The authors report the fabrication and characteristics of thin film transistors with ZnO channel layers (ZnO TFTs) having different oxygen contents. Also, the authors define the operation mechanism of ZnO TFTs as the variation of oxygen contents in the ZnO channel layer. The ZnO thin films were deposited on SiO2/n-Si substrate by dc magnetron sputtering at various oxygen partial pressures. Effects of oxygen contents in ZnO thin films on the electrical performance of ZnO TFTs with bottom gate structure were investigated. The ZnO thin films deposited at oxygen partial pressures of 40% exhibit a nonstoichiometric system in an oxygen rich state, resulting in resistivity as high as 10(5) Omega cm. ZnO TFTs with this channel layer exhibited depletion mode, turn on voltage of -15 V, on-off current ratio of similar to 10(6), and field effect mobility of 0.88 cm(2)/V s. This research implied that an attractive application for TFTs involves their use as select transistors in individual pixels of an active-matrix liquid-crystal display. (C) 2008 American Vacuum Society.