Two-dimensional electron gas (2DEG) formed at oxide heterointerfaces via atomic layer deposition (ALD) has attracted considerable interest toward fascinating electron-related physics and electronic device applications. The employment of oxide-based 2DEG in a confined channel in field-effect transistors (FETs) holds great promise for advanced electronic devices due to its high mobility, spatial confinement, and tunable conductivity. In this work, a 2DEG FET based on the Al₂O₃/ZnO heterostructure is fabricated with an optimized channel carrier density and oxide thickness. The carrier transport in the bulk and the oxide interface dominantly governed by percolation conduction, optical phonon scattering, and grain boundary scattering is comparatively studied through oxygen annealing and thickness engineering. A tunable carrier density from 4 × 10¹¹ cm⁻² to 2 × 10¹⁴ cm⁻² is achieved with a maximum Hall mobility of ∼62 cm² V⁻¹ s⁻¹. The electron distribution associated with the annealing process of the ZnO underlayer and the interface reaction during Al₂O₃ deposition are found to have an impact on the electrical characteristics of the devices. The fabricated Al₂O₃/ZnO-based 2DEG FET exhibits an on/off ratio over 10⁸, a subthreshold swing of 224 mV dec⁻¹, and a field-effect mobility of 5.7 cm² V⁻¹ s⁻¹ which can be promising for advanced oxide thin film-containing device and system applications.