Patent ID: 7800131

Claim:
A field effect transistor, comprising: a semiconductor substrate made of GaAs or InP; a layer structure made of compound semiconductor provided on the semiconductor substrate; a source electrode and drain electrode formed on the layer structure made of compound semiconductor with a space separating each other; a gate electrode placed between the source electrode and drain electrode; a first field plate placed over the layer structure made of compound semiconductor in a region between the gate electrode and the drain electrode, and isolated from the layer structure made of compound semiconductor; and a second field plate placed over the layer structure made of compound semiconductor, and isolated from the layer structure made of compound semiconductor and the first field plate, wherein: the compound semiconductor is compound semiconductor other than Group III nitride semiconductor; 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 the 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 gate length is designated as Lg; and a length in the gate length direction of an overlap region where the second field plate overlaps an upper part of a structure including the first field plate and the gate electrode is designated as Lol, the relation expressed below is satisfied: 0 <Lol/Lg≦ 1.