"Enhancement Mode Gallium Nitride (eGaN) FET Characteristics under Long Term Stress"

Enhancement Mode Gallium Nitride (eGaNTM) FET Characteristics
under Long Term Stress
Alexander Lidow
J. Brandon Witcher
Ken Smalley
Chief Executive Officer
Efficient Power Conversion
Corporation
El Segundo, California 90245
Senior Member
of the Technical Staff
Sandia National Laboratories
Albuquerque, New Mexico 87185
Product Engineer
Microsemi Corporation
Lawrence, Massachusetts 01841
Abstract: Enhancement mode HEMT transistors built
with Gallium-Nitride-on-silicon (eGaN) have been in the
commercial marketplace since 2009 as a replacement for
silicon power MOSFETs. Superior conductivity and
switching characteristics allow designers to greatly reduce
system power losses, size, weight, and cost. Military and
space applications could benefit from using eGaN FETs,
but the parts need to operate reliably under harsh
environmental conditions. In this paper we present results
demonstrating the stability of these devices at temperature
and under radiation exposure.
FET. Aluminum gallium nitride (AlGaN) is then grown on
top of the GaN to form a highly conductive two
dimensional electron gas layer (2DEG) immediately
underneath [1]. Additional layers are formed to create an
enhancement mode gate electrode as well as drain and
source electrodes (see Figure 1). This structure is repeated
many times to form a power device. Three layers of top
metal, isolated from each other by planarized insulators,
are then deposited to complete the circuit and bring the
terminals to the outside world. The end result is a simple,
cost-effective solution for power switching. This device
looks and behave similarly to a silicon MOSFET.
Keywords: Gallium nitride; GaN; eGaN; MOSFET; FET;
Radiation tolerance; SEE; Gamma radiation; Proton
radiation
Introduction:
The basic requirements for power semiconductors are
efficiency,
reliability,
controllability,
and
cost
effectiveness. Recent breakthroughs by Efficient Power
Conversion Corporation (EPC) in processing gallium
nitride have produced enhancement mode devices
(eGaNTM) with conductivity and switching speeds much
higher than Si power FETs. These improvements enable
power converters with higher efficiency and switching
frequency, as well as greater input voltage range, leading to
simpler, smaller power systems. Ranging in voltage from
40 V to 200 V, and on-state resistance from 4 mΩ to 100
mΩ, this new class of devices has also proved very capable
in a high radiation environment.
This paper is divided into four sections. The first discusses
the basic eGaN FET structure. The second section shows
the result of heavy ion testing with linear energy transfers
(LETs) ranging from 28.8 to 87.2 MeV*cm2/mg. The third
section gives pre-and post results for total dose testing up to
500 kRad (Si). The final section discusses the successful
multi-thousand hour stress testing under a variety of bias
and temperature conditions.
Structure: EPC’s eGaN FET process begins with silicon
wafers. A thin layer of Aluminum Nitride (AlN) is grown
on the silicon to isolate the device structure. On top of the
AlN, a thicker layer of highly resistive GaN is grown. This
layer provides a foundation on which to build the eGaN
Presented March 24, 2011
GOMACTech11
Figure 1. Simplified cross section of an eGaN FET
SEE Testing: Heavy-ion testing of EPC eGaN FETs was
performed at the Texas A&M cyclotron in September,
2010 following MIL-STD-750E, METHOD 1080. EPC’s
entire family of eGaN FET products was tested (See Table
1 for device parameters). EPC1005 (60 V, 7 mΩ),
EPC1014 (40 V, 16 mΩ), and EPC1015 (40 V, 4 mΩ) did
not exhibit any gate ruptures (SEGR) or drain ruptures
(SEB) at the highest LET available. For the remaining part
numbers, drain-ruptures were the primary mode of failure
and were typically observed as being a gradual transition
into failure.
In figure 2 is shown the applied drain-source voltages up to
which the parts remained within data sheet limits (dark
grey); the voltages up to which the devices exceeded data
sheet limits but remained functional (light grey), and the
voltage beyond which the devices failed catastrophically
(black). In general, the eGaN FETs demonstrated SEE
capabilities that exceed similar silicon MOSFETs currently
listed on the Qualified Military Listing (QML).
