Microelectronics Reliability 51 (2011) 1773–1777

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Microelectronics Reliability

journal homepage: www.elsevier .com/locate /microrel

A study of SiC Power BJT performance and robustness

A. Castellazzi a,⇑, T. Takuno b, R. Onishi c, T. Funaki c, T. Kimoto d, T. Hikihara b

a Power Electronics, Machines and Control Group, University of Nottingham, Nottingham NG7 2RD, UKb Power Conversion & System Control Laboratory, Kyoto University, 615-8510 Katsura, Kyoto, Japanc Power Systems Laboratory, Osaka University, 565-0871 Suita, Osaka, Japand Semiconductor Science & Engineering Laboratory, Kyoto University, 615-8510 Katsura, Kyoto, Japan

a r t i c l e i n f o

Article history:Received 30 May 2011Received in revised form 16 June 2011Accepted 27 June 2011Available online 23 July 2011

0026-2714/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.microrel.2011.06.046

⇑ Corresponding author.E-mail address: alberto.castellazzi@nottingham.ac

a b s t r a c t

This paper proposes an investigation of 1200 V rated transistors with the twofold purpose of assessingtheir performance and robustness under representative operational conditions and of extracting guide-lines for the design of reliable multi-chip power electronics modules based on SiC technology. It includesa thorough analysis of the devices steady-state and switching characteristics, as well as the investigationof short-circuit events. Taking into account operational conditions of real applications, this study consid-ers the dependence on ambient temperature, bias conditions and driver circuit parameters.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The interest in developing silicon carbide (SiC) based electricalpower conversion systems is by now clear to the specialist commu-nity (see [1], for example). In particular, the main drives are higherswitching frequencies, higher power densities and higher operat-ing temperatures than achievable with Si electronics. After yearsof research on material growth, device physics and manufacturing,SiC power transistors are now becoming available in sufficientquality and quantity to stimulate a concrete interest in the devel-opment of system solutions based entirely on this technology.That, in turn, motivates the concurrent initiation of performance,robustness and reliability studies of an application-orientednature.

In many commercial applications, the higher temperature capa-bility of SiC devices is still of secondary importance as comparedwith the possibility to increase both the volumetric and gravimet-ric power density of electrical power conversion equipment. This isthe case, in particular, of high reliability environments, such as, forinstance, the railway and avionic industry, where, on the otherhand, reduction of size and weight are major key aspects of tech-nology evolution and competitive product development. Next tothe diode, available SiC device types have included BJTs and JFETsfor some time now, and, more recently, SiC Power MOSFETs havealso been demonstrated [2–4]. From an industrialisation and com-mercialisation point of view, to date the BJT is quite a matureswitch technology type and its immunity from second breakdowntogether with recent breakthrough advancements in current gainfigures still make it an attractive and competitive candidate for

ll rights reserved.

.uk (A. Castellazzi).

the development of SiC based electrical power conversion systems[1,5–7].

For the time being, single chip current ratings are still limitedby the crystal growth and manufacturing process to a few tens ofamps maximum. Thus, for most power applications, multi-chipswitches are envisaged and, in view of significant behavioural dif-ferences as compared with Si devices, it is important to carry out adedicated study. So, this papers considers 1200 V rated SiC BJTs,with a nominal current rating of 6 A, and proposes an in-depthstudy of their characteristics, as well as an exhaustive investigationof their performance during stressful abnormal events. The devicesare commercially available engineering samples and were pack-aged in TO247 package, which is not qualified for high temperatureapplication [8]. Although the exact technological details are notknown, previous relevant publications reported a vertical devicestructure manufactured on a 4H–SiC wafer with low-resistive n-type substrate and a 15 lm thick nitrogen doped epitaxial collectorlayer (origin Cree, Inc. [9]), with base and emitter layers subse-quently grown, still epitaxially, in a continuous run to minimisethe density of defects. The base was doped with aluminium andthe emitter, comprising of a two layer mesa structure, was dopedwith nitrogen [10].

