Direct current(DC) and pulsed measurements are performed to determine the degradation mechanisms of Al-GaN /GaN high electron mobility transistors(HEMTs) under high temperature.The degradation of the DC characteristics is mainly attributed to the reduction in the density and the mobility of the two-dimensional electron gas(2DEG). The pulsed measurements indicate that the trap assisted tunneling is the dominant gate leakage mechanism in the temperature range of interest.The traps in the barrier layer become active as the temperature increases,which is conducive to the electron tunneling between the gate and the channel.The enhancement of the tunneling results in the weakening of the current collapse effects,as the electrons trapped by the barrier traps can escape more easily at the higher temperature. Direct current (DC) and pulsed measurements are performed to determine the degradation mechanisms of Al-GaN / GaN high electron mobility transistors (HEMTs) under high temperature. The degradation of the DC characteristics is primarily attributed to the reduction in the density and the mobility of the two-dimensional electron gas (2DEG). The pulsed measurements indicate that the trap assisted tunneling is the dominant gate leakage mechanism in the temperature range of interest. The traps in the barrier layer become active as the temperature increases, which is conducive to the electron tunneling between the gate and the channel. The enhancement of the tunneling results in the weakening of the current collapse effects, as the electron trapped by the barrier traps can escape more easily at the higher temperature.