Composite HfO 2 /Al 2 O 3 -dielectric In 0.2 Ga 0.8 As/Al 0.24 Ga 0.76 As metal–oxide–semiconductor high-electron-mobility transistors (MOS-HEMTs) by using RF sputtering/ozone water oxidization, respectively, are investigated. In comparison with a conventional Schottky-gate device on the same epitaxial structure, an Al 2 O 3 liner was chemically formed for the present MOS-HEMT to improve interfacial quality and decrease gate leakages. Moreover, a high-k HfO 2 layer was further deposited on the Al 2 O 3 liner to enhance the gate modulation capability. The present MOS-HEMT with the devised HfO 2 /Al 2 O 3 dielectric stack has demonstrated excellent switching characteristics, including superior subthreshold slope (S.S.) of 70mV/dec and high drain–source current (I DS ) on–off ratio of up to 6 orders. Improved direct-current (DC), radio-frequency (RF), and high-temperature device performances of the present design are also comprehensively studied in this work.