Low-voltage pentacene organic field-effect transistors are demonstrated (operating voltage of -3 V) with high- κ hafnium dioxide gate dielectrics grown by atomic layer deposition at 200 °C. A high hole mobility of 0.39 cm2 /V s with low threshold voltage (<-0.5 V) and low subthreshold slope of 120 mV/dec is achieved with a HfO2 dielectric layer modified with a phosphonic acid based treatment. A high value of 94.8 nF/V s is obtained for the product of mobility and capacitance density. The devices show excellent bias stress stability with or without the phosphonic acid at the HfO2 gate dielectric surface. © 2009 American Institute of Physics.