The undoped GaN (u-GaN) and two-dimensional electron gas (2DEG) metal-semiconductor-metal (MSM) photodetectors with semi-transparent Ni/Au Schottky barrier contact electrodes were fabricated. It was found that we could achieve a larger Ni/Au transmittance, higher Schottky barrier heights and larger photocurrent to dark current contrast ratios by photo-chemical annealing of these photodetectors in O-2. It was also found that the maximum quantum efficiencies were 13% and 57% for the photo-chemical annealing u-GaN and 2DEG photodetectors, respectively. Furthermore, it was found that we could achieve a larger responsivity, a lower noise level and a larger detectivity by using the 2DEG structure. (C) 2004 Published by Elsevier Ltd.