We demonstrate that the current-voltage (I-V) characteristics of In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) can be modeled by using the well-known physical model based on the exponential density of deep and tail states. The threshold voltage and the voltage-dependent field-effect mobility can be determined without ambiguity by using the newly proposed scheme. Both the transfer and the output curves of the device are well reproduced by sing the proposed modeling scheme with the obtained parameters.