Characterizing Threshold Voltage Shifts and Recovery in Schottky Gate and Ohmic Gate GaN HEMTs

Abstract

Threshold voltage shift in normally-OFF GaN High Electron Mobility Transistors (HEMTs) is an important reliability concern in GaN devices. Differences in device architecture between Schottky gate and Ohmic gate normally-OFF GaN HEMTs means that there are important differences in the physical mechanism behind threshold voltage shift due to gate stress. In this paper, a non-intrusive technique for the characterization of threshold voltage shift is applied to both technologies. The technique relies on using a sensing current to measure the third quadrant voltage before and after gate-voltage stress. The results show that in Schottky Gate GaN HEMTs, a positive threshold voltage shift occurs at low gate stress voltages due to electron trapping in the GaN/AlGaN interface while at higher gate stress voltages, the threshold voltage shift becomes negative due to hole trapping and accumulation. The stress time has a fundamental role on the measured threshold voltage shift at medium gate voltage levels and pulsed gate stresses are able to capture this phenomenon. For the Ohmic Gate GaN HEMTs, only a negative threshold voltage shift is observed for all stress currents with no apparent shift as the junction temperature is increased.