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MOS capacitors with an ultrathin aluminum oxide (Al2O3) gate dielectric were fabricated on n-type 4H-SiC. Al2O3 was prepared by room-temperature nitric acid (HNO3) oxidation of ultrathin Al film followed by furnace annealing. The effective dielectric constant of k∼9.4 and equivalent oxide thickness of 26 Å are produced, and the interfacial layer and carbon clusters are not observed in this paper. The electrical responses of MOS capacitor under heating and illumination are used to identify the conduction mechanisms. For the positively biased case, the conduction mechanism is shown to be dominated by Schottky emission with an effective barrier height of 1.12±0.13 eV. For the negatively biased case, the gate current is shown to be due to the generation-recombination process in depletion region and limited by the minority carrier generation rate. The feasibility of integrating alternative gate dielectric on SiC by a low thermal budget process is demonstrated.