A compound semiconductor MESFET is provided with a passivation layer (54) which is crystallographically similar to a compound semiconductor active layer (53) thereunder and has at least one constituent that is a constituent also of the compound semiconductor of the active layer. The passivation layer is an insulating or semi-insulating layer, and has a forbidden band gap wider than that of the compound semiconductor of the active layer. Such a MESFET can have excellent RF and other characteristics.
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