This paper reviews the most relevant mechanisms responsible for the degradation of GaN-based lateral and vertical electron devices. These components are almost ideal for application in power electronics, but the presence of semiconductor defects and the existence of degradation processes may limit their stability and lifetime. In this paper we focus on the following aspects: (i) the degradation processes induced by off-state conditions and leading to a time-dependent and/or catastrophic breakdown of the devices; (ii) the stability of the gate stack; (iii) the degradation of the electrical performance of vertical GaN transistors and diodes. To discuss these topics, we refer to case studies carried out in our laboratories.
GaN-HEMTs with p-GaN gate have recently demonstrated to be excellent normally-off devices for application in power conversion systems, thanks to the high and robust threshold voltage (V<sub>TH</sub>>1 V), the high breakdown voltage, and the low dynamic Ron increase. For this reason, studying the stability and reliability of these devices under high stress conditions is of high importance. This paper reports on our most recent results on the field- and time-dependent degradation of GaN-HEMTs with p-GaN gate submitted to stress with positive gate bias. Based on combined step-stress experiments, constant voltage stress and electroluminescence testing we demonstrated that: (i) when submitted to high/positive gate stress, the transistors may show a negative threshold voltage shift, that is ascribed to the injection of holes from the gate metal towards the p-GaN/AlGaN interface; (ii) in a step-stress experiment, the analyzed commercial devices fail at gate voltages higher than 9-10 V, due to the extremely high electric field over the p-GaN/AlGaN stack; (iii) constant voltage stress tests indicate that the failure is also time-dependent and Weibull distributed. The several processes that can explain the time-dependent failure are discussed in the following.