AN-1155

```Application Note AN-1155
Linear Mode Operation of Radiation Hardened
MOSFETS
By
Michael F. Thompson
Page
INTRODUCTION .........................................................................................................2
A Review of the Linear Mode of Operation and its Problems...............................2
Evolution of Device Gain and the Zero Temperature Crossover Point ...............4
Proposed Solutions...................................................................................................5
SOA Curves Based on Test Results........................................................................6
Conclusion .................................................................................................................11
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I. INTRODUCTION
I
NTERNATIONAL Rectifier would like to present this
document to explain our approach to the linear mode
operation of our high reliability (hi-rel) MOSFETS. We will
review the definition of this mode of operation and its
associated problems. The problems unique to this mode of
operation have been widely discussed in the literature for over
a decade, and this document will cover areas of interest to our
particular products. These areas include
--First, the problems that are particular to the
designers of MOSFET circuits that implement linear mode
operation.
--Second, a description of the gain of our hi-rel
MOSFETS and representative zero temperature gate threshold
crossovers.
--Third, a brief history of the proposed solutions by
various sources.
--Fourth, our proposed solution method involving
new curves of safe operating area based on test results.
II. A REVIEW OF THE LINEAR MODE OF OPERATION AND ITS
PROBLEMS
A. Definition of the Linear Mode
The linear mode of MOSFET operation is not to be
confused with the linear region, where the MOSFET drain
current is a linear function of the drain voltage. Linear mode
of operation is defined as operation of the MOSFET where
small changes of Vgs results in linear changes of the drain to
source current.
In the non-saturated region of MOSFET operation the drain
current is defined by [1] as
ID =
β
[2(V
2
GS
2
− VT 0 )VDS − VDS
]
labeled by various similar terms, but all refer to the same
basic problem. Any MOSFET is susceptible to this failure
mode, and this has been very well described in [1]-[4]. To
summarize the problem for discussion in this work, the failure
mode can be described qualitatively by examining the drain
current (Id) versus gate to source voltage (Vgs) graphs for any
given MOSFET.
Figure II-1 shows the transfer curve of the IR1405, an IR
part from commercial MOSFET trench technology product
line. In all graphs of this type that are discussed in this paper,
it is important to note the cross-over point where the gain of
the device is equal regardless of the MOSFET temperature.
This is called the gain zero temperature crossover point,
which is approximately 5.8V in this example. At values of
Vgs above this point, the gain of the device is less at higher
temperature. Cells of the MOSFET structure that are hotter
will channel less current than the surrounding, cooler cells.
The local hot spot can initially be caused by any number of
non-idealities such as small local solder voids under the
packaged parts or small nonuniformities in the silicon
structure. This is an example of a positive temperature
coefficient for Rds(on), which allows the hotter cells to
reduce their drain current and on-state conduction losses such
that they can cool off. Below this zero temperature crossover
point, any increase in cell temperature results in more drain
current, which allows the cell to pull in current from its
neighbors. Having more current conducted through the cell
makes its temperature rise due to on-state losses, so the gain
will then increase further, until the device temperature
maximum is reached and the part fails in a spectacular manner
at the hot spot.
(1)
where β is termed the device transconductance parameter
such that
⎛W ⎞ A
⎟[ V 2 ]
⎝L⎠
β = μ n COX ⎜
(2)
In saturation, the drain current is described by the equation
ID =
β
2
(VGS − VT 0 )2
(3)
B. The Problem
Operation of the MOSFET in linear mode can create what
has been called “thermal current focusing” [1], “electrothermal instability” [3], “thermal runaway”, and has been
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Figure II-1: Transfer Curve for IRF1405. This figure shows the
relationship between the applied gate voltage and the resulting drain
current through the transconductance gain of the device. The trouble with
linear mode operation begins when the designer operates in the region
below the zero temperature crossover (defined as the point where the
device’s gain is the same for all temperatures). In this case that is
approximately 5.8V from gate to source.
An example of a part failure in linear mode is shown in
Figure II-2 and Figure II-3. This is a MOSFET from the
commercial product line of IR (to date, we have not
confirmed any cases of hi-rel MOSFET failure due to linear
mode operation in any of our customers’ production
hardware). This part failed at a Vds of 16V and an Ids of
4.2A.
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Figure II-2: Thermal image of a failure of an IRF1405Z. The picture shows
the hotspot that develops when the problem of linear mode operation
manifests itself. The die temperature is not uniform and a severe gradient
forms where the gain of the local MOSFET cells have started to draw in the
current from neighboring cells.
3
1000
ID (A)
100
10
1
0.1
2
3
4
5
6
7
8
VGS (V)
25°C
Figure II-3: Visual Image of the IRF1405Z failure. This type of visual
pattern is more attractive when observed on the surface of the moon,
instead of on the surface of parts that are trying to land there.
Figure III-1: Transfer Curve for IRHM9260 (Generation 4, P channel
device). This figure shows the relationship between the applied gate
voltage and the resulting drain current through the transconductance gain of
the device. The trouble with linear mode operation begins when the
designer operates in the region below the zero temperature crossover
(defined as the point where the device’s gain is the same for all
temperatures). In this case that is approximately 3.9V from gate to source.
