Field effect transistor

A field effect transistor made of a nitride compound semiconductor, the field effect transistor comprising:

• a substrate;
• a buffer layer that is formed on the substrate;
• a high resistance layer or a foundation layer that is formed on the buffer layer;
• a carrier concentration controlling layer containing carbon that is formed on the high resistance layer or the foundation layer;
• a carrier traveling layer that is formed on the carrier concentration controlling layer;
• a carrier supplying layer that is formed on the carrier traveling layer;
• a recess formed at least in the carrier supplying layer and the carrier traveling layer;
• a source electrode and a drain electrode that are formed on the carrier supplying layer with the recess intervening therebetween;
• a gate insulating film that is formed on the carrier supplying layer so as to cover the recess;
• and
• a gate electrode that is formed on the gate insulating film in the recess, wherein
• the carrier traveling layer includes a two-dimensional electron gas generated therein due to a difference of bandgap energy between the carrier traveling layer and the carrier supplying layer, and
• a carbon concentration of the carrier concentration controlling layer is from 5×1017 cm−3 to 5×1019 cm−3.