A method for manufacturing a semiconductor device comprises, including forming a plurality of fins on a substrate, forming, a dummy gate stack on the fins forming a gate spacer on opposite sides of the dummy gate stack, forming source/drain trenches by etching the fins with the gate spacer and the dummy gate stack as a mask, forming source/drain extension regions on the bottom and sides of the trenches by performing lightly-doping ion implantation; and by performing epitaxial growth in and/or on the source/drain trenches, removing the dummy gate stack to form a gate trench; and forming a gate stack in the gate trench.