Janis M. Niedra

QSS Group, Inc., Cleveland, Ohio

Switching Characteristics of a 4H-SiC Based

Bipolar Junction Transistor to 200 °C

NASA/CR—2006-214257

November 2006
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NASA/CR—2006-214257

November 2006

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared under Contracts NAS3–98008 and NAS3–00145

Janis M. Niedra

QSS Group, Inc., Cleveland, Ohio

Switching Characteristics of a 4H-SiC Based

Bipolar Junction Transistor to 200 °C
Acknowledgments

This work was sponsored by the NASA Glenn Research Center under NAS3–98008 and NAS3–00145

with G.E. Schwarze as the Project Manager.

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NASA/CR—2006-214257 1

Switching Characteristics of a 4H-SiC Based 

Bipolar Junction Transistor to 200 °C 

Janis M. Niedra 

QSS Group, Inc. 

Cleveland, Ohio 44135 

Abstract 

Static curves and resistive load switching characteristics of a 600 V, 4 A rated, SiC-based NPN 

bipolar power transistor (BJT) were observed at selected temperatures from room to 200 °C. All testing 

was done in a pulse mode at low duty cycle (~0.1 percent). 

Turn-on was driven by an adjustable base current pulse and turn-off was accelerated by a negative 

base voltage pulse of 7 V. These base drive signals were implemented by 850 V, gated power pulsers, 

having rise-times of roughly 10 ns, or less. Base charge sweep-out with a 7 V negative pulse did not 

produce the large reverse base current pulse seen in a comparably rated Si-based BJT. This may be due to 

a very low charge storage time. The decay of the collector current was more linear than its exponentiallike rise. 

Switching observations were done at base drive currents (IB) up to 400 mA and collector currents (IC) 

up to 4 A, using a 100 Ω non-inductive load. At IB = 400 mA and IC = 4 A, turn-on times typically varied 

from 80 to 94 ns, over temperatures from 23 to 200 °C. As expected, lowering the base drive greatly 

extended the turn-on time. Similarly, decreasing the load current to IC = 1 A with IB = 400 mA produced 

turn-on times as short as 34 ns. Over the 23 to 200 °C range, with IB = 400 mA and IC = 4 A, turn-off 

times were in the range of 72 to 84 ns with the 7 V sweep-out.

Testing Circuits 

A constant voltage source feeding a resistive load in the collector circuit was chosen to make up a 

most simple power side of the circuit. A nearby charged capacitor served well as the power source, since 

operation was with sub-microsecond pulses at no more than 0.1 percent duty cycle. The BJT itself was 

mounted to a temperature-controlled plate, with a thermocouple attached directly to the BJT. A general 

circuit diagram is provided in figure 1. 

A fast rise, high voltage pulser is suitable for providing a very fast-rise current pulse to the base of a 

BJT. The DEI model HV-1000 is capable of pulse heights up to 850 V into a 50 Ω load at a low duty 

cycle, with a rise-time of 10 ns and durations up to 10 μs. Figure 2 shows the basic arrangement, omitting 

the required external high voltage supply and trigger pulser. Since the forward VBE is only of the order of 

10 V, good quality current pulses can be obtained into the base of the BJT. The duty cycle can be set as 

low as one pleases and about 0.1 percent was used. 

A turn-off circuit was devised to accelerate the charge carrier sweep-out from the base region by 

providing a negative VBE of about 7 V to terminate the collector current from its steady state value. In this 

way, turn-off transients comparable in duration, or even shorter, than the turn-on transients could be 

obtained. Such a circuit was implemented by adding a negative pulser (a DEI model HV-1000N) to the 

circuit in figure 2. A Zener diode was used to set the negative voltage level. The passive components in 

these circuits need not have a high power rating, due to the very low duty cycle. 
NASA/CR—2006-214257 2

Figure 1.—Basic resistive load turn-on switching test circuit. Currents 

IB and IC are sensed by high-speed current transformers (Tektronix 

CT1 and CT2). Potential VB is sensed by a 1 kΩ input impedance, 

fast-rise probe and potential VC by a Tektronix 10X probe. RC is a 

carbon composition resistor, normally set to 100Ω. 

Figure 2.—Block circuit diagram for a pulsed constant current base drive. RB values 

of 300 Ω to 1 kΩ were used. 

