The pentacene-based organic thin-film transistor (OTFT) with inserted lithium fluoride (LiF) thin layer (I nm) between pentacene layers was fabricated in ultrahigh vacuum (URV) condition with all in situ processes and improved electrical properties were achieved. The fabricated device structure is gold (30 nm)/pentacene/LiF (I nm)/pentacene/siO(2) (100 nm)/p+-si (substrate): the ratio of the upper pentacene layer thickness and the lower pentacene layer thickness was varied, keeping the sums of two pentacene layers of thickness with 100 nm. With varying the ratio of two pentacene layers of thickness, the field effect mobility and threshold voltage were affected. The field effect mobility increased when LiF layer was near the gold layer and decreased when LiF layer was near the SiO2 layer. In addition, the threshold voltage decreased as LiF layer come close to the gold layer. By inserting a thin LiF layer, the field effect mobility and threshold voltage increased, respectively, from 0.32 to 0.45 cm(2)/Vs and from -21.89 to - 4.16 V. (c) 2005 Elsevier B.V. All rights reserved.