Transparent amorphous oxide semiconductors have received much attention for such applications as thin film transistor (TFT) devices in liquid crystal displays (LCD), organic light emitting diodes (OLED), and transparent displays. In 2004, Hosono et al. made a breakthrough in replacing hydrogenated amorphous silicon (a-Si:H) and low-temperature polysilicon (LTPS) devices with amorphous oxide semiconductor in the fabrication of thin film transistors which were widely used in various display panels . More recently, a-Si:H semiconductor has been excluded by most manufacturers because of its poor mobility, degradation under electrical bias stress, and instability under illumination . In particular, LTPS is mostly used in active-matrix organic light emitting diode (AMOLED) displays with field effect mobility value of up to 100 cm2/Vs. However, despite its high mobility characteristics, LTPS usually showed relatively large threshold voltage variation . In contrast, TFTs based on metal oxide channel layer created a whole new area to explore with such advantages as simpler manufacturing process with good characteristics including high on-current and low off-current .