This thesis focuses on fabricating and studying (p-channel, n-channel and bilayer) field effect transistor devices under dark, steady state and transient illumination conditions. This thesis probes injection barrier at metal-semiconducting polymer interface that determine the overall charge injection property of the device. FET"s consisting on n-channel acceptor with a coating of optically active donor polymers is studied. Presence of D-A interface in FET showing n-channel transport is used to study the process of charge separation and charge transport occurring in bulk of the acceptor upon photoexciting the donor polymer.