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A study of SiC Power BJT performance and robustness
Microelectronics Reliability 51 (2011) 1773–1777
Contents lists available at ScienceDirect
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, UK Power Conversion & System Control Laboratory, Kyoto University, 615-8510 Katsura, Kyoto, Japan c Power Systems Laboratory, Osaka University, 565-0871 Suita, Osaka, Japan d Semiconductor Science & Engineering Laboratory, Kyoto University, 615-8510 Katsura, Kyoto, Japan b
a r t i c l e
i n f o
Article history: Received 30 May 2011 Received in revised form 16 June 2011 Accepted 27 June 2011 Available online 23 July 2011
a b s t r a c t This paper proposes an investigation of 1200 V rated transistors with the twofold purpose of assessing their performance and robustness under representative operational conditions and of extracting guidelines for the design of reliable multi-chip power electronics modules based on SiC technology. It includes a thorough analysis of the devices steady-state and switching characteristics, as well as the investigation of short-circuit events. Taking into account operational conditions of real applications, this study considers the dependence on ambient temperature, bias conditions and driver circuit parameters. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The interest in developing silicon carbide (SiC) based electrical power conversion systems is by now clear to the specialist community (see [1], for example). In particular, the main drives are higher switching frequencies, higher power densities and higher operating temperatures than achievable with Si electronics. After years of research on material growth, device physics and manufacturing, SiC power transistors are now becoming available in sufficient quality and quantity to stimulate a concrete interest in the development of system solutions based entirely on this technology. That, in turn, motivates the concurrent initiation of performance, robustness and reliability studies of an application-oriented nature. In many commercial applications, the higher temperature capability of SiC devices is still of secondary importance as compared with the possibility to increase both the volumetric and gravimetric power density of electrical power conversion equipment. This is the case, in particular, of high reliability environments, such as, for instance, the railway and avionic industry, where, on the other hand, reduction of size and weight are major key aspects of technology evolution and competitive product development. Next to the diode, available SiC device types have included BJTs and JFETs for some time now, and, more recently, SiC Power MOSFETs have also been demonstrated [2–4]. From an industrialisation and commercialisation point of view, to date the BJT is quite a mature switch technology type and its immunity from second breakdown together with recent breakthrough advancements in current gain figures still make it an attractive and competitive candidate for ⇑ Corresponding author. E-mail address: [email protected] (A. Castellazzi). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.06.046
the development of SiC based electrical power conversion systems [1,5–7]. For the time being, single chip current ratings are still limited by the crystal growth and manufacturing process to a few tens of amps maximum. Thus, for most power applications, multi-chip switches are envisaged and, in view of significant behavioural differences as compared with Si devices, it is important to carry out a dedicated study. So, this papers considers 1200 V rated SiC BJTs, with a nominal current rating of 6 A, and proposes an in-depth study of their characteristics, as well as an exhaustive investigation of their performance during stressful abnormal events. The devices are commercially available engineering samples and were packaged in TO247 package, which is not qualified for high temperature application [8]. Although the exact technological details are not known, previous relevant publications reported a vertical device structure manufactured on a 4H–SiC wafer with low-resistive ntype substrate and a 15 lm thick nitrogen doped epitaxial collector layer (origin Cree, Inc. [9]), with base and emitter layers subsequently grown, still epitaxially, in a continuous run to minimise the density of defects. The base was doped with aluminium and the emitter, comprising of a two layer mesa structure, was doped with nitrogen [10]. For illustration, reference is made to typical avionic operational conditions. More precisely, short-circuit and turn-off robustness are tested at 270 V over the temperature range between 15 °C to 130 °C. The parameters of the drive circuit are also varied to the aim of extracting useful information for system design optimisation. Applications of interest include not only high-frequency switching power conversion (e.g., for motor drive applications), but also current limiters and circuit-breakers. In the following, first the transistor steady-state characteristics are presented and discussed, then, its switching and short-circuit performance.
