(Invited) Ruggedness of SiC and GaN Power Transistors in Switching Based Tests
Authors: Zhang Ruizhe , Kozak Joseph , Liu Jingcun , Zhang Yuhao 
Published: December 2020

Wide bandgap (WBG) power devices are being implemented into power electronic applications at a growing rate to improve the weight, power density, and efficiency of the systems. However, the insufficient reliability and ruggedness data on both silicon carbide (SiC) and gallium nitride (GaN) power devices has hindered their wide adoption in the industrial applications that demand stressful device operations. Currently, a spectrum of reliability tests, e.g., accelerated lifetime tests, is being extensively performed by both device manufacturers and end users. Most of these qualification tests are operated within the device safe-operating-area (SOA). For example, high-temperature reverse bias (HTRB) tests are usually conducted at 80% of the rated voltage. However, the devices normally undergo dynamic events in converter applications that can exceed the SOA boundaries. As a result, further ruggedness studies need to be performed to quantify the out-of-SOA robustness of new power transistors in switching events. In this talk, we will present our recent work on the out-of-SOA ruggedness tests of SiC metal oxide semiconductor field effect transistors (MOSFETs) and GaN high electron mobility transistors (HEMT) under switching conditions (Fig. 1(a)).

To test the device ruggedness under switching conditions that are common to power applications, we developed hard-switching based ruggedness test where the device was stressed to withstand overvoltage and overcurrent beyond their voltage and current ratings in each switching cycle [1]. This “Switching Cycling” test has been conducted on 1200 V, 10 A rated SiC MOSFETs at 25 oC and 100 oC. The devices were stressed with a drain voltage equal to 90% the breakdown voltage (1450 V) and pulsed to conduct a peak of 20 A. Pulses were only 150 ns such that the conduction losses would induce minimum self-heating effects. The switching waveforms for both tests are shown in Fig. 1(b). Two independent failure mechanisms were identified where the first is seen in the changes in gate leakage current, and the second in drain leakage current [2]. The second mechanism has not been reported before in SiC MOSFETs and is believed to be a direct result of this out-of-SOA, switching cycling test. Both degradations were found to accelerate at the high ambient temperature. The physics of these two failure mechanisms were unveiled: the hot-electron induced gate-oxide degradation accounts for the increased gate leakage; the electron hopping through the defect states created in the semiconductor junction accounts for the increased drain leakage current (Fig. 1(c)).

The surge energy ruggedness (often referred to as avalanche ruggedness) have been heavily studied in SiC MOSFETs, but only recently been tested on power GaN HEMTS. Because the HEMT structure does not have inherent avalanching capabilities, it was uncertain how these devices would respond to the surge energy produced in the test. In this work, we developed two circuits to study the surge ruggedness of GaN HEMTs: an unclamped inductive switching (UIS) circuit [3] and a clamped inductive switching circuit with a controllable parasitic inductance [4] (Fig. 1(d)). Two commercial p-gate GaN HEMTs with Ohmic gate contacts (i.e., gate injection transistors) [3] and Schottky-type gate contacts [4] are studied. The two p-gate GaN HEMTs are found to both withstand surge energy through a resonant energy transfer between the device capacitance and the load/parasitic inductance, rather than a resistive energy dissipation as occurred in avalanche (Fig. 1(e)). The device failure occurs at the transient of peak resonant voltage and is limited by the device overvoltage capability. It is dominated by electric field rather than thermal runaway and the failure spot is consistent with the peak electric field location (Fig. 1(f)).

Through this work, a better understanding of the withstand capabilities, and switching based ruggedness of GaN devices and SiC devices have been demonstrated. The new degradation physics and characterization methods provide important references for the qualifications and applications of WBG power devices.

Reference: [1] J. P. Kozak, R. Zhang, H. Yang, K. D. T. Ngo, and Y. Zhang, “Robustness Evaluation and Degradation Mechanisms of SiC MOSFETs Overstressed by Switched Stimuli,” 2020 Applied Power Electronics Conference (APEC). [2] J. P. Kozak, R. Zhang, J. Liu, and Y. Zhang, “Physics of Degradation in SiC MOSFETs Stressed by Overvoltage and Overcurrent Switching,” 2020 IEEE International Reliability Physics Symposium (IRPS). [3] R. Zhang, J. P. Kozak, J. Liu, and Y. Zhang, “Surge Energy Robustness of GaN Gate Injection Transistors,” 2020 IEEE International Reliability Physics Symposium (IRPS). [4] R. Zhang, J. P. Kozak, M. Xiao, J. Liu, and Y. Zhang, “Surge-Energy and Overvoltage Ruggedness of p-Gate GaN HEMTs,” IEEE Transactions on Power Electronics, early access online.

Figure 1