In0.18Ga0.82As epitaxial layers having a thickness much greater than the established critical thickness of pseudomorphic layers have been grown on GaAs substrates. 0.25 &mgr;m gate metal‐semiconductor field‐effect transistors (MESFETs) are fabricated by silicon ion implantation into the epitaxial wafers. In spite of the large lattice mismatch and the high defect density, the devices show excellent device performance with a maximum extrinsic transconductance of 620 mS/mm and a current‐gain cutoff frequencyfTof 92.8 GHz. Furthermore, the Schottky gate characteristics of this device are shown to be comparable to those of GaAs MESFETs.