The physical and electrical properties of aluminum oxide films deposited on silicon by rf sputtering from an alumina target in an argon atmosphere were investigated as a function of sputtering power density in the range from 0.5 to 3 W/cm2. The deposition rates ranged from 20 to 80 Aå/min. The density, index of refraction, and dielectric constant of the films increased while the etch rate decreased with increasing power density. The surface charge at the aluminum oxide‐silicon interface was typically larger than 1012 e/cm2. This charge increased with increasing sputtering power density and could be reduced to by annealing. The films exhibited trapping instabilities at room temperature but no polarization was observed under bias‐temperature stress. The characteristics of composite layers of thermally grown silicon dioxide and sputtered aluminum oxide layers on silicon were also investigated and found to exhibit low surface charge densities, no hysteresis, and a “contact potential” as well as charge stored at the interface between the two insulators.