For illustration, reference is made to typical avionic operationalconditions. More precisely, short-circuit and turn-off robustnessare tested at 270 V over the temperature range between �15 �Cto 130 �C. The parameters of the drive circuit are also varied tothe aim of extracting useful information for system design optimi-sation. Applications of interest include not only high-frequencyswitching power conversion (e.g., for motor drive applications),but also current limiters and circuit-breakers. In the following, firstthe transistor steady-state characteristics are presented and dis-cussed, then, its switching and short-circuit performance.

0

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0 1 2 3 4 5VCE [V]

I C [A

]

IB=50 mA

IB=100 to 300 mA (step=50 mA)

T=-15 °C

0

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20

0 1 2 3 4 5

T=30°C

IB=100 mA

IB=200 mAIB=300 mA

0

5

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T=75°C

IB=300 mA

IB=200 mA

IB=100 mA

I C [A

]

VCE [V]

I C [A

]

(b)

(a)

(c)

1774 A. Castellazzi et al. / Microelectronics Reliability 51 (2011) 1773–1777

2. Steady-state characteristics

For the steady-state characterisation of the devices, a program-mable temperature regulated thermal chamber was used, togetherwith a Tektronix 371B high power curve tracer, adopting a four-terminal (Kelvin sense) measurement approach to isolate the influ-ence of interconnections on the results. The cabling used in themeasurement is nickel clad wire dedicated for electric furnaceusage.

Figs. 1a–d shows the measured output characteristics at differ-ent temperature values, �15, 30, 75 and 130 �C, respectively. Asopposed to Si ones, SiC BJTs exhibit a positive temperature coeffi-cient of their on-state resistance: this results mainly from the ab-sence of conductivity modulation phenomena, due to muchhigher doping levels and shorter lifetime of the charge carriers,which make the on-state resistance thermal performance essen-tially dependent upon the carriers’ mobility only (see [10–11],for example). This is a well-known feature of SiC BJTs and contrib-utes to make them attractive for novel power conversion applica-tions in the first place. The device exhibits good performance,with virtually temperature independent characteristics, for currentlevels up to about 5–6 A, which is the nominal maximum steady-state rating of the considered chips. Taking into account that it iscommon praxis in power applications to double such figure forthe maximum current in switched operation, the device perfor-mance becomes quite strongly temperature dependent: for in-stance, for a reference current value of 10 A, a nearly twofoldincrease in the VCE,SAT value is brought along by an increase in tem-perature from 30 to 130 �C. The base-drive current needs to be in-creased to keep good performance.

On the other hand, as is evident from Fig. 2, which shows themeasured BJT transfer characteristics at VCE =3 V, operation athigher current levels is associated not only with a positive temper-ature coefficient of the on-state resistance, but also with a negativetemperature coefficient of the current gain, favouring the paralleloperation of the devices (the forward bias base–emitter current in-creases with temperature for a given VBE value). This aspect is ofparticular interest for the implementation of protective functional-ities, such as, for instance, current limiters or solid-state circuitbreakers.

0

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0 1 2 3 4 5

T=130 °C

IB=100 mA

IB=200 mA

IB=300 mA

VCE [V]

I C [A

]

VCE [V]

(d)

Fig. 1. Measured output characteristics of a 1200 V SiC Power BJT at differenttemperature values: �15 �C (a), 30 �C (b), 75 �C (c), and 130 �C (d).

3. Switching performance

The switching performance of the transistors was tested bymeans of a double-pulse test circuit, for which a schematic anddriving sequence are shown in Fig. 3a and b, respectively. Duringthe first pulse, the device under test (DUT) is turned on and thecurrent rises proportionally to the ratio of bias voltage, VBIAS, andload inductance, LLOAD, values. When the transistor is turned off,the inductance current is diverted to the anti-parallel freewheelingdiode, DFW; in this case, the diode is also a commercial SiC device,rated at 600 V/10 A. During this time interval the load current re-mains essentially constant due to negligible power dissipation inthe diode. So, by properly controlling the duration of the first pulse,tON, the test current level for both the first turn-off and subsequentturn-on switching transitions can be accurately set to the desiredvalue.