Figure 3 provides a diagram of the combined circuit, used primarily for capturing the turn-off 

transient, once steady IC has been established by the positive base drive part. The two high voltage 

pulsers are timed by trigger signals from two channels of a delay trigger pulse generator, a Berkeley 

Nucleonics model 555. And the output from the HV-1000N overcomes that from the HV-1000 

sufficiently to activate the Zener diode, thus setting the negative base drive level. This circuit was 

constructed with low power resistors and diodes for use at very low duty cycle only. Moreover, the model 

555 was unable to gate the high voltage pulsers at duty cycles exceeding 10 percent. But using this 

somewhat elaborate setup, base voltage transition times, from positive to negative in less than 10 ns, 

could be achieved. 

Base drive 

 + V0

RC

+ VC

IC

IB

+ VB

HV-1000 

pulser 

 RB

To base of BJT 

 50 Ω
NASA/CR—2006-214257 3

Figure 3.—Block circuit diagram of a pulsed constant voltage base sweep-out drive 

added to the constant current base drive of figure 2. The reverse VBE is set by the 

Zener diode. The delay generator provides timed gating pulses, Channels A and B, 

to the positive and negative high voltage pulsers. 

High Temperature Switching Data Overview 

The data in table I presented below shows the amount of base drive needed to get acceptable 

switching at selected temperatures and load currents for the SiC-based BJT labeled as P2. This data was 

taken using the constant current base drive mode, primarily because it is easier to set the current to a 

given value. The values of currents indicated are their steady state values, reached typically in less than a 

microsecond. 

At the 0.1 percent duty cycle used, no temperature rise above ambient of the BJT’s case could be 

resolved. The case was, however, fastened to a BN substrate to provide good heat conductivity. An 

estimate of the switching loss of 10−4

 J per on-off cycle thus gives an estimate of an average power loss 

of only 0.1 W. Device temperature rise can then be estimated from either the specific heat (range 0.3 to 

1.0 J/(g K)) or the thermal conductivity (range 2 to 5 W/(cm K)) of SiC. Assuming a chip 1 mm thick and 

BNC 555 

delay 

generator 

HV-1000 

pulser 

470 Ω

To base of BJT 

Ch. A 50 Ω

Ch. B 

HV-1000N 

neg. pulser 

 50 Ω

470 Ω

(~6 V)

1N5819 

40 V, 1 A Schottky 
NASA/CR—2006-214257 4

3 mm on side, either estimate gives a temperature rise less than 1 K. Hence it seems safe to assume that 

the junction temperature is within a degree or so of that of the case in these pulse measurements. 

TABLE I—SWITCHING DATA, WITH 100 Ω LOAD RESISTOR AND –7 V BASE SWEEP-OUT 

(a) Data at 23 °C 

IB 

(mA) 

IC (A) V0 

(V) 

τIC, ON (ns) 

(10 to 90%) 

τIC, OFF (ns) 

(10 to 90%) 

400 4.0 402 80 84 

300 “ 402 116 

200 “ 402 198 

100 

 

400 3.0 308 65 74 

300 “ 308 92 

200 “ 308 156 

100 “ 304 >370 

 

400 2.0 208 49 64 

300 “ 206 71 

200 “ 206 116 

100 “ 204 288 

 

400 1.0 106 34 

300 “ 106 45 

200 “ 104 70 

100 “ 104 162 

(b) Data at 150 °C 

IB 

(mA) 

IC 

(A) 

V0 

(V) 

τIC, ON (ns) 

(10 to 90%) 

τIC, OFF (ns) 

(10 to 90%) 

400 4.0 410 90 72 

300 “ 410 132 

200 “ 408 248 

 

400 3.0 308 74 63 

300 “ 306 103 

200 “ 308 181 

100 “ 306 >361 

 

400 2.0 206 56 57 

300 “ 206 78 

200 “ 204 125 

100 “ 202 334 

 

400 1.0 106 36 

300 “ 106 49 

200 “ 104 76 

100 “ 104 170 
NASA/CR—2006-214257 5

TABLE I—Concluded. 

