Transparent amorphous oxide semiconductors (AOSs) have unique electron transport properties, such as high electron mobility (10-50 cm²/Vs) and absence of Hall voltage sign anomaly, which are not seen in conventional amorphous semiconductors. These materials are being used as channel layers in thin-film transistors (TFTs) due to their processing advantages, such as low-temperature deposition. The article reviews recent advances in AOS materials and their TFT applications, emphasizing the role of high ionicity and large spherical spread of unoccupied metal s orbitals in p-block metal oxides in enabling advantageous electronic structures for n-channel TFTs. AOSs are compared with conventional hydrogenated amorphous silicon (a-Si:H), which is widely used in current liquid-crystal displays. AOSs have superior characteristics to conventional semiconductors and have rapidly developed towards TFT backplanes for next-generation flat-panel displays. Recent prototype devices based on a-IGZO TFTs include flexible black-and-white e-paper, flexible AM-OLED, and front-drive color e-paper. AOS TFTs have been developed for various applications, including flexible electronics, system-on-glass, and system-on-panel technologies. AOS TFTs have several advantages, including low processing temperature, wide processing temperature window, large electron mobilities, low operation voltage, and simple electrode structure. These features are related to the electronic structure of a-IGZO, a typical AOS. The article also discusses the electronic structure of AOSs, the origin of large electron mobility, and the peculiar annealing effect. AOSs have a unique electronic structure that allows for high electron mobility even in amorphous structures. The article concludes that AOSs are expected to be the most promising technology for next-generation flat-panel displays due to their excellent performance and stability.Transparent amorphous oxide semiconductors (AOSs) have unique electron transport properties, such as high electron mobility (10-50 cm²/Vs) and absence of Hall voltage sign anomaly, which are not seen in conventional amorphous semiconductors. These materials are being used as channel layers in thin-film transistors (TFTs) due to their processing advantages, such as low-temperature deposition. The article reviews recent advances in AOS materials and their TFT applications, emphasizing the role of high ionicity and large spherical spread of unoccupied metal s orbitals in p-block metal oxides in enabling advantageous electronic structures for n-channel TFTs. AOSs are compared with conventional hydrogenated amorphous silicon (a-Si:H), which is widely used in current liquid-crystal displays. AOSs have superior characteristics to conventional semiconductors and have rapidly developed towards TFT backplanes for next-generation flat-panel displays. Recent prototype devices based on a-IGZO TFTs include flexible black-and-white e-paper, flexible AM-OLED, and front-drive color e-paper. AOS TFTs have been developed for various applications, including flexible electronics, system-on-glass, and system-on-panel technologies. AOS TFTs have several advantages, including low processing temperature, wide processing temperature window, large electron mobilities, low operation voltage, and simple electrode structure. These features are related to the electronic structure of a-IGZO, a typical AOS. The article also discusses the electronic structure of AOSs, the origin of large electron mobility, and the peculiar annealing effect. AOSs have a unique electronic structure that allows for high electron mobility even in amorphous structures. The article concludes that AOSs are expected to be the most promising technology for next-generation flat-panel displays due to their excellent performance and stability.