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A photoelectrochemical oxidation method was used to directly grow oxide layer on AlGaN surface. The annealed oxide layer exhibited beta-Ga2O3 and alpha-Al2O3 crystalline phases. Using a photoassisted capacitance-voltage method, a low average interface-state density of 5.1 times 1011 cm-2. eV-1 was estimated. The directly grown oxide layer was used as gate insulator for AlGaN/GaN MOS high-electron mobility transistors (MOS-HEMTs). The threshold voltage of MOS-HEMT devices is -5 V. The gate leakage currents are 50 and 2 pA at forward gate bias of VGS = 10 V and reverse gate bias of VGS = -10 V, respectively. The maximum value of gm is 50 mS/mm of VGs biased at -2.09 V.