A systematic comparison of the field-effect mobilities in polymer thin-film transistors (TFTs) and single-crystal organic field effect transistors (OFET) as a function of tunable gate insulator dielectric constant and gate-induced charge density, was studied. The channel space between the semiconductor and the gate with liquids of varying dielectric constants was filled. Single-crystal transistors were fabricated by placing the long axis of a single crystal across the PDMS gap. The metal-coated features on the PDMS stamp were used as source drain, and gate electrodes, while the gap served as a gate dielectric layer. Electrical characterization of the transistors was performed using a Lakeshore TTP4 probe station with Keithley 237 and 6517A electrometers. A wetting front propagated through the whole channel was clearly observed, which indicates the filling of the gap. The charge transport in single crystal transistors exhibit a completely different behavior, with the most striking phenomenon being the strong decrease in mobility with increased polarizability of the gate dielectric.