Constant voltage stress data have been used to extrapolate the lifetime for GaN HEMTs with p-type gate. The model, based on the relation between TTF and the charge-injected-to-breakdown (C2BD) confirms that: 1)E-model is not reliable for TTF modeling 2)impact ionization has a role in failure mechanism, as temperature-dependent TTF measurements show. 3)Failure is driven by the presence of hot-carriers in the p-GaN layer, with consequent degradation of the AlGaN barrier layer, as failure analysis exhibits.