Structures of a semiconductor device are provided. The semiconductor device includes a substrate, a gate structure over the substrate, and a first recess and a second recess in the substrate and at opposite sides of the gate structure. The semiconductor device also includes two source/drain structures over the first recess and the second recess respectively. At least one of the source/drain structures includes a first doped region partially filling in the first recess, a second doped region over the first doped region, and a third doped region over the second doped region. The second doped region contains more dopants than the first doped region or the third doped region.