A top-gate field-effect device with atomic-layer-deposited Al2O3 dielectric was fabricated to investigate magnetotransport properties of two-dimensional electron gas (2DEG) at a semi-insulating ZnO-Mg0.12Zn0.88O double heterostructure grown by laser molecular-beam epitaxy. Hall mobility monotonically increased as the density of accumulated electrons increased. The highest mobility at 2 K was recorded to be 5000 cm2 V-1 s-1 at a 2DEG density of 1.2x1012 cm-2, which is comparable to the previously reported value for a metallic ZnO/Mg0.2Zn0.8O heterostructure. Insulator-to-metal transition was observed at a critical density of 6x1011 cm-2. The metallic-state channel exhibited Shubnikov-de Haas oscillations, demonstrating an electric-field tunable quantum device based on transparent oxide semiconductor.
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