(c) Data at 175 °C 

IB 

(mA) 

IC 

(A) 

V0 

(V) 

τIC, ON (ns) 

(10 to 90%) 

τIC, OFF (ns) 

(10 to 90%) 

400 4.0 408 94 73 

300 “ 406 133 

200 “ 406 249 

100 “ >402 >360 

 

400 3.0 308 74 64 

300 “ 306 104 

200 “ 306 176 

100 “ >306 >373 

 

400 2.0 206 55 57 

300 “ 206 79 

200 “ 204 128 

100 “ 204 334 

 

400 1.0 106 36 

300 “ 106 47 

200 “ 104 75 

100 “ 102 171 

(d) Data at 200 °C 

IB 

(mA) 

IC 

(A) 

V0 

(V) 

τIC, ON (ns) 

(10 to 90%) 

τIC, OFF (ns) 

(10 to 90%) 

400 4.0 408 94 72 

300 “ 408 135 

200 “ 408 241 

 

400 3.0 308 74 65 

300 “ 306 107 

200 “ 304 181 

100 “ >302 >373 

 

400 2.0 204 56 58 

300 “ 204 78 

200 “ 204 126 

100 “ 204 340 

 

400 1.0 106 36 

300 “ 106 49 

200 “ 104 77 

100 “ 104 169 

Summary of Observations 

The data tables indicate that collector current turn-on transition times of 100 ns, or less, can be readily 

achieved with a 4 A current and a 100 Ω resistive load. This, however, requires a 400 mA base drive 

current step, at which the VBE steadies out to roughly 10 V, with 4 A of collector current flowing. As 

expected, this turn-on time is sensitive to the base drive and the load currents, with only a slight 

sensitivity to temperature, from 23 to 200 °C. If the load current is lowered, then similar times can be 

obtained at a lower base drive. Here the externally measured case temperature is thought to be a good 
NASA/CR—2006-214257 6

measure of also the junction temperature, because of the very low duty cycle pulse mode and the rather 

low switching loss per on-off cycle. 

An abrupt base turn-off was implemented by sending a negative voltage pulse to the base to 

overcome the positive base current pulse. Due to the low base charge storage time in this SiC device 

(crystal defects?), there was no need to limit the reverse base current. Without emitter junction reverse 

breakdown data, about 7 V was thought to be conservative. This was sufficient to give collector current 

decay times of the same order as its rise times. Faster times may be possible, depending on the emitter 

breakdown characteristics. Also, clamping the base voltage to the emitter level would certainly elongate 

the turn-off time. Although zero level clamping seems difficult with the pulsers, data points for a plot 

below a 1 V level may be feasible by omitting the Zener diode in figure 3. This has not yet been done. 

The setup shown in figure 3 produces a collector current turn-off waveform that is nearly linear in 

time, whereas its turn-on waveform approximates closer to exponential. This makes for a difference in the 

switching losses when calculating these from formulas to approximate the waveforms. 

Although the P2 device can switch up to at least 1.6 kW at 200 °C in a low duty cycle pulse mode, 

thermal conductance to case is expected to significantly reduce its capability at say 50 percent duty cycle. 

The estimation of junction temperature rise for a given average power dissipation in the device is 

problematic, at best. An experimental attempt will be described in another report. 
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12

Switching Characteristics of a 4H-SiC Based Bipolar Junction Transistor

to 200 °C

Janis M. Niedra

4H-SiC bipolar junction transistor; High temperature; Switching times; Base charge

sweep-out

Unclassified -Unlimited

Subject Category: 33

QSS Group, Inc.

21000 Brookpark Road

Cleveland, Ohio 44135

Project Manager, Gene E. Schwarze, Power and Propulsion Division, NASA Glenn Research Center, organization

code RPE, 216–433–6117.

Static curves and resistive load switching characteristics of a 600 V, 4 A rated, SiC-based NPN bipolar power transistor

(BJT) were observed at selected temperatures from room to 200 °C. All testing was done in a pulse mode at low duty

cycle (~0.1 percent). Turn-on was driven by an adjustable base current pulse and turn-off was accelerated by a negative

base voltage pulse of 7 V. These base drive signals were implemented by 850 V, gated power pulsers, having rise-times

of roughly 10 ns, or less. Base charge sweep-out with a 7 V negative pulse did not produce the large reverse base

current pulse seen in a comparably rated Si-based BJT. This may be due to a very low charge storage time. The decay

of the collector current was more linear than its exponential-like rise. Switching observations were done at base drive

currents (IB) up to 400 mA and collector currents (IC) up to 4 A, using a 100 Ω non-inductive load. At IB = 400 mA and

IC = 4 A, turn-on times typically varied from 80 to 94 ns, over temperatures from 23 to 200 °C. As expected, lowering

the base drive greatly extended the turn-on time. Similarly, decreasing the load current to IC = 1 A with IB = 400 mA

produced turn-on times as short as 34 ns. Over the 23 to 200 °C range, with IB = 400 mA and IC = 4 A, turn-off times

were in the range of 72 to 84 ns with the 7 V sweep-out.