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Enhancement-mode AlGaN/GaN high-electron mobility transistors (HEMTs) were realized by applying the O2 and N2O plasma oxidation method to the AlGaN Schottky layers. After the postannealing process, the threshold voltage Vth of conventional depletion-mode GaN HEMTs was -3.1 V, and this value was shifted to +0.3 V when either N2O or O2 plasma oxidation treatment was used. X-ray photoelectron spectroscopy (XPS) analysis revealed that both treatments formed an Al2O3/Ga2O3 compound oxide layer. The decomposition of N-O bonds in the N2O plasma suppressed the formation of nitrogen vacancy traps and ensured low roughness of the AlGaN surface during oxidation. This nitridation process also reduced the 1/f noise.