Table 1: EPC’s eGaN FET Electrical Characteristics
250
200
200
200
150
150
150
100
100
50
50
0
0
Voltage
250
Voltage
Voltage
SEE Heavy Ion Testing - Kr
SEE Heavy Ion Testing - Xe
SEE Heavy Ion Testing - Au
250
100
50
0
Figure 2. SEE Results for Au, Kr, and Xe bombardment. Dark grey bars represent the voltage range within which
devices remained inside data sheet limits. Light grey bars represent the range where devices continued to function
but drain-source leakage exceeded data sheet. Black bars represent regions where devices catastrophically failed
Total Dose Testing: Utilizing the “Gamma Cave” at the
University of Massachusetts, Lowell, six EPC1014 (40 V, 4
mΩ) were subjected to a total gamma dose of 500 kRads
(Si) at a dose rate of 96 Rads (Si)/sec. A 60Co source was
used and all testing was according to MIL-STD-750,
Method 1019. Two different test conditions were used.
The first test condition biased the drain-source at 80% of
rated VDS(MAX) (32 V in the case of the EPC1014). The
second test condition biased the gate-source at 5 V. Table
2 shows the pre and post electrical characteristics of these
devices. Very little change is seen in any of the
characteristics under either bias condition.
With 32 V from drain to source during irradiation, the
threshold voltage (VTH) changed less than 18% percent;
RDS(ON) changed less than 8 percent, and all parameters
remained well within the data sheet limits. With 5 V from
gate to source during irradiation, the threshold voltage
(VTH) changed less than 4 percent; RDS(ON) changed less
than 3 percent, and all parameters again remained well
within the data sheet limits.
Presented March 24, 2011
GOMACTech11
Whereas we can’t quantify how long the parts will last in a
given orbit since that depends on factors such as the
placement of the part within the payload and within the
spacecraft (This can make an order of magnitude to the
dose rate), we can make relative comparisons. The problem
designers run into with silicon MOSFETs is that they must
choose between radiation tolerance and electrical
performance. Commercial MOSFETs have thick gate
oxides and trap a lot of charge, resulting in large shifts in
the threshold voltage and eventual failure at relatively low
total-dose. Meanwhile, the rad-hard MOSFETs available
have on-resistance and device capacitance several times
higher than their commercial counterparts, leading to either
low efficiency or large size (due to the low switching
frequency). Now we have a new capability; eGaN FETs,
with electrical performance superior to the cutting edge Si
MOSFETs, and radiation tolerance at least as high as the
best rad-hard power MOSFETs available. These eGaN
FETs bring a combination of electrical and radiation
performance that cannot be matched.
Table 3. Reliability test results for eGaN FETs
Additional radiation testing is ongoing using different part
numbers, different bias conditions, and different dose rates.
These results will be reported in a later publication and are
expected to reinforce the conclusion that eGaN FETs are
resistant to very high doses of radiation.
Table 2. Pre and post irradiation data shows little
change after 500 kRad (Si)
EPC1014
kRAD
0k
VDS
0k
VGS
Group D
Limit
500k
VDS
500k
VGS
Test
Bias1
Bias2
Min
Max
1
2
3
4
5
6
Min
Max
1
2
3
4
5
6
IGSSr
5.00 V
IGSSf
5.00 V
IDSS
32.0 V
IDSS
40.0 V
500.0uA
12.60u
13.23u
11.27u
11.35u
11.49u
12.47u
2.000mA 100.0uA 100.0uA
38.87u 29.91u 29.06u
58.39u 26.26u 27.16u
34.95u 31.80u 30.24u
27.85u 31.42u 30.63u
27.90u 30.04u 29.69u
23.76u 30.29u 29.55u
500.0uA
5.119u
4.698u
3.860u
9.059u
6.500u
11.61u
2.000mA 100.0uA 100.0uA
24.95u 10.58u 11.93u
46.89u 8.679u 11.07u
28.35u 8.187u 10.75u
22.15u 33.16u 30.80u
32.39u 22.82u 23.51u
19.52u 35.05u 35.64u
VTH
2.00mA
700.0m
2.500 V
1.624
1.583
1.552
1.487
1.548
1.606
700.0m V
2.500 V
1.714
1.868
1.879
1.482
1.611
1.568
RDON
5.00 A
5.00 V
VDSON
500 mA
0.00 V
16.00mR 1.800 V
14.18m 2.304
14.64m 2.283
13.83m 2.254
13.68m 2.227
13.54m 2.251
13.81m 2.302
16.00mR 1.800 V
14.71m 2.401
15.83m 2.449
14.55m 2.386
13.47m 2.252
13.58m 2.243
13.41m 2.292
Long Term Reliability Testing: The key reliability
considerations for power transistors include: (a) device
stability in the on-state when the FET is fully enhanced
with voltage applied to the gate; (b) device stability in the
off-state when the FET is in voltage blocking mode
withstanding up to its rated drain-source voltage; and (c)
device stability in switching operation. Device stability is
impacted by device design, packaging technology, and
operating environment. Good reliability results have
previously been reported for depletion mode GaN FETs for
RF [2,3] applications and power switching applications [4].
