Pentacene field-effect transistor (FET) is analyzed as an injection-type element, assuming that carrier accumulation at the pentacene-gate insulator is due to the Maxwell-Wagner effect. The FET characteristics are derived based on a model in which carriers injected from electrodes are accumulated at the interface, and they are then conveyed along the channel by the force of electric field formed between source and drain electrodes. Optical second harmonic generation from the channel is dependent on the off- and on-states of the FET channel, and suggesting that carriers injected from source electrode make a significant contribution to the space charge field formation. (c) 2006 Elsevier B.V. All rights reserved.