Material characteristics and applications of transparent amorphous oxide semiconductors

Transparent amorphous oxide semiconductors have unique electron transport properties, such as large electron mobility (10-5.0cm(2)/Vs) and the absence of a Hall voltage sign anomaly, that are not seen in conventional amorphous semiconductors. This class of materials has been attracting much attention as a channel layer in thin-film transistors (TFTs) utilizing the above features along with the processing advantage that thin films can be deposited at low temperatures by conventional sputtering methods. The primary driving force for this trend is a rapidly emerging demand for backplane TFTs that can drive the next generation of flat-panel displays. This article reviews the recent advances in fundamental science of these materials and their TFT applications. Emphasis is placed on the view that high ionicity in chemical bonding and large spherical spread of unoccupied metal s orbitals in p-block metal oxides lead to the realization of electronic structures that are advantageous for n-channel TFT applications. Amorphous oxide semiconductors are compared with conventional hydrogenated amorphous silicon, which is used widely as the channel material for backplane TFTs in current liquid-crystal displays.