Patent ID: 8569794

Claim:
A Group III nitride semiconductor device which is obtained by laminating at least a buffer layer made of a Group III nitride compound and a ground layer made of a Group III nitride semiconductor, which is formed on the buffer layer, on a substrate, wherein the buffer layer is formed of a single crystalline structure, the buffer layer is made of AlN, a thickness of the buffer layer is within a range of 20 to 100 nm, a lattice constant of a-axis of the buffer layer is smaller than a lattice constant of a-axis of AlN in a bulk state, the ground layer is made of GaN and is in contact with the buffer layer, and lattice constants of the buffer layer satisfy a relationship represented by a formula (1): ( c 0 −c )/( a 0 −a )≧−1.4 (1) wherein in the formula (1), c 0 represents a lattice constant of c-axis of a bulky AlN, c represents a lattice constant of c-axis of the buffer layer, a 0 represents a lattice constant a-axis of bulky AlN, and a represents a lattice constant of a-axis of the buffer layer.