A semiconductor device comprising: an electrical conductor layer, an SiC layer formed on a surface of the electrical conductor layer, an AIN layer formed on a surface of the SiC layer, a thickness of the AIN layer is larger than 0 and equal to or less than 15 nm, a first GaN layer of a first conductivity type, formed on a surface of the AIN layer, a composite layer formed on a surface of the first GaN layer, a first electrode formed at a surface side of the first GaN layer, and a second electrode formed on a reverse face side of the electrical conductor layer, wherein the magnitude of electrical current which flows between the first electrode and the second electrode depends on the electrical voltage between the first electrode and the second electrode, the composite layer includes an Al m Ga 1-m N (0 < m ≦ 1, y < m) layer, and a second GaN layer formed on a surface of the Al m Ga 1-m N layer, a distance between the first electrode and the electrical conductor layer is larger than a distance between the second GaN layer and the electrical conductor layer, the first electrode is placed in Schottky contact with the second GaN layer, each of the first GaN layer and the second GaN layer has n-type conductivity, and each of the first GaN layer and the second GaN layer is doped with Si, a Si dope density of the second GaN layer is different from a Si dope density of the first GaN layer, an n-type impurity concentration of the second GaN layer is lower than an n-type impurity concentration of the first GaN layer, and the n-type impurity concentration of an upper part of the second GaN layer is lower than the n-type impurity concentration of the lower part of the second GaN layer. an electrical conductor layer, an SiC layer formed on a surface of the electrical conductor layer, an AIN layer formed on a surface of the SiC layer, a thickness of the AIN layer is larger than 0 and equal to or less than 15 nm, a first GaN layer of a first conductivity type, formed on a surface of the AIN layer, a composite layer formed on a surface of the first GaN layer, a first electrode formed at a surface side of the first GaN layer, and a second electrode formed on a reverse face side of the electrical conductor layer, wherein the magnitude of electrical current which flows between the first electrode and the second electrode depends on the electrical voltage between the first electrode and the second electrode, the composite layer includes an Al m Ga 1-m N (0 < m ≦ 1, y < m) layer, and a second GaN layer formed on a surface of the Al m Ga 1-m N layer, a distance between the first electrode and the electrical conductor layer is larger than a distance between the second GaN layer and the electrical conductor layer, the first electrode is placed in Schottky contact with the second GaN layer, each of the first GaN layer and the second GaN layer has n-type conductivity, and each of the first GaN layer and the second GaN layer is doped with Si, a Si dope density of the second GaN layer is different from a Si dope density of the first GaN layer, an n-type impurity concentration of the second GaN layer is lower than an n-type impurity concentration of the first GaN layer, and the n-type impurity concentration of an upper part of the second GaN layer is lower than the n-type impurity concentration of the lower part of the second GaN layer.