Properties and doping limits of amorphous oxide semiconductors

The paper reviews our understanding of the conduction, localisation, instability and doping properties of amorphous oxide semiconductors. Amorphous oxide semiconductors (AOSs) are characterized by a larger Bohr radius than a-Si:H, so that the density of states at the mobility edge is much lower than in a-Si:H. Combined with a sharper conduction band tail, this leads to a much small total density of tail states in AOSs, leading to a sharper turn-on characteristic for their thin film transistors. The band gaps of most semiconducting oxides are wide, which is valuable for transparent electronics, but means that they are unlikely to be bipolar dopable, reducing their prospects for solar cells.