the
DUT rather than at the output of the instrument. Inductance in
connections resists changes in current and tries to hold back the
current by creating a voltage drop. This is similar to resistance
in the leads. However, inductance only plays a role while the
current is changing, whereas resistance plays a role even when
current is steady.
The inductance of connections between the SMUs’ outputs
and the DUT should be kept as low as possible to minimize
impacting SMU performance. To drive fast rising pulses, the
Model 2651A must have enough voltage overhead to compensate
for the voltage drop created by the inductance. If the supply does
not have enough overhead, the inductance can slow the rise time
of the pulse.
Another reason why the inductance of connections between
the SMUs’ outputs and the DUT should be kept as low as
possible is that if the inductance causes a voltage drop large
enough to exceed the 3V source-sense lead drop limit of the
Kelvin connections, readings could be affected. If the 3V limit
is exceeded, readings taken during the rising or falling edge of
www.keithley.com63
the pulse could be invalid. However, readings taken during the
stable part of the pulse will not be affected.
On the Model 2651A, the amount of overhead in the
power supply varies depending on the operating region
in the power envelope (see Model 2651A datasheet at
www.keithley.com/data?asset=55786 for more detail); but, in
general, the amount of voltage drop caused by inductance
should be kept under the 3V source-sense lead drop limit of the
Kelvin connections. We can calculate the maximum amount of
inductance allowed in our connections by using the equation:
di
V = L · ___
dt
where: V is the voltage in volts, L is the inductance in henries,
and di/dt is the change in current over the change in time. If we
rewrite the equation solving for L we get:
di
L = V / ___
dt
As an example, let’s assume that with zero inductance the
Model 2651A produces a 50A pulse through our DUT with a
rise time of 35µs. In order to not exceed the 3V limit while
maintaining this rise time, the max amount of inductance per
test lead is:
50A
3V /______= 2.1µH
35µs
In this example (35µs rise time for a 50A pulse), to not exceed
the 3V limit we must ensure that our test leads have less than
2.1µH of inductance per lead.
NOTE: The Model 2651A specifications indicate a maximum
inductive load of 3μH, thus the total inductance for both HI
and LO leads must be less than 3μH under all conditions.
Set the Compliance
In parallel configurations, like the one shown in Figure 5, the
voltage limit of one SMU should be set 10% lower than the
voltage limit of the other SMU. This allows only one SMU to go
into compliance and become a voltage source.
Setting Correct Voltage Limits
In a parallel SMU configuration, setting voltage limits properly
is important. If both SMUs were to go into compliance and
become voltage sources, then we would have two voltage sources
in parallel. If this condition occurs, an uncontrolled amount
of current could flow between the SMUs, possibly causing
unexpected results and/or damage to the DUT. This condition
can also occur if the DUT becomes disconnected from the
test circuit. Fortunately, this condition can easily be avoided
by setting the compliance for one of the SMUs lower than the
compliance of the other SMU.
For example, in Figure 6 we have two Model 2651As configured
as 20A current sources that are connected in parallel to create a
40A current source. The voltage limit of SMU #1 is configured to
10V and the voltage limit of SMU #2 is configured to 9V and they
are sourcing into a 10mW load. If one of the leads disconnects
from the DUT during the test, each SMU would ramp up its
output voltage trying to force 20A until SMU #2 reaches its
voltage limit of 9V and goes into compliance. SMU #1 continues
to raise its output voltage until 20A are flowing from it into SMU
#2. This condition can be seen in Figure 7. Because the SMUs
are the same model, SMU #2 can sink the 20A current SMU #1
is delivering to it. Note that operating in this condition will cause
SMU #2 to heat up quickly and will cause it to shut off if it heats
up too much. This over-temperature protection is a safety feature
built into the Model 2651A to help prevent accidental damage
to the unit.
40A
20A
20A
+
10mΩ
SMU #1
SMU #2
Limit: 10V
Limit: 9V
400mV
–
Figure 6: Example of two current source SMUs connected in parallel and
functioning under normal operation.
Definition
An SMU, or any real current source for that matter, has a limit as
to how much voltage it can output in order to deliver the desired
current. When the voltage limit in an SMU is reached, the SMU
goes into compliance and becomes a voltage source set to that
voltage limit. When the compliance on one SMU is set lower than
the compliance on the other SMU, the voltage limit can only be
reached by one of the SMUs. In other words, when the SMU with
the lower voltage limit goes into compliance, it becomes a voltage
source with low impedance and begins to sink the current from
the other SMU. With the SMU in compliance sinking current, the
other SMU can now source its programmed current level and
thus never go into compliance.
64
0A
20A
20A
+
–
SMU #1
9V
10mΩ
SMU #2
Limit: 10V
Figure 7: Example of two current source SMUs connected in parallel and
functioning under compliance operation (for example, if a lead disconnects).
www.keithley.com
Set the Output Off-Mode
Introduced with the Model 2651A are new features to the
NORMAL output off-mode of Series 2600A instruments.
Previously, under the NORMAL output off-mode, when the
output was turned off, the SMU was reconfigured as a voltage
source set to 0V. This would happen whether the SMU’s on
state was configured as a current source or a voltage source.
This is still the default configuration for the NORMAL output
off-mode; however, the NORMAL output off-mode can now have
its off function configured as a current source. With the off
function set to current, when the output is turned off the SMU is
reconfigured as a 0A current source. This happens whether the
SMU’s on state was configured as a current or voltage source.
When putting two SMUs configured as current sources in
parallel, the SMU whose On State voltage limit is set lower
should be configured using an output off-mode of NORMAL
with an off function of voltage and its Off State current limit
should be set to 1mA. The other SMU, whose On State voltage
limit is higher, should be configured using an output off-mode
of NORMAL with an off function of current and its Off State
voltage limit should be set to 40V. To illustrate this, let’s use
Figure 6 as an example. For this configuration, both SMU’s
output off-mode should be set to NORMAL. Also, SMU #1
should have its off function set to current with an off limit of
40V and SMU #2 should have its off function set to voltage with
an off limit of 1mA. (The 40V and 1mA off limits are provided
in the configuration guidelines in the reference manual of the
Model 2651A.)
Setup of this new output off mode configuration is done
through two new ICL commands:
• smua.source.offfunc
• smua.source.offlimitv
smua.source.offfunc is used to select the off function,
for example:
smua.source.offfunc = smua.OUTPUT_DCVOLTS
-- Sets the off function to voltage
smua.source.offfunc = smua.OUTPUT_DCAMPS
-- Sets the off function to current
smua.source.offlimitv is used to set the voltage limit of the
Off State configuration when the off function is current. It is
similar to the command smua.source.offlimiti, which sets
the current limit for the off state when the off function is voltage.
Example usage follows:
smua.source.offlimitv = 10
-- Sets the off state voltage limit to 10V
Correctly Setting the Output Off-Mode
If you configure an SMU as a current source and do not change
the off-mode, then when you turn the output off, the SMU will
switch its source function from current to voltage and begin
sourcing 0V. If you did not anticipate this switch, you could
have a problem as the SMU essentially becomes a short to
whatever is connected to it. If you had two SMUs in parallel
and the SMU whose output was still on was operating as a
voltage source when the other SMU’s output was turned off, you
would have two voltage sources in parallel, which could result
in excessive current flow and could potentially damage the SMU.
Figure 7 shows what would happen if a connection to the
DUT were severed. SMU #2, whose voltage limit is lower, would
go into compliance and SMU #1, with a higher voltage limit,
would deliver all of its current to SMU #2. If SMU #1’s output
were to be shut off unexpectedly and its output mode turned
it into a 0V voltage source, then we would have a 0V voltage
source in parallel with a 9V voltage source. In this case, SMU
#2 would come out of compliance and switch back to a current
source. However, uncontrolled current may flow before this
switch occurs.
If SMU #1 had its output off function configured as a current
source, the unexpected shut off of SMU #1’s output would not
have resulted in two voltage sources in parallel. Instead, SMU #1
would have simply dropped to a 0A current source. Because SMU
#1’s voltage limit was set higher than SMU #2’s voltage limit,
SMU #2 would remain in compliance but now no current would
flow in the system since SMU #1 is still in control and forcing 0A.
If the opposite situation were to occur and SMU #2’s output
turned off unexpectedly, the situation would still be safe. SMU
#2, whose off function was configured as a voltage source,
would simply drop down from the 9V state to 0V. This is not
a problem as SMU #1 is still a current source and holds the
current to the 20A it was sourcing. The system is still not settled,
however, since SMU #2 is configured with an off limit of 1mA.
Because of this, SMU #2 goes into compliance, becomes a 1mA
current source, and begins to raise its output voltage to try to
limit current to 1mA. At this state, we have two current sources
in parallel. As SMU #2 continues to ramp its output voltage,
SMU #1 goes into compliance at 10V and becomes a 10V voltage
source. In this state, SMU #2, a current source at this time, is in
control and only 1mA of current is flowing.
Example
This example is designed to collect Rds(on) measurement data
from a power MOSFET device by using a pulsed current sweep
to test up to 100A, however, it can be easily modified for use in
other applications.
Required Equipment
This example requires the following equipment:
• Two Model 2651A High Power System SourceMeter
Instruments that will be connected in parallel to source up
to 100A pulsed through the drain of the DUT
• One Model 26xxA System SourceMeter Instrument to control
the gate of the DUT
• Two TSP-Link® cables for communications and precision
timing between instruments
www.keithley.com65
NOTE: You can also perform a TSP-Link reset from the remote
command interface by sending tsplink.reset() to Model
2651A #1.
0.020
Vgs = 10V
Vgs = 6V
Rds (ohms)
0.018
1.Press MENU.
0.016
2.Select TSPLink.
3.Select RESET.
0.014
NOTE: If error 1205 is generated during the TSP-Link reset,
ensure that Model 2651A #2 and Model 26xxA have unique
TSP-Link node numbers.
0.012
0.010
0
10
20
30
40
50
60
70
80
90
100
Ids (Amps)
Figure 8: Example results.
• One GPIB cable or one Ethernet cable to connect the
instruments to a computer
Communications Setup
The communication setup is illustrated in Figure 9. GPIB is
being used to communicate with the PC, but this application can
be run using any of the supported communication interfaces.
The TSP-Link connection enables communication between the
instruments, precision timing, and tight channel synchronization.
To configure the TSP-Link communication interface, each
instrument must have a unique TSP-Link node number.
Configure the node number of Model 2651A #1 to 1, Model
2651A #2 to 2, and Model 26xxA to 3.
GPIB
Model 2651A SMU #1
(TSP-Link Node #1)
Model 2651A SMU #2
(TSP-Link Node #2)
Controller
TSP-LINK
Series 2600A SMU
(TSP-Link Node #3)
Figure 9: Communications setup for examples.
To set the TSP-Link node number using the front panel
interface of either instrument:
1.Press MENU.
Device Connections
Connections from the SourceMeter instruments to the DUT can
be seen in Figure 10. Proper care should be taken to ensure
good contact through all connections.
NOTE: For best results, all connections should be left floating
and no connections should be tied to ground. Also, all
connections should be made as close to the device as possible
to minimize errors caused by voltage drops between the DUT
and the points in which the test leads are connected.
Gate Resistor
(if required)
D
G
HI
SHI
S
Model
26xxA
SMU
HI
SHI
HI
SHI
Model
2651A
SMU #1
Model
2651A
SMU #2
SLO
LO
SLO
LO
SLO
LO
Figure 10: Connections for dual SMU Rds(on) sweep.
NOTE: During high current pulsing, the gate of your DUT may
begin to oscillate, creating an unstable voltage on the gate and
thus unstable current through the drain. To dampen these
oscillations and stabilize the gate, a resistor can be inserted
between the gate of the device and the Force and Sense Hi
leads of the Model 26xxA. If the gate remains unstable after
inserting a dampening resistor, enable High-C mode on the
Model 26xxA (leaving the dampening resistor in place).
2.Select TSPLink.
Configuring the Trigger Model
3.Select NODE.
In order to achieve tight timing and 100A pulses with two Model
2651As, the advanced trigger model must be used. Using the
trigger model, we can keep the 50A pulses of the two Model
2651As synchronized to within 500ns to provide a single 100A
pulse. Figure 11 illustrates the complete trigger model used in
this example.
4.Use the navigation wheel to adjust the node number.
5.Press ENTER to save the TSP-Link node number.
On Model 2651A #1, perform a TSP-Link reset to alert Model
2651A #1 to the presence of Model 2651A #2 and Model 26xxA:
66
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In this example, Model 2651A #1 is configured to control the
overall timing of the sweep while Model 2651A #2 is configured
to wait for signals from Model 2651A #1 before it can generate a
pulse. The Model 26xxA is controlled by script in this example,
so its trigger model is not used.
Model 2651A #1 Trigger Model Operation
In Model 2651A #1’s trigger model (Figure 12), Timer 1 is used
to control the period of the pulse while Timer 2 is used to
control the pulse width. TSP-Link Trigger 1 is used to tell Model
2651A #2 to output its pulse.
When the trigger model of Model 2651A #1 is initialized, the
following occurs:
1.The SMU’s trigger model leaves the Idle state, flows through
the Arm Layer, enters the Trigger Layer, outputs the ARMED
event trigger, and then reaches the Source Event where it
waits for an event trigger.
2.The ARMED event trigger is received by Timer 1, which
begins its countdown and passes the trigger through to be
received by TSP-Link Trigger 1, and the SMU’s Source Event.
3.TSP-Link Trigger 1 receives the event trigger from Timer 1
and sends a trigger through the TSP-Link to Model 2651A #2
to instruct it to output the pulse.
4.The SMU’s Source Event receives the event trigger from Timer
1, begins to output the pulse, waits the programmed source
delay, if any, outputs the SOURCE_COMPLETE event to Timer
2, and then lets the SMU’s trigger model continue.
5.Timer 2 receives the SOURCE_COMPLETE event trigger from
Timer 1 and begins to count down.
6.The SMU’s trigger model continues to the Measure Event
where it waits a programmed measure delay, if any, takes a
measurement, and then continues until it hits the End Pulse
Event where it waits for an event trigger.
7.Timer 2’s countdown expires and Timer 2 outputs an event
trigger to the SMU’s End Pulse Event.
8.The SMU’s End Pulse Event receives the event trigger from
Timer 2, outputs the falling edge of the pulse, then lets the
SMU’s trigger model continue.
9.The SMU’s trigger model then compares the current Trigger
Layer loop iteration with the trigger count.
Model 2651A #2 Trigger Model Operation
In Model 2651A #2’s trigger model (Figure 13), Timer 1 is
used to control the pulse width and is programmed with the
same delay as Model 2651A #1’s Timer 2. The pulse period is
controlled by TSP-Link Trigger 1, which receives its triggers
from Model 2651A #1’s Timer 1, thus the pulse period for Model
2651A #2 is controlled by the same timer as the Model 2651A #1.
When the trigger model of Model 2651A #2 is initialized, the
following occurs:
1.The SMU’s trigger model leaves the Idle state, flows through
the Arm Layer, enters the Trigger Layer, and then reaches the
Source Event where it waits for an event trigger.
2.TSP-Link Trigger 1 receives a trigger from TSP-Link and
outputs an event trigger to the SMU’s Source Event.
