Organic semiconductors exhibiting p-, n-type, or ambipolar majority charge transport are grown on six different bilayer dielectric structures consisting of various spin-coated polymers / HMDS on 300 nm SiO2/p+-Si, and are characterized by AFM, SEM, and WAXRD, followed by field-effect transistor (FET) electrical characterization. It is observed that in case of air-sensitive n-type semiconductors, dielectric surface modifications induce large variations in the corresponding OTFT performance parameters although the film morphologies and microstructures remain similar. In marked contrast, the device performance of air-stable n-type and p-type semiconductors is not significantly affected by the same dielectric surface modifications. This study provides key information on the chemical origin of the charge trapping sites at the FET dielectric-semiconductor interface. In parallel, bottom-contact FETs of n-type oligothiophenes were investigated by a combination of microscopy/electrical measurements and new insights for the poor electron injection efficiency from the Au contacts are presented