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High-current level ZnO thin-film transistors (TFTs) on transparent substrates are demonstrated using low-temperature RF sputtering. Oxygen passivation induced fill of oxygen vacancies within the sputtered n-type ZnO thin films on glass substrates is investigated to manipulate the performance of top-gate ZnO TFTs. The surface oxygen passivation effectively enlarges grain size of the ZnO on glass substrates from 7 nm to 20 nm and increases the oxygen composition ratio from 30% to 35%, which essentially yields a TFT with its significantly increase of drain-source current and Ion/Ioff ratio, as compared with a typical ZnO based TFT. The optimum duration of oxygen passivation in this study yields a device with a drain-source current level 0.87 mA under a bias condition VGS = 5 V and VDS = 15 V, Ion/Ioff ratio 1.4 times 106. We further demonstrate high-performance top-gate ZnO TFTs by applying similar low-temperature process on a flexible polymer substrate. The device shows an IDS 26 muA under a bias condition VGS = 15 V and VDS = 25 V with gate size W/L = 600 mum/300 mum. The average optical transmission of the entire flexible TFT structure in the visible spectrum range is about 82% while the transmission at 550 nm is 88%.