3.The SMU’s Source Event receives the event trigger from
TSP-Link Trigger 1, begins to output the pulse, waits for
a programmed source delay, if any, outputs the SOURCE_
COMPLETE event to Timer 1, and then lets the SMU’s trigger
model continue.
4.Timer 1 receives the SOURCE_COMPLETE event trigger from
TSP-Link Trigger 1 and begins its countdown.
5.The SMU’s trigger model continues until it reaches the
Measure Event where it waits for a programmed measure
delay, if any, takes a measurement, and then continues until
it hits the End Pulse Event where it stops and waits for an
event trigger.
6.Timer 1’s countdown expires and Timer 1 outputs an event
trigger to the SMU’s End Pulse Event.
7.The SMU’s End Pulse Event receives the event trigger from
Timer 1, outputs the falling edge of the pulse, then lets the
SMU’s trigger model continue.
8.The SMU’s trigger model compares the current Trigger Layer
loop iteration with the trigger count.
a. If the current iteration is less than the trigger count, then
the trigger layer repeats and the SMU’s trigger model
reaches Source Event where it waits for another trigger
from TSP-Link Trigger 1. The trigger model then repeats
from Step 2.
b.If the current iteration is equal to the trigger count, then
the SMU’s trigger model exits the Trigger Layer, passes
through the Arm Layer, and returns to the Idle state.
a. If the current iteration is less than the trigger count, then
the trigger layer repeats and the SMU’s trigger model
reaches Source Event where it waits for another trigger
from Timer 1. Because Timer 1 had its count set to the
trigger count minus one, Timer 1 will continue to output
a trigger for each iteration of the Trigger Layer loop. The
trigger model then repeats from Step 3.
Example Program Code
b.If the current iteration is equal to the trigger count, then
the SMU’s trigger model exits the Trigger Layer, passes
through the Arm Layer, and returns to the Idle state.
The TSP script for this example contains all the code
necessary to perform a pulsed Rds(on) sweep up to 100A using
two Model 2651A High Power System SourceMeter instruments
NOTE: The example code is designed to be run from Test
Script Builder or TSB Embedded. It can be run from other
programming environments such as Microsoft® Visual Studio
or National Instruments LabVIEW®, however, modifications
may be required.
www.keithley.com67
Model 2651A (master)
node[1].
smua.trigger.
Idle
Arm layer
IDLE_EVENT_ID
SWEEPING_EVENT_ID
arm.count = 1
arm.stimulus
trigger.timer[1].
stimulus
50e-3 s
passthrough = true
EVENT_ID
Trigger layer
count = 99
stimulus
500e-6 s
passthrough = false
EVENT_ID
ARMED_EVENT_ID
count = 100
source.stimulus
trigger.timer[2].
SWEEP_COMPLETE_EVENT_ID
measure.stimulus
endpulse.stimulus
SOURCE_COMPLETE_EVENT_ID
MEASURE_COMPLETE_EVENT_ID
PULSE_COMPLETE_EVENT_ID
count = 1
tsplink.trigger[1].
EVENT_ID
mode = tsplink.TRIG_FALLING
stimulus
Model 2651A (subordinate)
node[2].
smua.trigger.
Idle
Arm layer
tsplink.trigger[1].
EVENT_ID
mode = tsplink.TRIG_FALLING
IDLE_EVENT_ID
SWEEPING_EVENT_ID
arm.count = 1
arm.stimulus
Trigger layer
stimulus
SWEEP_COMPLETE_EVENT_ID
ARMED_EVENT_ID
count = 100
source.stimulus
trigger.timer[1].
stimulus
500e-6 s
passthrough = false
EVENT_ID
measure.stimulus
endpulse.stimulus
SOURCE_COMPLETE_EVENT_ID
MEASURE_COMPLETE_EVENT_ID
PULSE_COMPLETE_EVENT_ID
count = 1
Figure 11: Example of a complete trigger model for Rds(on) sweep up to 100A.
68
www.keithley.com
node[1] (Model 2651A)
node[1].
smua.trigger.
Idle
Arm layer
IDLE_EVENT_ID
SWEEPING_EVENT_ID
arm.count = 1
arm.stimulus
trigger.timer[1].
stimulus
50e-3 s
passthrough = true
EVENT_ID
Trigger layer
count = 99
stimulus
500e-6 s
passthrough = false
EVENT_ID
ARMED_EVENT_ID
count = 100
source.stimulus
trigger.timer[2].
SWEEP_COMPLETE_EVENT_ID
measure.stimulus
endpulse.stimulus
SOURCE_COMPLETE_EVENT_ID
MEASURE_COMPLETE_EVENT_ID
PULSE_COMPLETE_EVENT_ID
count = 1
tsplink.trigger[1].
EVENT_ID
mode = tsplink.TRIG_FALLING
stimulus
Figure 12: Example of a trigger model for 2651A #1 for Rds(on) sweep up to 100A.
node[2] (Model 2651A)
node[2].
smua.trigger.
Idle
Arm layer
tsplink.trigger[1].
EVENT_ID
mode = tsplink.TRIG_FALLING
IDLE_EVENT_ID
SWEEPING_EVENT_ID
arm.count = 1
arm.stimulus
Trigger layer
stimulus
SWEEP_COMPLETE_EVENT_ID
ARMED_EVENT_ID
count = 100
source.stimulus
trigger.timer[1].
stimulus
500e-6 s
passthrough = false
EVENT_ID
measure.stimulus
endpulse.stimulus
SOURCE_COMPLETE_EVENT_ID
MEASURE_COMPLETE_EVENT_ID
PULSE_COMPLETE_EVENT_ID
count = 1
Figure 13: Example of a trigger model for 2651A #2 for Rds(on) sweep up to 100A.
www.keithley.com69
and a Model 26xxA System SourceMeter instrument. This script
can also be downloaded from Keithley’s website at
www.keithley.com/base_download?dassetid=55808.
The script performs the following functions:
• Initializes the TSP-Link connection
• Configures all the SMUs
• Configures the trigger models of the two Model 2651As
• Prepares the readings buffers
• Initializes the sweep
• Processes and returns the collected data in a format that can
be copied and pasted directly into Microsoft Excel®
The script is written using TSP functions rather than a single
block of inline code. TSP functions are similar to functions in
other programming languages such as C or Visual Basic and
must be called before the code contained in them is executed.
Because of this, running the script alone will not execute the
test. To execute the test, run the script to load the functions
into Test Script memory and then call the functions. Refer to
the documentation for Test Script Builder or TSB Embedded for
directions on how to run scripts and enter commands using the
instrument console.
Within the script, you will find several comments describing
what is being performed by the lines of code as well as
documentation for the functions contained in the script. Lines
starting with
node[2].
are commands that are being sent to Model 2651A #2 through
the TSP-Link interface. Lines starting with
node[3].
70
are commands that are being sent to the Model 26xxA through
the TSP-Link interface. All other commands are executed on the
Model 2651A #1.
Example Program Usage
The functions in this script are designed such that the sweep
parameters of the test can be adjusted without needing to
rewrite and re-run the script. A test can be executed by calling
the function
DualSmuRdson()
with the appropriate values passed in its parameters.
Parameters of the function DualSmuRdson()
Parameter
gateLevel
dstart
dstop
dsteps
pulseWidth
pulsePeriod
pulseLimit
Units
Description
Voltage level to which the gate will be held
Volts
during the test
Amps
Level of the first step in the drain sweep
Amps
Level of the last step in the drain sweep
N/A
Number of steps in the drain sweep
Seconds Width of the pulse in the drain sweep
Time between the start of consecutive pulses in the
Seconds
drain sweep
Volts
Voltage limit of the pulses in the drain sweep
This is an example call to function DualSmuRdson().
DualSmuRdson(10, 1, 100, 100, 500e-6, 50e-3, 10)
This call sets the gate SMU output to 10V, then sweeps the drain
of the DUT from 1A to 100A in 100 points. The points of this
sweep will be gathered using pulsed measurements with a pulse
width of 500µs and a pulse period of 50ms for a 1% duty cycle.
These pulses are limited to a maximum voltage of 10V. At the
completion of this sweep, all SMU outputs will be turned off
and the resulting data from this test will be returned in an Excel
compatible format for graphing and analysis.
www.keithley.com
Example Test Script Processor (TSP®) Script
--[[
Title:
Combining SMUs for 100A Example
Description:
This script is designed to perform an Rds(on)sweep on a power
MOSFET device. It combines two 2651A SMUs in parallel to perform a current
sweep up to 100A. Data collected from the sweep is then returned in a
Microsoft Excel compatible format for plotting and analysis.
Equipment needed:
2x 2651A
1x 26xxA
2x TSP-Link Cable
TSP-Link Configuration:
----------------------Unit
|
Node #
2651A #1
|
1
2651A #2
|
2
26xxA
|
3
]]
Master Node (PC Interface): Node 1
--[[
Name:
DualSmuRdson(gateLevel, dstart, dstop, dsteps, pulseWidth,
pulsePeriod, pulseLimit)
Description:
This function uses two 2651A SMUs to perform a pulsed Rds(on)
sweep with currents up to 100A.
Parameters:
gateLevel:
The gate level to be used during the sweep
dstart:
The starting current level of the drain sweep
dstop:
The ending current level of the drain sweep
The number of steps in the drain sweep
dsteps:
pulseWidth: The width of the drain pulse in seconds
pulsePeriod: The time from the start of one drain pulse to
the next in seconds
pulseLimit: The voltage limit of the drain pulse in volts
Note: Actual pulse limit will be 10% lower than setting
to protect SMUs in a compliance condition
]]
Example Usage:
DualSmuRdson(10, 1, 100, 100, 500e-6, 50e-3, 10)
function DualSmuRdson(gateLevel, dstart, dstop, dsteps, pulseWidth, pulsePeriod, pulseLimit)
tsplink.reset(3) -- Verify that at least three nodes are present
reset()
-- Configure 2651A #1 (Drain SMU 1)
----------------------------------smua.reset()
smua.source.func
= smua.OUTPUT_DCAMPS
smua.sense
= smua.SENSE_REMOTE
smua.source.offmode= smua.OUTPUT_NORMAL
smua.source.offfunc= smua.OUTPUT_DCVOLTS
= 1e-3 -- Set off limit
smua.source.offlimiti
-- SMU #1 will be a 0V voltage source with 1mA limit when its
-- output is turned off. SMU #2 will be a 0A current source with
-- a 10V limit when the output is turned off. These settings keep
-- the parallel combination safe in case one SMU is turned off.
smua.source.rangei = math.max(math.abs(dstart / 2), math.abs(dstop / 2))
smua.source.leveli
= 0
-- Sets the DC bias level
smua.source.limitv
= 9
-- Sets the DC bias limit
-- SMU #2 will have a voltage limit of 10V. By setting the voltage
-- limit 10% lower than that of SMU #2, we can ensure that only
-- one of the two SMUs will ever go into compliance and become a
-- voltage source. This is desirable, because if both SMUs went
-- into compliance, there would be two voltage sources in parallel,
-- which is an unsafe condition.
www.keithley.com71
smua.measure.nplc
= 0.005
= pulseLimit
smua.measure.rangev
smua.measure.autozero
= smua.AUTOZERO_ONCE
smua.measure.delay
=
pulseWidth - ((1 / localnode.linefreq) * smua.measure.nplc)) - 20e-6
-- Set the delay so that the measurement is near the end of the pulse
-- Prepare the reading buffers
smua.nvbuffer1.clear()
smua.nvbuffer1.collecttimestamps =
smua.nvbuffer1.collectsourcevalues =
smua.nvbuffer1.fillmode
smua.nvbuffer2.clear()
smua.nvbuffer2.collecttimestamps =
smua.nvbuffer2.collectsourcevalues =
smua.nvbuffer2.fillmode
1
1
1
1
= smua.FILL_ONCE
= smua.FILL_ONCE
-- Configure TSP-Link Trigger 1
tsplink.trigger[1].clear()
tsplink.trigger[1].mode
= tsplink.TRIG_FALLING
tsplink.trigger[1].stimulus
= trigger.timer[1].EVENT_ID
-- TSP-Link Trigger 1 signals 2651A #2 to pulse
-- Timer 1 controls the pulse period by triggering the pulse to begin
= dsteps - 1
trigger.timer[1].count
trigger.timer[1].delay
= pulsePeriod
trigger.timer[1].passthrough = true
trigger.timer[1].stimulus
= smua.trigger.ARMED_EVENT_ID
trigger.timer[1].clear()
-- Timer 2 controls the pulse width
trigger.timer[2].count
= 1
= pulseWidth - 3e-6
trigger.timer[2].delay
trigger.timer[2].passthrough = false
trigger.timer[2].stimulus
= smua.trigger.SOURCE_COMPLETE_EVENT_ID
trigger.timer[2].clear()
-- Configure SMU Trigger Model for Sweep
-- Each unit will source half the current, so divide the start
-- and stop values by 2
smua.trigger.source.lineari(dstart / 2, dstop / 2, dsteps)
smua.trigger.source.limitv = pulseLimit - (pulseLimit * 0.1)
-- Again, keep the limit SMU #1 lower than the limit of SMU #2
-- to prevent parallel V-sources
smua.trigger.measure.iv(smua.nvbuffer1, smua.nvbuffer2)
smua.trigger.measure.action = smua.ENABLE
-- Return to the bias level at the end of the pulse/sweep
smua.trigger.endpulse.action = smua.SOURCE_IDLE
smua.trigger.endsweep.action = smua.SOURCE_IDLE
smua.trigger.count
=
smua.trigger.arm.stimulus
=
smua.trigger.source.stimulus =
smua.trigger.measure.stimulus
smua.trigger.endpulse.stimulus
smua.trigger.source.action =
dsteps
0
trigger.timer[1].EVENT_ID
= 0
= trigger.timer[2].EVENT_ID
smua.ENABLE
-- Configure 2651A #2 (Drain SMU 2)
----------------------------------node[2].smua.reset()
node[2].smua.source.func
= node[2].smua.OUTPUT_DCAMPS
node[2].smua.sense
= node[2].smua.SENSE_REMOTE
node[2].smua.source.offmode= node[2].smua.OUTPUT_NORMAL
node[2].smua.source.offfunc= node[2].smua.OUTPUT_DCAMPS
node[2].smua.source.offlimitv
= 10 -- Set off limit
-- SMU will be a 0A current source with 10V limit when output is turned off
node[2].smua.source.rangei
=
math.max(math.abs(dstart / 2), math.abs(dstop / 2))
node[2].smua.source.leveli = 0
-- Sets the DC bias level
node[2].smua.source.limitv = 10 -- Sets the DC bias limit
node[2].smua.measure.nplc
72
= 0.005
www.keithley.com
node[2].smua.measure.rangev= pulseLimit
= node[2].smua.AUTOZERO_ONCE
node[2].smua.measure.autozero
node[2].smua.measure.delay = (pulseWidth ((1 / node[2].linefreq) * node[2].smua.measure.nplc)) - 20e-6
-- Set the delay so that the measurement is near the end of the pulse
-- Prepare the reading buffers
node[2].smua.nvbuffer1.clear()
node[2].smua.nvbuffer1.collecttimestamps
node[2].smua.nvbuffer1.collectsourcevalues
node[2].smua.nvbuffer1.fillmode
node[2].smua.nvbuffer2.clear()
node[2].smua.nvbuffer2.collecttimestamps
node[2].smua.nvbuffer2.collectsourcevalues
node[2]. smua.nvbuffer2.fillmode
-- Configure TSP-Link Trigger 1
node[2].tsplink.trigger[1].clear()
node[2].tsplink.trigger[1].mode
= 1
= 1
= node[2].smua.FILL_ONCE
= 1
= 1
= node[2].smua.FILL_ONCE
= node[2].tsplink.TRIG_FALLING
-- Timer 1 controls the pulse width
node[2].trigger.timer[1].count
= 1
node[2].trigger.timer[1].delay
= pulseWidth - 3e-6
node[2].trigger.timer[1].passthrough
= false
node[2].trigger.timer[1].stimulus
=
node[2].smua.trigger.SOURCE_COMPLETE_EVENT_ID
node[2].trigger.timer[1].clear()
-- Configure SMU Trigger Model for Sweep
node[2].smua.trigger.source.lineari(dstart / 2, dstop / 2, dsteps)
= pulseLimit
node[2].smua.trigger.source.limitv
node[2].smua.trigger.measure.iv(node[2].smua.nvbuffer1, node[2].smua.nvbuffer2)
node[2].smua.trigger.measure.action
= node[2].smua.ENABLE
-- Return the output to the bias level at the end of the pulse/sweep
node[2].smua.trigger.endpulse.action
= node[2].smua.SOURCE_IDLE
node[2].smua.trigger.endsweep.action
= node[2].smua.SOURCE_IDLE
node[2].smua.trigger.count
= dsteps
node[2].smua.trigger.arm.stimulus
= 0
node[2].smua.trigger.source.stimulus
= node[2].tsplink.trigger[1].EVENT_ID
node[2].smua.trigger.measure.stimulus = 0
node[2].smua.trigger.endpulse.stimulus = node[2].trigger.timer[1].EVENT_ID
node[2].smua.trigger.source.action
= node[2].smua.ENABLE
-- Configure the 26xxA (Gate SMU)
--------------------------------node[3].smua.reset()
node[3].smua.source.func
= node[3].smua.OUTPUT_DCVOLTS
node[3].smua.sense
= node[3].smua.SENSE_REMOTE
node[3].smua.source.levelv = gateLevel
node[3].smua.source.highc = node[3].smua.ENABLE
-- If you find your gate oscillating even with a dampening resistor
-- in place, try enabling high-C mode to help stabilize the gate.
