Patent ID: 7863648

Claim:
A field effect transistor, comprising: a layer structure made of Group III nitride semiconductor comprising hetero-junction; a source electrode and a drain electrode formed on the layer structure made of Group III nitride semiconductor with a space separating each other; a gate electrode placed between the source electrode and the drain electrode; a first field plate placed over the layer structure made of Group III nitride semiconductor in a region between the gate electrode and the drain electrode, and isolated from the layer structure made of Group III nitride semiconductor; and a second field plate placed over the layer structure made of Group III nitride semiconductor, and isolated from the layer structure made of Group III nitride semiconductor and the first field plate, wherein the first field plate is biased at the same electric potential as that of the gate electrode, the second field plate is biased at the same electric potential as that of the source electrode, the second field plate comprises a shielding part located in a region between the first field plate and the drain electrode, and serving to shield the first field plate from the drain electrode, and an upper end of the shielding part is located above an upper surface of the first field plate, whereby, in a cross sectional view in the gate length direction, when a length in a gate length direction of an overlap region where the second field plate overlaps an upper part of a structure comprising the first field plate and the gate electrode is designated as Lol and a gate length is designated as Lg, the relation expressed below is satisfied: 0 <Lol/Lg â‰¦1; wherein the field effect transistor further comprises a first insulating film for covering a surface of the layer structure made of Group III nitride semiconductor in the region between the gate electrode and the drain electrode, and wherein the first insulating film is a film containing nitrogen.