Insertion of a thin layer of the Lewis acid molybdenum trioxide (MoO3) improves the apparent mobility of charge carriers in organic field-effect transistors (OFETs). In order to identify the dominant mechanism responsible for this, we characterized devices having a bottom pentacene (70 - z nm)/MoO3 (1 nm)/top pentacene (z nm) structure and those having pentacene (69 nm)/MoO3 (1 nm) only under the gold source and drain electrodes. The former devices exhibited large drain currents (I-DS)and mobilities, regardless of the z value. Moreover, the latter devices exhibited comparable I-DS and mobilities to those of conventional pentacene OFETs, suggesting that the formation of CT complexes just above the conduction channel is the critical mechanism. Carriers generated by dissociation of CT complexes at the pentacene/MoO3 interface contribute to the formation of an effective channel and the apparent mobility. (C) 2012 The Japan Society of Applied Physics