A method of manufacturing a semiconductor element includes a first step of epitaxially growing an AlN layer on a substrate, a second step of forming a buffer layer on the AlN layer by epitaxially growing AlxGayInzN where x, y, and z satisfy x+y+z=1 and y is not zero without adding Fe, a third step of forming a resistance layer on the buffer layer by epitaxially growing AlxGayInzN where x, y, and z satisfy x+y+z=1 and y is not zero while adding Fe, a step of epitaxially growing a channel layer on the resistance layer, a step of epitaxially growing an electron supply layer above the channel layer, and a step of forming an electrode above the electron supply layer.