US Patent 7459718 Field effect transistor

A FET includes a nitride semiconductor in which leak current is reduced and breakdown voltage is improved. The FET is formed from a substrate, a buffer layer made of a nitride semiconductor, a first semiconductor layer made of a nitride semiconductor, and a second semiconductor layer made of a nitride semiconductor, wherein at least the buffer layer and the first semiconductor layer include a p-type dopant. The concentration of the p-type dopant is higher in the buffer layer than that in the first semiconductor layer, and the concentration of the p-type dopant is higher in the first semiconductor layer than that in the second semiconductor layer.