We have analyzed the on-state gate current of an AlGaN/GaN high-electron-mobility transistor (HEMT) by applying electrical and thermal stresses. The gate current shows a positive temperature dependence and its activation energy is less than the gate Schottky barrier height. This indicates that the gate current is a thermionic field emission and flows through a thinned AlGaN Schottky depletion layer by a tunneling or hopping mechanism. The depletion layer is considered to be thinned by the crystal defects created in the gate fabrication process. On the other hand, we have found that the gate current is decreased by applying a high electrical power under a high thermal stress. Strong electroluminescence can be observed by applying stress. These experimental results imply that the crystal defects are annealed out by the recombination-enhanced defect reaction mechanism induced by generated hot carriers. Therefore, the gate current is decreased by the reductions in the tunneling and hopping probability due to the expansion of the depletion layer.