A semiconductor device includes a first gate stack structure over a substrate, a source/drain epitaxial layer, a lightly doped region, and a silicide region. The source/drain epitaxial layer is disposed in the substrate and adjacent to the first gate stack structure. The lightly doped region is located in the substrate to be electrically connected to the source/drain epitaxial layer. The lightly doped region includes a first portion protrudes from a sidewall of the source/drain epitaxial layer. The silicide region is in contact with a top surface and sidewalls of a top portion of the source/drain epitaxial layer and a top surface of the first portion of the lightly doped region. The top portion of the source/drain epitaxial layer is higher than the top surface of the first portion of the lightly doped region.