A compound semiconductor device comprises a substrate, comprising a top surface, a bottom surface, a side surface connecting the top surface and the bottom surface; and a semiconductor stack formed on the top surface, wherein the side surface comprises a first deteriorated surface, a second deteriorated surface, a first crack surface between the first and second deteriorated surfaces, a second crack surface between the first deteriorated surface and the top surface, and a third crack surface between the second deteriorated surface and the bottom surface, wherein the first and second deteriorated surfaces are rougher than at least one of the first crack surface, the second crack surface and the third crack surface; and wherein the second crack surface is about perpendicular to the top surface, and the third crack surface is about perpendicular to the bottom surface.