Al2O3 thin films were deposited by a metal organic chemical vapour deposition on InAlN/GaN heterostructures using Ar or NH3 as a carrier gas. Effects of NH3 and Ar carrier gases on the electrical and structural properties of Al2O3/InAlN/GaN HEMT devices were investigated by current voltage, current collapse, and Auger electron spectroscopy measurements. Al2O3 deposited using Ar as a carrier gas leads to a substantial gate leakage current reduction with no increase of the current collapse compare to Schottky gate based HEMT. On the other hand, HEMT electrical performance shows degradation if NH3 carrier gas is used. Similarly, Auger electron spectroscopy revealed presence of carbon on InAlN surface when NH3 carrier gas was applied. It is suggested that the formation of carbon-related traps on the InAlN surface takes place in the early stages of the Al2O3 deposition with NH3 carrier gas that strongly influences the HEMT performance.