A simple method to extract the density of interface trap states (D-it) from polycrystalline-silicon thin-film transistors with a doped active layer is proposed. In this method, an equation describing the interface trap charge density (Q(it)) as functions of the front- and back-side surface potentials is derived, taking into account the dopant concentration in the channel region. Using both the front-side surface potential (phi(s)) determined by the low frequency capacitance method, and the back-side surface potential (phi(b)) obtained by using a compact modeling technique where phi(b) is analytically calculated as a function of phi(s) instead of solving the Poisson equation numerically, D-it is calculated by taking the derivative of Q(it) with respect to phi(s) without assuming any specific distribution of interface trap states in the bandgap. The validity of the extraction method is confirmed using a two-dimensional device simulator. In addition, the results of applying this method to a real device are presented.