A. Castellazzi et al. / Microelectronics Reliability 51 (2011) 1773–1777
7
IB=100 to 300 mA 6
(step=50 mA)
5
IC [A]
For the steady-state characterisation of the devices, a programmable temperature regulated thermal chamber was used, together with a Tektronix 371B high power curve tracer, adopting a fourterminal (Kelvin sense) measurement approach to isolate the influence of interconnections on the results. The cabling used in the measurement is nickel clad wire dedicated for electric furnace usage. Figs. 1a–d shows the measured output characteristics at different temperature values, 15, 30, 75 and 130 °C, respectively. As opposed to Si ones, SiC BJTs exhibit a positive temperature coefficient of their on-state resistance: this results mainly from the absence of conductivity modulation phenomena, due to much higher doping levels and shorter lifetime of the charge carriers, which make the on-state resistance thermal performance essentially dependent upon the carriers’ mobility only (see [10–11], for example). This is a well-known feature of SiC BJTs and contributes to make them attractive for novel power conversion applications in the first place. The device exhibits good performance, with virtually temperature independent characteristics, for current levels up to about 5–6 A, which is the nominal maximum steadystate rating of the considered chips. Taking into account that it is common praxis in power applications to double such figure for the maximum current in switched operation, the device performance becomes quite strongly temperature dependent: for instance, for a reference current value of 10 A, a nearly twofold increase in the VCE,SAT value is brought along by an increase in temperature from 30 to 130 °C. The base-drive current needs to be increased to keep good performance. On the other hand, as is evident from Fig. 2, which shows the measured BJT transfer characteristics at VCE =3 V, operation at higher current levels is associated not only with a positive temperature coefficient of the on-state resistance, but also with a negative temperature coefficient of the current gain, favouring the parallel operation of the devices (the forward bias base–emitter current increases with temperature for a given VBE value). This aspect is of particular interest for the implementation of protective functionalities, such as, for instance, current limiters or solid-state circuit breakers.
(a)
IB=50 mA
4
T=-15 °C 3 2 1 0 0
1
2
3
4
5
VCE [V]
(b) 20 IB=300 mA
IB=200 mA
15
T=30°C
IC [A]
2. Steady-state characteristics
10
IB=100 mA 5
0 0
1
2
3
4
5
VCE [V]
(c)
20
IB=300 mA
15 IB=200 mA
IC [A]
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T=75°C
10 IB=100 mA
5
0 0
1
2
3
4
5
VCE [V] 3. Switching performance
(d)
20
IB=300 mA 15
IC [A]
The switching performance of the transistors was tested by means of a double-pulse test circuit, for which a schematic and driving sequence are shown in Fig. 3a and b, respectively. During the first pulse, the device under test (DUT) is turned on and the current rises proportionally to the ratio of bias voltage, VBIAS, and load inductance, LLOAD, values. When the transistor is turned off, the inductance current is diverted to the anti-parallel freewheeling diode, 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 remains essentially constant due to negligible power dissipation in the diode. So, by properly controlling the duration of the first pulse, tON, the test current level for both the first turn-off and subsequent turn-on switching transitions can be accurately set to the desired value. To enable for easy test of both double-pulse turn-on and tun-off transitions and, subsequently, of short-circuit events, the driver design was based on the ADuM1233 IC (Analog Devices), an isolated, precision half-bridge driver. This IC provides two independent and galvanically isolated output channels that can be paralleled for higher turn-on current pulses during the short-circuit test. The driver, for which a simplified schematic is given in Fig. 4, enables switching frequencies well into the MHz range;
10
IB=200 mA
T=130 °C
5
IB=100 mA 0 0
1
2
3
4
5
VCE [V] Fig. 1. Measured output characteristics of a 1200 V SiC Power BJT at different temperature values: 15 °C (a), 30 °C (b), 75 °C (c), and 130 °C (d).
moreover, it is suitable for use with JFETs and MOSFETs, too, and as such can be used for benchmarking tests of different device technologies under similar operating conditions [12]. Following the results from the previous section, in these tests the main parameter of study was the device temperature. The DUT was mounted on a hot-plate and its case temperature moni-
A. Castellazzi et al. / Microelectronics Reliability 51 (2011) 1773–1777
16 14 12
IC [A]
10 8
30 °C 75 °C 130 °C -15 °C
6 4 2
Increasing temp.
0 2,5
3,0
3,5
VBE [V] Fig. 2. Measured transfer characteristics for four different temperature values (VCE = 3 V in these measurements).
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so as to bias the device with 300 mA base current. The parasitic inductance between bias voltage source and devices was intentionally kept to a minimum: in contrast to standard past double-pulse test circuits, in which different values of parasitic inductance are tried out to define design parameters in relation to the device limits of safe-operation, the parasitic inductance is minimised here mainly for three reasons: because the device is being used quite far away from its maximum breakdown voltage, because multichip SiC based power electronics will need designs with strongly reduced 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 performance. In (a), the collector current waveform for the double-pulse is shown: actually, three curves corresponding to the three different temperatures are plotted, but they can hardly be distinguished, as the device switching performance in the considered temperature range does not show appreciable changes; it is worth noting
(a) DFW
variable TCASE
R BASE
+
VBIAS 270V
LLOAD
DUT
-
VPULSE
(a) (b) VPULSE t ON
t OFF t
(b) Fig. 3. Circuit schematic (a), and driving sequence (b), of the experimental setup for switching performance characterisation of the SiC BJT.
