Ultra-thin gate oxides with thickness smaller than 3 nm were prepared by anodic oxidation (anodization) in deionized water under direct-current biasing superimposed with alternating-current signal (DAC-ANO). It is experimentally observed that the DAC-ANO oxides after suitable high temperature annealing have better electrical characteristics than conventional rapid thermal oxides (RTO). Other advantages of DAC-ANO oxides include lower leakage current, higher time-zero dielectric breakdown and time-dependent dielectric breakdown endurance, and less stress-induced leakage current. The charge trapping behavior under high field stress is less significant in DAGANO oxides than in RTO ones. The improved reliability of DAC-ANO oxides can be explained by the nature of AC switching effect. DAGANO oxide is a potential candidate in the application of ultra-thin gate oxide to deep sub-micron devices. (C) 2003 Elsevier Ltd. All rights reserved.