Low temperature (LT) and high pressure oxidized (HPO) Al2O3 is investigated as a gate dielectric for AlInN/GaN MIS-HEMTs. The time and temperature of the oxidation process was optimized for best performance. X-ray photoelectron spectroscopic (XPS) studies confirmed the near complete oxidation of Al to form Al2O3. MIS-HEMTs with 7 nm thick LT-HPO Al2O3 showed six orders reduction in gate leakage current and five orders improvement in ID,ON/ID,OFF ratio compared to the reference HEMT. Also, these MIS-HEMTs proved to be significantly better than HEMTs in terms of maximum drain current, subthreshold slope and off-state breakdown voltage. Reliability studies under constant voltage stress conditions show that the threshold voltage variation is within acceptable limits. Interface trap charges were estimated with the help of dynamic capacitance dispersion technique. The improved current collapse in MIS-HEMTs over HEMTs indicates the good quality of the interface between the dielectric and barrier layer. © 2001-2011 IEEE.