The transfer characteristics of amorphous indium zinc oxide thin-film transistors were investigated. The active layer in the bottom gate structure of the transistor was fabricated using room-temperature rf-magnetron sputtering. The device operated as an n-type enhancement mode exhibited a clear pinch-off behavior and an on/off ratio of ∼106. The field-effect mobility of 9.6cm2/Vs and subthreshold slope of 0.3V/decade were obtained. The positive threshold voltage shift was observed under the positive gate bias stress. The field-effect mobility and subthreshold slope remained nearly unchanged within the time of the gate bias stress. The time dependence of the threshold voltage shift was well matched with the stretched-exponential time dependence model.