Based on density-functional theory calculations, the effects of cation compositions on electronic structures of In-Sn-Zn-O amorphous semiconductors were investigated. We considered various composition ratios of In, Sn, and Zn in O stoichiometric condition, and found that the conduction band minimum (CBM) energy level decreases and the valence band tail (VBT) energy level extent increases as the sum of In and Sn ratios (R-In + R-Sn) increases. The CBM lowering is attributed to the increased overlap of the In-5s and Sn-5s orbitals as the R-In + R-Sn increases, and correspondingly the electron effective masses (m(e)*) are found to be reduced. The VBT increase is found to be due to the increased density of the In and Sn atoms, near which the O-2p inter-site pp sigma* coupling is larger than that near the Zn atoms. The acute O-(In,Sn)-O angles are suggested to be structurally important, giving the stronger O-O pp sigma* coupling. (C) 2013 AIP Publishing LLC.