Summary of Inductive SiC BJT Switching 

by Steven L. Kaplan 

ARL-TR-3678 November 2005 

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Army Research Laboratory 

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ARL-TR-3678 November 2005 

Summary of Inductive SiC BJT Switching 

Steven L. Kaplan 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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Summary of Inductive SiC BJT Switching 

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Steven L. Kaplan 

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14. ABSTRACT 

Significant development of silicon carbide (SiC) for device applications allows its uniquely favorable properties to be exploited in circuit 

designs. The 4H-SiC structure has several characteristics that provide optimal speed and power handling. These include wide bandgap 

(3 cV), high dielectric breakdown (3.5 MV/cm), and high thermal conductivity (5 W/cm-K) [1]. By combining these properties, SiC bipolar 

junction transistors can achieve fast switching at high voltages (1.2 kV). New generation devices are also being developed with increased 

current handling capability (above 10 A). However, in order to meet the power handling requirements of FCS systems, such as hybrid-electric 

vehicle (HEV) power conditioning, it is necessary to configure these devices in parallel and validate their operation at high temperatures 

(package temperatures of 90 °C, and junction temperatures to over 150 °C). This report documents experimental characterization of SiC BJTs 

fabricated by CREE in various applications, including parallel switching. The results presented here indicate that SiC is well suited to meet 

these goals. 

15. SUBJECT TERMS

Silicon carbide, bipolar, transistor, switching 

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Contents 

List of Figures iv

Introduction 1

Procedure 2

Results 4

Conclusions 9

References 10

Distribution 11 

iii
List of Figures 

Figure 1. Test module contains a pair of silicon carbide BJT devices, mounted on a common 

collector pad. Parallel emitter leads extend to the left and base leads to the right. A 

doughnut shaped RTD to enable temperature monitoring is located above the devices. ..........3 

Figure 2. Inductive switching test setup. ........................................................................................4 

Figure 3. Current Sharing Between a Parallel Pair of 2 mm SiC BJTs at 150 °C. .........................5 

Figure 4. Oscilloscope trace showing load current and voltage (top) along with the individual 

emitter currents measured for a 2 mm parallel BJT pair during inductive switching................5 

Figure 5. Switching loss during turn-off of parallel BJT pair.........................................................6 

Figure 6. 400 V 100 °C turn-on. .....................................................................................................7 

Figure 7. 400 V 100 °C turn-off. ....................................................................................................7 

Figure 8. Energy loss during turnoff for 3 mm BJT module at 300 V and 400 V collector 

voltages. .....................................................................................................................................8 

Figure 9. Energy loss during turn-on for 3 mm BJT module at 300 V and 400 V collector 

voltages. .....................................................................................................................................8 

iv
Introduction 

Several factors determine the overall power handling capability of any system that employs 

switching devices. These include: the device current ratings, the device gain, as well as the 

switching and conduction losses. The current handling for each switch in the system can be 

leveraged by placing devices in parallel. This is essential for power electronics applications. 

Silicon carbide (SiC) devices are particularly well-suited to such applications, due to their unique 

combination of characteristics. These include fast recovery and good voltage blocking, along 

with excellent high temperature performance. This should allow SiC bipolar junction (BJT) 

devices to operate effectively in parallel at package temperatures up to 150 °C. These device 

properties result from the wide bandgap (3 eV), high dielectric breakdown (3.5 MV/cm), and 

high thermal conductivity (5 W/cm-K) of SiC (1). However, the effectiveness of a parallel 

configuration can be compromised by unequal current sharing between devices. To evaluate the 

potential of SiC devices to meet the requirements of a high temperature power converter, SiC 

bipolar junction transistors (BJT) fabricated by CREE were packaged in parallel pair modules at 

ARL. Both 2 by 2 mm and 3 by 3 mm BJT devices were fabricated and evaluated. The data 

presented here focuses on recently delivered 3 mm by 3 mm devices, as well as older 2 mm 

device modules. The 2 mm devices have gains of about 10, while the newer 3 mm devices have 

gains above 20 and block voltages to about 1.2 kV. A Tektronix 370 curve tracer was used to 

measure the current gain of each BJT, for use in matching the devices as well as post-packaging 

measurements to determine temperature dependence. The results detailed here are being used to 

apply SiC BJT devices to development of an all SiC 3-phase inverter, which provides versatile 

power conditioning. This inverter has been demonstrated at power levels up to 10 kW. 