-- Prepare the reading buffers
node[3].smua.nvbuffer1.clear()
node[3].smua.nvbuffer1.collectsourcevalues
if node[3].smua.nvbuffer1.fillmode
node[3].smua.nvbuffer1.fillmode
end
node[3].smua.nvbuffer2.clear()
node[3].smua.nvbuffer2.collectsourcevalues
if node[3].smua.nvbuffer2.fillmode
node[3].smua.nvbuffer2.fillmode
end
= 1
~= nil then
= node[3].smua.FILL_ONCE
= 1
~= nil then
= node[3].smua.FILL_ONCE
--------------------------- Ready to begin the test
--------------------------- Outputs on
node[3].smua.source.output
= node[3].smua.OUTPUT_ON
node[2].smua.source.output
= node[2].smua.OUTPUT_ON
smua.source.output
= smua.OUTPUT_ON
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if errorqueue.count > 0 then
print(“Errors were encountered”)
reset()
return
end
-- Give the gate some time to settle before starting the sweep
delay(0.001)
node[3].smua.measure.iv(node[3].smua.nvbuffer1, node[3].smua.nvbuffer2)
-- Start the 2651A #2 trigger model
node[2].smua.trigger.initiate()
-- Start the 2651A #1 trigger model
smua.trigger.initiate()
-- Wait until test is complete
waitcomplete()
-- Outputs off
= node[3].smua.OUTPUT_OFF
node[3].smua.source.output
smua.source.output
= smua.OUTPUT_OFF
node[2].smua.source.output
= node[2].smua.OUTPUT_OFF
end
-- Print back data
PrintDualSmuRdsonData()
--[[
Function:
PrintDualSmuRdsonData()
Description:
This function processes the data stored in the SMU reading buffers by
function DualSmuRdson() and prints back the individual SMU data and the
combined SMU data and Rds(on) readings in a format that is copy and paste
compatible with Microsoft Excel.
]]
function PrintDualSmuRdsonData()
-- Print the gate SMU readings
print(“Gate SMU\r\nSource Value\tVoltage\tCurrent”)
print(string.format(“%0.2f\t%g\t%g\r\n”,
node[3].smua.nvbuffer1.sourcevalues[1],
node[3].smua.nvbuffer2[1],
node[3].smua.nvbuffer1[1]))
-- Print column headers
print(“Timestamp\tSource Value\tVoltage 1\tCurrent 1\tVoltage
2\tCurrent 2\tVoltage\tCurrent\tRds(on)”)
-- Loop through the reading buffer printing one row at a time
for i = 1,smua.nvbuffer1.n do
-- Combined Source Level = SMU1 source level + SMU2 source level
sourceLevel = smua.nvbuffer1.sourcevalues[i] +
node[2].smua.nvbuffer1.sourcevalues[i]
-- Combined Voltage = Average(SMU1 Voltage reading, SMU2 Voltage reading)
combinedVoltage = (smua.nvbuffer2[i] + node[2].smua.nvbuffer2[i]) / 2
-- Combined Current = SMU1 Current reading + SMU2 Current reading
combinedCurrent = smua.nvbuffer1[i] + node[2].smua.nvbuffer1[i]
-- Rds(on) = Combined Voltage / Combined Current
rdson = combinedVoltage / combinedCurrent
end
74
-- Print a row of data
print(string.format(“%g\t%g\t%g\t%g\t%g\t%g\t%g\t%g\t%g”,
smua.nvbuffer1.timestamps[i],
sourceLevel,
smua.nvbuffer2[i],
smua.nvbuffer1[i],
node[2].smua.nvbuffer2[i],
node[2].smua.nvbuffer1[i],
combinedVoltage,
combinedCurrent,
rdson))
end
74
www.keithley.com
Optimizing Reliability Testing of Power
Semiconductor Devices and Modules with
Keithley SMU Instruments and Switch Systems
Introduction
To minimize early defect rates and to continuously improve the
overall reliability and lifetime of power semiconductors, a variety
of important tests are performed by both manufacturers and
end-use designers. Many of these tests are outlined in JEDEC
Standards such as JESD22-A108D “Temperature, Bias, and
Operating Life,” JESD22-A110D “Highly Accelerated Temperature
and Humidity Stress Test (HAST),” or JESD236 “Reliability
Qualification of Power Amplifier Modules.” This application brief
discusses methods to optimize reliability testing of silicon and
wide band gap (WBG) power semiconductor devices, modules,
and materials by using Keithley SourceMeter® Source Measure
Unit (SMU) Instruments and Switch Systems (Figures 1 and 2).
voltages while subjecting them to temperatures that are well
beyond normal operating conditions. During this stress, a variety
of key operating parameters are measured at specific time
intervals. Some of the more popular reliability tests for power
semiconductors are HTOL (High Temperature Operating Life),
ELFR (Early Life Failure Rate), HTFB (High Temperature Forward
Bias), HTRB (High Temperature Reverse Bias), and HAST (Highly
Accelerated Temperature & Humidity Stress Test). These tests will
either use a continuous bias (Figure 3) or cycled bias (Figure 4).
A continuous bias can be a fixed voltage or a staircase ramp. A
cycled bias will typically vary the duty cycle and/or frequency
of the bias voltage. In both cases, key device parameters will be
tested continuously or at specific time intervals.
Reliability Testing Challenges
Figure 1. Keithley Series 2650A High Power SourceMeter SMU Instruments.
Reliability testing of today’s WBG power semiconductors presents
several key challenges for engineers and test system designers.
Most importantly, since most of these devices are being targeted
for energy-efficiency applications, they have much lower leakage
and on-resistance specifications compared to traditional silicon.
The test instrumentation must therefore be capable of providing
the necessary accuracy, resolution, and stability to meet the
electrical requirements of these devices. In addition, since
WBG devices exhibit failure mechanisms that are different from
silicon, effective reliability testing per JEDEC standards requires
larger sample sizes and longer stress durations to adequately
predict important reliability parameters. This requires test
instrumentation that is capable of supplying enough power to
test many devices in parallel, while maintaining the accuracy and
resolution mentioned above. Finally, the test instrumentation
must be able to respond to the high speed behaviors associated
with these devices and produce the masses of data associated
with testing devices in parallel. Each instrument in the
system must be fast, and all units must operate in a highly
synchronized manner.
Keithley Solutions
Typical Reliability Tests
Keithley reliability test solutions based on the Series 2600B
and 2650A System SourceMeter SMU Instruments meet these
demanding requirements (Figure 5). Keithley’s Series 2600B and
2650A are modular, independent, isolated SMUs that provide
up to 200W per channel with measurement resolutions to
sub-pA levels.
Typical reliability tests involve stressing a batch or batches
of sample devices for hundreds or thousands of hours with bias
voltages that are greater than or equal to their normal operating
In addition, all Series 2600B and 2650A SMU Instruments
contain Keithley’s Test Script Processor (TSP®) and TSP-Link®
technology for ultra- high speed operation and parallel test.
Figure 2. Keithley Series 3700A and 707B Series Switch Systems.
www.keithley.com75
Vd = 200V
Vd = 1320V
+5V steps every 8 hours
Vd = 50V
Vd = 1200V
±120V steps every 10 hours
Temp = 140°C
Temp = 130°C
Temp = 80°C
0
48
96
144
192
240
0
Hours
240
480
720
960
Hours
Figure 3. Continuous Bias Test. Vd=50V. Step temperature 15°C every 48
hours from 80°C to 140°C. Step Vd 5V every 8 hours from 50V to 200V. Collect
I-V curve data before and after each voltage step. Test 72 devices per batch.
Figure 4. Cycled Bias Test. Vd cycles between 1200V to 1320V every 10 hours
with a 50% duty cycle. Maintain temperature of 130°C for 1000 hours, and
continuously measure Id. Test 240 devices per batch.
Keithley’s TSP technology enables the instrument to perform
advanced tests without PC intervention through the use
of embedded test scripts. These scripts are complete test
subroutines executed from the instrument’s non-volatile memory
that can perform conditional branching, flow control, advanced
calculations, pass-fail testing, and more. On-board memory
buffers can store over 140,000 readings, and can be queried
while data is being written to them. For larger, multi-channel
applications, Keithley’s TSP-Link technology works together with
TSP technology to enable high-speed, multi-channel, SMU-perpin parallel testing or seamless integration with Keithley’s Series
3700A or Series 700 Switch Mainframes (Figure 6). The Keithley
Series 3700A Mainframes also have a temperature measurement
option that can be used for monitoring internal oven
temperature at multiple locations. Synchronization with external
equipment such as temperature chambers is enabled by the
built-in digital I/O capability. Because Series 2600B and 2650A
instruments have fully isolated channels that do not require a
mainframe, they can easily be reconfigured and re-deployed as
reliability test requirements evolve.
This test cycles the drain voltage (Vd) from 1200V to 1320V every
10 hours with a 50% duty cycle, and maintains a temperature of
130°C for 1000 hours while continuously measuring the devices’
off-state leakage current (Id). A batch size of 240 is tested, so the
most economical approach is to use a switch system to route the
power of one SMU instrument to all DUTs.
Solution Details
Reliability test algorithms are typically very specific to the device
under test (DUT) and its market requirements. In this application
brief, we are referring to the cycled bias test example in Figure 4.
An example system block diagram is shown in Figure 7*.
The Keithley Model 2657A High Voltage SMU Instrument
provides power to the drain terminal of all 240 DUTs, which
are connected in parallel. Maximum current available from the
Model 2657A at voltages of 1200V–1320V is 120mA, so there is
plenty of capacity. In fact, the Model 2657A can supply power to
multiple batches. Current limiting resistors are placed in series
with each DUT in case of device failure.
The Keithley Model 2635B SMU Instrument is used to
measure the leakage current (Id) of each device at the source
terminal, and is connected to each device through the Keithley
Model 3706A Switch System with Model 3720 Switch Cards. A
low-leakage diode is also placed at the source terminal in parallel
with each switch. When a given switch is open, any leakage
current from the DUT will flow through this diode. When a
given switch is closed, the leakage current for that DUT will flow
through the Keithley Model 2635B (since its output is 0V when
measuring current). The Model 3720 Switch Cards are configured
Model 2657A: 180W, High Voltage
Model 2636B: Dual Channel, Low Current
Model 2651A: 200W, High Current
• Source and measure up to 3000V @ 20mA or
1500V @ 120mA.
• Basic voltage source accuracy of 0.03% with
5mV programming resolution.
• Basic current measure accuracy of 0.02%.
1fA resolution.
• Source and measure up to 200V @ 100mA or
20V @ 1.5A per channel.
• Basic voltage source accuracy of 0.03% with 5µV programming resolution.
• Basic current measure accuracy of 0.02%.
0.1fA resolution.
• Source and measure up to 40V @ 5A
or 10V @ 20A
• Basic voltage source accuracy of 0.02% with
5µV programming resolution.
• Basic current measure accuracy of 0.02%.
1pA resolution.
Figure 5. Key specifications of Keithley SourceMeter SMU instruments.
76
www.keithley.com
for single-pole switching, and each card
has a 60-channel capacity; therefore, four
switch cards are used (the Model 3706A
Switch System chassis has a total capacity
of six switch cards). The scanning speed
of the Model 3720 Switch Card is 120
channels per second.
Model 3706A: High Speed Switch System
Model 707B: Low Current Switch System
• Up to 576 channels of 2-pole switching
• Multiplexer or matrix configurations with 300V,
1A capability
• Built-in temperature measurement option.
• Up to 576 matrix cross-points
• 1300V, 1A switching with <1pA offset
• 200V, 2A switching with <100fA offset
Figure 6. Key specifications of Keithley switch systems.
GPIB
PC
or LAN
2657A
TSP-Link
2635B
TSP-Link
3706A
I Limit
DUT #1
G
D
I Limit
DUT#240
G
S
D
S
3720 Cards
Ch.1
Low Leakage Diode
Ch.240
Low Leakage Diode
For maximum system speed and
programming simplicity, the TSP-Link
intercommunication bus is used to
connect the three instruments together.
Only one connection is needed to the PC
controller, and this is through the Model
2657A over GPIB or LAN (LXI). The PC
controller contains the main test program
that calls the TSP scripts (subroutines)
that reside inside the Model 2657A’s nonvolatile memory. Keithley’s TSP technology
enables all instrument control and most
data management to be performed at
the instrument level, thus eliminating
the typical GPIB or LAN traffic delays
that slow system-level throughput in
instrument-based systems.
In this example, Keithley’s Series
2600B and 2650A System SourceMeter
SMU Instruments and Series 3700A
Switch Systems are used to meet the
accuracy, resolution, power, and speed
requirements of today’s semiconductor
reliability test applications.
Figure 7.