(c)
Fig. 4. Detail of gate-driver circuit enabling for both short-circuit and double-pulse test in half-bridge configuration (here, only one channel is shown for simplicity).
tored with a thermometer. Three positive values were considered in this case, 30, 75 and 130 °C. The diode temperature was not changed and also not controlled. However, due to the specific mounting approach, it was not influenced by the changing temperature of the DUT and equals ambient temperature during all measurements. Also, the gate resistance value was not changed, but set
Fig. 5. Experimental current and voltage waveforms for the double-pulse switching transitions: (a) collector current profile for three different temperatures (30, 75 and 130 °C); (b) collector–emitter voltage profile, also for three different temperatures; (c) detail of turn-on transition.
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A. Castellazzi et al. / Microelectronics Reliability 51 (2011) 1773–1777
also that virtually no diode reverse-recovery current is observed at turn-on. In (b), the collector–emitter voltage waveform is reported:
Fig. 6. Experimental current and voltage waveforms for overload turn-off at 130 °C in double-pulse switching test.
variable TCASE R BASE
+
VBIAS
DUT
-
in this case, too, it is actually three waveforms being superimposed and showing virtually no difference; this waveform clearly shows that the parasitic inductance is kept at a minimum value; possible differences in the on-state VCE value could not be detected in this measurement, due to the resolution in measuring the relatively high voltage value. In (c), a zoom of the turn-on transition is proposed, revealing that the device actually switches faster as the temperature increases; a slight increase in the same direction was also noted in the turn-on transition; however, in view of the entity of the change in relation to the difference in temperature between profiles, the effect is deemed not relevant to the envisaged goals and the overall focus of this study; on the other hand, it should be noted that the current value at the second turn-off transition is about three times the nominal device current rating and the switching transition is completed in around 100 ns, without any indication of charge-extraction phenomena even at the higher temperature value (see previous discussion about the absence of conductivity modulation). Finally, in Fig. 6, the collector current and collector–emitter voltage profiles are shown for a hotplate temperature of 130 °C: here, the collector current before turn-off equals four times the nominal device current and the device can still turn-off safely and with very clean waveforms. As a whole, these results indicate a very good switching performance and turn-off overload robustness of the BJT for the specific application considered and indicate that very high switching speeds can be achieved, provided care is taken in minimising stray inductance in both the power and driving loops.
VPULSE 4. Short-circuit test
(a) VPULSE tSC t
(b) Fig. 7. Schematic of the experimental setup for the short-circuit robustness test.
Fig. 7 summarises the test conditions and describes the experimental set-up: the driving pulse width was set at 10 ls and the actual circuit also included a high voltage 2.2 mF input filter capacitor and low inductive planar interconnections to ensure negligible voltage drop during the test and enable fast current rise. The temperature of the DUT was set by means of a hotplate and the driver circuit parameters (e.g., base-resistance) could be varied to investigate their influence on the device performance. The driving sequence in this case consists of a single pulse whose duration was set to 10 ls in accordance with standard values used in Si-based power electronics: this is the minimum time required for protection circuitry to detect and remove the faulty condition; for reference, short-circuit capable Si IGBTs typically have a withstand
14,5
Increasing temp.
9,5
IC [A]
30°C 75°C 130°C 4,5
-0,5 -1,0E-06
1,0E-06
3,0E-06
5,0E-06
7,0E-06
9,0E-06
1,1E-05
1,3E-05
1,5E-05
time [s] Fig. 8. Measured short-circuit current waveforms at VBIAS = 270 V, for three different temperature values; IBASE = 150 mA.
