AN ANALYTICAL MODEL FOR ORGANIC-BASED THIN-FILM TRANSISTORS

A model describing the current-voltage characteristics of organic thin-film transistors (TFTs) is presented. The model is based on the trap distribution deduced from temperature-dependent current-voltage measurements on Au/alpha-sexithienyl (alpha-6T)/Au symmetrical structures, which comprises a dominant single shallow trap level located near the valence-band edge. Numerical and approximate analytical derivations of the saturation current density as a function of the gate voltage have been made. From these calculations, the dependence of the threshold voltage on the parameters of the trap level (density and energy) is deduced. It appears that the threshold voltage corresponds to the filling of traps, and is a surface equivalent of the trap-filled limit voltage in bulk space-charge-limited current. The model is in good agreement with experimental data on alpha-6T TFTs. The energy of the trap level compares well with that obtained from the temperature-dependent conductivity. However, the mobility is much lower in the TFT than in a bulk structure. This is tentatively explained by the strong influence of the state of the insulator-semiconductor interface on the characteristics of a TFT.