Pentacene organic thin-film transistors (OTFTs) were fabricated on a polyethersulfone (PES) substrate with an Al2O3 gate dielectric grown at 150 degrees C by plasma enhanced atomic layer deposition. Due to the roughness of the Al2O3 surfaces, the OTFTs with Al2O3 of 150 nm showed better performance compared to that with Al2O3 of 120 nm. When OTFTs with a 150 nm thick Al2O3 layer were fabricated on PES substrate, excellent electrical characteristics were obtained, including carrier mobility as large as 0.62 cm(2)/V s, a subthreshold slope as low as 0.4 V/dec, and on-off current ratio as large as 10(7). (c) 2007 The Electrochemical Society.