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Amorphous oxide semiconductors (AOSs) are expected as new channel materials in thin-film transistors (TFTs) for large-area and/or flexible flat-panel displays and other giant-microelectronics devices. So far, many prototype displays have been demonstrated in these four years since the first report of AOS TFT. The most prominent feature of AOS TFTs is that they operate with good performances even if they are fabricated at low temperatures without a defect passivation treatment. The TFT mobilities exceed 10 cm2/(Vmiddots), which are more than ten times larger than those of conventional amorphous semiconductor devices. In addition, they operate at low voltages, e.g., < 5 V owing to their small subthreshold voltage swings. These features indicate that electron transport in oxide semiconductors are insensitive to random structures and these oxides do not form high-density defects that affect electron transport and TFT operation. In this paper, we discuss the origins of the prominent features of AOS devices from the viewpoint of materials science of AOS.