We report detailed current-transport studies of ultrathin Al2O3 dielectrics on GaAs grown by atomic layer deposition (ALD) as a function of film thickness, ambient temperature and electric field. The leakage current in ultrathin Al2O3 on GaAs is comparable to or even lower than that of the state-of-the-art SiO2 on Si, not counting on high dielectric constant for Al2O3. By measuring leakage current at a wide range of temperatures from 133 K to 475 K, we are able to identify the electron transport mechanism and measure the thermal-activation energy of the ultrathin oxide films. This thermal-activation energy is proposed as a parameter to generally characterize the quality of ultrathin dielectrics on semiconductors. (c) 2006 Elsevier Ltd. All rights reserved.