Field effect transistor and manufacturing method thereof

The invention provides a structure with high compression between the field plate and the insulating film without the GaN-serial FET of the filam between the gate electrode and the semiconductor and the manufacturing method. An opening for forming a gate electrode (2) is provided by a first photoresist pattern formed on an insulating film (21). Reactive ion etching by inductively coupled plasma is applied to the insulating film (21) through the first photoresist pattern as a mask to thereby expose the surface of a GaN semiconductor layer (10-13), evaporating thereon a gate metal such as NiAu, thereby forming the gate electrode (2) by self-aligned process. This prevents an oxidized film from being formed on the surface of the semiconductor layer. After the gate electrode (2) is formed, a second photoresist pattern is formed to form a field plate (6) on the gate electrode (2) and the insulating film (21) through the second photoresist pattern as a mask. Thereby, Ti having a high adhesiveness with an insulating film made of SiN or the like can be used as a field plate metal.