A. Castellazzi et al. / Microelectronics Reliability 51 (2011) 1773–1777
(a)
34 30
Increasing VBIAS
26
VBIAS = 200 V VBIAS = 100 V
IC [A]
22 18 14
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current of 300 mA, this time for a nominal bias voltage value of 270 V and for two different temperature values, 30 and 130 °C, respectively. In this case too, let alone a small difference in the initial current peak, more pronounced at lower temperature values, there is virtually no difference in the short-circuit current profiles. 5. Conclusion
10 6 2 -2
-1
1
3
5
7
9
11
13
15
time [µs]
(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 different temperature values at VBIAS = 270 V and IBASE = 300 mA.
capability between 10 and 20 ls depending on the bias voltage as compared to the maximum device rating. Fig. 8 shows the short-circuit current waveforms, measured at three different temperature values and a bias voltage of 270 V. Here, the base current values was kept relatively low, around 150 mA. These results demonstrate the device capability to well withstand the abnormal event and points out that, in marked contrast to its Si counterpart, it exhibits a negative temperature coefficient of the collector current in these conditions (i.e. a Si–MOSlike behaviour), too, making it a suitable candidate for the implementation of multi-chip power modules. The transient decrease in the short-circuit current level was interpreted as the result of self-heating effects on carriers’ mobility, more pronounced for higher initial current and power values (i.e., lower temperatures). However, in separate tests, the base current was increased to 300 mA and the nominal power dissipation level was changed by intervening on the bias voltage, while leaving the hotplate temperature fix at 75 °C. The results, Fig. 9a, indicate that the physical operation of the BJT is actually more complex and cannot be interpreted merely on the basis of changes in mobility value with temperature; however, it contributes to make the device performance virtually independent from static and transient temperature variations even in the case of short-circuit events, in which significant heat is generated: this is further confirmed from the results of Fig. 9b, which show the short circuit current still for a base-drive
This paper presented an application related study of performance and robustness of a 1200 V–6 A SiC Power BJT. The interest of the study resides in the maturity of the particular transistor technology, possibly not too far from industrialisation, at least in the lower voltage range (indicatively, <2.5 kV). The results included here already provide interesting information for the development of multi-chip modules, showing, on the one side, that SiC Power BJTs are a competitive device choice and on the other, that bespoke drive circuit designs could yield optimised performance, both in switching and regulation applications. In particular, no indication 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 and rapid improvement in SiC device fabrication capability and well complement concurrent studies on package related thermomechanical issues (see [13], for example). Further activities will include an accurate analysis of parallel device performance and include SOA limit operation to determine the energy level associated with destruction under various operating conditions. References [1] Millan J. Wide band-gap power semiconductors devices. IET Circ Dev Syst 2007;1(5):372–9. [2] Funaki T, Matsushita M, Sasagawa M, Kimoto T, Hikihara T. A study on SiC devices in synchronous rectification of DC–DC converter. In: Proc APEC2007, Anaheim, CA, USA. [3] Matocha K, Losee P, Arthur S, Nasadoski J, Glaser J, Dunne G. 1400 V, 5 mW cm2 SiC MOSFETs for high-speed switching. In: Proc ISPSD2010, Hiroshima, Japan; June 2010. [4] Nakamura T, Sasagawa M, Nakano Y, Otsuka T, Miura M. Large current SiC power devices for automobile applications. In: Proc IPEC2010, Sapporo, Japan; June 2010. [5] Gao Y, Huang A, Agarwal AK, Zhang Q. Theoretical and experimental analyses of safe operating area (SOA) of 1200-V 4H–SiC BJT. IEEE Trans Electron Dev 2008;55(8). [6] Domeij M, Zaring1 C, Konstantinov AO, Nawaz1 M, Svedberg1 J-O, Gumaelius K, et al. 2.2 kV SiC BJTs with low VCESAT fast switching and short-circuit capability. In: Material science forum, 2010; vols. 645–648. [7] Miyake H, Kimoto T, Suda J. 4H–SiC bipolar junction transistors with record current gains of 257 on (0 0 0 1) and 335 on (0 0 0-1). In: Proc ISPSD2011, San Diego, CA, USA. [8] http://www.transic.com. [9] http://www.cree.com. [10] Domeij M, Lee H-S, Danielsson E, Zetterling C-M, Östling M, Schöner A. Geometrical effects in high current gain 1100-V 4H–SiC BJTs. IEEE Electron Dev Lett 2005;26(10):743–5. [11] Zhang Q, Agarwal AK. Design and technology considerations for SiC bipolar devices: BJTs, IGBTs, and GTOs. Phys Status Solidi A 2009;206(10):2431–56. [12] Takuno T, Hikihara T, Tsuno T, Hatsukawa S. HF gate drive circuit for a normally-on SiC JFET with inherent safety. In: Proc EPE2009, Barcelona, Spain. [13] Escrouzailles V, Castellazzi A, Solomalala P, Mermet-Guyennet M. Finiteelement analysis of the thermo-mechanical stresses affecting SiC power switches. In: Proc ESREF2010, Gaeta, Italy.
