The hysteresis of the capacitance–voltage (C–V) characteristics of an Al2O3/n-GaN metal–insulator–semiconductor structure was evaluated under light (white LED)-irradiation and dark conditions. The hysteresis was not observed under the dark condition but was observed under the light-irradiation condition. The GaN surface was completely depleted in negative bias under the dark condition. The C–V characteristics indicated that the hysteresis is caused by hole trapping under the LED irradiation condition and that the holes are generated in the n-GaN surface by LED irradiation and subsequently injected into the Al2O3 films. When the holes are generated in the depletion region of GaN for any reason, such as a short generation lifetime, they can be trapped in the Al2O3 films.