A compact surface potential based model for organic thin-film transistors (OTFTs), including both tail and deep trap states across the band gap, is reported. The model has been developed on the basis of a complete surface potential approach for undoped-body OTFTs. Accurate surface potentials are calculated by explicitly including the floating backside potential that varies with applied biases. A pseudo-2D resistor model is developed to capture the structural features of the OTFT. The resistor model considers, in particular, the effects originating from a bias dependent 2D current flow in the channel region and results in accurate reproduction of the electrical characteristics. The fitting capability of the developed OTFT model is verified against measured high-performance dinaphtho thieno thiophene (DNTT) based field-effect transistor data. Accurate reproduction of the current characteristics of the OTFT test structures is verified from a week to a strong inversion regime.