We present the characteristics of GaN ultraviolet (UV) photodetectors (PDs) with a low-temperature (LT)-AlN cap layer. With the LT-AlN cap layer, it was found that reverse leakage current became smaller by four orders of magnitude. With an incident wavelength of 360 nm, it was found that the responsivity under zero bias for the proposed PD was 0.063 A/W, which corresponds to a quantum efficiency of 21.7%. Furthermore, it was found that the UV to visible rejection ratio of the PD with a LT-AlN cap layer was larger than that of a conventional Schottky barrier PD. (c) 2007 The Electrochemical Society.