Degradation of GaN-HEMTs with p-GaN Gate: Dependence on temperature and on geometry

This paper investigates the time-dependent degradation of normally-off transistors with p-GaN gate submitted to constant voltage stress. Based on combined dc characterization and temperature-dependent analysis, we study the dependence of time-To-failure on stress temperature and device geometry. The results of this analysis indicate that: (i) normally-off transistors with p-GaN gate have a good stability, reaching a 20 years lifetime with a 7.2 V gate bias; (ii) at higher stress voltages, a time-dependent failure is observed. Time-To-failure (TTF) depends exponentially on stress voltage, while failure is ascribed to a localized breakdown process that takes place in the p-GaN/AlGaN stack; (iii) TTF scales with device area only if the area is changed by increasing the gate width (and not if area is increased by modifying gate length). This result suggests that degradation occurs mostly in proximity of the gate edge, rather than at the center of the gate. (iv) finally, stress tests carried out at different temperature levels indicate that TTF is dependent on temperature, with activation energy of 0.48-0.50 eV.