We report on ZnO-based thin-film transistors (TFTs) fabricated using SiO2/P-Si substrates on which their ZnO channel layers have been deposited by rf sputtering at various temperatures: room temperature (RT), 100, and 200 degreesC. When they went through rapid thermal annealing in forming gas ambient (H-2:N-2 = 1: 10) for n-type doping, the highest field effect mobility of similar to1.93 cm(2)/V S was achieved from ZnO-TFTs prepared using the deposition temperature of 200 degreesC while a low mobility (similar to0.2 cm(2)/V s) was from TFTs using RT-deposited ZnO. However, the TFT sample using 200degreesC deposited ZnO, unlike the others, revealed a very large amount of off-state current resulting in the on/off current ratio of only similar to10(2). It is because the ZnO layer deposited at 200 degreesC is too conductive to act as an optimum TFT channel. It is concluded that optimal ZnO-based TFTs are obtained using the medium deposition temperature of 100 degreesC. (C) 2004 American Vacuum Society.