Inversion-channel metal-oxide-semiconductor field-effect-transistors (MOSFETs) have been fabricated using in-situ molecular beam deposited Al2O3 as a gate dielectric directly on freshly molecular beam epitaxy grown Ga-stabilized (4 × 6) and As-covered c(4 × 4) GaAs(100) reconstructed surfaces. The MOSFET using the former surface gives a drain current (Id) of 92 μA/μm and a transconductance (Gm) of 43 μS/μm in an 1 μm gate length configuration; these values are more than 100 times higher than those attained in the MOSFET using the latter surface, which has an Id of 0.47 μA/μm and a Gm of 0.45 μS/μm. The enhancement of the inversion currents and Gm may indicate Fermi-level unpinning at the oxide/GaAs(100) interface. The result further confirms that the mid-gap interfacial trap densities of 2 × 1012 eV−1 cm−2 and of exceeding 1013 eV−1cm−2 in the samples on the Ga-stabilized and the As-covered GaAs(100) surfaces, respectively, are correlated to the inversion-channel device performance.