This paper focuses on the effects of surface states on the performance of hydrogenated amorphous silicon based thin-film transistors. An analytical model which simultaneously accounts for surface states at the gate insulator/amorphous silicon interface besides the localized states in the gap of amorphous silicon is derived for a coplanar structure. Through illustrative simulations, the current-voltage characteristics are derived. The field effect mobility-to-the band mobility ratio, the

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... f drain current ratio, the threshold voltage and the subthreshold swing are investigated to give a clear indication of these effects. An analytical approach to determine the characteristic temperature of the deep states in the presence of surface states at the insulator/a-Si interface is also proposed. It is demonstrated that surface states considerably affect transistor characteristics and accurate treatment of the device performance cannot neglect the presence of these states.