We have shown that the off-state degradation in GaN based metal-oxide-semiconductor high electron mobility transistors has three field dependent regimes. We have considered Al2O3 and AlOxNy as the gate dielectrics. The degradation is dominated by electron trapping near the drain edge at relatively low electric field; hole trapping followed by structural defects tends to dominate at high electric field. The structural defects may potentially be caused by an inverse piezoelectric effect when the AlGaN/GaN interface may relax partially. The hole trapping, in particular, at moderate to high electric field can lead to an anomalous trend. The off-state drain current bears the signature of the degradation. The transistor performance is found to deteriorate for all the cases independent of the degradation mechanism. The trapping contribution toward the degradation is partially recoverable; however, the structural damage is found to be permanent. It is also observed that both the gate dielectrics qualitatively suffer from the same degradation mechanism with oxynitride high electron mobility transistors showing higher resilience than their oxide counterpart. Published by AIP Publishing.