EPC’s eGaN FET reliability test results are summarized in
Table 3, in which the type of test, stress conditions, part
numbers, sample size, stress hours, and number of fails are
listed. JEDEC standards were followed where applicable.
Even very high levels of environmental stress applied to
hundreds of parts generated no failures thus demonstrating
the basic capability of the technology and the product to
survive over many years in terrestrial or space
environments.
Presented March 24, 2011
GOMACTech11
# of fails
1000HR
3000HR
Stress Test
Test Condition
Part Number
Sample Size
HTRB
100Vds, 125oC
EPC1001
45
0
-
HTRB
40Vds, 125oC
EPC1014
50
0
-
o
HTRB
200Vds, 125 C
EPC1012
50
0
HTRB
200Vds, 125oC
EPC1010 with underfill
50
0
-
HTRB
200Vds, 150oC
EPC1010
50
0
0
Stress Test
Test Condition
Part Number
Sample Size
HTGB 5V
5Vgs, 125oC
EPC1001
45
0
0
HTGB 5.4V
5.4Vgs, 125oC
EPC1001
45
0
0
HTGB 5V
5 Vgs, 150oC
EPC1010
45
0
-
HTGB -5V
-5Vgs, 125oC
EPC1001
50
0
-
Stress Test
Test Condition
Part Number
Sample Size
TC
-40C to 125oC
EPC1001
-
# of fails
1000HR
3000HR
# of fails
1000cys
45
0
TC
-40C to 125 C
EPC1014
50
0
TC
-40C to 125oC
EPC1012
45
0
-
TC
-40C to 125oC
EPC1012 with underfill
45
0
-
Stress Test
Test Condition
Part Number
Sample Size
THB
85oC/85RH, 40Vds
EPC1014
45
0
-
THB
85oC/85RH, 40Vds
EPC1015
45
0
-
THB
85oC/85RH, 100Vds
EPC1010
25
0
-
EPC1010 with underfill
25
0
-
THB
o
o
85 C/85RH, 100Vds
Stress Test
Test Condition
Part Number
Sample Size
MSL1
85oC/85RH, 168HR
EPC1001
50
Stress Test
Test Condition
Part Number
Sample Size
Power Supply Life Test
10A, 250 kHz, 30oC
EPC1001
10
-
# of fails
1000HR
# of fails
168HR
0
-
# of fails
1200HR
0
-
Conclusions:
EPC’s eGaN FETs have been tested under heavy ion
bombardment,
gamma
irradiation,
and
various
environmental stress factors. These devices demonstrate
their ability to be used in the most stringent of
environmental as well as radiation environments and
exceed the capabilities of silicon power MOSFETs.
References
1.
M. Asif Khan, A. Bhattarai, J.N. Kuznia, and D.T.
Olson, “High Electron Mobility Transistor Based on a
GaN-AlxGa1-xN Heterojunction,” Appl. Phys. Lett.,
vol. 63, no. 9, 1993, pp. 1214-1215.
2.
S. Singhal, J.C. Roberts, P. Rajagopal, T. Li, A.W.
Hanson, R. Therrien, J.W. Johnson, I.C. Kizilyalli,
K.J. Linthicum, “GaN-on-Si failure mechanisms and
reliability improvements,” in Proc. IEEE Int. Rel.
Phys. Symp.,
3.
P. Saunier, C. Lee, A. Galistreri, D. Dumka, J.
Jimenez, H.Q. Tserng, M.Y. Kao, P.C. Chao, K. Chu,
A. Souzis, I. Eliashevich, S. Guo, J. del Alama, J.
Joh, M. Shur, “Progress in GaN Performances and
Reliability,” in Proc.
4.
M. Briere, “GaN on Si Based Power Devices: An
Opportunity to Significantly to Impact Global Energy
Consumption,” CS MANTECH, May, 2010.
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