An organic thin film transistor (OTFT) with a back gate structure on a patterned pentacene active region was fabricated. The variations of electrical properties as a function of polyvinylcinnamate (PVCN) concentration used as a gate dielectric were evaluated. In addition, the morphology of the pentacene thin film was characterized by scanning probe microscope by simultaneously obtaining the topology and current image. Within the pentacene thin film, the current was observed to flow through grain rather than through the grain boundaries. Within marginal variations, an OTFT fabricated using 8% PVCN revealed the best electrical properties in terms of mobility, threshold voltage (V-T), subthreshold swing, and on/off ratio. (C) 2008 American Vacuum Society.