A method for manufacturing a semiconductor device is provided. The method includes forming, on a substrate, a plurality of fins extending along a first direction; forming, on the fins, a dummy gate stack extending along a second direction; forming a gate spacer on opposite sides of the dummy gate stack in the first direction; epitaxially growing raised source/drain regions on the top of the fins on opposite sides of the gate spacer in the first direction; performing lightly-doping ion implantation through the raised source/drain regions with the gate spacer as a mask, to form source/drain extension regions in the fins on opposite sides of the gate spacer in the first direction; removing the dummy gate stack to form a gate trench; and forming a gate stack in the gate trench.