Breakdown and Leakage Current Measurements on High Voltage Semiconductor Devices

Breakdown and Leakage Current Measurements on High Voltage Semiconductor Devices
Number 3249
Application Note
Se­ries
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,
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.
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
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
Appendix
# Import pyVisa and time modules into the Python
environment
import visa
import time
# 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")
# Define sweep variables for the programmed output
voltage
# and measured current readings
voltage = 0
currReading = ""
currRdgList = []
# Turn on the output of the 2290
time.sleep(1)
ki2290.write("HVON")
print "Running Sweep . . . "
# 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")
# 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")
# 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")
# 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
currReading = float(currReading)
currRdgList.append(currReading)
voltage = voltage + 100
# Set the voltage of the 2290 to 0V and turn off its
output
ki2290.write("VSET 0")
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
Specifications are subject to change without notice. All Keithley trademarks and trade names are the property of Keithley Instruments, Inc.
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© Copyright 2013 Keithley Instruments, Inc.
Printed in the U.S.A
No. 3249
1.17.14
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