metal-oxide-semiconductor high electron mobility transistors (HEMTs) grown on Si substrates by using ozone water oxidation method are investigated. Superior improvements of 52.2% in two-terminal gate-drain breakdown voltage , 30.3% in drain-source current density at , 43.6% in maximum , 34.7% in maximum extrinsic transconductance , and 52.7%/34.3% in unity-gain cutoff/maximum oscillation frequency are achieved as compared with a reference Schottky-gated HEMT. Thermal stability is studied by conducting temperature-dependent characterizations of devices at ambient temperatures of 300–550 K. Time-dependent electrical reliability analyses for the devices stressed in off-state ( and ) for 0–60 h and on-state ( and ) for 0–20 h are also made to physically investigate the dominant degradation mechanisms. Excellent reliability and thermal stability at 300–550 K are achieved by the present design.