Contents lists available at ScienceDirect
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, UK Power Conversion & System Control Laboratory, Kyoto University, 615-8510 Katsura, Kyoto, Japan c Power Systems Laboratory, Osaka University, 565-0871 Suita, Osaka, Japan d Semiconductor Science & Engineering Laboratory, Kyoto University, 615-8510 Katsura, Kyoto, Japan b
a r t i c l e
i n f o
Article history: Received 30 May 2011 Received in revised form 16 June 2011 Accepted 27 June 2011 Available online 23 July 2011
a b s t r a c t This paper proposes an investigation of 1200 V rated transistors with the twofold purpose of assessing their performance and robustness under representative operational conditions and of extracting guidelines for the design of reliable multi-chip power electronics modules based on SiC technology. It includes a thorough analysis of the devices steady-state and switching characteristics, as well as the investigation of short-circuit events. Taking into account operational conditions of real applications, this study considers the dependence on ambient temperature, bias conditions and driver circuit parameters. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The interest in developing silicon carbide (SiC) based electrical power conversion systems is by now clear to the specialist community (see [1], for example). In particular, the main drives are higher switching frequencies, higher power densities and higher operating temperatures than achievable with Si electronics. After years of research on material growth, device physics and manufacturing, SiC power transistors are now becoming available in sufficient quality and quantity to stimulate a concrete interest in the development of system solutions based entirely on this technology. That, in turn, motivates the concurrent initiation of performance, robustness and reliability studies of an application-oriented nature. In many commercial applications, the higher temperature capability of SiC devices is still of secondary importance as compared with the possibility to increase both the volumetric and gravimetric power density of electrical power conversion equipment. This is the case, in particular, of high reliability environments, such as, for instance, the railway and avionic industry, where, on the other hand, reduction of size and weight are major key aspects of technology evolution and competitive product development. Next to the diode, available SiC device types have included BJTs and JFETs for some time now, and, more recently, SiC Power MOSFETs have also been demonstrated [2–4]. From an industrialisation and commercialisation point of view, to date the BJT is quite a mature switch technology type and its immunity from second breakdown together with recent breakthrough advancements in current gain figures still make it an attractive and competitive candidate for ⇑ Corresponding author. E-mail address: [email protected] (A. Castellazzi). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.06.046
the development of SiC based electrical power conversion systems [1,5–7]. For the time being, single chip current ratings are still limited by the crystal growth and manufacturing process to a few tens of amps maximum. Thus, for most power applications, multi-chip switches are envisaged and, in view of significant behavioural differences as compared with Si devices, it is important to carry out a dedicated study. So, this papers considers 1200 V rated SiC BJTs, with a nominal current rating of 6 A, and proposes an in-depth study of their characteristics, as well as an exhaustive investigation of their performance during stressful abnormal events. The devices are commercially available engineering samples and were packaged in TO247 package, which is not qualified for high temperature application [8]. Although the exact technological details are not known, previous relevant publications reported a vertical device structure manufactured on a 4H–SiC wafer with low-resistive ntype substrate and a 15 lm thick nitrogen doped epitaxial collector layer (origin Cree, Inc. [9]), with base and emitter layers subsequently grown, still epitaxially, in a continuous run to minimise the density of defects. The base was doped with aluminium and the emitter, comprising of a two layer mesa structure, was doped with nitrogen [10]. For illustration, reference is made to typical avionic operational conditions. More precisely, short-circuit and turn-off robustness are tested at 270 V over the temperature range between 15 °C to 130 °C. The parameters of the drive circuit are also varied to the aim of extracting useful information for system design optimisation. Applications of interest include not only high-frequency switching power conversion (e.g., for motor drive applications), but also current limiters and circuit-breakers. In the following, first the transistor steady-state characteristics are presented and discussed, then, its switching and short-circuit performance.
