A heterojunction field effect transistor realizing a high performance by a significant decrease. in source resistance while maintaining a sufficiently high gate resistivity to voltage is provided. Sequentially stacked on a c-face sapphire substrate via a buffer layer are an undoped GaN layer, undoped Al.sub.0.3 Ga.sub.07 N layer, undoped GaN channel layer, undoped Al.sub.0.15 Ga.sub.0.85 N spacer layer, n-type Al.sub.0.15 Ga.sub.0.85 N electron supply layer, graded undoped Al.sub.z Ga.sub.1-z N barrier layer and n-type Al.sub.0.06 Ga.sub.0.94 N contact layer, and a gate electrode, source electrode and drain electrode are formed on the n-type Al.sub.0.06 Ga.sub.0.94 N contact layer to form a AlGaN/GaN HEMT. The Al composition z in the graded undoped Al.sub.z Ga.sub.1-z N barrier layer continuously decreases from 0.15 to 0.06, for example, from the n-type Al.sub.0.15 Ga.sub.0.85 N electron supply layer toward the n-type Al.sub.0.06 Ga.sub.0.94 N contact layer. An n.sup.++ -type GaN contact layer may be formed on the n-type Al.sub.0.06 Ga.sub.0.94 N contact layer in the region for the source electrode and the drain electrode, and the source electrode and the drain may be formed on it.