Semiconductor device

A semiconductor device, comprising:

• a supporting substrate having a first main surface and a second main surface;
• a first GaN layer of a first conductivity type provided on the side of the first main surface of the supporting substrate;
• a second GaN layer of the first conductivity type provided on the first GaN layer;
• an AlxGa1−xN (0<x<1) layer provided on the second GaN layer;
• a third GaN layer of a second conductivity type provided on the AlxGa1−xN (0<x<1) layer;
• a fourth GaN layer of the first conductivity type provided on the third GaN layer;
• an insulating film covering at least a top of the fourth GaN layer;
• a trench gate reaching the inside of the second GaN layer from an upper surface of the fourth GaN layer;
• a gate electrode provided in the trench gate with a gate insulating film interposed therebetween;
• a first main electrode connected to the third GaN layer;
• and
• a second main electrode paired with the first main electrode,
• wherein the donor concentration of the third GaN layer is lower than that of the fourth GaN layer.