A semiconductor device includes a substrate, a gate stack, and an epitaxy structure. The gate stack over the substrate and includes a gate dielectric layer, a bottom work function (WF) metal layer, a top WF metal layer, and a filling metal. The bottom WF metal layer is over the gate dielectric layer. The top WF metal layer is over and in contact with the bottom WF metal layer. At least one of the top and bottom WF metal layers includes dopants, and the top WF metal layer is thicker than the bottom WF metal layer. The filling metal is over the top WF metal layer. The epitaxy structure is over the substrate and adjacent the gate stack.