AlGaN/GaN High Electron Mobility Transistors (HEMTs) are emerging as next-generation semiconductors optimized for high-power and high-frequency applications, with their performance highly dependent on the surface and interface quality of the AlGaN/GaN structure. In particular, the 2-Dimensional Electron Gas (2DEG) formed in the AlGaN layer is susceptible to trapping by surface defects, which degrades electrical characteristics and makes the device vulnerable to degradation. In this study, we propose an approach to enhance device reliability and performance by forming a gallium oxynitride (GaON) interfacial layer through O₂ plasma treatment on the AlGaN surface. This method effectively suppresses interface defects, resulting in improved electrical properties such as reduced interface trap density (Dit), threshold voltage (Vth) shift, increased drain current density (Id), and enhanced transconductance density (gm). Furthermore, this surface treatment demonstrates the potential for process simplification by improving the electrical characteristics of power semiconductor devices without the need for complex deposition steps.