A method of manufacturing a semiconductor device is provided. The method includes forming a semiconductor layer on a substrate, forming an electron transit layer and an electron supply layer, and forming openings in the layers. A first source region and a first drain region are formed in the openings, and a second source region and a second drain region are formed thereon. A gate electrode, a source electrode, and a drain electrode are provided, wherein the source and drain electrodes are formed on the second source region and the second drain region, respectively. A concentration of an electrically conductive impurity in the second source region is lower than that in the first source region, and a concentration of the electrically conductive impurity in the second drain region is lower than that in the first drain region.