Tunnel current in organic field-effect transistors

Several recent papers on field-effect transistors made of vapor deposited organic semiconductors reported that charge carrier mobility might become temperature independent at low temperature. To account for this behavior, we develop a model based on the polycrystalline nature of the organic films where charge transport is limited by grain boundaries. The free carrier density in these intergrain regions is controlled by traps, which leads to the formation of back-to-back Schottky barriers at both sides of the grain boundary. A resolution of Poisson's equation shows that in the conducting channel of the transistor, the barrier height and width decreases as the gate bias increases. In addition, the barrier width rapidly becomes negligible as compared to the physical dimension of a grain boundary, so that the barrier can be approximated to a rectangular barrier. At low temperature, tunneling through this barrier becomes predominant, leading to temperature independent mobility. A quantitative check of the model is made by least square fitting the gate voltage dependent current, which gives reasonable determination of the trap density and size of the grain boundaries. (C) 2003 Elsevier Science B.V. All rights reserved.