Organic Thin-Film Transistors (OTFTs) in top-contact configuration and Metal-Insulator-Semiconductor (MIS) structures with different organic dielectrics as the gate insulator have been fabricated using the same organic semiconductor layer, pentacene, in order to investigate the changes in the electrical behavior by varying the interface properties. A gold bottom gate electrode was sputtered on a glass substrate whereas gold source and drain were thermally evaporated onto the pentacene layer. Several organic dielectrics have been tested as insulating layers, namely were poly(vinyl alcohol) (PVA), polyvinyl alcohol with ammonium dichromate (PVAad) as a cross-linking agent, poly (4-vinyl phenol) (PVP), poly(dimethylsiloxane) (PDMS) and poly(methylsilsesquioxane) (pMSSQ). The interesting differences found out by varying the interface confirm that the chemical-physical interaction between semiconductor and dielectric is crucial for the conduction mechanisms of the charge carriers. In particular we observed that the electron traps can influence not only the electron transport, therefore allowing a more or less marked ambipolar behavior, but also affect the hole transport.