A contact structure for a semiconductor device includes a substrate comprising a major surface and a cavity. A bottom surface of the cavity is lower than the major surface. The contact structure also includes a strained material in the cavity, and a lattice constant of the strained material is different from lattice constant of the substrate. The contact structure also includes a first metal layer over the strained material, a dielectric layer over the first metal layer, and a second metal layer over the dielectric layer. The dielectric layer has a thickness ranging from 1 nm to 10 nm.