A. Castellazzi et al. / Microelectronics Reliability 51 (2011) 1773–1777
7
IB=100 to 300 mA 6
(step=50 mA)
5
IC [A]
For the steady-state characterisation of the devices, a programmable temperature regulated thermal chamber was used, together with a Tektronix 371B high power curve tracer, adopting a fourterminal (Kelvin sense) measurement approach to isolate the influence of interconnections on the results. The cabling used in the measurement is nickel clad wire dedicated for electric furnace usage. Figs. 1a–d shows the measured output characteristics at different temperature values, 15, 30, 75 and 130 °C, respectively. As opposed to Si ones, SiC BJTs exhibit a positive temperature coefficient of their on-state resistance: this results mainly from the absence of conductivity modulation phenomena, due to much higher doping levels and shorter lifetime of the charge carriers, which make the on-state resistance thermal performance essentially dependent upon the carriers’ mobility only (see [10–11], for example). This is a well-known feature of SiC BJTs and contributes to make them attractive for novel power conversion applications in the first place. The device exhibits good performance, with virtually temperature independent characteristics, for current levels up to about 5–6 A, which is the nominal maximum steadystate rating of the considered chips. Taking into account that it is common praxis in power applications to double such figure for the maximum current in switched operation, the device performance becomes quite strongly temperature dependent: for instance, for a reference current value of 10 A, a nearly twofold increase in the VCE,SAT value is brought along by an increase in temperature from 30 to 130 °C. The base-drive current needs to be increased to keep good performance. On the other hand, as is evident from Fig. 2, which shows the measured BJT transfer characteristics at VCE =3 V, operation at higher current levels is associated not only with a positive temperature coefficient of the on-state resistance, but also with a negative temperature coefficient of the current gain, favouring the parallel operation of the devices (the forward bias base–emitter current increases with temperature for a given VBE value). This aspect is of particular interest for the implementation of protective functionalities, such as, for instance, current limiters or solid-state circuit breakers.
(a)
IB=50 mA
4
T=-15 °C 3 2 1 0 0
1
2
3
4
5
VCE [V]
(b) 20 IB=300 mA
IB=200 mA
15
T=30°C
IC [A]
2. Steady-state characteristics
10
IB=100 mA 5
0 0
1
2
3
4
5
VCE [V]
(c)
20
IB=300 mA
15 IB=200 mA
IC [A]
1774
T=75°C
10 IB=100 mA
5
0 0
1
2
3
4
5
VCE [V] 3. Switching performance
(d)
20
IB=300 mA 15
IC [A]
The switching performance of the transistors was tested by means of a double-pulse test circuit, for which a schematic and driving sequence are shown in Fig. 3a and b, respectively. During the first pulse, the device under test (DUT) is turned on and the current rises proportionally to the ratio of bias voltage, VBIAS, and load inductance, LLOAD, values. When the transistor is turned off, the inductance current is diverted to the anti-parallel freewheeling diode, 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 remains essentially constant due to negligible power dissipation in the diode. So, by properly controlling the duration of the first pulse, tON, the test current level for both the first turn-off and subsequent turn-on switching transitions can be accurately set to the desired value. To enable for easy test of both double-pulse turn-on and tun-off transitions and, subsequently, of short-circuit events, the driver design was based on the ADuM1233 IC (Analog Devices), an isolated, precision half-bridge driver. This IC provides two independent and galvanically isolated output channels that can be paralleled for higher turn-on current pulses during the short-circuit test. The driver, for which a simplified schematic is given in Fig. 4, enables switching frequencies well into the MHz range;
10
IB=200 mA
T=130 °C
5
IB=100 mA 0 0
1
2
3
4
5
VCE [V] Fig. 1. Measured output characteristics of a 1200 V SiC Power BJT at different temperature values: 15 °C (a), 30 °C (b), 75 °C (c), and 130 °C (d).
moreover, it is suitable for use with JFETs and MOSFETs, too, and as such can be used for benchmarking tests of different device technologies under similar operating conditions [12]. Following the results from the previous section, in these tests the main parameter of study was the device temperature. The DUT was mounted on a hot-plate and its case temperature moni-
A. Castellazzi et al. / Microelectronics Reliability 51 (2011) 1773–1777
16 14 12
IC [A]
10 8
30 °C 75 °C 130 °C -15 °C
6 4 2
Increasing temp.
0 2,5
3,0
3,5
VBE [V] Fig. 2. Measured transfer characteristics for four different temperature values (VCE = 3 V in these measurements).
1775
so as to bias the device with 300 mA base current. The parasitic inductance between bias voltage source and devices was intentionally kept to a minimum: in contrast to standard past double-pulse test circuits, in which different values of parasitic inductance are tried out to define design parameters in relation to the device limits of safe-operation, the parasitic inductance is minimised here mainly for three reasons: because the device is being used quite far away from its maximum breakdown voltage, because multichip SiC based power electronics will need designs with strongly reduced 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 performance. In (a), the collector current waveform for the double-pulse is shown: actually, three curves corresponding to the three different temperatures are plotted, but they can hardly be distinguished, as the device switching performance in the considered temperature range does not show appreciable changes; it is worth noting
(a) DFW
variable TCASE
R BASE
+
VBIAS 270V
LLOAD
DUT
-
VPULSE
(a) (b) VPULSE t ON
t OFF t
(b) Fig. 3. Circuit schematic (a), and driving sequence (b), of the experimental setup for switching performance characterisation of the SiC BJT.
