In this work, a technology computer-aided design (TCAD) device model based on an fabricated amorphous InGaZnO thin-film transistor (a-IGZO TFT) was established to study the correlation between carrier concentration distribution, current–voltage (I–V) and capacitance–voltage (C–V) characteristics of a-IGZO TFTs. The equilibrium carrier concentration of the a-IGZO layer influenced by defect states, interface fixed charges, gate and S/D electrodes were studied systematically and quantitatively. The a-IGZO thickness was varied from 10 nm to 140 nm. The doping concentration in the bulk a-IGZO layer and source/drain (S/D) contact regions were changed from to . The physical mechanisms underlying the I–V and C–V variation caused by above-mentioned parameters were explored by analyzing the energy band diagram and carrier concentration distribution in the a-IGZO layer. The total number of electrons in the channel region of the TFT was calculated by numeric integration for further investigation of its correlation with the a-IGZO thickness, I–V, and C–V characteristics of the a-IGZO TFTs.