*This example is for illustrative purposes only. Full test system design and adherence to proper safety
guidelines and procedures are the responsibility of the designer and operator.
www.keithley.com77
78
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VDS Ramp and HTRB Reliability Testing of High
Power Semiconductor Devices with Automated
Characterization Suite (ACS) Software
Introduction
Wide bandgap semiconductor materials such as silicon carbide
(SiC) and gallium nitride (GaN) offer physical properties superior
to those of silicon (Si) for power device applications, enabling
devices based on these materials to withstand high voltages and
temperatures, as well as permitting higher frequency response,
greater current density, and faster switching [1]. These emerging
power devices have great potential; however, the technologies
necessary to create and refine them are still under development
and therefore less mature than silicon technology. This creates
some big challenges associated with designing and characterizing
these devices, as well as process monitoring and reliability
issues [2].
Before they can gain commercial acceptance, the reliability
of wide bandgap devices must be proven and there is a demand
for higher reliability requirements. The continuous drive for
greater power density at the device and package levels creates
consequences in terms of higher temperatures and temperature
gradients across the package. New application areas often mean
more severe ambient conditions. For example, in automotive
hybrid traction systems, the cooling liquid for the combustion
engine may reach temperatures as high as 120°C. In order to
provide sufficient margin, this means the maximum junction
temperature (TJMAX ) must be increased from 150°C to 175°C
[4]. In safety-critical applications such as aircraft, the zerodefect concept has been proposed to meet stricter reliability
requirements.
VDS Ramp and HTRB Reliability Tests
The V DS ramp and the High Temperature Reverse Bias (HTRB)
tests are among the most common reliability tests for power
devices. In a V DS ramp test, as the drain-source voltage is stepped
from a low voltage to a voltage that’s higher than the rated
maximum drain-source voltage, specified device parameters
are evaluated. The test is useful for tuning the design and
process conditions, as well as verifying that devices deliver
the performance specified on their data sheets. For example,
Dynamic R DS(ON), monitored using a V DS ramp test, provides a
measurement of how much a device’s ON-resistance increases
after being subjected to a drain bias [5]. Although a V DS ramp test
is generally used as a quick form of parametric verification, an
HTRB test evaluates long-term stability under high drain-source
bias. During an HTRB test, the device samples are stressed at or
slightly less than the maximum rated reverse breakdown voltage
(usually 100% or 80% of V RRM) at an ambient temperature close
to their maximum rated junction temperature (TJMAX ) over a
Figure 1. Automated Characterization Suite (ACS) graphical user interface
period of time (usually 1,000 hours)[3][5][6][7]. The leakage
current is continuously monitored throughout the test and a
fairly constant leakage current is generally required to pass
the test. Because it combines electrical and thermal stress, this
test can be used to check the junction integrity, crystal defects
and ionic-contamination level, which can reveal weaknesses or
degradation effects in the field depletion structures at the device
edges and in the passivation [8].
Test Instrumentation and
Measurement Considerations
Power device characterization and reliability testing require test
instrumentation with higher voltage as well as more sensitive
current measurement capability than ever before [2]. During
operation, the devices undergo both electrical and thermal stress:
when in the ON state, they have to pass tens or hundreds of amps
with minimal loss (low voltage, high current); when they are
OFF, they have to block thousands of volts with minimal leakage
currents (high voltage, low current). Additionally, during the
switching transient, they are subjected to a brief period of both
high voltage and high current. The high current experienced
during the ON state generates a large amount of heat, which may
degrade device reliability if it is not dissipated efficiently [1].
Reliability tests typically involve high voltages, long test times,
and often multiple devices under test (wafer level testing). As a
result, well-designed test systems and measurement plans are
essential to avoid breaking devices, damaging equipment, and
losing test data. Consider the following factors when executing
V DS ramp and HTRB reliability tests:
www.keithley.com79
• Device connections: When testing vertical devices with a
common drain, proper connections are required to prevent
stress termination in case of a single device breakdown.
• Current limit control: Current limit should allow for adjustment
at breakdown to avoid damage to the probe card and device.
• Stress control: The high voltage stress must be well controlled
to avoid overstressing the device, which can lead to
unexpected device breakdown.
• Proper test abort design: The test program must be designed
in a way that allows the user to abort the test (that is, terminate
the test early) without losing the data already acquired.
• Data management: Effective data collection is essential
to accommodate the large datasets due to long time and
multi-site testing
A comprehensive hardware and software solution is
essential to address these test considerations effectively.
Keithley’s Automated Characterization Suite (ACS) software
supports semiconductor characterization at the device, wafer,
and cassette levels. It offers users maximum flexibility for
performing applications, so they can switch easily between
manual operation for lab use and fully automated operation
for production settings, using the same test plan. Its integrated
test plan and wafer description function allow setting up single
or multiple test plans on one wafer and selectively executing
them later, either manually or automatically. ACS is compatible
with many of Keithley’s most advanced Source Measure Unit
(SMU) instruments, including the Model 2636B (capable of
sourcing up to 200V and measuring with 0.1fA resolution) and
the high power Model 2657A (capable of sourcing up to 3kV and
measuring with 1fA resolution).
Depending on the needs of the test environment, ACS
systems can be configured to create anything from a simple
system with a few instruments on a benchtop to an integrated,
fully automated rack of instruments on a production floor.
In addition to controlling Keithley SourceMeter® SMUs, other
instruments, and switching hardware, ACS can control most
standard automatic probers, execute the tests and return the
resulting data for parameter extraction and analysis, all through
a graphical user interface (Figure 1). The V DS breakdown
reliability test module for V DS ramp and HTRB testing
described in this application note is included in ACS
Version 5.0 [9].
Single device
a.
b.
Drain
Gate
Source
a. Drain: Connected to an SMU for VDS stress and measure
Gate: Connected to GND (ground)
Source: Connected to GND
Drain
Gate
SMU
SMU
Source
SMU
SMU
b. Drain: Connected to an SMU for VDS stress and measure
Gate: Connected to an SMU to control the state of the device
Source: Connected to an SMU to extend the range of the VDS
stress
Multi-devices
c.
Drain
Gate
Drain
Gate
Source
SMU
Source
SMU
SMU
e.
Gate
Drain
Gate
Source
SMU
SMU
Source
SMU
d. Drain: Connected to an SMU to extend the range of the
VDS stress
Gate and Source shorted: Connected to an SMU for VDS
stress and measure
e. Drain: Connected to GND
Gate: Connected to an SMU to control the state of the device
Source: Connected to an SMU for VDS stress and measure
f.
Drain
SMU
c. Drain: Connected to GND
Gate and Source shorted: Connected to an SMU for VDS
stress and measure
d.
SMU
f. Drain: Connected to an SMU to extend the range of the
VDS stress
Gate: Connected to an SMU to control the state of the device
Source: Connected to an SMU for VDS stress and measure
Figure 2. Various device connection options used with V DS breakdown test module
80
www.keithley.com
ACS VDS Breakdown Test Module
ACS’s V DS breakdown test module applies two different stress
tests across the drain and source of the MOSFET structure (or
across the collector and emitter of an IGBT) for V DS ramp and
HTRB reliability assessment:
Vds_Vramp – Applies ramped stress. This test sequence can
be used as a quick method of parametric verification. The devices
tested are evaluated on whether they meet the performance
specifications listed on their data sheets.
Vds_Constant – Applies constant stress. This test sequence
can be set up for reliability testing over an extended period and
at elevated temperature, such as an HTRB test.
Device Connection
Depending on the number of instruments and devices, or
the probe card type, users can implement various connection
schemes (Figure 2) to achieve the desired stress configurations.
When testing a single device, a user can apply voltage at the
drain only for V DS stress and measure, which requires only one
SMU per device. Alternatively, a user can connect each gate and
source to a SMU for more control in terms of measuring current
at all terminals, extend the range of V DS stress, and set voltage
on the gate to simulate a practical circuit situation. For example,
to evaluate the device in the OFF state (including HTRB test),
VGS might be set to VGS < 0 for an N-channel depletion device,
VGS > 0 for a P-channel device, or VGS = 0 for an enhancementmode device.
For multi-device testing, careful consideration of device
connections is essential. In a vertical device structure, the drain
is common; therefore, it is not used for stress sourcing so that
stress will not be terminated in case a single device breaks down.
Instead, the source and gate are used to control stress.
Test Sequence
The Vds_Vramp test sequence has three stages: pre-test, main
stress-measure, and post-test. Pre-test and post-test are optional.
During the pre-test, a constant voltage is applied to verify the
initial integrity of the body diode of the MOSFET; if the body
diode is determined to be good, the test proceeds to the main
stress-measure stage. Starting at a lower level, the drain-source
voltage stress is applied to the device and ramps linearly to a
point higher than the rated maximum voltage or until the userspecified breakdown criteria is reached. If the tested device is
not broken at the main stress stage, the test proceeds to the
next step, the post-test, in which a constant voltage is applied
to evaluate the state of the device, similar to the pre-test. The
measurements throughout the test sequence are made at both
source and gate for multi-device testing (or drain for the single
device case) and the breakdown criteria will be based on the
current measured at source (or drain for a single device). The
Vds_Vramp test sequence is useful for evaluating the effect of a
drain-source bias on the device’s parameters. Figure 3 shows an
example of a stress vs. time diagram for the main stress-measure
stage of a Vds_Vramp test. (Pre-test and post-test are not shown.)
Although the Vds_Vramp sequence is generally used as a
quick method of parametric verification, the Vds_Constant test
sequence is suitable for long-term stability evaluations, such as
HTRB, which normally run for an extended period at an elevated
temperature. The Vds_Constant test sequence has a structure
similar to that of the Vds_Vramp (that is, optional pre-test, main
stress-measure, and optional post-test), with a constant voltage
stress applied to the device during the stress stage and different
breakdown settings. The stability of the leakage current IDSS
is monitored throughout the test. Figure 4 shows an example
stress vs. time diagram for the main stress-measure stage of
a Vds_Constant test. (Pre-test and post-test are not shown.)
Because this test requires evaluation over an extended period,
the test module includes two features developed to prevent
damage to the probe and device and to reduce the size of the
dataset created: dynamic limit change and data compression.
Features to Solve Test Challenges
Several features of the ACS V DS breakdown test module are
designed to address the challenges commonly associated with
V DS ramp and HTRB reliability testing, namely the risk of device
and equipment damage, device overstress and unexpected device
breakdowns, the potential for data loss when aborting a test, and
the management of large datasets. The following features offer
solutions to these challenges: dynamic limit change, soft abort/
bias, real-time post data, data compression, and graphic update
suppression.
• Dynamic limit change (current limit control)
A current limit can be set for the SMUs when applying the
voltage. The output current will be clamped to the limit
(compliance value) to prevent damage to the DUT. The highlevel limit is usually set by estimating the maximum current
during stress (for example, the current at the beginning of
the stress). For most of the stress time, however, the current
is much lower than the limit. When a breakdown occurs,
there is no need to keep a high-level limit current running
to that particular device because, over an extended time,
the high-level current may melt the probe card tips and
damage the devices. Dynamic limit change allows the current
limit to vary according to the settings. The limit value can be
set as a multiple of the measured current; in some cases, it
can reduce the current by three orders of magnitude, which
corresponds to 106 times lower in terms of power. This feature
is implemented in the Vds_Constant test sequence (Figure 6).
• Soft bias and soft abort (stress control and proper test
abort design)
Soft bias/abort is an enhanced feature for protecting the
measured device. This function allows the forced voltage or
current to reach the desired value by ramping gradually at
the start or the end of the stress, or when aborting the test,
instead of changing suddenly. It helps to prevent in-rush
currents and unexpected device breakdowns. In addition, it
serves as a timing control over the process of applying stress.
www.keithley.com81
Initial bias
Vds_ramp
Post bias
Voltage
Drain
Vds
0V
Gate
Source
Time
Figure 3. Stress vs. time diagram for Vds_Vramp test for a single device. Drain, gate and source are each connected to a Series 2600B SMU respectively. The
drain is used for VDS stress and measure; the VDS range is extended by a positive bias on drain and a negative bias on source. A soft bias (gradual change
of stress) is enabled at the beginning and end of the stress (initial bias and post bias). This test is used in conjunction with the device connection shown in
Figure 2b. Measurements are performed at the “x” points.
Initial bias
Vds constant
Post bias
Voltage
Drain
Soft bias enabled
0V
Gate
Source
Vds
Measure
Time
Figure 4. Stress vs. time diagram for Vds_Constant test sequence. The example is shown for vertical structure and multi-device case. Common drain, gate and
source are each connected to a Series 2600B SMU respectively. The source is used for V DS stress and measure; the V DS range is extended by a positive bias
on the drain and a negative bias on the source. A soft bias (gradual change of stress) is enabled at the beginning and end of the stress (initial bias and post
bias). This test is used in conjunction with the device connection shown in Figure 2f. Measurements are performed at the “x” points.
As seen in Figures 3 and 4, the soft bias is enabled at the
beginning and end of the stress.
82
The soft abort function works with a digital I/O module
installed and connected to the test instrument. The digital
I/O bit is set to LO after the user presses the abort button,
and the LO bit input is sent to the software to invoke the
soft abort function. The data will not be lost due to the data
formatting at the termination of the test program. This is
especially useful when the user does not want to continue
the test as planned. For instance, imagine that 20 devices are
being evaluated in a breakdown test for 10 hours and one
of the tested devices exhibits abnormal behavior (such as
substantial leakage current). The user will want to stop the
test and redesign the test plan without losing the data that was
taken for 10 hours. This feature is implemented in both the
Vds_Vramp and Vds_Constant test modules (Figures 5 and 6).
• Real-time post data (data management and stress control
This feature enables the user to select the test data to be
posted and sent from the instruments to the ACS software
after a specified time interval or when the desired number
of points have been accumulated. When the Vds_Vramp
sequence is used in a large-scale ramp test, which may use
up to eight SMUs or other instruments, these settings ensure
safe data logging and transfer. This is because data transfer
via GPIB takes longer in multi-site testing than in single device
testing. In addition, the master instrument controls data
transactions between all other instruments and the host PC,
as well as controlling the ramping rate of the stress. The time
needed for communication might prevent ramping at the userspecified ramping rate.
Initially, test data is stored in instrument memory during
testing, then transferred after the test is completed, which
helps ensure the system uses the desired ramp rate. However,
in some instances, the user will want to see the data graphed
www.keithley.com
Figure 5. Vds_Vramp test sequence
Figure 6. Vds_Constant test sequence
www.keithley.com83
in real time, in order to check for irregularities and avoid
human error or accidents.
The “real-time post data” option allows for manual control
of data transfer. This balanced solution gives the user the
advantage of both a real-time data check and ramp rate
control. This feature is implemented in the Vds_Vramp test
sequence (Figure 5).
• Data compression (data management)
Reliability tests can run over many hours, days, or weeks,
and have the potential to amass enormous datasets. Instead
of collecting all the data produced, this function allows users
to log only the data important to their particular work. Users
can choose when to start data compression and how the data
will be recorded by using the measured value or time settings.
Data points can be logged when the current shift exceeds a
specified percentage as compared to previously logged data
and when the current is higher than a specified noise level. If
the “time” setting is selected, the mean value of the data over
the specified period or each value of the specified number of
points per decade will be logged. This feature is implemented
in the Vds_Constant test sequence (Figure 6).
