We report metal-oxide-semiconductor (MOS) capacitors and field-effect transistors (MOSFET's) in the GaAs semiconductor system using an unpinned interface. The structures utilize plasma-enhanced chemical vapor deposition (PECVD) for the silicon dioxide insulator on GaAs that has been terminated with a few monolayers of silicon during growth by molecular beam epitaxy. Interface densities in the structures have been reduced to ~ 1012 cm 2-eV High-frequency characteristics indicate strong inversion of both p-type and n-type GaAs. The excellent insulating quality of the oxide has also allowed demonstration of quasi-static characteristics. MOSFET's operating in depletion mode with a transconductance of 60 mS/mm at 8.0-pm gate lengths have been fabricated. © 1988 IEEE.