(c)
Fig. 4. Detail of gate-driver circuit enabling for both short-circuit and double-pulse test in half-bridge configuration (here, only one channel is shown for simplicity).
tored with a thermometer. Three positive values were considered in this case, 30, 75 and 130 °C. The diode temperature was not changed and also not controlled. However, due to the specific mounting approach, it was not influenced by the changing temperature of the DUT and equals ambient temperature during all measurements. Also, the gate resistance value was not changed, but set
Fig. 5. Experimental current and voltage waveforms for the double-pulse switching transitions: (a) collector current profile for three different temperatures (30, 75 and 130 °C); (b) collector–emitter voltage profile, also for three different temperatures; (c) detail of turn-on transition.
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also that virtually no diode reverse-recovery current is observed at turn-on. In (b), the collector–emitter voltage waveform is reported:
Fig. 6. Experimental current and voltage waveforms for overload turn-off at 130 °C in double-pulse switching test.
variable TCASE R BASE
+
VBIAS
DUT
-
in this case, too, it is actually three waveforms being superimposed and showing virtually no difference; this waveform clearly shows that the parasitic inductance is kept at a minimum value; possible differences in the on-state VCE value could not be detected in this measurement, due to the resolution in measuring the relatively high voltage value. In (c), a zoom of the turn-on transition is proposed, revealing that the device actually switches faster as the temperature increases; a slight increase in the same direction was also noted in the turn-on transition; however, in view of the entity of the change in relation to the difference in temperature between profiles, the effect is deemed not relevant to the envisaged goals and the overall focus of this study; on the other hand, it should be noted that the current value at the second turn-off transition is about three times the nominal device current rating and the switching transition is completed in around 100 ns, without any indication of charge-extraction phenomena even at the higher temperature value (see previous discussion about the absence of conductivity modulation). Finally, in Fig. 6, the collector current and collector–emitter voltage profiles are shown for a hotplate temperature of 130 °C: here, the collector current before turn-off equals four times the nominal device current and the device can still turn-off safely and with very clean waveforms. As a whole, these results indicate a very good switching performance and turn-off overload robustness of the BJT for the specific application considered and indicate that very high switching speeds can be achieved, provided care is taken in minimising stray inductance in both the power and driving loops.
VPULSE 4. Short-circuit test
(a) VPULSE tSC t
(b) Fig. 7. Schematic of the experimental setup for the short-circuit robustness test.
Fig. 7 summarises the test conditions and describes the experimental set-up: the driving pulse width was set at 10 ls and the actual circuit also included a high voltage 2.2 mF input filter capacitor and low inductive planar interconnections to ensure negligible voltage drop during the test and enable fast current rise. The temperature of the DUT was set by means of a hotplate and the driver circuit parameters (e.g., base-resistance) could be varied to investigate their influence on the device performance. The driving sequence in this case consists of a single pulse whose duration was set to 10 ls in accordance with standard values used in Si-based power electronics: this is the minimum time required for protection circuitry to detect and remove the faulty condition; for reference, short-circuit capable Si IGBTs typically have a withstand
14,5
Increasing temp.
9,5
IC [A]
30°C 75°C 130°C 4,5
-0,5 -1,0E-06
1,0E-06
3,0E-06
5,0E-06
7,0E-06
9,0E-06
1,1E-05
1,3E-05
1,5E-05
time [s] Fig. 8. Measured short-circuit current waveforms at VBIAS = 270 V, for three different temperature values; IBASE = 150 mA.
