Varying thicknesses of the HfO2 films prepared by atomic layer deposition (ALD) were compared in terms of electrical properties and reliability. Scaling the physical thickness of the HfO2 dielectric did not increase the capacitance as expected due to the thicker interfacial SiOX layer obtained by the oxidation process used for thermal annealing. After thermal annealing, HfO2 crystallization increased with the thickness of HfO2 film, which then increases the dielectric constant for the bulk HfO2 film. The breakdown behaviors of the HfO2 gate dielectric film were not scaled to thickness, improved with HfO2 thickness until saturation. The reliability characteristics of the HfO2 dielectric under unipolar AC stressing were also evaluated. Dielectric breakdown failure time of unipolar AC stressing becomes longer in comparison to that stressed in constant voltage stress. As the thickness of the HfO2 dielectric increases, a larger lifetime enhancement is detected due to the effective charge de-trapping for thicker dielectrics under AC stressing. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved.