Abstract: A simulation model of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) was established, and the influences of two parameters including channel band tail state density and channel layer thickness on the characteristics of the a-IGZO TFTs were simulated and analyzed. The results show that when the density of tail states in the active channel increases to 1.5×1019 cm^-3·eV^-1, the device performances deteriorate with increased subthreshold swing. The results also show that with channel layer thickness below 40 nm, the TFTs show good transfer characteristics, whereas the TFTs with channel layer thickness over 40 nm show deteriorated performances with increased off-state current, decreased on/off current ratio, and increased subthreshold swing. The numerical simulation analysis method of this work can provide theoretical basis and experimental guidance for design and fabricate of a-IGZO TFTs.