• Suppress redundant graphic update (data management)
This practical function reduces the risks of working with large
datasets by allowing users to specify that results graphics
should NOT be refreshed automatically. This offers an effective
way to avoid continuously and redundantly plotting and
replotting a huge amount of data. The graph is updated only
when the user requests it by pressing the “Refresh” button.
Conclusion
The V DS ramp and HTRB tests are important power device
reliability tools for parametric verification and long-term
stability evaluation. The V DS breakdown test module in ACS
84
Version 5.0 includes two stress test sequences for V DS ramp
and HTRB reliability evaluation, which are enhanced with
features that address common test challenges. The software and
instrumentation in an ACS system are configurable for use in
a wide variety of applications at the device, wafer, or cassette
level, and in settings from a simple benchtop test setup to an
automated, integrated rack of instruments. An optional ACS2600-RTM package from Keithley offers additional reliability
tests, including Hot Carrier Injection (HCI), Negative Bias
Temperature Instability (NBTI), Time Dependent Dielectric
Breakdown (TDDB), voltage ramp (V Ramp) test, and ramped
current (JRamp) tests. Refer to the ACS-2600-RTM User’s Manual
for more information on these tests [10].
References
1. R. J. Kaplar, M. J. Marinella, S. DasGupta, M. A. Smith, S. Atcitty, M. Sun, and
T. Palacios, “Characterization and Reliability of SiC- and GaN Based Power
Transistors for Renewable Energy Applications” IEEE EnergyTech, 2012
2. L. Stauffer and M. Cejer, “Demand for Higher Power Semi Devices Will
Require Pushing Instrumentation to New Extremes,” Keithley Instruments
documentation 2012
3. J. Lutz, H Schlangenotto, U. Scheuermann, and R. De Doncker,
Semiconductor Power Devices: Physics, Characteristics, Reliability,
Springer, 2011.
4. R. Singh, A. R. Hefner, and T. R. McNutt, “ Reliability Concerns in
Contemporary SiC Power Devices”
5. EPC GaN Transistor Application Reliability Report (Readiness: Phase
One Testing)
6. International Rectifier RBV BRIDGE SERIES QUALIFICATION REPORT
7. Cree Z-RECTM 1200V Qualification Report
8. Power Semiconductor Reliability Handbook. Alpha and Omega
Semiconductor
9. ACS Reference Manual
10.ACS-2600-RTM User’s Manual
www.keithley.com
Testing High Brightness LEDs under Pulse
Width Modulation Using the Model 2651A
High Power System SourceMeter® Instrument
Introduction
With a worldwide focus on energy conservation, the high
power consumption of traditional light sources has come under
heavy scrutiny and governments are demanding improvements
in the energy efficiency of lighting sources. This demand has
lead to heavy investment in and development of alternatives to
the incandescent bulb. Compact Fluorescent (CFL) bulbs have
become prevalent in the marketplace and, although they are
much more efficient and last longer than traditional bulbs, they
still fall short of being ideal replacements. High Brightness Light
Emitting Diodes (HBLEDs), however, have proven themselves
to be a much better option. Like incandescent bulbs, they reach
full brightness immediately and do not contain any difficultto-dispose-of chemicals. They also offer advantages over
incandescents: they have incredibly long lifetimes and efficiency
levels are continuously being increased.
Unfortunately, the cost of HBLEDs is still too high for most
consumers to select them over cheaper technologies, despite
the large reduction in energy consumption they offer. In order
to reduce the cost of HBLEDs to consumers, manufacturers
are constantly working to improve yields and further increase
efficiency levels. Meeting this demand requires proper testing
and proper testing requires the proper test equipment.
Manufacturers demand a lot from their test equipment today
to test high brightness LEDs properly. This application note
explores some of the electrical test requirements and how the
Model 2651A High Power System SourceMeter® instrument can
be applied to meet these demands.
For many applications, combining multiple LEDs into a
single luminaire works well when it’s desirable to have the
light spread in multiple directions and the size of the luminaire
provides sufficient space to fit multiple LEDs. However, in other
applications where space is limited and/or the light must be
directional, this approach is either undesirable or simply will not
work. This demand for a lot of light in a small package has lead
to the development of high power LED modules, which consist
of one or more large-die LEDs. When multiple die are present,
they’re either wired in parallel or in series, depending on the
application and the available power source. The die of these
LEDs are much larger than the die of typical HBLEDs and can
handle much larger currents as well. It’s common for a single die
to be required to withstand current levels as high as 10A.
Testing high power HBLED modules properly demands test
equipment that can deliver a lot of power to the DUT. Although
SMUs, given their ability to source and measure in a single
instrument, are the best type of test equipment for testing LEDs,
most SMUs on the market simply can’t deliver the level of power
required. High power HBLED modules often require 100W of
power or more, but most instrument-based SMUs are capable
of delivering only 40W or less. Keithley’s Model 2651A High
Power System SourceMeter instrument is capable of delivering
up to 200W of continuous DC power and up to 2000W of pulsed
power, making it more than powerful enough to test both
today’s and tomorrow’s high power modules (Figure 1).
The Demand for More Power
HBLEDs are commonly defined as LEDs that operate at 1W of
power or higher, with typical operating ranges of from 1W to 3W.
Instead of running on 10–30mA of current and being packaged
in small 3mm or 5mm plastic domes, HBLEDs run at 300mA–1A
or more and are mounted on a small, thermally conductive
board designed to draw heat away from the LED’s junction.
Despite how bright a single HBLED can be, it is typically not
bright enough to be used by itself in most lighting applications.
Instead, multiple HBLEDs are commonly combined to create
a single luminaire, whether for an LED light bulb for a retrofit
application or an entire lighting fixture. Even though they are
combined in actual use, production testing is typically performed
at the individual package level, requiring modest power delivery
capability, which is well within the capabilities of most modern
instrument-based source-measure units (SMUs), such as the
Keithley Series 2400, 2600B and 2651A SourceMeter instruments.
+50A
+20A
+10A
+5A
0A
–5A
–10A
DC and
Pulse
Pulse
only
–20A
–50A
–40V
–20V
–10V
0V
+10V
+20V
+40V
Figure 1: Power envelope for the Model 2651A High Power System
SourceMeter Instrument.
www.keithley.com85
Pulse Width Modulation
Pulse width modulation is a common method of controlling
the brightness of LEDs. When using this technique, the current
through the LED is pulsed at a constant frequency with a
constant pulse level, but the width of the pulse is varied
(Figure 2). Varying the width of the pulse changes the amount
of time the LED is in the ON state, as well as the perceived
level of brightness. In this drive scheme, the LED is actually
flashing, but the frequency of the flashing is so high that it’s
indistinguishable to the human eye from a constant light level.
Although it’s possible to control the brightness of an LED simply
by lowering the forward drive current, pulse width modulation is
the preferable technique for a number of reasons.
50% Duty Cycle
75% Duty Cycle
25% Duty Cycle
Figure 2: In pulse width modulation, the pulse level and frequency remain
constant but the duty cycle is varied.
The first and arguably the most important reason for using
pulse width modulation is to maintain consistency of the color of
the light as the LED’s brightness is reduced. In an LED, the color
of the light it emits is related to the forward voltage at which it
operates. Although the forward voltage of an LED will remain
relatively constant as the forward current is changed, it actually
does vary by as much as tens to even hundreds of millivolts. This
occurs especially at lower current levels (Figure 3). This slight
variation in forward voltage equates to a slight variation in light
color, which is undesirable for the end user. If heating effects
are ignored, in the pulse width modulation technique, the LED
is pulsed using exactly the same current level on every pulse, so
Another important reason pulse width modulation is
preferable is because this technique provides linear control over
brightness. The amount of light an LED emits is not linearly
related to the amount of current used to drive it. In other words,
reducing the drive current by 50% will not cut the light output
by 50%; instead, it will drop by some other amount. This would
make a dimming scheme based on varying current difficult to
apply because it would be necessary to characterize each LED’s
light output vs. forward current then calibrate the drive scheme
to that curve. Using pulse width modulation is a much simpler
way to get linear control over brightness. With pulse width
modulation, in order to make the LEDs output 50% as much
light, all that’s necessary is to reduce the duty cycle by 50%. If
the LEDs are only ON for half as long, only half as much light
will be produced.
Power efficiency is another advantage of the pulse width
modulation approach. Because pulse width modulation uses
a constant current level for each pulse, it’s possible to select
a pulse level where the LED operates most efficiently, that is,
where the lumen output per watt is the greatest. That means
the LED is operating at maximum efficiency no matter what
brightness level is used. Another way in which pulse width
modulation enhances efficiency is that LEDs will actually output
more light for a given drive current when pulsed rather than at
DC. Many manufacturers’ datasheets include a graph of forward
current vs. luminous flux. If the manufacturer has characterized
the LED under both pulsed and DC drive currents, one can
observe that the pulsed characterization curve lies above the DC
characterization curve. This is due to the reduced self-heating
that the pulsed drive current produces. Finally, pulse width
modulation enhances power efficiency even in the circuitry
that drives it. The switching circuitry used in Pulsed Width
Modulation wastes very little power. When the switch circuitry
is turned off, virtually no current flows and virtually zero power
is being used. When the switch is turned on, due to the very low
on state resistances, nearly all the power is delivered to the LED
and very little is consumed by the switch. In a variable current
drive scheme, power to the LED is often reduced by consuming
the excess power elsewhere in the circuit.
Pulse Width Modulation with the Model 2651A
High Power System SourceMeter Instrument
NOTE:The techniques described here for outputting a pulse
width modulated waveform with the Model 2651A are
applicable to all members of the Series 2600B System
SourceMeter instrument family.
IF
VF
Figure 3: Forward voltage changes significantly when the forward current is
low and becomes relatively constant as forward current becomes large.
86
the forward voltage is the same for every pulse; therefore, the
color of the light emitted will not vary.
Given that LEDs are often used with pulse width modulation,
it’s only appropriate that they be tested with pulse width
modulation techniques. As part of Pulsed Width Modulation
testing, an LED is usually tested by running a series of pulses
through it while using a spectrometer to take an integrated
www.keithley.com
measurement of the light output over the course of many pulses.
This measurement may take tens or hundreds of milliseconds
to complete. During the pulsed output, the forward voltage is
measured on every pulse to look for changes as the temperature
of the LED rises. Figure 4 illustrates this test.
• 8-pin Signal Control Cable
• 2-pin terminal block extender for use with the Model
2651A-KIT-1
• 8-in terminal block extender for use with 8-pin signal
control cable
• TSP-Link RJ45 LAN Crossover Cable
Spectrometer integration time
...
VF measurements on every pulse
Figure 4: The LED is pulsed with a series of pulses and a V F measurement
is taken at each pulse. A spectrometer concurrently takes optical
measurements.
The Model 2651A High Power System SourceMeter instrument
is capable of outputting a pulse width modulated waveform with
up to 100% duty cycle from 0–20A, 50% duty cycle from 20–30A,
and 35% duty cycle from 30–50A. The Model 2651A’s advanced
trigger model allows for precision pulse widths and duty
cycles and tight synchronization with other instruments. These
synchronization features can be used to combine two Model
2651As to achieve a Pulsed Width Modulation waveform with
pulse current levels twice as high as a single Model 2651A allows
with the same duty cycle. This note details how to configure
the Model 2651A to output a 30A, 50% duty cycle waveform
with a digital I/O output trigger for triggering a spectrometer. It
also details how to combine two Model 2651As to increase the
current to 60A for this same test.
Communications
Single SourceMeter Instrument
To perform this test using a single 2651A SourceMeter instrument
configuration, connect the instrument to the computer via GPIB
or Ethernet as illustrated in Figure 5.
GPIB/Ethernet
Controller
Figure 5: Communications setup for a single SourceMeter instrument.
Dual SourceMeter Instruments
For a dual 2651A SourceMeter instrument configuration, connect
the first Model 2651A to the computer via GPIB or Ethernet.
Connect the second Model 2651A to the first Model 2651A via the
TSP-Link connection. Assign the first Model 2651A to Node #1
and the second Model 2651A to Node #2. These connections are
illustrated in Figure 6.
Required Equipment
GPIB/Ethernet
Performing this test requires the following equipment:
• PC with GPIB or Ethernet adapter
TSP-LINK
Model 2651A SMU #1
(TSP-Link Node #1)
Model 2651A SMU #2
(TSP-Link Node #2)
• GPIB cable or RJ45 LAN Crossover Ethernet cable
• Model 2651A High Power System SourceMeter Instrument
Model 2651A SMU #1
(TSP-Link Node #1)
Controller
• Model 2651A-KIT-1 Low-Impedance/High-Current
Coaxial Cable
Figure 6: Communications setup for two Model 2651A instruments.
• 8-pin signal control cable
Connecting the Spectrometer to the Digital I/O
• 2-pin terminal block extender for use with the Model
2651A-KIT-1
In order for the Model 2651A to trigger the spectrometer, the
spectrometer’s start of test trigger line must be connected to the
Model 2651A’s digital I/O port. Connect the trigger line from
the spectrometer to pin #1 of the 25-pin D-Sub connector on
the back panel of the Model 2651A. Ground connections can be
• 8-pin terminal block extender for use with 8-pin signal
control cable
• Digital I/O DB-25 Male Connector Kit Hardware
• 12 AWG or thicker cabling to connect from terminal
blocks to device
Add the following equipment to perform this test at up to
100A with a 35% duty cycle or at up to 60A with a 50% duty cycle:
• Model 2651A High Power System SourceMeter Instrument
• Model 2651A-KIT-1 Low-Impedance/High-Current
Coaxial Cable
Model 2651A
13
DIGITAL I/O
Trigger Line
1
Trigger In
25
GND
14
Spectrometer
Ground
Figure 7: Connections from Model 2651A digital I/O port to spectrometer.
www.keithley.com87
found on any of pins 15 through 21. The proper connections are
illustrated in Figure 7.
NOTE:In dual SourceMeter instrument configurations, connect
only to the digital I/O port of SMU #1. The digital I/O
port of SMU #2 is not used.
Device Connections
Figures 8 and 9 illustrate the connections from the SourceMeter
instruments to the LED device under test. Figure 8 illustrates
the connections for a single SourceMeter instrument; Figure 9
illustrates the connections for dual SourceMeter instruments.
S LO
G
G
G
G
S HI
–
From
Model 2651A
Figure 8: Connections from a single Model 2651A SourceMeter instrument to
LED device under test.
S LO
G
G
G
G
S HI
The trigger model shown in Figure 10 will perform a pulse
width modulation test on an LED using a single Model 2651A.
In this configuration, Timer 1 controls the pulse period, Timer
2 controls the pulse width and Timer 3 inserts a delay between
the start of the pulse and the start of the measurement. Timer 4
creates a delay between the start of the waveform output and the
start of the spectrometer measurement. When Timer 4 expires, it
triggers Digital I/O Trigger 1, which sends the start trigger to the
spectrometer.
Configuring the Trigger Model for Dual Model
2651A SourceMeter Instruments
12AWG or thicker
recommended
From
Model 2651A
#1
–
+
S LO
G
G
G
G
S HI
From
Model 2651A
#2
12AWG or thicker
recommended
Figure 9: Connections from dual Model 2651A SourceMeter instruments to
LED device under test.
