Al2O3 formed by oxidation of Al at high pressure (3 atmosphere) is investigated as a gate dielectric for AlInN/GaN MIS-HEMTs.The oxidation has been carried out at low temperature (~200 °C) resulting in improved reliability.The time and temperature of the low temperature and high pressure oxidation (LT-HPO) 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 (IG) 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 (ID,Max), subthreshold slope and off-state breakdown voltage. Reliability studies under constant voltage stress conditions showed that the threshold voltage variation was within acceptable limits. Interface trap charges were estimated as 1.5×1013 cm-2 eV-1 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. The fT and fMax of MIS-HEMTs were higher than those of HEMTs. HEMTs and MIS-HEMTs with LT-HPO Al2O3 showed repeatable results over different fabrication runs. After evaluating the performance of LT-HPO Al2O3 MIS-HEMTs, the optimized process was extended to oxidize Ti and Ni. TiO2 and NiO were investigated as gate dielectrics for AlInN/GaN MIS-HEMTs. Dielectric formation was confirmed by XPS studies. Increase in gate voltage swing, gm and ID,Max was observed for both TiO2 and NiO MIS-HEMTs compared to HEMTs. LT-HPO NiO MIS-HEMTs showed three orders reduction in IG in reverse bias and four orders reduction in forward bias compared to HEMTs. TiO2 MIS-HEMTs also showed four orders reduction in IG in the reverse bias and three orders reduction in the forward bias. Dit is extracted for both NiO (2.3×1013 cm-2 eV-1) and TiO2 (2×1013 cm-2 eV-1) MIS-HEMTs using capacitance dispersion technique. Our work shows that high pressure oxidation of metals to obtain gate dielectrics in MIS-HEMTs is a viable technology.