Planar GaAs(100) depletion-mode (D-mode) metal-oxide-semiconductor field-effect transistors (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 (Vg
) increased from Vg
= Vfb
= 0.5 V to Vg
= 2 V and Vg
= 4 V, where Vfb
is the flat-band voltage, respectively, much higher than those in the previously published GaAs-based D-mode MOSFETs. In addition, we have achieved a high Ion/Ioff
of 107 and a subthreshold slope (SS) of 63 mV/dec, which approaches the thermal limit of 60 mV/dec 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 (Dit
) of 4.1 × 1011 eV-1cm-2 from our Al2O3/Y2O3/GaAs MOSFET, the lowest among the planar (In)GaAs MOSFETs.