The goal of this effort is to study the SiC devices under conditions that simulate switching of an 

actual motor. It is also crucial that these conditions include module temperatures up to at least 

90 oC, since this is required for the mobile power conditioning applications underlying this 

effort. Various methods of monitoring temperature have been employed, including RTDs and 

thermal imaging. The RTD only gives temperature values where it is mounted, which can be 

40 o

C or more below the device temperature. Thermal imaging provides more accurate 

temperature measurement. However, the thermal camera is highly sensitive to changing 

emissivities of the subject materials. Methods of accounting for disparate emissivities during 

thermal imaging are being developed. Detailed thermal data is not presented here, but will be 

included in upcoming publications of results from the continued SiC device evaluation effort. 

Previous measurements of similar SiC BJT devices have demonstrated their excellent high power 

switching performance (2). The switching results presented here provide further evidence of the 

superior high current and high temperature capabilities of these devices to comparable silicon 

devices. 

1
Procedure 

Parallel BJT modules were switched inductively, at voltages up to 750 V and device currents up 

to 10 A. The addition of more parallel devices with higher gains will increase the current 

switching capability. In order to achieve the 10 kW power handling required, each inverter 

phase leg must ultimately switch 30 A (RMS). Modules corresponding to each phase leg contain 

multiple BJTs configured with parallel base inputs. These are matched prior to packaging so that 

base current sharing is maximized. The collectors of these devices are fed in common by the 

high power source. Measurement results (shown in figures 3 and 4) indicate excellent on-state 

current sharing between the devices. The emitter currents of the parallel devices were within 

10% of one another for all the BJT modules tested. The most impressive results during parallel 

switching indicate that the individual device currents are within 3% of each other. The device 

current and voltage data was measured at package temperatures from 25 C to 150 C. Switching 

was performed up to 1 kHz and 10% duty cycle, with maximum power handling up to 4 kW per 

device. Figure 1 shown above, is a photograph of a parallel BJT module containing a matched 

pair of devices. 

2
Figure 1. Test module contains a pair of silicon carbide BJT devices, mounted on a common 

collector pad. Parallel emitter leads extend to the left and base leads to the right. A 

doughnut shaped RTD to enable temperature monitoring is located above the devices. 

Figure 2 is the basic inductive BJT switching circuit schematic for this experiment. The BJT 

indicated in this circuit actually represents the parallel pair referred to in this document. The 

inductance used here is nominally 4 mH, while the load was varied to produce the desired 

collector current values. Base current was adjusted in order to accommodate the high 

temperature device gains, while limiting the maximum base input to a safe level of 1.0 A per 

device. The on-state BJT current densities were conservatively limited to the presumed safe 

maximum value of 110 A/cm2

. This means, for example, that each 2 mm device can safely 

handle an on-state current of 4.5 A at temperatures up to 150 C, while the base currents of these 

devices are 1.0 A. Gain measurements of the 2 mm devices at 150 °C indicated gains of 

approximately 4 or 5. Therefore, the gain of these devices at high temperature is sufficient to 

accommodate the maximum base and collector currents. The 3 mm devices are able to handle 

10 A of collector-emitter current. The high temperature gain of these devices is well above 10, 

so that the maximum base current is sufficient to drive them at the maximum collector-emitter 

current and temperature. 

3
600V

Figure 2. Inductive switching test setup. 

Results 

Figure 3 is an expanded view of the individual device currents for a parallel pair of 2 mm BJT 

devices in the inductive switching test setup. The high degree of sharing achieved by the BJT 

pair is clearly illustrated in this figure. Total emitter current was found to be 6.32 Amps. The 

on-state current of device 1 is 3.06 Amps, while the on-state current through device 2 is 3.26 

Amps. This translates into 48.5% and 51.5% of the total emitter current, respectively. Other 

device pairs gave similar results, with current sharing within 8% for each of the four device pairs 

tested. Figure 4 shows the repetitive switching of the BJT device pair at 10% duty cycle. The 

base input pulses are 100 microseconds each, delivered at a frequency of 1 kHz. Figure 5 

represent the switching losses experienced by the devices at turn-off. The devices had losses in 

the 700 to 850 W range during turn-off (losses during turn-on were small compared to those 

during turn-off.). The calculated energy loss during turn-off is below 1mJ for each device. 

