In this paper, based on the surface potential model, taking into account both the deep and tail state distributions simultaneously, and using the simplified Fermi-Dirac function, a unified local-state model is obtained. Using the effective characteristic temperature, the unified current-voltage (I-V) model for a-Si:H thin-film transistor a-Si:H TFT is developed. This model can describes all operating regions including subthreshold region, linear area and saturated zone through a single equation. By comparison with the experimental data, it is shown that this model can accurately describe the current voltage characteristic of the a-Si:H TFT.