Heavy ion-induced electrical degradations and mechanisms of high-electron-mobility transistors (HEMTs) were investigated in this article. AlGaN/GaN HEMTs were irradiated by 800-MeV Bi ions with total fluence of 5.23×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The results show that the saturation current decreased by 13.1%, the threshold voltage negatively shifted by 0.21 V, the peak transconductance decreased by 26.6%, the reverse gate-leakage current increased much more than 16 times, and forward gate-leakage current increased more than three times. Furthermore, all electrical performances experienced partial recovery after room temperature annealing (RTA) and 175 °C thermal annealing except gate-leakage current. To further explore the mechanism of gate-leakage degradation, AlGaN/GaN heterojunction wafers were exposed to 1400-MeV Ta ions with fluence of 6.86×107 ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and characterized by conductive atomic force microscope (CAFM) before and after the irradiation. The results show that the leakage current significantly increased in AlGaN material areas after the irradiation. Comparing the increase in gate leakage of the AlGaN/GaN heterojunction and AlGaN/GaN HEMT induced by heavy ions irradiation, it can be seen that the gate-leakage current of AlGaN/GaN HEMT after heavy irradiation almost entirely was caused by latent tracks. The mechanism for the irreversible increase in gate leakage after heavy ions irradiation could be attributed to the latent tracks along the ion trajectories through the heterojunction.