Trapping phenomena and degradation mechanisms in GaN-based power HEMTs

The second part of the paper deals with device degradation: based on a wide set of experimental results, we describe the physical mechanisms responsible for the worsening of the properties of the devices. More specifically, we demonstrate that stress in off-state conditions may result in measurable changes in the pinch-off voltage, mostly consisting in a negative-threshold instability (NBTI). The origin of this shift is discussed in detail; we also demonstrate that in a real-life cascode configuration (where a low, subthreshold leakage current flows through the device in the off-state), NBTI effects are mitigated. Finally, we discuss the stability of the gate-stack, induced by the exposure to positive gate bias.