10 kV SiC BJTs - GeneSiC Semiconductor

Proceedings of The 25th International Symposium on Power Semiconductor Devices & ICs, Kanazawa
10 kV SiC BJTs – static, switching and reliability
Siddarth Sundaresan, Stoyan, Jeliazkov, Brian Grummel, Ranbir Singh
GeneSiC Semiconductor, Inc.
43670 Trade Center Pl, Suite 155,
Dulles, VA 20166, USA.
[email protected]
emitter and base epilayers were appropriately designed for
maximizing the current gain of the BJTs. In addition to
discrete BJTs, two-stage Darlington BJTs were also
fabricated on the same wafer with an output to driver
transistor ratio of 3:1. After device fabrication, static
electrical characterization of the 10 kV BJTs were
performed with a Tek 371 curve tracer and a custom-built
high-voltage measurement system with a 20 kV limit. The
switching characteristics were measured with an inductive
load and a standard double-pulse scheme. An off-the-shelf
IGBT gate driver (IXDD614) was used for driving the BJTs
with a 18 nF dynamic capacitor connected in parallel with
the gate resistor for fast charging and discharging of the
BJT’s internal capacitances. GeneSiC’s 8 kV/10 A SiC JBS
rectifier was used as the free-wheeling diode for the
switching measurements. A schematic of the gate driver
circuit used for testing the BJTs is provided in Fig. 1.
Abstract— Open-base breakdown voltages as high as 10.5 kV
(91% of theoretical avalanche limit and 125 V/µm), onresistance of 110 mΩ-cm2 close to the unipolar limit of 94 mΩcm2, and current gain as high as 75 are measured on 10 kV-class
SiC BJTs. Monolithic Darlington-connected BJTs fabricated on
the same wafer yield current gains as high as 3400, and show Si
BJT-like output characteristics with a differential on-resistance
as low as 44 mΩ-cm2 in the saturation region and a distinct
quasi-saturation region. Switching measurements performed at
a DC link voltage of 5 kV and collector current of 8 A feature a
collector current rise time as low as 30 ns during turn-on and
collector voltage recovery time as low as 100 ns during turn-off.
Very low turn-on and turn-off switching energies of 4.2 mJ and
1.6 mJ, respectively, are extracted from the switching transients,
which are 19 and 25 times smaller than the corresponding
switching energies reported on 6.5 kV Si IGBTs. When turnedon to a short-circuited load at a collector bias of 4500 V, the 10
kV BJT shows a temperature-invariant, withstand time in excess
of 20 µs. Leakage currents < 1µA (system limit) are measured,
even after 234 hours of operation under a DC collector bias of
5000 V at elevated temperatures.
10 kV-class SiC BJTs are extremely attractive for
significantly reducing the size, weight, cooling
requirements and increasing the efficiency of power
conversion electronics for the medium-voltage range of
applications. Previous reports on ≥ 10 kV SiC BJTs
describe either un-optimized device designs with very low
(< 30) current gain [1] or small-area (100 µm diameter) test
BJTs with mA current capability [2]. None of these
reported devices are suitable for insertion into actual power
electronics systems. This article presents a comprehensive
analysis of the experimental characteristics of single-stage
and monolithic Darlington SiC npn BJTs recently fabricated
at GeneSiC Semiconductor with large chip sizes of 3.65
mm x 3.65 mm (active area = 2.7 mm2) and 7.3 mm x 7.3
mm (active area = 28 mm2).
The SiC BJTs presented in this article were fabricated
on 84 µm thick, 7x1014 cm-3 doped n- collector layers. The
Figure 1. Simplified schematic of the gate drive configuration used in this
work for driving the high-gain SiC BJTs. A typical CG of 18 nF and a RG of
11 Ω were used for the switching measurements.
A. Blocking Characteristics
The on-wafer blocking I-V characteristics measured on
one 100 mm SiC wafer populated with 10 kV BJTs is shown
in Fig. 2(a). The on-wafer testing was performed up to to 6
kV, which is the limit of the on-wafer probing system for
three terminal devices. Extremely low leakage currents are
observed for these devices up to the testing limit. The high
background leakage observed at voltages < 1000 V is due to
the capacitive contribution of the testing apparatus.
Part of this work was performed under an U.S. Office of Naval Research
(ONR) SBIR contract N00014-C-10-0104, and the support from Dr. H. Scott
Coombe is gratefully acknowledged.
