This paper reports on a comprehensive on-state reliability evaluation on depletion-mode (VTH∼-4V) AlGaN/GaN Metal-Insulator-Semiconductor High Electron Mobility Transistors (MIS-HEMTs) with a bi-layer dielectric (in-situ Si3N4/Al2O3). We have studied the strength and the lifetime of the dielectric to breakdown by means of a Time Dependent Dielectric Breakdown (TDDB) experiment performed at 200°C and the trapping effects induced by applying a positive gate voltage stress. Additionally, for the first time, we have studied the effect of the on-state stress as a function of the drain voltage. The results show that 1) Based on a Time Dependent Dielectric Breakdown (TDDB) evaluation, an applied gate voltage stress of +6V for the lifetime of 20 years can be extrapolated at 200°C. 2) By fitting with a power law, applying +1V gate voltage for 20 years leads to a threshold voltage shift of 0.2V. This guarantees a good reliability margin when these devices are used in cascode switching circuit applications. 3) A new mechanism of high junction temperature thermal de-trapping was observed during a high drain bias stress.