We studied transfer characteristics of pentacene thin film transistors, fabricated by using polymethylmetacrylate (PMMA) buffer layer, in order to evaluate the parasitic series resistance in devices with different active layer thickness (10 - 80 nm) and contact architectures (top and bottom contacts). For bottom contact TFTs, the highest series resistance (1.7 x 10(4) Omega cm) was found for the thinnest pentacene films, probably related to step coverage problems of the thin pentacene film over the gold contacts. In contrast, for the top contact TFTs, the 10 nm pentacene films had the lowest resistance (similar to 1.8 x 10(3) Omega cm) and the resistance increases to similar to 8 x 10(4) Omega cm for the thicker films. The results can be related to the effect of the series resistance induced by the vertical transport through the pentacene film. (c) 2007 Elsevier Ltd. All rights reserved.