Investigation of bottom-contact organic field effect transistors by two-dimensional device modeling

We present device simulations for p-channel organic field effect transistors. The current conservation equation and Poisson's equation are solved self consistently in two dimensions in the drift-diffusion approximation. We focus on modeling transistor structures consisting of a p(+) Si gate electrode, a silicon dioxide gate insulator, and a polymer layer as the active (channel) material. The source and drain contacts are taken to be deposited directly on the gate insulator (bottom contact structure). We focus particularly on calculations of the surface potential for which experimental data have recently been published [L. Burgi, H. Sirringhaus, and R. H. Friend, Appl. Phys. Lett. 80, 2913 (2002)]. We find that the surface potential on the polymer layer closely matches the channel potential as argued by Burgi Furthermore, we show that the experimentally observed drops in the surface potential above the source and drain contacts can be explained as a consequence of polymer material defects near the contacts and, significantly, that these results cannot be explained solely by large contact Schottky energy barriers. We also present results for the transistor output characteristics and examine the effects associated with changes in charge carrier injection for different source and drain contact materials and different models for the injection process. These results elucidate the device physics of ideal organic field effect transistors and model the behavior of current experimental devices including the most likely parasitic effects. (C) 2003 American Institute of Physics.