Field effect transistor

A field effect transistor, comprising:

• a semiconductor stack formed on a substrate, and having a first nitride semiconductor layer, and a second nitride semiconductor layer formed on the first nitride semiconductor layer and having a wider bandgap than that of the first nitride semiconductor layer;
• a source electrode and a drain electrode, which are formed on the semiconductor stack so as to be separated from each other;
• a gate electrode formed between the source electrode and the drain electrode so as to be separated from the source electrode and the drain electrode;
• and
• a hole injection portion formed on the semiconductor stack at a position closer to the drain electrode than to the gate electrode, wherein
• the hole injection portion has a p-type third nitride semiconductor layer, and a hole injection electrode formed on the third nitride semiconductor layer, and
• the hole injection electrode and the drain electrode have substantially a same potential.