The Ni/Au/Al2O3/n-GaN metal-oxide-semiconductor structure with circular transparent electrode has been fabricated by using atomic layer deposition technique. Effects of ultra-violet (UV) light illumination on the capacitance characteristics and deep interface states are analyzed. Physical origin of bias-induced capacitance drop in the accumulation region of some non-ideal devices is explored. Due to the extremely long electron emission time and the extremely slow minority carrier generation rate, a typical deep depletion behavior can be observed in the dark room-temperature capacitance-voltage sweep curve, and the deep-level interface state occupancy above the electron quasi-Fermi level remains unchanged. Under the UV illumination, photo-induced holes will empty the deep interface traps above the electron quasi-Fermi level, and also de-charge the deep donor-like traps in the oxide layer. The anomalous capacitance drop in the accumulation region is attributed to the bias-dependent excessive leakage conductance across the dielectric layer, which might be induced by a charge-to-breakdown process related to electrical traps in the oxide and the inferior interface quality.