To test the 10 kV BJTs up to avalanche breakdown,
selected die were packaged in special test coupons, which
have a 13 kV isolation rating. As shown in Fig. 2(b), the SiC
BJTs display a breakdown voltage in the range 10000-10500
V, which corresponds to 91% of the avalanche breakdown
limit, calculated by direct integration of the 4H-SiC impact
ionization co-efficients [3] for the 84 µm thick/7x1014 cm-3
doped n-collector layer.
the output characteristics of the Darlington BJTs shown in
Fig. 3 shows distinct saturation (up to IC = 8 A) and quasisaturation regions, reminiscent of a Si BJT, combined with
a very high current gain of 3400. The differential ron,sp for
the Darlington BJT in the saturation region is calculated as
44.8 mΩ-cm2, which is 69% lower than the ron,sp observed
on the discrete BJT, and also significantly lower than the
unipolar limit for the n- collector region.
Collector Voltage (V)
Figure 3. Output characteristics measured on (Top,a): 10 kV/2.7 mm2 SiC
BJTs and (Bottom,b): 10 kV/28 mm2 Discrete and Monolithic Darlington
Figure 2. (Top,a): Collector-Emitter (BVCEO) blocking characteristics
measured on-wafer to the 6 kV limit and (Bottom,b): BVCEO characteristics
measured on packaged SiC BJTs up to avalanche breakdown at 10-10.5 kV.
B. Output Characteristics
Fig. 3(a) shows the output characteristics measured on a
2.7 mm2 SiC BJT, while Fig. 3(b) shows the output
characteristics measured on 28 mm2 discrete and two-stage,
monolithic Darlington BJTs at 25°C. The 2.7 mm2 BJT in
Fig. 3(a) has an on-resistance (ron,sp) as low as 110 mΩ-cm2
in the saturation region, which is only slightly larger than
the drift resistance of the collector region, calculated as 94
mΩ-cm2, using an electron mobility of 800 cm2/V.s and
assuming 100% ionization of the nitrogen donors in the ncollector region. The discrete BJTs also show distinctly
majority carrier-like output characteristics with the different
base current curves overlapped in the saturation region, and
the absence of a quasi-saturation region. The high
ionization energy (≈190 meV) of the Al acceptors in the pbase layer, combined with the short minority carrier
lifetimes (≈1-2 µs) in 4H-SiC makes it difficult to achieve
conductivity modulation of the collector region in SiC BJTs
[4]. A high current gain (β) of 75 is also measured for both
the 2.7 mm2 and 28 mm2 discrete BJTs. On the other hand,
It is noteworthy that 1200 V-rated SiC Darlington BJTs
reported earlier by our group [5] on 1x1016 cm-3 doped ncollector layers with similarly high values of β did not show
quasi-bipolar characteristics (see Fig. 4).
Figure 4. Comparison of output characteristics measured on 4 mm2, 1200 V
and 53 mm2, 10 kV SiC Darlington BJTs. The 10 kV Darlington BJT shows
distinct saturation and quasi-saturation regions resulting from minority
carrier storage in the collection region, which are absent in the 1200 V
results in fast recovery of the collector voltage in 100 ns.
The collector current fall time is about 150 ns. There is no
tail in the collector current waveform, which indicates
purely majority carrier operation with no minority carrier
storage in the collector region. There was no difference in
switching waveforms measured at 25°C (not shown) and
150°C, which is further proof of the majority carrier
operation of the 10 kV discrete SiC BJTs. The ledge
observed in the collector current waveform at ≈ 3.5 A is
caused by the parasitic capacitance of the test setup, while
the periodic oscillations are caused by resonance between
the parasitic circuit reactances. The turn-off energy loss is
calculated as 1.6 mJ by integrating the product of the
collector current and voltage waveforms.
Since the base layer doping concentrations were the
same in both the 1200 V and 10 kV BJTs, the conductivity
modulation observed in the 10 kV transistor is attributed to
the lower collector doping (7x1014 cm-3) in the 10 kV
devices, which results in minority carrier storage in the
collector region.
Turn-on and turn-off waveforms from clamped inductive
load switching of 10 kV/28 mm2 SiC BJTs, under a doublepulse scheme are shown in Fig. 5 for a DC link voltage of 5
kV, and collector current of 8 A. The measurements were
performed at a base-plate temperature of 150°C. The BJT
was driven with a constant base current of ≈ 1 A by
connecting a 11 Ω gate resistor at the output of the gate
driver. Peak turn-on and turn-off currents of 2.5 A and -3 A,
respectively, were supplied by connecting a 18 nF capacitor
in parallel with the gate resistor, as shown in Fig. 1. The
voltage output of the gate driver was switched from -8 V to
15 V. The high dynamic peak base currents enable rapid
charging and discharging of the BJT’s base-emitter and
base-collector capacitances, resulting in fast switching
The turn-on and turn-off losses obtained from the 10
kV/8A SiC BJT switching measurements are compared with
a 6.5 kV Si IGBT from ABB [6] in Table I. It can be seen
that the SiC BJT achieves 19 times lower turn-on energy
loss and 25 times lower turn-off energy loss as compared to
the Si IGBT, in-spite of operating at a higher temperature
(150°C) as compared to the Si IGBT (125°C).
