AlGaN/GaN high-electron mobility transistors (HEMTs) and metal–oxide–semiconductor (MOS)-HEMTs using HfO 2 as a gate dielectric have been analyzed at room temperature, after short thermal annealing (STA) and thermal cycle tests, during off-state electrical step stress, high-temperature reverse bias (HTRB), and positive bias temperature instability (PBTI) tests. The results showed that the leakage current in as-fabricated MOS-HEMTs decreased by 10 6 and the on/off ratio increased by over 10 4 than the HEMTs. Moreover, it was even higher after an STA test, up to 10 8 , in the MOS-HEMTs, and the surface trapping effects were mitigated, especially if a KOH cleaning was used before HfO 2 deposition. The MOS-HEMTs also showed higher electrical stability after off-state step electrical stress, HTRB, and PBTI tests.