The current transport mechanism at metal gate/p-GaN interface in p-GaN HETMs has been investigated. Space Charge Limited Current (SCLC) well describes the behaviour of current density (J G ) at lower applied bias (V G < 6 V), while Thermionic Field Emission (TFE) represents the dominant current mechanism at higher V G . Then, p-GaN gate reliability was investigated by time-to-failure (TTF) analysis carried out at constant positive V G . In particular, the devices’ lifetime as function of the applied V G was described considering the J G -V G dependence according the TFE model. In this way, a maximum V G for 10-year lifetime (V Gmax 10 years ) of 8.5 V has been estimated, significantly higher than that extracted by conventional E-model (7 V).