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An important parameter which characterizes thin-film transistors (TFTs) is the threshold voltage. Various methods have been proposed to extract the threshold voltage in amorphous InGaZnO (a-IGZO) TFTs, but few models have been presented based on material characteristics and the carrier transport. With regard to a-IGZO films, under low carrier concentrations, current conduction would be dominated by trap-limited conduction, while it switches to the percolation conduction mechanism as the carrier concentration increased and the degenerate conduction is achieved at higher carrier concentrations. In this case, the threshold voltage can be defined as the gate voltage when the degenerate conduction comes into existence, and then a physics-based method of threshold voltage extraction for a-IGZO TFTs is developed. The accuracy of the proposed model was proved by comparison with the measured data. As the new definition makes it possible to combine the threshold voltage with the material property, it is expected to play a significant role in the device modeling for the simulation of circuits based on a-IGZO TFTs.