The spectral responses of back-illuminated p-i-n AlGaN ultraviolet photodetector under different bias voltage are investigated. The device structure is composed of p-GaN/ p-Al(0.3)Ga(0.7)N/i-Al(0.3) Ga(0.7)N/n-Al(0.6)Ga(0.4)N and Ohmic contacts, where the p-GaN layer is very thin. There are two peaks in the spectral response of photocurrent located at 350 and 290 nm, respectively. It is found that in some devices the relative intensity and phase of these two peaks may change strongly with applied forward bias. A detailed analysis suggests that the possible Schottky-type-like behavior of metal and p-GaN contact and an unsatisfactory doping of p-GaN and p-AlGaN are responsible for the abnormal phenomenon. (C) 2011 American Institute of Physics.