Display omitted •We provide give an overview on the most common degradation processes of GaN-based HEMTs.•We describe the time-dependence of gate degradation mechanism during off-state tests.•The influence of the reverse voltage as degradation accelerating factor is also provided.•We present a combined electrical, thermal and EL analysis of hot electrons degradation.•We define a methodology for the evaluation hot electrons effects. This paper describes a deep investigation of the degradation mechanisms induced by off-state and on-state stress in AlGaN/GaN HEMTs. Concerning reverse-bias degradation, results underline that the exposure to reverse-bias stress can induce (i) a recoverable change in the gate current due to the accumulation of negative charges under the gate, (ii) and a permanent degradation of gate characteristics due to the generation of vertical parasitic leakage paths through the AlGaN layer. Further analysis of the kinetics of this degradation mechanism, correlated with time-resolved Electroluminescence (EL) measurements, allowed to define a model that explains the time-dependence of the phenomenon and the role of gate voltage as accelerating factor, providing an interpretation for both recoverable and permanent modifications of the main device characteristics induced by reverse-bias stress. On the other hand, on devices that have shown an improved robustness against the reverse-bias gate degradation, we investigated the origin of the degradation under on-state stress. In this case, results obtained with a combined electrical and optical analysis, showed that on-state stress may induce a significant decrease in drain saturation current and Electroluminescence (EL) signal, with a degradation rate that strongly depends on the EL intensity measured before stress, which is representative of the presence of hot-electrons in the channel. On-state degradation can be ascribed to a decrease in the electric-field, due to the trapping of electrons within the barrier or at the surface induced by hot-electrons. Therefore, by using the EL signal as measure of the stress accelerating factor, it was possible to derive an acceleration law for hot-electron degradation on GaN HEMTs.