Nitride semiconductor device

A nitride semiconductor device, comprising:

• a first nitride layer, said first nitride layer including a binary nitride compound;
• a second nitride layer disposed on the first nitride layer, said first nitride layer having a first surface and a second surface that are opposite to each other, said second surface being at an interface of said first nitride layer and said second nitride layer, a thickness direction being defined from said first surface to said second surface, said second nitride layer including a first element, a second element, and a third element, said first element being an aluminum element, said second element being a nitrogen element, said third element being a gallium element, a concentration of said first element increasing along the thickness direction;
• and
• a third nitride layer disposed on said second nitride layer, said third nitride layer being a layer of ternary nitrogen compound or a quaternary nitrogen compound, said third nitride layer including said first element;
• a band gap of said third nitride layer being greater than a band gap of said second nitride layer, a two-dimensional electron gas being generated in said first nitride layer;
• a maximum concentration value of said first element of said second nitride layer being no greater than a concentration value of said first element of said third nitride layer;
• along the thickness direction, a thickness said first nitride layer being represented by d1, a thickness of said second nitride layer being represented by d2, a thickness of said third nitride layer being represented by d3;
• wherein d2<d1, and 5 Å≤d2≤60 Å.