A semiconductor device having an AlGaN-GaN heterojunction structure including an AlGaN layer and a GaN layer which device exhibits no changes over time in sheet resistance. As shown in FIG. 1, in a semiconductor device having an AlGaN-GaN heterojunction structure including an AlGaN layer 1 and a GaN layer 2, when the Al molar fraction of AlGaN (x%) and the thickness of the AlGaN layer (y nm) satisfy the relations: x + y