Field effect transistor

A field effect transistor comprising:

• a substrate;
• a buffer layer laminated on the substrate and having a multilayer structure formed by AlxGa1-xN and AlyGa1-yN (0≦x<1, 0≦y<1, and x≠y);
• a high resistance layer laminated on the buffer layer and made of GaN containing carbon;
• a channel layer laminated on the high resistance layer;
• a nitride semiconductor layer laminated on the channel layer and serving as a barrier layer;
• a source electrode and a drain electrode disposed on the nitride semiconductor layer in an arrangement spaced from each other;
• and
• a gate electrode formed between the source electrode and the drain electrode and on the nitride semiconductor layer, the gate electrode being connected to the nitride semiconductor layer directly or with an insulating film interposed therebetween,
• wherein a carbon concentration in the buffer layer at side closer to the high resistance layer is not less than 0.8×1019/cm3 and not more than 1.0×1021/cm3,
• a carbon concentration in the high resistance layer at side closer to the buffer layer is not less than 3.7×1018/cm3 and not more than 1.0×1021/cm3, and
• a carbon concentration in the high resistance layer at side closer to the channel layer is not less than 1.4×1019/cm3 and not more than 1.0×1021/cm3, and
• the carbon concentration in the high resistance layer gradually increases toward the channel layer from side close to the buffer layer.