Ion‐implanted In0.1Ga0.9As metal‐semiconductor field‐effect transistors (MESFETs) have been fabricated on 3 in. GaAs (100) substrates. The structure comprises an undoped InGaAs epitaxially layer grown directly on a GaAs (100) substrate by the metalorganic chemical vapor deposition (MOCVD) technique. Si+28is ion implanted into the InGaAs layer to form an active channel layer. MESFETs with 100 &mgr;m gate width and 0.5 &mgr;m gate length are fabricated using standard process techniques. The best device shows a maximum transconductance of 426 mS/mm. FromS‐parameter measurements, the current‐gain cutoff frequencyftis 37 GHz and the maximum available gain cutoff frequencyfmaxis 85 GHz. These results are comparable to InGaAs MESFETs grown by MBE.