Oxide surface passivation grown by atomic layer deposition (ALD) has been applied to GaAs metal-semiconductor field-effect transistors (MESFETs). The breakdown characteristic of a MESFET is greatly improved by both Al2O3 and HfO2 passivation. Three-terminal transistor breakdown voltage is improved to a maximum level of 20 V with Al2O3 passivation from 11 V without any surface passivation. With the removal of native oxide and passivation on GaAs surface at drain-gate (D-G) and source-gate (S-G) spacings, the device breakdown characteristics are significantly improved. (c) 2005 Elsevier Ltd. All rights reserved.