Figure 6. High-resolution switching waveforms measured on a 10 kV/28
mm2 BJT turning on from a DC link voltage of 5 kV to a collector current of
8 A, at a base-plate temperature of 150°C. A turn-on dI/dt as high as 530
A/µs is extracted from the collector current waveform.
Figure 5. Clamped inductive load switching of 10 kV/28 mm2 SiC BJTs at
a DC link voltage of 5000 V, collector current up to 8 A, and base-plate
temperature of 150°C.
High-resolution switching waveforms from the BJT
turn-on portion are shown in Fig. 6. The BJT shows ultrafast turn-on capability with collector current rise times < 30
ns, and collector voltage fall times of < 200 ns. The
collector current overshoot up to 15 A in Fig. 6 is due to the
capacitive charge stored in the free-wheeling SiC JBS
rectifier, which adds to the turn-on current of the BJT. The
oscillations observed in the collector current are caused by
resonances between the reactive elements of the test circuit
and the wire bonds of the power devices. The turn-on
energy loss is calculated as 4.2 mJ by integrating the
product of the collector current and collector voltage
High-resolution switching waveforms from the BJT turnoff portion are shown in Fig. 7. Rapid extraction of the
minority carriers stored in the base region of the BJT is
enabled by the peak negative base current of – 3 A, which
Figure 7. High-resolution switching waveforms measured on a 10 kV/28
mm2 BJT turning off to a DC link voltage of 5 kV from a collector current of
8 A, at a base-plate temperature of 150°C. A turn-off dV/dt as high as 44
V/ns is extracted from the collector voltage waveform.
10 kV
6.5 kV
10 A
Eon (mJ)
Eoff (mJ)
A. Short Circuit Safe Operating Area
The time-to-failure under short circuit conditions is an
important reliability parameter, which needs to be
experimentally determined for any power device
technology. When a 10 kV/2.7 mm2 BJT was turned-on to a
short-circuited load at a collector bias of 4500 V, with a
gate current of 20 mA, a short-circuit current (ISC) of 1 A,
and a withstand time (tSC) in excess of 20 µs were
measured, which is invariant of base-plate temperatures in
the range of 25 °C – 125 °C (see Fig. 8). A tSC of 20 µs is
well in excess of the response time of typical VCE,SAT based
short-circuit protection circuitry [7].
from 1400 V to 4500 V, at a base plate temperature of 125
°C. This observation confirms lack of any short channel
effects in the BJT output characteristics, unlike SiC
B. Stability of leakage currents under long-term highvoltage operation
Another important reliability parameter is the long-term
stability of the leakage current, when the BJT is biased at
high-voltages at elevated base plate temperatures. To
simulate these conditions, a 10 kV/2.7 mm2 SiC BJT was
subjected to a DC collector bias (BVCES) of 5 kV, at a baseplate temperature of 125°C for 162 hours, followed by 72
hours at 175°C.The leakage current flowing through the
BJTs was monitored for this test (system limit = 1 µA), as a
function of time and is shown in Fig. 9.
Figure 9. Time evolution of leakage currents under a DC collector bias
(BVCEO) of 5 kV impressed upon 10 kV/2.7 mm2 SiC BJT for 162 hours at a
base-plate temperature of 125°C followed by 72 hours at 175°C.
It can be seen from Fig. 9 that the leakage current is
extremely stable for the duration of this long-term HTRBlike test. Leakage currents < 1µA (system limit) are
measured at 175°C on even after 234 hours of DC operation
under a collector bias of 5000 V.
Figure 8. Short circuit switching measurements peformed on 10 kV/2.7
mm2 SiC BJTs. (Top,a): Temperature-invariant short circuit switching with
tSC = 20 µs, and (Bottom,b): Near collector bias-invariance of short circuit
currents resulting from perfectly flat output characteristics in the active
region of 10 kV SiC BJTs
A near-∞ Early voltage can be inferred in Fig. 8(b) by
the invariance of ISC, when the collector bias is increased
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