PSPICE models that are available from IR (& that can be
obtained by browsing to http://www.irf.com/productinfo/models/) do not predict this failure as they assume a
uniform die temperature to simulate MOSFET behavior.
The IR radiation hardened MOSFETs were optimized to be
used in switching applications, not linear applications. In
switching power devices, it is desired to improve the device
gain as new technology is introduced. Older devices therefore
have lower gains than the new devices. Unfortunately for the
designers of circuits that implemented the linear mode, that
means that this problem gets worse with newer parts (and
could be the reason why this has not been so much of a
Figure III-1 through Figure III-4 show the improvement in
hard) MOSFETs offered from IR. The figures also show the
zero temperature crossover point for representative parts from
each generation of design. The slope of the curves show how
the gain was increased for each generation, and the crossover
points give some idea of the range of gate voltages which
would be of concern for linear mode operation.
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1000
100
ID (A)
III. EVOLUTION OF DEVICE GAIN AND THE ZERO
TEMPERATURE CROSSOVER POINT
150°C
10
1
0.1
2
3
4
5
6
7
8
VGS (V)
25°C
150°C
Figure III-2: Transfer Curve for IRHMS597260 (R5, P channel device).
Note the increased slope of the curve versus the previous Generation 4
device, showing greater potential for instability with respect to device gain,
along with a higher zero temperature crossover voltage.
4
commercial grade of IR parts, but for our hi-rel components
the best method is an SOA curve derived from test data.
1000
ID (A)
100
10
1
0.1
2
3
4
5
6
7
8
VGS (V)
25°C
150°C
Figure III-3: Transfer Curve for IRHNJ67130 (R6). Note the significantly
larger zero temperature crossover voltage as compared to previous
generations. There is an increase in gain as well. However some of this is
due to the inherently larger gain of N channel over P channel devices, due
to the higher mobility of electrons as compared to holes.
A. Placing a Knee in the SOA Curves
The failure of some types of MOSFETs would lend one to
the conclusion that a “knee” could be artificially inserted into
the SOA curve in order to prevent incorrect linear mode
operation of MOSFETs, or perhaps even placing a guard
banded version of an SOA line would be sufficient. Figure
IV-1 shows a typical SOA curve from [5] and it can be seen
why one would be tempted to modify the SOA boundaries
with new lines or inserting a knee in the curve. However, the
accuracy of either of these approaches would depend on the
overall sample size and/or the accuracy of the data point at the
extreme of Vds. Not to mention that the straight-line guard
band would also cut out a large area of SOA unnecessarily,
and could create false perceptions of application problems
that do not exist. For certain parts where the parts fail at even
lower levels of Vds than shown in Figure IV-1, the SOA
curve would loss an unacceptably large percentage of its area.
1000
ID (A)
100
10
1
0.1
2
3
4
5
6
7
8
VGS (V)
25°C
Figure IV-1: A Generic MOSFET SOA. Observed failures of the parts are
plotted against the standard SOA curve. Possible guard-banded versions of
the curve are also shown.
150°C
Figure III-4: Transfer Curve for IRHLNA77064 (R7). This is a logic-leveldriven device, so the gain shifts even higher for this generation of devices.
Note that the gain at the zero temperature crossover point is one order of
magnitude higher than the Generation 4 devices.
IV. PROPOSED SOLUTIONS
There have been a number of proposed solutions to this
problem. The first has been shown to be fairly ineffective [5],
the thermal instability method has been in use for the
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B. Spirito Curves
In any device, thermal instability can occur when the
electrically generated power exceeds the thermally dissipated
power [6]. Spirito et al. used this as the basis for their
approach [5] of an augmented safe operating area (SOA)
curve. The condition of instability as defined in this approach
is
5
ΔI D
1
≥
ΔT VDS Z θ
(4)
where Zθ is the thermal impedance of the part from junction
to case.
This approach, while it is useful for certain MOSFET
designs, is not universally applicable. It does provide a
boundary, but in the case of IR’s hi-rel radiation hardened
MOSFETS the device structure is such that this is not a good
application. The Spirito curves would, in some cases, generate
new SOA curves that are too conservative and would remove
too much of the effective SOA for hi-rel applications. This
would lead some of our customers to the incorrect conclusion
that their circuit applications will create a failure of the part.
IR has determined that a test method is best for generating
SOA curves of our rad hard parts. The following section will
discuss the Spirito Curves and the test-based curves with
specific examples.
C. Testing
The third method proposed for predicting the safe operating
area of any given device, is the most basic and perhaps the
most reliable one: test results. By characterizing a product
family through actual testing, new curves can be generated. It
has been the experience of International Rectifier Hi-Rel
division that the most accurate way to characterize our parts is
through individual testing instead of applying a representative
curve across an entire product family.
