1200 V-class 4H-SiC “Super” Junction Transistors with Current Gains of 88 and Ultra-fast Switching capability Ranbir Singha,*, Stoyan Jeliazkov and Eric Lieser GeneSiC Semiconductor, 43670 Trade Center Pl, Suite 155, Dulles, Virginia 20166, USA. a [email protected], *corresponding author Keywords: SiC transistor, current gain, RBSOA, switching, breakdown voltage. Abstract: 1200 V-Class Super-High Current Gain Transistors or SJTs developed by GeneSiC are distinguished by low leakage currents of < 100 µA at 325 °C operating temperature, turn-on and turn-off switching transients of < 15 ns at 250 °C, maximum Common Source current gains of 88 and low on-resistance of 5.8 mΩ-cm2. Two-stage cascaded SJTs display a record high current gain of 3475. Results from detailed on-state, blocking, switching and reliability characterization of 1200 V-class 4 mm2 and 16 mm2 SiC SJTs are presented in this paper. Introduction Silicon Carbide “Super” Junction Transistors (SJTs) are “Super-High” current gain SiC NPN BJTs currently developed by GeneSiC in 1200 V – 10 kV ratings. The SiC SJTs are Gateoxide free, normally-off, quasi-majority carrier devices with a square reverse biased safe operating area (RBSOA) and a slightly positive temperature co-efficient of on-resistance. The current driven SJTs are capable of 250 °C operation, offer < 20 ns turn-on and turn-off capability, current gains as high as 88, low on-state voltage drop and high-current operation, due to the absence of a channel region and near zero Drain-Source offset voltage. Incorporating high voltage, high frequency and high-temperature capable SiC SJTs will increase the power conversion efficiency and reduce the size/weight/volume of commercial power electronics. This paper will report on an in-depth study of the static, switching and long-term reliability characteristics of the 1200 V-class SiC SJTs recently designed and fabricated at GeneSiC. Device Design and Fabrication The SiC SJTs fabricated in this work have n- drift epilayers designed for blocking 1200 V, and optimally designed Source and Channel epilayers for achieving high current gains and low onresistance. Two SJT chip sizes of 4 mm2 and 16 mm2 are fabricated in this work and are packaged in both standard TO-220 packages and high-temperature metal-can packages. On-state and Blocking Voltage Characteristics Optimized edge termination and surface passivation schemes were used for SJT fabrication to achieve near-theoretical breakdown voltages and temperature independent, low reverse leakage currents up to 325 °C (Figure 1(a)). The output I-V characteristics of a 16 mm2 SiC SJT shown in Figure 1(b) feature a near-zero Drain-Source offset voltage, distinct lack of a quasi-saturation region and the merging of the different Gate current I-V curves in the saturation region. The last two features imply lack of charge storage in the drift region of the SiC SJT and clearly distinguish it from a “bipolar” Si BJT. Note that the NPN terminology is replaced with FET terminology for describing the SJT terminals. This inherent property of the SiC SJT enables temperature independent, fast switching transients. Appropriate metallization schemes along with an optimized epilayer design resulted in VDS,SAT values as low as 1.35 V at 25 °C and 2.7 V at 175 °C at 30 A of Drain current. A low on-resistance of 5.8 mΩ-cm2 is extracted from the 25 °C output I-V characteristics at 30 A. The positive temperature co-efficient of on-resistance observed in Figure is desirable for paralleling multiple devices for high-current configurations. Figure 1: (Left, a) Open-Base Blocking I-V characteristics measured on 4 mm2 SJTs up to 325 °C and (Right, b) Output characteristics of a 16 mm2 SJT measured up to 175 °C. Single, 5 µs wide pulsed current measurements were used to investigate the variation of the common-Source current gain (β) with increasing Drain current with minimal self-heating. For a 4 mm2 SJT, a β as high as 88 is measured at Drain current levels ranging from 4 A to 10 A at 25 °C, as shown in Figure 2(a). High-level injection effects in the base layer decrease the β at Drain currents > 300 A/cm2, which is notably greater than the thermal dissipation capability of most commercial packaging. It can be noted from Figure 2(b), that the β shows a negative temperature co-efficient, decreasing from a maximum value of 81 at 25 °C