This work reports on the fabrication and characterization of the organic thin film transistors OTFTs. Pentacene transistors are fabricated in top and bottom S/D contact configurations on preoxidized Si wavers. Three different metals – Pt, Au and Al – are used for the contacts. Similar performances are reported for Au and Pt bottom contact devices. Dielectric surface engineering leads to a subthreshold swing of 600mV/dec and a 0.9V threshold voltage. Top contact devices show an improvement in the mobility of two decades compared to the bottom contact devices. The mobility extracted on Au and Pt top contact devices is 0.51cm2/(Vs) and 0.12cm2/(Vs) respectively. It is shown that for top contact devices, the ION current depends strongly on the pentacene thickness. A three decades increase in IOFF current is reported for Au top contact devices.