1566-1199/$ see front matter 2010 Elsevier B.V doi:10.1016/j.orgel.2010.07.026 * Corresponding author at: Department of Elec Engineering, The Ohio State University, Columbus, +1 614 247 6235; fax: +1 614 292 7596. E-mail address: firstname.lastname@example.org (P.R. Berger). Here we report on low-supply voltage and high-performance regioregular poly-(3-hexythiophene) (P3HT) organic thin-film transistors (OTFT) using a thin atomic layer deposited (ALD) HfO2 gate dielectric. For the capacitance–voltage characteristics, the measured gate capacitance was 392 nF/cm at 1 kHz and the dielectric constant was estimated 18.5 for a 40 nm thick ALD HfO2 gate dielectric. The gate dielectric showed breakdown fields >5 MV/cm with the leakage current density less than 10 7 A/cm at 2 MV/cm. With octadecyltrichlorosilane-treated ALD HfO2 gate dielectrics, P3HT OTFTs demonstrated high fieldeffect mobilities up to 0.01 ± 0.002 cm/V s in the saturation region with the drain voltage of 5 V, which is at least one order of magnitude higher than typically reported mobilities particularly for low-voltage regioregular P3HT OTFTs using high-k inorganic gate dielectrics. The threshold voltage in the saturation region and on–off current ratio were measured to be 1.2 ± 0.2 V and 1.1(±0.1) 10, respectively. 2010 Elsevier B.V. All rights reserved.