US Patent 7720124 Semiconductor device and fabrication method thereof

A semiconductor device includes a first nitride semiconductor layer formed on a substrate and a second nitride semiconductor layer formed on the first nitride semiconductor layer so as to be in contact with the first nitride semiconductor layer. The first nitride semiconductor layer contains a p-type impurity. The second nitride semiconductor layer contains an n-type impurity and a p-type impurity. In the second nitride semiconductor layer, the concentration of the n-type impurity is higher than the concentration of the p-type impurity.