In this study, gallium nitride (GaN)-based metal-oxide-semiconductor diodes with a gallium oxide (GaOx) gate layer formed by ac bias-assisted photoelectrochemical oxidation of n-type GaN in H2O were demonstrated. A typical flatband voltage of 4.35 V was obtained by capacitance-voltage (C-V) measurement. The observed C-V curves shifted toward the negative voltage side when the GaOx/n-GaN heterostructures were annealed in ambient oxygen (O-2). This result could be tentatively attributed to the fact that the negative effective charges existing in the GaOx layer and/or in the n-GaN/GaOx interface could be reduced due to the ambient O-2 annealing process. A negative value of the effective charge density of similar to 2.01 X 10(-7) C/cm(-2), which corresponds to an effective insulator charge number density of around 1.25 X 10(12) cm(-2), was obtained. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3489264] All rights reserved.