Patent ID: 8247842

Claim:
A semiconductor device comprising: a substrate; a buffer layer formed on the substrate; and a device formation layer formed on the buffer layer, and containing a nitride-based semiconductor, wherein the buffer layer is formed by sequentially stacking, a plurality of times, a first nitride-based semiconductor layer made of a material having a lattice constant lower than a lattice constant of a material of the substrate; a first composition graded layer formed on the first nitride-based semiconductor layer, and made of a material having a lattice constant gradually higher than the lattice constant of the first nitride-based semiconductor layer in a thickness direction; a second nitride-based semiconductor layer formed on the first composition graded layer, and made of a material having a lattice constant higher than the lattice constant of the first nitride-based semiconductor layer; and a second composition graded layer formed on the second nitride-based semiconductor layer, and made of a material having a lattice constant gradually lower than the lattice constant of the second nitride-based semiconductor layer in the thickness direction, wherein the second composition graded layer is thicker than the first composition graded layer, and a rate of change of aluminum content of the second composition graded layer is smaller than a rate of change of aluminum content of the first composition graded layer.