The variation of electrical conductivity and generation current in the active layer of polycrystalline silicon thin-film transistors (TFTs) was analysed as a function of the thickness of this layer. A simple model for the active layer and numerical solution of Poisson's equation in two dimensions has shown that the electrical conductivity, together with the generation current, does depend on the carrier traps at the grain boundaries and on the electrostatic coupling between the oxide/silicon interface or between the interfaces and the parallel grain boundary when present.