We reported on a two-dimensional simulation of electrical properties of the radio frequency (rf) sputter amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). The a-IGZO TFT used in this work has the following performance: field-effect mobility (μeff) of ∼12 cm2/V s, threshold voltage (Vth) of ∼1.15 V, subthreshold swing (S) of ∼0.13 V/dec, and on/off ratio over 1010. To accurately simulate the measured transistor electrical properties, the density-of-states model is developed. The donorlike states are also proposed to be associated with the oxygen vacancy in a-IGZO. The experimental and calculated results show that the rf sputter a-IGZO TFT has a very sharp conduction band-tail slope distribution (Ea=13 meV) and Ti ohmic-like source/drain contacts with a specific contact resistance lower than 2.7×10-3 Ω cm2.