When using dual Model 2651A SourceMeter instruments, the
two SMUs are connected in parallel to create a single source
with twice as much current capacity. Read and understand the
section titled “Combining SMU outputs” in the Model 2651A High
Power System SourceMeter Instrument Reference Manual before
performing this test.
88
The advanced trigger model must be used to source current
levels above those available in the DC operating regions of the
Model 2651A. This same trigger model gives the Model 2651A
its precise pulse widths and tight synchronization and makes
accurate pulse width modulation possible. The following sections
illustrate how to configure the trigger model to output a pulse
width modulated waveform and trigger a spectrometer with the
Model 2651A.
Configuring the Trigger Model for a Single
Model 2651A SourceMeter Instrument
12AWG or thicker
recommended
From
Model 2651A
#2
Configuring the Trigger Model
+
From
Model 2651A
From
Model 2651A
#1
NOTE:Wiring from the 2-pin terminal block extender that is
connected to the end of the low-impedance/high-current
coaxial cable provided with the Model 2651A to the
device under test should be made with 12 AWG wire or
heavier in order to support the high current levels. Also,
the length of these wires should be kept to a minimum to
minimize inductance.
The trigger model shown in Figure 11 will perform a pulse
width modulation test on an LED using two Model 2651A
SourceMeter instruments with their outputs connected together
in parallel. Timer 1 of Model 2651A #1 controls the pulse period
of both SMUs. It does this by relaying its output trigger signal
on TSP-Link® Trigger 1, which is then sent to Model 2651A #2.
Due to the extremely low latency of TSP-Link triggering, Model
2651A #2 is kept in sync with Model 2651A #1 to within 500ns.
Because this signal is sent once every time Timer 1 expires, longterm synchronization between the SMUs is ensured because the
two SMUs are synchronized at the start of every pulse cycle.
As in the single SMU configuration, Timer 2 controls the
pulse width and Timer 3 inserts a delay between the start of the
pulse and the start of the measurement. Each SMU uses its own
Timer 2 and Timer 3 to control its pulse width and measure
delay. Given the accuracy of the timers and the synchronization
at the start of every pulse, it’s certain that both SMUs will output
a pulse with the same pulse width and take measurements at
the same time. Finally, once again Timer 4 on Model 2651A #1
creates a delay between the start of the pulse waveform and the
start of the spectrometer measurement by delaying the output of
Digital I/O Trigger 1.
www.keithley.com
2651A Trigger Model
Controls Pulse Period
trigger.timer[1].
stimulus
passthrough = true
smua.trigger.
Arm Layer
SWEEP_COMPLETE_EVENT_ID
arm.stimulus
Trigger Layer
trigger.timer[3].
490μs
SWEEPING_EVENT_ID
arm.count = 1
count = 99
Controls Measure Delay
stimulus
IDLE_EVENT_ID
Idle
EVENT_ID
1ms
count = 100
source.stimulus
EVENT_ID
measure.stimulus
MEASURE_COMPLETE_EVENT_ID
endpulse.stimulus
passthrough = false
ARMED_EVENT_ID
SOURCE_COMPLETE_EVENT_ID
PULSE_COMPLETE_EVENT_ID
count = 1
Controls Pulse Width
trigger.timer[2].
stimulus
500μs
passthrough = false
digio.trigger[1].
To
Spectrometer
EVENT_ID
EVENT_ID
count = 1
Delays Start of Spectrometer
Measurement
trigger.timer[4].
mode = digio.TRIG_FALLING
EVENT_ID
stimulus
1ms
stimulus
count = 1
passthrough = false
Figure 10: Trigger model for pulsed width modulation test on LED with a single Model 2651A System SourceMeter instrument.
2651A #1 Trigger Model
Controls Pulse Period
trigger.timer[1].
tsplink.trigger[1].
stimulus
smua.trigger.
EVENT_ID
1ms
Arm Layer
passthrough = true
mode = tsplink.TRIG_FALLING
IDLE_EVENT_ID
Idle
EVENT_ID
count = 99
SWEEPING_EVENT_ID
arm.count = 1
SWEEP_COMPLETE_EVENT_ID
arm.stimulus
stimulus
Controls Measure Delay
Trigger Layer
trigger.timer[3].
stimulus
source.stimulus
EVENT_ID
490μs
measure.stimulus
count = 100
MEASURE_COMPLETE_EVENT_ID
endpulse.stimulus
passthrough = false
ARMED_EVENT_ID
SOURCE_COMPLETE_EVENT_ID
PULSE_COMPLETE_EVENT_ID
count = 1
Controls Pulse Width
trigger.timer[2].
stimulus
500μs
passthrough = false
count = 1
Delays Start of Spectrometer
Measurement
digio.trigger[1].
To
Spectrometer
EVENT_ID
EVENT_ID
trigger.timer[4].
mode = digio.TRIG_FALLING
EVENT_ID
stimulus
count = 1
1ms
stimulus
passthrough = false
2651A #2 Trigger Model
tsplink.trigger[1].
EVENT_ID
mode = tsplink.TRIG_FALLING
stimulus
smua.trigger.
stimulus
490μs
passthrough = false
IDLE_EVENT_ID
Idle
trigger.timer[3].
Arm Layer
EVENT_ID
count = 1
SWEEPING_EVENT_ID
arm.count = 1
SWEEP_COMPLETE_EVENT_ID
arm.stimulus
Trigger Layer
Controls Pulse Width
source.stimulus
trigger.timer[2].
stimulus
500μs
passthrough = false
measure.stimulus
EVENT_ID
count = 100
ARMED_EVENT_ID
SOURCE_COMPLETE_EVENT_ID
MEASURE_COMPLETE_EVENT_ID
endpulse.stimulus
PULSE_COMPLETE_EVENT_ID
count = 1
Figure 11: Trigger model for pulsed width modulation test on LED with dual Model 2651A System SourceMeter instruments.
www.keithley.com89
Configuring the Frequency and Duty Cycle
In both the single and dual SMU configurations, configuring
the waveform for a particular frequency and duty cycle requires
setting the appropriate pulse period and pulse width in the
trigger model. Frequency ( f ) is related to pulse period (P) by
the equation P = 1 / f; therefore, it’s possible to control the
frequency of the waveform by setting the appropriate pulse
period. For a 1kHz waveform, P = 1/1kHz = 1ms. Duty cycle
(D.C.) is the ratio of pulse width (PW) over pulse period (P) or
D.C. = PW / P; therefore, the duty cycle can be set by setting the
appropriate value for pulse width; this value can be calculated
using the equation D.C. * P = PW.
Modulating the Pulse Width
The trigger model diagrams in Figures 10 and 11 show the pulse
width being controlled by Timer 2, which has a fixed timeout
value. This is represented in code by calling the ICL command
trigger.timer[2].delay = pulseWidth
where pulseWidth is a fixed delay value in seconds. Having a
fixed timeout value means that the pulse width for every cycle in
a single waveform output will be the same for the entire length
of the waveform. Modulating the pulse width during a single
waveform output requires that the timeout value of Timer 2 be
variable. To facilitate this, the timers of the advanced trigger
model can be assigned a delay list rather than a single value. This
can be done by calling the ICL command
trigger.timer[2].delaylist = pulseWidthTable
where pulseWidthTable is a table containing multiple delay
values in seconds. With a delay list assigned to Timer 2, each
pulse in the waveform can be assigned a different pulse width;
therefore, the waveform can be pulse width modulated.
NOTE:The example script in this application note allows for
both fixed and variable pulse widths. See the function
documentation for details.
Example Program Code
NOTE:The Test Script Processor (TSP®) Script in this application
note is for demonstration purposes only and is not
optimized for fastest production throughput. Please
contact a Keithley Applications Engineer for system
throughput optimization considerations.
NOTE:The TSP Script in this application note is designed to be
run from Test Script Builder. It can be run from other
programming environments such as Microsoft® Visual
Studio or National Instruments LabVIEW®; however,
modifications may be required.
The TSP script provided in this application note contains all the
code necessary to perform a pulse width modulation test with
optical measurement on a high brightness LED using one or two
Model 2651A High Power System SourceMeter instruments. The
code for this script can be found in Appendix A: Source Code.
The script performs the following functions:
90
• Initializes the TSP-Link connection (only when using
two units)
• Configures the SMU(s) ranges and measurement settings
• Configures the trigger model(s)
• Prepares the readings buffers for data
• Outputs the PWM waveform
• Returns the collected data to the instrument console in a
format that can be copied and pasted directly into a Microsoft
Excel® spreadsheet.
The script is written using TSP functions rather than a single
block of inline code. TSP functions are similar to functions in
other programming languages such as C or Visual Basic and
must be called before the code contained in them is executed.
Because of this, running the script alone will not execute the
test. To execute the test, first run the script to load the functions
into Test Script memory and then call the functions. Refer to the
documentation for Test Script Builder for directions on how to
run scripts and enter commands using the instrument console.
Within the script, there are several comments describing
what is being performed by the lines of code, as well as
documentation for the functions contained in the script. Lines
starting with
node[2].
are commands that are being sent to Model 2651A #2 through
the TSP-Link interface. All other commands are executed on
Model 2651A #1.
Example Program Usage
This script contains two functions for outputting a pulse width
modulated waveform: one for use with a single Model 2651A
High Power System SourceMeter instrument and one for use with
two Model 2651A High Power System SourceMeter instruments.
These functions contain parameters whose values are used to
configure the waveform, allowing the user to set properties
of the waveform such as the frequency and duty cycle without
needing to rewrite any code. The following sections provide
explanations of each function and its parameters.
PWM_Test_Single()
PWM_Test_Single(pulseLevel, pulseLimit, frequency,
dutyCycle, numpulses, specDelay)
This function will output a pulse width modulated waveform
using a single Model 2651A High Power System SourceMeter
instrument. A forward voltage measurement will be taken with
the Fast ADC on every pulse in the waveform and measurements
will be placed 10μs before the falling edge of the pulse. Use the
parameters of this function to configure the properties of the
Pulsed Width Modulation waveform and to set the delay between
the start of the waveform and the start of the spectrometer
measurement. If any parameters are left blank, a default value
will be used.
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Parameter
pulseLevel
Units
Amps
Description
The current level to pulse to during the test
Min:–50
Max:+50
Default:1
Comments:
The value set here goes into determining what operating region the SMU is in and will therefore have an effect on the maximum duty cycle
value that can be set without error.
pulseLimit
Volts
Voltage limit of the pulses during the test
Max:40
Default:1
Comments:
The value set here goes into determining what operating region the SMU is in and will therefore have an effect on the maximum duty cycle
value that can be set without error.
frequency
Hz
The number of pulses per second
Min:0.1
Max:10,000
Default:100
Comments:
Frequency and Duty Cycle determine the pulse width of the waveform. The minimum frequency that can be used without error may be
higher than the value shown if the operating region for the test is outside of the DC operating area.
dutyCycle
%
The on time of the pulse as a percentage of the pulse period
Min:0.01
Max:99
Default:1
Comments:
This parameter may be assigned a single value or a table of values. If it is assigned a single value, then all pulses in the waveform will
have the same duty cycle. If this parameter is assigned a table of values, then the duty cycle for each pulse will be determined by a
corresponding entry in the table. For example, if the table has the values 50, 25, and 40, then the first pulse in the waveform will have a
duty cycle of 50%, the second pulse will have a duty cycle of 25%, and the third pulse a duty cycle of 40%. If there are more pulses in the
waveform than there are values in the table, then when the last value in the table is reached, the values in the table will be reused from the
beginning of the table starting with the next pulse. Using the previous example, if the number of pulses to output is 5, then the duty cycles
of the pulses will be 50%, 25%, 40%, 50%, and 25%.
Frequency and Duty Cycle determine the pulse width of the waveform. Pulse and duty cycle of the instrument are limited depending on
the region of the power envelope the SMU is operating in. The minimum and maximum duty cycle that can be used without error may be
higher or lower than the values shown depending on the operating region and selected frequency for the test. If the duty cycle specified
results in a pulse width/duty cycle that is too large for the operating region, the SMU will limit the pulse width/duty cycle itself to its
maximum allowable value. This will appear in the output waveform in the form of pulses that are cut short or are missing.
See the Model 2651A Specification for details on maximum pulse widths and duty cycles.
numpulses
N/A
specDelay
The number of pulses in the waveform
Min:2
Max:
100,000 or greater
Default:10
Seconds The time between the start of the PWM output and the output of the Digital I/O trigger to start the spectrometer measurement.
Min:0
Default:0
www.keithley.com91
The following is an example call to function
PWM _ Test _ Single().
PWM_Test_Single(30, 10, 1000, 50, 100, 1e-3)
This call will output a pulse width modulated waveform
of 100 pulses with a pulse level of 30A, a 10V voltage limit,
a frequency of 1kHz, and a duty cycle of 50%. Spectrometer
measurements will begin 1ms after the start of the waveform
output. At the completion of the test, the SMU output will be
turned off and the forward voltage measurements collected
during the test will be printed to the instrument console in a
format compatible with copying and pasting into a Microsoft
Excel spreadsheet. An example of this output can be seen in
Figure 12.
will begin 1ms after the start of the waveform output. At the
completion of the test, the SMU output will be turned off and
the forward voltage measurements collected during the test will
be printed to the instrument console in a format compatible for
copy and pasting into Microsoft Excel. An example of this output
can be seen in Figure 13.
Figure 13: 20A 1kHz pulse width modulated waveform from a Keithley Model
2651A into a high power LED module.
PWM_Test_Dual()
PWM_Test_Dual(pulseLevel, pulseLimit, frequency,
dutyCycle, numpulses, specDelay)
Figure 12: 30A 1kHz pulsed waveform with 50% duty cycle from a Keithley
Model 2651A into a high power LED module.
In the previous example, the duty cycle of the waveform was
fixed at 50%. To create a waveform where the pulse width varies
for each pulse, it’s necessary to pass the function a table of duty
cycles. The following example calls demonstrate this.
dutyTable = {20, 40, 60, 80, 60, 40, 20, 40, 60
PWM_Test_single(20, 10, 1000, dutyTable, 9, 1e-3)
This call will output a pulse width modulated waveform of 9
pulses with a pulse level of 20A, a 10V voltage limit, a frequency
of 1kHz and a varying duty cycle. The duty cycle for each pulse
will be read from the table. The first pulse will have a duty cycle
of 20%, the second pulse a duty cycle of 40%, the third pulse
a duty cycle of 60% and so on. Spectrometer measurements
92
This function uses two Model 2651A High Power System
SourceMeter instruments to output a pulse width modulated
waveform with as much as twice as much current as a single
Model 2651A is capable of.