To enable for easy test of both double-pulse turn-on and tun-offtransitions and, subsequently, of short-circuit events, the driverdesign was based on the ADuM1233 IC (Analog Devices), an iso-lated, precision half-bridge driver. This IC provides two indepen-dent and galvanically isolated output channels that can beparalleled for higher turn-on current pulses during the short-cir-cuit test. The driver, for which a simplified schematic is given inFig. 4, enables switching frequencies well into the MHz range;

moreover, it is suitable for use with JFETs and MOSFETs, too, andas such can be used for benchmarking tests of different devicetechnologies under similar operating conditions [12].

Following the results from the previous section, in these teststhe main parameter of study was the device temperature. TheDUT was mounted on a hot-plate and its case temperature moni-

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30 °C75 °C130 °C-15 °C

Increasing temp.

I C [A

]

VBE [V]

Fig. 2. Measured transfer characteristics for four different temperature values(VCE = 3 V in these measurements).

270V

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-VBIAS DUT

VPULSE

RBASE

variable TCASE

LLOADDFW

VPULSE

t

tON tOFF

(a)

(b) Fig. 3. Circuit schematic (a), and driving sequence (b), of the experimental setup forswitching performance characterisation of the SiC BJT.

Fig. 4. Detail of gate-driver circuit enabling for both short-circuit and double-pulsetest in half-bridge configuration (here, only one channel is shown for simplicity).

(a)

(b)

(c)

Fig. 5. Experimental current and voltage waveforms for the double-pulse switchingtransitions: (a) collector current profile for three different temperatures (30, 75 and130 �C); (b) collector–emitter voltage profile, also for three different temperatures;(c) detail of turn-on transition.

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tored with a thermometer. Three positive values were consideredin this case, 30, 75 and 130 �C. The diode temperature was notchanged and also not controlled. However, due to the specificmounting approach, it was not influenced by the changing temper-ature of the DUT and equals ambient temperature during all mea-surements. Also, the gate resistance value was not changed, but set

so as to bias the device with 300 mA base current. The parasiticinductance between bias voltage source and devices was intention-ally kept to a minimum: in contrast to standard past double-pulsetest circuits, in which different values of parasitic inductance aretried out to define design parameters in relation to the device lim-its of safe-operation, the parasitic inductance is minimised heremainly for three reasons: because the device is being used quitefar away from its maximum breakdown voltage, because multi-chip SiC based power electronics will need designs with stronglyreduced stray inductance (e.g., for higher switching frequencies)and to get better insight into the very device performance (e.g.,switching speed and its dependence on temperature).

Fig. 5 summarises representative results of the device perfor-mance. In (a), the collector current waveform for the double-pulseis shown: actually, three curves corresponding to the three differ-ent temperatures are plotted, but they can hardly be distinguished,as the device switching performance in the considered tempera-ture range does not show appreciable changes; it is worth noting

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also that virtually no diode reverse-recovery current is observed atturn-on. In (b), the collector–emitter voltage waveform is reported:

Fig. 6. Experimental current and voltage waveforms for overload turn-off at 130 �Cin double-pulse switching test.

+

-VBIAS DUT

VPULSE

RBASE

variable TCASE

VPULSE

t

tSC

(a)

(b) Fig. 7. Schematic of the experimental setup for the short-circuit robustness test.

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4,5

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14,5

-1,0E-06 1,0E-06 3,0E-06 5,0E-06 7,0Etim

Incr

I C [A

]

Fig. 8. Measured short-circuit current waveforms at VBIAS = 270

in this case, too, it is actually three waveforms being superimposedand showing virtually no difference; this waveform clearly showsthat the parasitic inductance is kept at a minimum value; possibledifferences in the on-state VCE value could not be detected in thismeasurement, due to the resolution in measuring the relativelyhigh voltage value. In (c), a zoom of the turn-on transition is pro-posed, revealing that the device actually switches faster as thetemperature increases; a slight increase in the same directionwas also noted in the turn-on transition; however, in view of theentity of the change in relation to the difference in temperature be-tween profiles, the effect is deemed not relevant to the envisagedgoals and the overall focus of this study; on the other hand, itshould be noted that the current value at the second turn-off tran-sition is about three times the nominal device current rating andthe switching transition is completed in around 100 ns, withoutany indication of charge-extraction phenomena even at the highertemperature value (see previous discussion about the absence ofconductivity modulation). Finally, in Fig. 6, the collector currentand collector–emitter voltage profiles are shown for a hotplatetemperature of 130 �C: here, the collector current before turn-offequals four times the nominal device current and the device canstill turn-off safely and with very clean waveforms.

