Nitride semiconductor

A nitride semiconductor comprising:

• a substrate;
• an initial growth layer formed on the substrate;
• a buffer layer formed on the initial growth layer;
• a superlattice buffer layer formed on the buffer layer;
• a channel layer which is formed on the superlattice buffer layer and which is composed of a plurality of layers;
• and
• a barrier layer formed on the channel layer,
• wherein the superlattice buffer layer is formed by alternately stacking high-Al content layers which have the composition AlxGa1-xN (0.5≦x≦1.0) and a thickness a and low-Al content layers which have the composition AlyGa1-yN (0≦y≦0.3) and a thickness b, the channel layer is joined to the superlattice buffer layer and is formed by stacking at least an AlzGa1-zN layer and a GaN layer in that order from the superlattice buffer layer side, and the Al composition of the AlzGa1-zN layer is the same as the average Al composition of the superlattice buffer layer.