ZnO-based thin film transistors (TFTs) have been fabricated on a SiO2/p-Si substrate by rf magnetron sputtering at room temperature and they exhibited a saturation current level of a few muA under a gate bias of 40V, electron mobility of less than 0.05 cm(2)/V s, and on/off current ratio of similar to10(5) in the dark. Illuminated by ultraviolet (UV, lambda=340 nm) light with an optical power density of 1.26 mW/cm(2), our TFT displayed a high photocurrent gain of more than 50 muA at the same gate bias of 40 V and it also showed that the photocurrent decreases with lowering the UV intensity. In a channel depletion state with a gate bias of -40 V, the photo-detecting sensitivity becomes much higher than in the accumulation state with the gate bias of 40 V. (C) 2004 Elsevier B.V. All rights reserved.