Manufacturing method of semiconductor device

A manufacturing method of a semiconductor device, comprising:

• providing a substrate having a first side and a second side opposite to the first side;
• forming a first III-V compound layer at the first side of the substrate;
• forming a drain trench and a contact trench at the second side of the substrate, wherein the drain trench extends from the second side of the substrate toward the first side of the substrate and penetrates the substrate, the contact trench extends from the second side of the substrate toward the first side of the substrate and penetrates the substrate, and the drain trench and the contact trench are formed concurrently by a same process;
• forming a drain electrode in the drain trench;
• forming a back contact structure in the contact trench;
• forming a buffer layer at the first side of the substrate, wherein at least a part of the buffer layer is located between the substrate and the first III-V compound layer;
• and
• forming a second III-V compound layer on the buffer layer, wherein the second III-V compound layer is located between the first III-V compound layer and the buffer layer, the first III-V compound layer comprises an n-type lightly doped gallium nitride layer, and the second III-V compound layer comprises an n-type heavily doped gallium nitride layer.