Failure analysis of normally-off GaN HEMTs under avalanche conditions

Gallium nitride (GaN) high electron-mobility transistors (HEMTs) are promising devices in the power electronics field owing to their wide bandgap (WBG). However, all the potential advantages provided by their WBG require reliability improvement. In industrial applications, robustness is one of the main factors considered by circuit designers. This study focuses on the observation of the degradation behavior of the main waveforms of unclamped inductive-switching (UIS) test circuits of two different commercial GaN HEMT structures. The relevance of this study lies in the potential applications of these devices to high-voltage applications and automotive systems where they are subjected to many UIS events over their lifetime. This study shows that avalanche does not occur on these devices; therefore, the breakdown is caused by the high voltage. A deeper analysis of the breakdown mechanism is achieved using a curve/tracer analyzer, lock-in thermography, and focused ion beam. These experiments reveal that impact ionization is the main failure mechanism that causes breakdown in both structures.