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An analytical one-piece drain current model of ZnO-based amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) is presented in this article. This model is able to solve the main obstacle in a lack of analytical and physics-based nonregional device modeling for AOSTFTs and is suitable to be implemented into circuit simulators. Based on the effective-charge-density approach, free carrier density and localized carrier density, including tail and deep states, are unified into total carrier density. Then, one-piece surface potential is derived analytically from equivalent 1-D Poisson’s equation described in an effective-charge-density approach. It is also possible that channel carrier mobility is calculated from the ratio between free carrier and effective total carrier densities. Furthermore, drain current expression is derived analytically and validated by experimental data. Good agreements of ${I}$ – ${V}$ characteristics prove that the proposed model could serve as a useful simulation tool implemented into circuit simulators.