GaN-based HFET having a surface-leakage reducing cap layer

A semiconductor device comprising:

• a substrate;
• a buffer layer comprising GaN formed on the substrate, wherein surfaces of the buffer layer are c facets of Ga atoms;
• a channel layer comprising GaN or InGaN formed on the buffer layer, wherein surfaces of the channel layer are c facets of Ga or In atoms;
• an electron donor layer comprising AlGaN formed on the channel layer, wherein surfaces of the electron donor layer are c facets of Al or Ga atoms;
• a source electrode and a drain electrode formed on the electron donor layer;
• a cap layer comprising InGaAlN formed between the source electrode and the drain electrode without being in contact with the source electrode and the drain electrode, wherein surfaces of the cap layer are c facets of Ga or In atoms and at least a portion of the cap layer is in contact with the electron donor layer;
• and
• a gate electrode formed at least a portion of which is in contact with the cap layer, wherein
• the cap layer has a composition which is substantially lattice matched with the buffer layer in a c facet, and
• the electron donor layer is formed so that an absolute value of magnitude of a polarization occurring within the cap layer is smaller than that of a magnitude of a polarization occurring within the electron donor layer.