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This letter investigates the electrical properties of SiO2 gate dielectric on GaN heterostructures deposited by atomic layer deposition (ALD). ALD SiO2 has a dielectric constant of 3.9 and a bandgap of 8.8 eV. ALD SiO2 provides a good interface to GaN and minimizes the interfacial layer growth. The threshold voltage of metal-oxide-semiconductor heterojunction field-effect transistors with ALD SiO2 dielectric is -1.5 V, owing to a fixed charge concentration of -1.6 × 1012 cm-2. It was also found that devices with ALD SiO2 dielectric exhibit three orders of magnitude reduction in gate leakage current compared to conventional Schottky gate HFETs.