This work shows that placement of source/drain contacts on top of organic semiconductor film in top-contact organic thin film transistors (OTFT) can offer significant improvement in device speed as compared to bottom-contact devices. 2D numerical simulations and analytical models for unity gain frequency are used to quantify the differences between the top and bottom-contact devices. It is shown that while transconductance is lower in top-contact devices, the gate-drain capacitance is significantly lower as well resulting in improved frequency performance. It is also shown that there is an optimum semiconductor film thickness at which maximum unity gain frequency in top-contact OTFT is obtained. (C) 2009 Elsevier Ltd. All rights reserved.