The objective of this paper is to analyze the impact of the high temperature on the on-state and the reverse leakage currents (gate, drain and bulk) of AlGaN/GaN HEMTs grown on Si(111) in the temperature range of 25–310°C. After intensive testing, it has been observed that the (drain and gate–to-source) leakage currents exhibit a weak increase with the temperature up to 150°C. The leakage through the silicon substrate has a primordial role, as the drain reverse leakage appears to be due to electron injection from the Si substrate/AlN/GaN buffer layers into the GaN buffer. At higher temperatures, the activation energy (E a ), extracted from Arrhenius plots, is much higher being E a =0.20eV and E a =0.40eV for drain and gate leakage currents, respectively. We suggest that this relevant thermal activation is related to temperature-dependent thermionic currents over different barriers.