As a whole, these results indicate a very good switching perfor-mance and turn-off overload robustness of the BJT for the specificapplication considered and indicate that very high switchingspeeds can be achieved, provided care is taken in minimising strayinductance in both the power and driving loops.

4. Short-circuit test

Fig. 7 summarises the test conditions and describes the experi-mental set-up: the driving pulse width was set at 10 ls and the ac-tual circuit also included a high voltage 2.2 mF input filtercapacitor and low inductive planar interconnections to ensure neg-ligible voltage drop during the test and enable fast current rise. Thetemperature of the DUT was set by means of a hotplate and the dri-ver circuit parameters (e.g., base-resistance) could be varied toinvestigate their influence on the device performance. The drivingsequence in this case consists of a single pulse whose duration wasset to 10 ls in accordance with standard values used in Si-basedpower electronics: this is the minimum time required for protec-tion circuitry to detect and remove the faulty condition; for refer-ence, short-circuit capable Si IGBTs typically have a withstand

-06 9,0E-06 1,1E-05 1,3E-05 1,5E-05e [s]

30°C

75°C

130°C

easing temp.

V, for three different temperature values; IBASE = 150 mA.

-226

10141822263034

-1 1 3 5 7 9 11 13 15time [µs]

VBIAS = 200 V

VBIAS = 100 V

Increasing VBIAS

I C [A

] (a)

(b)

Fig. 9. Experimental short-circuit current waveforms for different conditions: (a)two different values of VBIAS at T = 75 �C and IBASE = 300 mA and (b) two differenttemperature values at VBIAS = 270 V and IBASE = 300 mA.

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capability between 10 and 20 ls depending on the bias voltage ascompared to the maximum device rating.

Fig. 8 shows the short-circuit current waveforms, measured atthree different temperature values and a bias voltage of 270 V.Here, the base current values was kept relatively low, around150 mA. These results demonstrate the device capability to wellwithstand the abnormal event and points out that, in marked con-trast to its Si counterpart, it exhibits a negative temperature coef-ficient of the collector current in these conditions (i.e. a Si–MOS-like behaviour), too, making it a suitable candidate for the imple-mentation of multi-chip power modules. The transient decreasein the short-circuit current level was interpreted as the result ofself-heating effects on carriers’ mobility, more pronounced forhigher initial current and power values (i.e., lower temperatures).However, in separate tests, the base current was increased to300 mA and the nominal power dissipation level was changed byintervening on the bias voltage, while leaving the hotplate temper-ature fix at 75 �C. The results, Fig. 9a, indicate that the physicaloperation of the BJT is actually more complex and cannot be inter-preted merely on the basis of changes in mobility value with tem-perature; however, it contributes to make the device performancevirtually independent from static and transient temperature varia-tions even in the case of short-circuit events, in which significantheat is generated: this is further confirmed from the results ofFig. 9b, which show the short circuit current still for a base-drive

current of 300 mA, this time for a nominal bias voltage value of270 V and for two different temperature values, 30 and 130 �C,respectively. In this case too, let alone a small difference in the ini-tial current peak, more pronounced at lower temperature values,there is virtually no difference in the short-circuit current profiles.

5. Conclusion

This paper presented an application related study of perfor-mance and robustness of a 1200 V–6 A SiC Power BJT. The interestof the study resides in the maturity of the particular transistortechnology, possibly not too far from industrialisation, at least inthe lower voltage range (indicatively, <2.5 kV). The results in-cluded here already provide interesting information for the devel-opment of multi-chip modules, showing, on the one side, that SiCPower BJTs are a competitive device choice and on the other, thatbespoke drive circuit designs could yield optimised performance,both in switching and regulation applications. In particular, noindication of degradation has been found on the tested devices,which showed the same performance before and after the tests.

These investigations are necessary in view of the steady andrapid improvement in SiC device fabrication capability and wellcomplement concurrent studies on package related thermo-mechanical issues (see [13], for example). Further activities willinclude an accurate analysis of parallel device performance andinclude SOA limit operation to determine the energy level associ-ated with destruction under various operating conditions.

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