A semiconductor device includes a semiconductor substrate, a pair of source/drain regions, and a gate stack. The pair of source/drain regions is on the semiconductor substrate. The gate stack is laterally between the source/drain regions and includes a gate dielectric layer over the semiconductor fin, a metal element-containing layer over the gate dielectric layer, and a fill metal layer over the metal element-containing layer. The metal element-containing layer has a dopant, and a concentration of the dopant in an upper portion of the metal element-containing layer is higher than a concentration of the dopant in a bottom portion of the metal element-containing layer.