4
0

0.5

1

1.5

2

2.5

3

3.5

4

0 10 20 30 40 50 60 70 80 90 100

Time (microsec)

Current (Amps)

Device 1

Device 2

Figure 3. Current Sharing Between a Parallel Pair of 2 mm SiC BJTs at 150 °C. 

Figure 4. Oscilloscope trace showing load current and voltage (top) along with the individual 

emitter currents measured for a 2 mm parallel BJT pair during inductive switching. 

5
Switching Loss of Parallel Devices 

0

100

200

300

400

500

600

700

800

900

1000

90 91 92 93 94 95

Time (microsec)

Power (W)

device 1

device 2

Figure 5. Switching loss during turn-off of parallel BJT pair. 

6
Figures 6 and 7 show the transient response during inductive switching of a three device parallel 

module containing 3 mm SiC BJTs at 100 o

C. The oscilloscope traces in these figures show the 

base currents, along with the total collector current and voltage at turn-on and turn-off, 

respectively. These data were taken while each device had a collector-emitter potential of 

400 V, with 30 A total current through the module. 

Figure 6. 400 V 100 °C turn-on. 

Figure 7. 400 V 100 °C turn-off. 

7
The energies associated with turn-on and turn-off as a function of temperature, for both 300 V 

and 400 V, are shown in figures 8 and 9. The turn-on energies ranged from 1 to 4 mJ, while the 

turn-off energies were 1 to 6 mJ. The temperature dependence of these results is relatively flat, 

confirming the excellent high temperature performance of these BJTs in particular, as well as the 

expected performance of an all SiC inverter operating at a heat sink temperature of 90 oC. 

Figure 8. Energy loss during turnoff for 3 mm BJT module at 300 V and 400 V 

collector voltages. 

Turnoff Energy vs Temperature

0

1

2

3

4

5

6

0 25 50 75 100 125

Temp (C)

Energy (mJ)

300 V

400 V

Turnon Energy vs Temperature

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 25 50 75 100 125

Temp (C)

Energy (mJ)

300 V

400V

Figure 9. Energy loss during turn-on for 3 mm BJT module at 300 V and 400 V 

collector voltages. 

8
Conclusions 

Based on the results of these experiments, it appears that the CREE SiC BJT devices examined 

here effectively share current during inductive switching, and maintain this sharing at high 

temperature (up to a device package temperature of 150 o

C). Low energy losses and power 

dissipations during inductive switching were indicated by the measured transient data. These 

results bode well for the application of these devices to the high current parallel inverter circuits 

necessary for power conditioning systems. The high temperature performance of these devices 

should allow for the flexible, portable system applications envisioned in the Future Combat 

Systems (FCS) program. Efforts to optimize device efficiency and high temperature 

performance are continuing. Experimental results will be forthcoming for the next generation of 

the SiC BJT devices in FY06. We are also focusing on understanding the operation and failure 

mechanisms of these devices. This information is used to maximize circuit performance and 

provided as feedback to CREE so that they can continue to improve the fabrication process for 

subsequent generations of devices. 

9
References 

1. Choyke, W. J.: Matsunami, H.; Pensl, G. “Silicon Carbide – A Review of Fundamental 

Questions and Applications to Current Device Technology,” Wiley-VCH, Berlin 1997. 

2. Agarwal, A.; Ryu, S.; Capell, C.; Richmond, J.; Palmour, J. W.; Phan, B.; Stambaugh, J.; 

Bartlow, H.; Brewer, K. “SiC BJT’s for High Power Switching and RF Applications,” 

Proceedings MRS Silicon Carbide 2002 – Materials, Processing and Devices, Vol.742, K7.3. 

10
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