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(a)
34 30
Increasing VBIAS
26
VBIAS = 200 V VBIAS = 100 V
IC [A]
22 18 14
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current of 300 mA, this time for a nominal bias voltage value of 270 V and for two different temperature values, 30 and 130 °C, respectively. In this case too, let alone a small difference in the initial current peak, more pronounced at lower temperature values, there is virtually no difference in the short-circuit current profiles. 5. Conclusion
10 6 2 -2
-1
1
3
5
7
9
11
13
15
time [µs]
(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 different temperature values at VBIAS = 270 V and IBASE = 300 mA.
capability between 10 and 20 ls depending on the bias voltage as compared to the maximum device rating. Fig. 8 shows the short-circuit current waveforms, measured at three different temperature values and a bias voltage of 270 V. Here, the base current values was kept relatively low, around 150 mA. These results demonstrate the device capability to well withstand the abnormal event and points out that, in marked contrast to its Si counterpart, it exhibits a negative temperature coefficient of the collector current in these conditions (i.e. a Si–MOSlike behaviour), too, making it a suitable candidate for the implementation of multi-chip power modules. The transient decrease in the short-circuit current level was interpreted as the result of self-heating effects on carriers’ mobility, more pronounced for higher initial current and power values (i.e., lower temperatures). However, in separate tests, the base current was increased to 300 mA and the nominal power dissipation level was changed by intervening on the bias voltage, while leaving the hotplate temperature fix at 75 °C. The results, Fig. 9a, indicate that the physical operation of the BJT is actually more complex and cannot be interpreted merely on the basis of changes in mobility value with temperature; however, it contributes to make the device performance virtually independent from static and transient temperature variations even in the case of short-circuit events, in which significant heat is generated: this is further confirmed from the results of Fig. 9b, which show the short circuit current still for a base-drive
This paper presented an application related study of performance and robustness of a 1200 V–6 A SiC Power BJT. The interest of the study resides in the maturity of the particular transistor technology, possibly not too far from industrialisation, at least in the lower voltage range (indicatively, <2.5 kV). The results included here already provide interesting information for the development of multi-chip modules, showing, on the one side, that SiC Power BJTs are a competitive device choice and on the other, that bespoke drive circuit designs could yield optimised performance, both in switching and regulation applications. In particular, no indication 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 and rapid improvement in SiC device fabrication capability and well complement concurrent studies on package related thermomechanical issues (see [13], for example). Further activities will include an accurate analysis of parallel device performance and include SOA limit operation to determine the energy level associated with destruction under various operating conditions. References [1] Millan J. Wide band-gap power semiconductors devices. IET Circ Dev Syst 2007;1(5):372–9. [2] Funaki T, Matsushita M, Sasagawa M, Kimoto T, Hikihara T. A study on SiC devices in synchronous rectification of DC–DC converter. In: Proc APEC2007, Anaheim, CA, USA. [3] Matocha K, Losee P, Arthur S, Nasadoski J, Glaser J, Dunne G. 1400 V, 5 mW cm2 SiC MOSFETs for high-speed switching. In: Proc ISPSD2010, Hiroshima, Japan; June 2010. [4] Nakamura T, Sasagawa M, Nakano Y, Otsuka T, Miura M. Large current SiC power devices for automobile applications. In: Proc IPEC2010, Sapporo, Japan; June 2010. [5] Gao Y, Huang A, Agarwal AK, Zhang Q. Theoretical and experimental analyses of safe operating area (SOA) of 1200-V 4H–SiC BJT. IEEE Trans Electron Dev 2008;55(8). [6] Domeij M, Zaring1 C, Konstantinov AO, Nawaz1 M, Svedberg1 J-O, Gumaelius K, et al. 2.2 kV SiC BJTs with low VCESAT fast switching and short-circuit capability. In: Material science forum, 2010; vols. 645–648. [7] Miyake H, Kimoto T, Suda J. 4H–SiC bipolar junction transistors with record current gains of 257 on (0 0 0 1) and 335 on (0 0 0-1). In: Proc ISPSD2011, San Diego, CA, USA. [8] http://www.transic.com. [9] http://www.cree.com. [10] Domeij M, Lee H-S, Danielsson E, Zetterling C-M, Östling M, Schöner A. Geometrical effects in high current gain 1100-V 4H–SiC BJTs. IEEE Electron Dev Lett 2005;26(10):743–5. [11] Zhang Q, Agarwal AK. Design and technology considerations for SiC bipolar devices: BJTs, IGBTs, and GTOs. Phys Status Solidi A 2009;206(10):2431–56. [12] Takuno T, Hikihara T, Tsuno T, Hatsukawa S. HF gate drive circuit for a normally-on SiC JFET with inherent safety. In: Proc EPE2009, Barcelona, Spain. [13] Escrouzailles V, Castellazzi A, Solomalala P, Mermet-Guyennet M. Finiteelement analysis of the thermo-mechanical stresses affecting SiC power switches. In: Proc ESREF2010, Gaeta, Italy.