A unified drain current model of complementary (p- and n-type) organic thin film transistors (OTFTs) is presented. The model is physically based and takes into account the detailed properties of the organic semiconductor through the density of states (DOS). The drain current depends on the geometrical and physical parameters of the transistor, on the applied gate, drain and source voltages, and on the surface potential at the source and drain contacts. An analytical expression of the surface potential is derived. The proposed model is validated with the numerical calculations and the measurements of both p- and n-type OTFTs fabricated in a printed complementary technology. The provided analyses show that the model is continuous, accurate, and includes the main physical effects taking place in complementary organic transistors. Thanks to its analytical and symmetric formulation, it is suitable for the design of …