This paper presents a detailed analysis of the electroluminescence (EL) and short-term degradation processes related to hot electrons that occur in AlGaN/GaN-based high-electron mobility transistors (HEMTs) submitted to on-state stress. Based on optical and electrical characterization, we demonstrate that: 1) when biased in on-state, HEMTs emit a luminescence signal, which is uniformly distributed along gate width, and related to intraband transitions of hot electrons; the intensity of the luminescence has bell-shaped dependence on gate voltage; 2) when submitted to on-state stress (with VD = 30V and several VG levels), HEMTs show a significant degradation, mostly consisting in an increase in on-resistance and in a decrease in drain current; and 3) stress tests carried out at several VG levels (with V D = 30V) indicate that the degradation rate does not increase monotonically with VG, as would be expected if temperature and/or power dissipation were the main driving forces for degradation. On the contrary, degradation rate was found to have bell-shaped dependence on VG, similarly to what was found for the intensity of the EL signal. The observed degradation process is ascribed to trapping of negative charge in the gate-drain access region, activated by hot electrons. The degradation mechanism cannot be recovered at room temperature but only through exposure to UV light.