V. SOA CURVES BASED ON TEST RESULTS
The method for creating new SOA curves for rad hard IR
MOSFETs is based on testing actual parts. The device failure
mechanism is tied to the gain of the device, which has low
variability across the lots produced. Random samples of
MOSFET die were chosen from a particular part deign, and
the die was then assembled into a package with a lid and then
tested. The parts were tested by applying specific Vds and
increasing the Ids in steps until either the junction temperature
reaches 150 degrees Celsius or instability is detected. As the
power dissipation in the device is increased, we measure the
temperature of the die and watch for signs of non-linear
temperature increases. For high drain biases, this would
usually happen well short of the maximum rated die
temperatures for the parts. This data served as the basis for the
new SOA curves.
For the first parts tested, we created a new SOA curve and
plotted it with the existing SOA curves and the Spirito Curve,
for the sake of comparison.
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Figure V-1: IRHM9260 Preliminary SOA Curve. This shows a Gen 4 P-channel device, with the existing SOA curves for shorter pulse widths, the Spirito
curve, and the measured curve derived from test results. It is important to note a few things in this figure. First, the Gen 4 device is relatively unaffected by
Linear Mode operation, as noted by both the Spirito Curve and the Measured Curve. Second, the Spirito Curve in this case is not far removed from the
Measured Curve. Finally, the updated SOA curve approved for use is available on the IR web site.
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Figure V-2: IRHM597260 Preliminary SOA Curve. This shows an R5 P-channel device, with the existing SOA curves for shorter pulse widths, the Spirito
curve, and the measured curve derived from test results. This figure shows how the R5 generation would seem to be more affected by the Spirito Curve. Yet the
test data shows otherwise. The updated SOA curve approved for use is available on the IR web site.
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Figure V-3: IRHNJ 67130 Preliminary SOA Curve. This shows an R6 N-channel device, with the existing SOA curves for shorter pulse widths, the Spirito
curve, and the measured curve derived from test results. This figure shows how the R6 generation would seem to be more affected by the Spirito Curve. Again
the test data shows otherwise. The updated SOA curve approved for use is available on the IR web site.
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Figure V-4: IRHLNA77064 Preliminary SOA Curve. This shows an R7 N-channel device, with the existing SOA curves for shorter pulse widths, the Spirito
curve, and measured curve derived from test results. This figure shows how the R7 generation would seem to be more affected by the Spirito Curve. Again the
test data shows otherwise. The updated SOA curve approved for use is available on the IR web site.
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VI. CONCLUSION
The IR hi-rel MOSFETs, while designed for switching
applications, can be used in other application if the proper
SOA curves are followed. The most applicable method for
creating a new SOA curve is via test characterization of the
packaged part. The Spirito method, while it does yield a
conservative curve of the SOA, does not accurately enough
reflect the performance of IR’s rad hard MOSFETs. At the
time of this publication (August 2009), IR has completed new
SOA curves for approximately 30% of its rad hard
MOSFETs., and continues to publish new curves on the web
original industry alert can be found there, as well as the latest
compilation of SOA curves.
ACKNOWLEDGMENT
Michael F. Thompson (IEEE M’05–SM’07) became a Member (M) of IEEE
in 2005, and a Senior Member (SM) in 2007. Born in Pittsburgh, PA in 1972,
the author received a Bachelor of Science degree from the University of
Pennsylvania in Philadelphia, PA, USA in 1994, and a Master of Science
degree (with a specialty in power electronics and controls) from the State
University of New York in 1998.
He has worked for Lockheed Martin and Northrop Grumman. His
design experience covers AC-DC, DC-AC, DC-DC power conversion at
power levels that range from milliwatts to megawatts. He is familiar with the
construction and characteristics of energy storage devices and the
complexities of digital controls for power systems. Low noise output ripple
designs and distributed power systems are of interest to him. He is presently
working for International Rectifier as the Hi-Rel Field Application Engineer
for the Eastern United States and is based out of his office in Maryland.
I would like to thank Milt Boden, Sandra Liu, Max Zafrani,
and Lala Dhaimade (all of whom are employed by IR) for
providing many answers to the questions I uncovered as this
problem was being investigated, and particularly for imparting
to me some of their considerable knowledge of MOSFET
technology.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
J.P. Uyemura (1988). Fundamentals of MOS Integrated Circuits, pp. 2324
H. R. Ronan, “MOSFET SOA: Thermal current focusing affects power
MOSFET operation”, PCIM August 1998, pp.51-55.
G. Breglio, F. Frisina, A. Magri, and P. Spirito, “Electro-thermal
Instability in low voltage Power MOS: experimental characterization”,
IEEE ISPSD 1999, pp. 233-236.
P. Spirito, G. Breglio, and V. d’Alessandro, “Modeling the Onset of
Thermal Instability in Low Voltage Power MOS: an Experimental
Validation”, IEEE ISPSD 2005, pp. 159-162
A. Kwan, K. Teasdale, N. Nguyen, J. Ambrus, & T. McDonald,
“Improved SOA analysis for trench MOSFETs using the Spirito
approach”, 9th Annual Automotive Electronics Reliability Workshop,
Nashville, TN, April 21, 2004
P. Hower, C-Y Tsai, S. Merchant, T. Efland, S. Pendharkar, R.
Steinhoff, and J. Brodsky, “Avalanche-induced thermal instability in
LDMOS transistors”, IEEE ISPSD Osaka 2001, p 153
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