Abstract Planar GaAs(100) depletion-mode (D-mode) MOSFETs as passivated with in situ deposited Al₂O₃/Y₂O₃ dielectric have shown enhancement of the drain current by 167% and 333% as the gate voltage (V _g) increased from flat-band voltage (V _fb), namely V _g = V _fb = 0.5 V to V _g = 2 V and V _g = 4 V, respectively, much higher than those in the previously published GaAs-based D-mode MOSFETs. In addition, we have achieved a high I _on/I _off of 10⁷ and a subthreshold slope (SS) of 63 mV dec⁻¹, which approaches the thermal limit of 60 mV dec⁻¹ at 300 K and is the record-low value among planar (In)GaAs MOSFETs. Moreover, using the measured SS data, we have deduced an interfacial trap density (D _it) of 4.1 × 10¹¹ eV⁻¹ cm⁻² from our Al₂O₃/Y₂O₃/GaAs MOSFET, the lowest value among the planar (In)GaAs MOSFETs.