A semiconductor device includes a substrate, a gate structure, semimetallic source/drain structures, and source/drain contacts. The gate structure is over the substrate. The semimetallic source/drain structures are respectively on opposite sides of the gate structure, in which a band structure of each of the semimetallic source/drain structures has a valence band and a conduction band at different symmetry k-points. The source/drain contacts land on top surfaces of the semimetallic source/drain structures, respectively.