to 44 at 200 °C, due to the increase in ionization of the p-type Al acceptors in the base layer of the SJT. Figure 2 (Left, a) Single-pulse (5 µs) measurements to extract the common-Source current gain, β of a 4 mm2 SJT as a function of Drain current and (Right, b) negative temperature co-efficient exhibited by the β of a 4 mm2 SJT, decreasing from a maximum of 81 at 25 °C to 44 at 200 °C. Monolithic 2-stage, 4 mm2 and 16 mm2 cascaded SJTs were also fabricated on the same wafer as discrete SJTs using an output chip area to driver chip area ratio of ≈ 3. An 8 µm wide isolation trench provided the necessary isolation between the driver and output transistors. From the output characteristics shown in Figure 3, a record high β of 3475 is observed at a Drain current of 14 A for the 4 mm2 device and a β of 2423 is observed a Drain current of 72 A for the 16 mm 2 device. These are the highest current gains recorded on any SiC BJT reported to-date. Figure 3: 25 °C Output characteristics of a (Left, a) 4 mm2 2-stage on-chip cascaded SJT and a (Right, b) 16 mm2 2-stage on-chip cascaded SiC SJT. The current gain of 3475 displayed by the 4 mm 2 device is the highest current gain ever recorded on any SiC current-controlled transistor to-date. Switching characteristics of discrete and cascaded SJTs Switching measurements on the 4 mm2 and 16 mm2 SiC SJTs were performed with an inductive load and free-wheeling 1200 V/ 7 A or 1200 V/30 A GeneSiC SiC Schottky rectifiers. A commercially available IGBT gate driver with an output voltage swing from -8 V to 15 V was used for driving the SJTs. For the 4 mm2 SJTs, a 100 nF dynamic capacitor connected in parallel with the Gate resistor generated high initial dynamic Gate currents of 4.5 A and -1 A during turn-on and turn-off switching respectively, while maintaining a constant Gate current of 0.52 A during its turnon pulse. These large initial dynamic Gate currents charge/discharge the device input capacitance rapidly, yielding a faster switching performance. Temperature-independent, ultra-low Drain current rise and fall times of 12 ns and 13 ns, respectively, were recorded (Figure 4) for switching 8 A and 800 V by the 4 mm2 SJT. Switching measurements on 4 mm2 Cascaded SJT devices using a similar test setup yielded relatively higher current rise and fall times of 27 ns and 37 ns, respectively (Figure 5). The higher switching times observed for the Cascaded SJTs as compared to the discrete device are due to the lack of Gate control over the switching of the output transistor. Figure 4: (Left, a) Turn-On and (Right, b) Turn-Off Drain Current and Voltage transients recorded for switching 800 V and 8 A through a 4 mm2 SiC SJT. There is no difference in switching speed between 25 °C and 250 °C, due to the unipolar nature of the SJT device design. Figure 5: (Left, a) Turn-On and (Right, b) Turn-Off Drain Current and Voltage transients recorded for switching 500 V and 7 A through a 4 mm2, 2-stage on-chip cascade SiC SJT. The significantly higher rise time for the Drain Bias as compared to a discrete device during device turn-off is due to the “Open-Base” turn-off of the output transistor. Device Robustness Measurements Preliminary results from the short-circuit capability and avalanche ruggedness of the 4 mm2 SJTs fabricated in this work are shown in Figure 6(a) and (b). When the SJT is turned on to a short circuit at a Drain voltage of 800 V with 0.2 A of Gate current, a short circuit current of 13 A and a short circuit withstand time of 22 µs are observed in Figure , which is considerably higher than the 10 µs reported on SiC MOSFETs. Under these short circuit conditions, device destruction occurred at 25 µs. A single-pulse avalanche energy (EAS) of 20.4mJ was extracted from Unclamped Inductive Switching (UIS) performed on the 4 mm2 SJT at the rated current of 7 A with a 1 mH inductor. Figure 6: (Left) A short circuit withstand time of 22 µs is obtained, when a 4 mm 2 SJT is switched on to a shortcircuited load at a Drain bias of 800 V and a Gate current of 0.2 A. (Right) A single-pulse avalanche energy of 20.4 mJ is obtained for unclamped Inductive switching (UIS) of a 4 mm2 SJT at the rated Drain current of 7 A. Acknowledgement: The funding support from Office of Naval Research (ONR) under SBIR Contract N00014-C-10-0104, and support from Dr. Scott Coombe is gratefully acknowledged.