Transport properties of a field effect transistor based on pentacene have been investigated by experimental and numerical analysis. Moreover, thin film transistors based on porphyrin have been realized and characterized. In order to derive from basic principles the transport properties of the organic semiconductors, we used a Monte Carlo (MC) simulator to calculate the field-dependent mobility. The overall device simulation is performed by using two-dimensional drift-diffusion simulations taking into account field-dependent mobility obtained from MC simulations, interface/bulk trap states and fixed charge density at the organic/oxide interface. We demonstrate how carrier mobility can be extracted from experimental device characteristics, without using the conventional MOSFET theory that could lead to the wrong result. Finally we demonstrate the realization of simple organic logic circuitry, namely an organic inverter with a different configuration of load transistor.