This function will output a pulse width modulated waveform
using two Model 2651A High Power System SourceMeter
instruments connected together via TSP-Link. By combining the
SMU’s outputs in parallel, twice the current of a single Model
2651A can be delivered to the device under test. Just like the
single SMU test, a forward voltage measurement will be taken
on every pulse in the waveform and measurements will be
taken with the Fast ADC, placed 10μs before the falling edge of
the pulse. Use the parameters of this function to configure the
properties of the Pulsed Width Modulation waveform and to set
the delay between the start of the waveform and the start of the
spectrometer measurement. If any parameters are left blank, a
default value will be used.
www.keithley.com
Parameter
pulseLevel
Units
Amps
Description
The current level to pulse to during the test
Min:–100
Max:+100
Default:1
Comments:
The value set here goes into determining what operating region the SMU is in and will therefore have an effect on the maximum duty
cycle value that can be set without error.
pulseLimit
Volts
Voltage limit of the pulses during the test
Max:40
Default:1
Comments:
The value set here goes into determining what operating region the SMU is in and will therefore have an effect on the maximum duty
cycle value that can be set without error.
frequency
Hz
The number of pulses per second
Min:0.1
Max:10,000
Default:100
Comments:
Frequency and Duty Cycle determine the pulse width of the waveform. The minimum frequency that can be used without error may be
higher than the value shown if the operating region for the test is outside of the DC operating area.
dutyCycle
%
The on time of the pulse as a percentage of the pulse period
Min:0.01
Max:99
Default:1
Comments:
This parameter may be assigned a single value or a table of values. If it is assigned a single value then all pulses in the waveform will
have the same duty cycle. If this parameter is assigned a table of values then the duty cycle for each pulse will be determined by a
corresponding entry in the table. For example, if the table has the values 50, 25, 40 then the first pulse in the waveform will have a duty
cycle of 50%, the second pulse will have a duty cycle of 25% and the third pulse a duty cycle of 40%. If there are more pulses in the
waveform than there are values in the table, then when the last value in the table is reached, the values in the table will be reused from
the beginning of the table starting with the next pulse. Using the previous example, if the number of pulses to output is 5 then the duty
cycles of the pulses will be 50%, 25%, 40%, 50%, 25%.
Frequency and Duty Cycle determine the pulse width of the waveform. Pulse and duty cycle of the instrument are limited depending on
the region of the power envelope the SMU is operating in. The minimum and maximum duty cycle that can be used without error may be
higher or lower than the values shown depending on the operating region and selected frequency for the test. If the duty cycle specified
results in a pulse width/duty cycle that is too large for the operating region, the SMU will limit the pulse width/duty cycle itself to its
maximum allowable value. This will appear in the output waveform in the form of pulses that are cut short or are missing.
See the Model 2651A Specification for details on maximum pulse widths and duty cycles.
numpulses
N/A
specDelay
The number of pulses in the waveform
Min:2
Max:
100,000 or greater
Default:10
Seconds The time between the start of the PWM output and the output of the Digital I/O trigger to start the spectrometer measurement.
Min:0
Default:0
www.keithley.com93
The following is an example call to function
PWM _ Test _ Dual().
PWM_Test_Dual(60, 10, 1000, 50, 100, 1e-3)
the forward voltage measurements collected during the test will
be printed to the instrument console in a format compatible for
copy and pasting in to Microsoft Excel. An example of this output
can be seen in Figure 15.
This call will output a pulse width modulated waveform
of 100 pulses with a pulse level of 60A, a 10V voltage limit,
a frequency of 1kHz, and a duty cycle of 50%. Spectrometer
measurements will begin 1ms after the start of the waveform
output. At the completion of the test, the SMU output will be
turned off, and the forward voltage measurements collected
during the test will be printed to the instrument console in a
format compatible with copy and pasting into Microsoft Excel. An
example of this output can be seen in Figure 14.
Figure 15: 40A 1kHz pulse width modulated waveform from dual Keithley
Model 2651As into a high power LED module.
Conclusion
Figure 14: 60A 1kHz pulsed waveform with 50% duty cycle from dual
Keithley Model 2651As into a high power LED module.
Just like the single SMU function, this function allows for
a variable duty cycle as well. To create a waveform where the
pulse width varies for each pulse it’s necessary to pass the
function a table of duty cycles. The following example calls
demonstrate this.
dutyTable = {20, 40, 60, 80, 60, 40, 20, 40, 60}
PWM_Test_Dual(40, 10, 1000, dutyTable, 9, 1e-3)
This call will output a pulse width modulated waveform of 9
pulses with a pulse level of 40A, a 10V voltage limit, a frequency
of 1kHz, and a varying duty cycle. The duty cycle for each pulse
will be read from the table. The first pulse will have a duty cycle
of 20%, the second pulse a duty cycle of 40%, the third pulse
a duty cycle of 60% and so on. Spectrometer measurements
will begin 1ms after the start of the waveform output. At the
completion of the test, the SMU output will be turned off and
94
HBLEDs are advancing at an incredible pace and manufacturers
are working hard to make them the lighting source choice of
the future. In order to get there, LED manufacturers must jump
several hurdles, ever trying to reduce the cost of manufacturing
LEDs while simultaneously trying to increase their efficiency
and light output. At the center of meeting these objectives,
manufacturers require accurate, reliable, and repeatable source
and measurement equipment with the power and flexibility to
adapt to their ever-changing testing needs.
Series 2600B System SourceMeter instruments offer the
features and flexibility to keep up with LED manufacturers’
testing needs. Innovative features like the advanced trigger
model allow Series 2600B instruments to make repeatable
measurements accurately and reliably using complex drive
schemes such as pulse width modulated waveforms, AC
waveforms, and even arbitrary waveforms. The newest
addition to this product line, the Model 2651A High Power
System SourceMeter Instrument, provides additional power
to handle even the brightest high power LED modules while
still maintaining low current accuracy. Series 2600B System
SourceMeter instruments, with their flexible output capabilities
and their ability to source and measure accurately in a single
instrument, make them the perfect choice for testing HBLEDs.
www.keithley.com
Appendix A: Source Code
NOTE: The code in this script will work without modification only with the Model 2651A High Power System SourceMeter
instrument. However, this script can also work with other Series 2600B System SourceMeter instruments with only minor
modifications.
--[[
Title: Pulse Width Modulation Script
Desription: The purpose of this script is to generate a pulse width
modulated waveform for use in testing High Brightness LED modules.
Users of this script should call the functions in the User Functions
section. Functions in the Utility Functions section are used by the
User Functions to execute the test.
System Setup:
PWM_Test_Single()
1x Model 2651A
PWM_TEST_Dual()
2x Model 2651A
1x TSP-Link Cable
Node 1: 2651A #1 (Master)
Node 2: 2651A #2 (Slave)
]]---================
-- User Functions
--================
--[[PWM_Test_Single()
This function uses a single SMU to output a pulse width modulated waveform.
--]]
function PWM_Test_Single(pulseLevel, pulseLimit, frequency, dutyCycle, numPulses, specDelay)
if (pulseLevel == nil) then pulseLevel = 1 end
if (pulseLimit == nil) then pulseLimit = 1 end
if (frequency == nil) then frequency = 100 end
if (dutyCycle == nil) then dutyCycle = 1 end
if (numPulses == nil) then numPulses = 10 end
if (specDelay == nil) then specDelay = 0 end
local pulsePeriod
local pulseWidth
local measDelay
-- Calculate the timing parameters from the frequency and duty cycle
pulsePeriod,pulseWidth,measDelay = CalculateTiming(frequency, dutyCycle)
-- Do a quick check on the input parameters
f,msg = SimpleRegionCheck(pulseLevel, pulseLimit, dutyCycle, 1)
if (f == false) then
print(msg)
quit()
end
reset()
smua.reset()
smua.source.func
= smua.OUTPUT_DCAMPS
smua.sense= smua.SENSE_REMOTE
smua.source.autorangei= 0
smua.source.rangei
= pulseLevel
smua.source.leveli
= 0
-- Set the DC bias limit. This is not the limit used during the pulses.
smua.source.limitv
= 1
www.keithley.com95
smua.measure.autozero = smua.AUTOZERO_ONCE
smua.measure.autorangev= 0
smua.measure.rangev
= pulseLimit
-- The fast ADC allows us to place the measurements very close to the falling edge of
-- the pulse allowing for settled measurements even when pulse widths are very small
smua.measure.adc
= smua.ADC_FAST
smua.measure.count
= 1
smua.measure.interval= 1e-6
-- Uncomment the following lines to turn on measure filtering. When enabled, the SMU
-- will take multiple measurements and average them to produce a single reading.
-- Because the Fast ADC can take one measurement every microsecond, several measurements
-- can be aquired in a small time to produce an averaged reading.
--smua.measure.filter.count
= 5
--smua.measure.filter.enable
= smua.FILTER_ON
-- This measure delay sets the delay between the measurement trigger being received
-- and when the actual measurement(s) start. This is set to 0 because we will be
-- delaying the trigger itself and do not need additional delay.
smua.measure.delay
= 0
-- Setup the Reading Buffers
smua.nvbuffer1.clear()
smua.nvbuffer1.appendmode
= 1
smua.nvbuffer1.collecttimestamps= 1
smua.nvbuffer2.clear()
smua.nvbuffer2.appendmode
= 1
smua.nvbuffer2.collecttimestamps= 1
-- Configure the Trigger Model
--============================
-- Timer 1 controls the pulse period
trigger.timer[1].count = numPulses > 1 and numPulses - 1 or 1
trigger.timer[1].delay = pulsePeriod
trigger.timer[1].passthrough= true
trigger.timer[1].stimulus
= smua.trigger.ARMED_EVENT_ID
-- Timer 2 controls the pulse width
trigger.timer[2].count= 1
if (type(pulseWidth) == “table”) then
-- Use a delay list if the duty cycle will vary for each pulse
trigger.timer[2].delaylist
= pulseWidth
else
-- else every pulse will be the same duty cycle
trigger.timer[2].delay
= pulseWidth
end
trigger.timer[2].passthrough= false
trigger.timer[2].stimulus
= smua.trigger.SOURCE_COMPLETE_EVENT_ID
-- Timer 3 controls the measurement
trigger.timer[3].count= 1
if (type(measDelay) == “table”) then
-- If the duty cycle is variable then the measure delay will be as well
trigger.timer[3].delaylist
= measDelay
else
trigger.timer[3].delay
= measDelay
end
trigger.timer[3].passthrough= false
trigger.timer[3].stimulus
= smua.trigger.SOURCE_COMPLETE_EVENT_ID
-- Configure SMU Trigger Model for Sweep
96
www.keithley.com
smua.trigger.source.lineari(pulseLevel, pulseLevel, numPulses)
smua.trigger.source.limitv
= pulseLimit
smua.trigger.measure.action
= smua.ASYNC
smua.trigger.measure.iv(smua.nvbuffer1, smua.nvbuffer2)
smua.trigger.endpulse.action= smua.SOURCE_IDLE
smua.trigger.endsweep.action= smua.SOURCE_IDLE
smua.trigger.count
= numPulses
smua.trigger.arm.stimulus
= 0
smua.trigger.source.stimulus= trigger.timer[1].EVENT_ID
smua.trigger.measure.stimulus
= trigger.timer[3].EVENT_ID
smua.trigger.endpulse.stimulus
= trigger.timer[2].EVENT_ID
smua.trigger.source.action
= smua.ENABLE
-- Configure the Digital I/O trigger
ConfigureSpectrometerTrigger(specDelay)
-- Start the Test
--===============
-- Turn the output on
smua.source.output
-- Start the trigger model execution
smua.trigger.initiate()
= 1
-- While the trigger model is outputing the waveform and collecting the
-- measurements, the script will scan the status model for any overruns
-- that may occur as a result of using impropper settings.
local ovr = false
local msg = “”
while ((status.operation.sweeping.condition ~= 0) and (ovr == false)) do
ovr, msg = CheckForOverRun(localnode)
end
if (ovr == true) then
smua.abort()
print(msg)
end
-- Turn the output off
smua.source.output
= 0
-- Return the data
PrintData()
end
--[[PWM_Test_Dual()
This function uses two SMUs connected together in parallel to ouput a pulse width
modulated wavform. By using two SMUs higher current levels/duty cycles can be achieved.
--]]
function PWM_Test_Dual(pulseLevel, pulseLimit, frequency, dutyCycle, numPulses, specDelay)
if (pulseLevel == nil) then pulseLevel = 1 end
if (pulseLimit == nil) then pulseLimit = 1 end
if (frequency == nil) then frequency = 100 end
if (dutyCycle == nil) then dutyCycle = 1 end
if (numPulses == nil) then numPulses = 10 end
if (specDelay == nil) then specDelay = 0 end
local pulsePeriod
local pulseWidth
local measDelay
-- Calculate the timing parameters from the frequency and duty cycle
pulsePeriod,pulseWidth,measDelay = CalculateTiming(frequency, dutyCycle)
-- Do a quick check on the input parameters
www.keithley.com97
f,msg = SimpleRegionCheck(pulseLevel, pulseLimit, dutyCycle, 2)
if (f == false) then
print(msg)
quit()
end
-- Initialize the TSP-Link
errorqueue.clear()
tsplink.reset()
errcode,errmsg,stat = errorqueue.next()
if (errcode ~= 0) then
print(errmsg)
exit()
end
reset()
ConfigureLocalSMU(pulseLevel, pulseLimit, pulsePeriod, pulseWidth, measDelay, numPulses)
ConfigureRemoteSMU(pulseLevel, pulseLimit, pulsePeriod, pulseWidth, measDelay, numPulses)
-- Start the Test
--===============
-- Turn the output on
smua.source.output= 1
node[2].smua.source.output
= 1
-- Start the trigger model execution
node[2].smua.trigger.initiate()
smua.trigger.initiate()
-- While the trigger model is outputing the waveform and collecting the
-- measurements, the script will scan the status model for any overruns
-- that may occur as a result of using impropper settings.
local ovr1 = false
local ovr2 = false
local msg1 = “”
local msg2 = “”
-- Loop until the sweep is either complete, or an overrun condition is detected
while (((status.operation.sweeping.condition ~= 0) or (node[2].status.operation.sweeping.condition ~=
0)) and (ovr1 == false) and (ovr2 == false)) do
ovr1, msg1 = CheckForOverRun(localnode)
ovr2, msg2 = CheckForOverRun(node[2])
end
if ((ovr1 == true) or (ovr2 == true)) then
smua.abort()
node[2].smua.abort()
print(“SMU#1:”, msg1)
print(“SMU#2:”, msg2)
end
-- Turn the output off
node[2].smua.source.output
= 0
smua.source.output= 0
-- Return the data
PrintDataDual()
end
--===================
-- Utility Functions
--===================
function ConfigureLocalSMU(pulseLevel, pulseLimit, pulsePeriod, pulseWidth, measDelay, numPulses)
smua.reset()
smua.source.func
= smua.OUTPUT_DCAMPS
smua.sense= smua.SENSE_REMOTE
smua.source.autorangei= 0
98
www.keithley.com
smua.source.rangei
= pulseLevel/2
smua.source.leveli
= 0
-- Set the DC bias limit. This is not the limit used during the pulses.
smua.source.limitv
= 1
smua.source.offmode
= smua.OUTPUT_NORMAL
smua.source.offfunc
= smua.OUTPUT_DCVOLTS
smua.source.offlimiti= 1e-3
smua.measure.autozero = smua.AUTOZERO_ONCE
smua.measure.autorangev= 0
smua.measure.rangev
= pulseLimit
-- The fast ADC allows us to place the measurements very close to the falling edge of
-- the pulse allowing for settled measurements even when pulse widths are very small
smua.measure.adc
= smua.ADC_FAST
smua.measure.count
= 1
smua.measure.interval= 1e-6
-- Uncomment the following lines to turn on measure filtering. When enabled, the SMU
-- will take multiple measurements and average them to produce a single reading.
