Patent ID: 8779438

Claim:
A field-effect transistor comprising: a substrate; a first nitride semiconductor layer formed above the substrate having a first band gap energy; a second nitride semiconductor layer formed on the first nitride semiconductor layer and having a second band gap energy, the second band gap energy being larger than the first band gap energy; a third nitride semiconductor layer formed on the second nitride semiconductor layer and containing a p-type impurity; a source electrode formed on the second nitride semiconductor layer; a drain electrode formed on the second nitride semiconductor layer; a gate electrode formed on or above the third nitride semiconductor layer and disposed between the source electrode and the drain electrode; and an isolation region comprised of a first portion of the first nitride semiconductor layer and a second portion of the second nitride semiconductor layer, wherein: the first portion and the second portion include an impurity, respectively, a resistance of the first portion is higher than a resistance of a remaining portion of the first nitride semiconductor layer other than the first portion, and a resistance of the second portion is higher than a resistance of a remaining portion of the second nitride semiconductor layer other than the second portion.