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 gate electrode, a silicon dioxide gate insulator, and a pentacene or a conjugated 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 examine the effects associated with differences in charge carrier injection for different source and drain contact materials. It is also shown that, if the organic material immediately adjacent to the contacts has poor conduction properties, ‘parasitic’ source and drain series resistances that depend on the contact/organic injection barrier height as well as the channel material mobility can result.