-- Because the Fast ADC can take one measurement every microsecond, several measurements
-- can be aquired in a small time to produce an averaged reading.
--smua.measure.filter.count
= 5
--smua.measure.filter.enable
= smua.FILTER_ON
-- This measure delay sets the delay between the measurement trigger being received
-- and when the actual measurement(s) start. This is set to 0 because we will be
-- delaying the trigger itself and do not need additional delay.
smua.measure.delay
= 0
-- Setup the Reading Buffers
smua.nvbuffer1.clear()
smua.nvbuffer1.appendmode
= 1
smua.nvbuffer1.collecttimestamps= 1
smua.nvbuffer2.clear()
smua.nvbuffer2.appendmode
= 1
smua.nvbuffer2.collecttimestamps= 1
-- Configure the Trigger Model
--============================
-- Timer 1 controls the pulse period
trigger.timer[1].count = (numPulses > 1) and numPulses - 1 or 1
trigger.timer[1].delay = pulsePeriod
trigger.timer[1].passthrough= true
trigger.timer[1].stimulus
= smua.trigger.ARMED_EVENT_ID
-- Timer 2 controls the pulse width
trigger.timer[2].count= 1
if (type(pulseWidth) == “table”) then
-- Use a delay list if the duty cycle will vary for each pulse
trigger.timer[2].delaylist
= pulseWidth
else
-- else every pulse will be the same duty cycle
trigger.timer[2].delay
= pulseWidth
end
trigger.timer[2].passthrough= false
trigger.timer[2].stimulus
= smua.trigger.SOURCE_COMPLETE_EVENT_ID
-- Timer 3 controls the measurement delay
trigger.timer[3].count= 1
if (type(measDelay) == “table”) then
-- If the duty cycle is variable then the measure delay will be as well
trigger.timer[3].delaylist
= measDelay
www.keithley.com99
else
trigger.timer[3].delay
= measDelay
end
trigger.timer[3].passthrough= false
trigger.timer[3].stimulus
= smua.trigger.SOURCE_COMPLETE_EVENT_ID
-- TSP-Link Trigger 1 is used to synchronize the SMUs by telling
-- the second SMU when to pulse.
tsplink.trigger[1].clear()
tsplink.trigger[1].mode = tsplink.TRIG_FALLING
tsplink.trigger[1].stimulus
= trigger.timer[1].EVENT_ID
-- Configure SMU Trigger Model for Sweep
smua.trigger.source.lineari(pulseLevel/2, pulseLevel/2, numPulses)
smua.trigger.source.limitv
= pulseLimit
smua.trigger.measure.action
= smua.ASYNC
smua.trigger.measure.iv(smua.nvbuffer1, smua.nvbuffer2)
smua.trigger.endpulse.action= smua.SOURCE_IDLE
smua.trigger.endsweep.action= smua.SOURCE_IDLE
smua.trigger.count
= numPulses
smua.trigger.arm.stimulus
= 0
smua.trigger.source.stimulus= trigger.timer[1].EVENT_ID
smua.trigger.measure.stimulus
= trigger.timer[3].EVENT_ID
smua.trigger.endpulse.stimulus
= trigger.timer[2].EVENT_ID
smua.trigger.source.action
= smua.ENABLE
end
function ConfigureRemoteSMU(pulseLevel, pulseLimit, pulsePeriod, pulseWidth, measDelay, numPulses)
node[2].smua.reset()
node[2].smua.source.func
= node[2].smua.OUTPUT_DCAMPS
node[2].smua.sense= node[2].smua.SENSE_REMOTE
node[2].smua.source.autorangei
= 0
node[2].smua.source.rangei
= pulseLevel/2
node[2].smua.source.leveli
= 0
-- Set the DC bias limit. This is not the limit used during the pulses.
node[2].smua.source.limitv
= 1
node[2].smua.source.offmode
= node[2].smua.OUTPUT_NORMAL
node[2].smua.source.offfunc
= node[2].smua.OUTPUT_DCAMPS
node[2].smua.source.offlimitv
= 40
node[2].smua.measure.autozero
= node[2].smua.AUTOZERO_ONCE
node[2].smua.measure.autorangev
= 0
node[2].smua.measure.rangev
= pulseLimit
-- The fast ADC allows us to place the measurements very close to the falling edge of
-- the pulse allowing for settled measurements even when pulse widths are very small
node[2].smua.measure.adc
= node[2].smua.ADC_FAST
node[2].smua.measure.count
= 1
node[2].smua.measure.interval
= 1e-6
-- Uncomment the following lines to turn on measure filtering. When enabled, the SMU
-- will take multiple measurements and average them to produce a single reading.
-- Because the Fast ADC can take one measurement every microsecond, several measurements
-- can be aquired in a small time to produce an averaged reading.
--node[2].smua.measure.filter.count = 5
--node[2].smua.measure.filter.enable = node[2].smua.FILTER_ON
-- This measure delay sets the delay between the measurement trigger being received
-- and when the actual measurement(s) start. This is set to 0 because we will be
-- delaying the trigger itself and do not need additional delay.
node[2].smua.measure.delay
= 0
-- Setup the Reading Buffers
node[2].smua.nvbuffer1.clear()
100
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node[2].smua.nvbuffer1.appendmode= 1
node[2].smua.nvbuffer1.collecttimestamps= 1
node[2].smua.nvbuffer2.clear()
node[2].smua.nvbuffer2.appendmode= 1
node[2].smua.nvbuffer2.collecttimestamps= 1
-- Configure the Trigger Model
--============================
-- Timer 2 controls the pulse width
node[2].trigger.timer[2].count
= 1
if (type(pulseWidth) == “table”) then
-- Use a delay list if the duty cycle will vary for each pulse
node[2].trigger.timer[2].delaylist= pulseWidth
else
-- else every pulse will be the same duty cycle
node[2].trigger.timer[2].delay = pulseWidth
end
node[2].trigger.timer[2].passthrough= false
node[2].trigger.timer[2].stimulus = node[2].smua.trigger.SOURCE_COMPLETE_EVENT_ID
-- Timer 3 controls the measurement delay
node[2].trigger.timer[3].count
= 1
if (type(measDelay) == “table”) then
-- If the duty cycle is variable then the measure delay will be as well
node[2].trigger.timer[3].delaylist= measDelay
else
node[2].trigger.timer[3].delay = measDelay
end
node[2].trigger.timer[3].passthrough= false
node[2].trigger.timer[3].stimulus = node[2].smua.trigger.SOURCE_COMPLETE_EVENT_ID
-- TSP-Link Trigger 1 is used to synchronize the SMUs. SMU #2 receives
-- its trigger to pulse from SMU #1
node[2].tsplink.trigger[1].clear()
node[2].tsplink.trigger[1].mode
= node[2].tsplink.TRIG_FALLING
-- Release the trigger line when the pulse is complete
node[2].tsplink.trigger[1].stimulus = 0
-- Configure SMU Trigger Model for Sweep
node[2].smua.trigger.source.lineari(pulseLevel/2, pulseLevel/2, numPulses)
node[2].smua.trigger.source.limitv = pulseLimit
node[2].smua.trigger.measure.action = node[2].smua.ASYNC
node[2].smua.trigger.measure.iv(node[2].smua.nvbuffer1, node[2].smua.nvbuffer2)
node[2].smua.trigger.endpulse.action= node[2].smua.SOURCE_IDLE
node[2].smua.trigger.endsweep.action= node[2].smua.SOURCE_IDLE
node[2].smua.trigger.count
= numPulses
node[2].smua.trigger.arm.stimulus= 0
node[2].smua.trigger.source.stimulus= node[2].tsplink.trigger[1].EVENT_ID
node[2].smua.trigger.measure.stimulus = node[2].trigger.timer[3].EVENT_ID
node[2].smua.trigger.endpulse.stimulus = node[2].trigger.timer[2].EVENT_ID
node[2].smua.trigger.source.action = node[2].smua.ENABLE
end
function ConfigureSpectrometerTrigger(specDelay)
-- Digital I/O line 1 triggers the spectrometer measurements
-- Timer 4 puts a delay between the start of the pulse train and the
-- output of the digital IO trigger on Digital I/O line 1
digio.trigger[1].clear()
digio.trigger[1].mode
= digio.TRIG_FALLING
-- If the delay value is > 0 then configure a timer to provide the delay
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if specDelay > 0 then
trigger.timer[4].count= 1
trigger.timer[4].delay = specDelay
trigger.timer[4].passthrough= false
trigger.timer[4].stimulus
= smua.trigger.ARMED_EVENT_ID
digio.trigger[1].stimulus
= trigger.timer[4].EVENT_ID
else
-- Else bypass the timer and trigger the digital I/O immediately
-- Configure the Digital I/O pin that will trigger the spectrometer
digio.trigger[1].stimulus
= smua.trigger.ARMED_EVENT_ID
end
end
function CheckForOverRun(pNode)
-- Check SMUA Trigger Overruns
if (bit.bitand(pNode.status.operation.instrument.smua.trigger_overrun.condition,
return true, “smua arm trigger is overrun”
end
if (bit.bitand(pNode.status.operation.instrument.smua.trigger_overrun.condition,
return true, “smua source trigger is overrun”
end
if (bit.bitand(pNode.status.operation.instrument.smua.trigger_overrun.condition,
return true, “smua measure trigger is overrun”
end
if (bit.bitand(pNode.status.operation.instrument.smua.trigger_overrun.condition,
return true, “smua endpulse trigger is overrun”
end
2) == 2) then
4) == 4) then
8) == 8) then
16) == 16) then
local CFORi = 0
-- Check Timers for Overrun
if (pNode.status.operation.instrument.trigger_timer.trigger_overrun.condition > 0) then
return true, string.format(“Timer trigger is overrun: 0x%x”, CFORi)
end
-- Check Blenders for Overrun
if (pNode.status.operation.instrument.trigger_blender.trigger_overrun.condition > 0) then
return true, string.format(“blender trigger is overrun: 0x%x”, CFORi)
end
-- Check TSP-Link Triggers for Overrun
if (pNode.status.operation.instrument.tsplink.trigger_overrun.condition > 0) then
return true, string.format(“TSP-Link trigger is overrun: 0x%x”, CFORi)
end
-- Check DIGIO Triggers for Overrun
if (pNode.status.operation.instrument.digio.trigger_overrun.condition > 0) then
return true, string.format(“digio trigger is overrun: 0x%x”, CFORi)
end
-- Check LAN Triggers for Overrun
if (pNode.status.operation.instrument.lan.trigger_overrun.condition > 0) then
return true, string.format(“LAN trigger is overrun: 0x%x”, CFORi)
end
return false, “no overrun detected”
end
function PrintData()
print(“Timestamp\tVoltage\tCurrent”)
for i=1,smua.nvbuffer1.n do
print(smua.nvbuffer1.timestamps[i], smua.nvbuffer2[i], smua.nvbuffer1[i])
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end
end
function PrintDataDual()
local voltage
local current
print(“Timestamp\tVoltage\tCurrent”)
for i=1,smua.nvbuffer1.n do
voltage = (smua.nvbuffer2[i] + node[2].smua.nvbuffer2[i])/2
current = smua.nvbuffer1[i] + node[2].smua.nvbuffer1[i]
print(smua.nvbuffer1.timestamps[i], voltage, current)
end
end
function
local
local
local
CalculateTiming(frequency, dutyCycle)
pulsePeriod
pulseWidth
measDelay
= 1/frequency
-- If duty cycle was a table then we need to create delay lists for the timers
if (type(dutyCycle)==”table”) then
pulseWidth= {}
measDelay = {}
for i=1,table.getn(dutyCycle) do
if ((dutyCycle[i] > 99) or (dutyCycle[i] < 0.01)) then
print(string.format(“Error: dutyCycle[%d] must be between 0.01% and 99%.”, i))
exit()
end
-- Calculate pulse width from period and duty cycle. Subtract 3us of overhead
pulseWidth[i]
= pulsePeriod * (dutyCycle[i]/100) - 3e-6
-- Set measure delay so measurement happen 10us before the falling edge of the pulse
measDelay[i]
= pulseWidth[i] - 10e-6
end
else -- Duty cycle was a single value so we only need a single delay value for the timers
if ((dutyCycle > 99) or (dutyCycle < 0.01)) then
print(“Error: dutyCycle must be between 0.01% and 99%.”)
exit()
end
pulseWidth
= pulsePeriod * (dutyCycle/100) - 3e-6
measDelay
= pulseWidth - 10e-6
end
return pulsePeriod, pulseWidth, measDelay
end
function SimpleRegionCheck(pulseLevel, pulseLimit, dutyCycle, SMUs)
-- This function only serves as a quick check that the entered parameters are
-- within the max allowable duty cycles for the operating regions. This function
-- does not check that the pulse widths are within the maximums as well.
local pLev = math.abs(pulseLevel)
f = true
msg = “Checks passed.”
if ((pulseLimit >= 10e-3) and (pulseLimit <= 10)) then
if ((pLev > 30*SMUs) and (dutyCycle > 35)) then
msg = string.format(“Duty Cycle too high for pulse region 5. Duty cycle must be 35%% or less
for pulse levels above %dA.”, 30*SMUs)
f = false
elseif (((pLev > 20*SMUs) and (pLev <= 30*SMUs)) and (dutyCycle > 50)) then
msg = string.format(“Duty Cycle too high for pulse region 2. Duty cycle must be 50%% or less
for pulse levels between %dA and %dA.”, 20*SMUs, 30*SMUs)
f = false
end
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elseif ((pulseLimit > 10) and (pulseLimit <= 20)) then
if ((pLev > 20*SMUs) and (dutyCycle > 10)) then
msg = string.format(“Duty Cycle too high for pulse region 6. Duty cycle
for pulse levels above %dA.”, 20*SMUs)
f = false
elseif (((pLev > 10*SMUs) and (pLev <= 20*SMUs)) and (dutyCycle > 40)) then
msg = string.format(“Duty Cycle too high for pulse region 3. Duty cycle
for pulse levels between %dA and %dA.”, 10*SMUs, 20*SMUs)
f = false
end
elseif (pulseLimit > 20) and (pulseLimit <= 40) then
if ((pLev > 10*SMUs) and (dutyCycle > 1)) then
msg = string.format(“Duty Cycle too high for pulse region 7. Duty cycle
for pulse levels above %dA.”, 10*SMUs)
f = false
elseif (((pLev > 5*SMUs) and (pLev <= 10*SMUs)) and (dutyCycle > 40)) then
msg = string.format(“Duty Cycle too high for pulse region 4. Duty cycle
for pulse levels between %dA and %dA.”, 5*SMUs, 10*SMUs)
f = false
end
else
msg = “Error: pulseLimit out of range. pulseLimit must be between 10mV and
f = false
end
must be 10%% or less
must be 40%% or less
must be 1%% or less
must be 40%% or less
40V.”
return f,msg
end
--PWM_Test_Single(1, 2, 100, 1, 10, 0)
-- duty = {20, 40, 60, 80, 60, 40, 20, 40, 60}
--PWM_Test_Single(20, 10, 1000, duty, 9, 1e-3)
--PWM_Test_Dual(40, 10, 1000, duty, 9, 1e-3)
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© Copyright 2014 Keithley Instruments, Inc.
Printed in the U.S.A
No. 3260
4.15.14
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