A semiconductor device includes a substrate, a 2-D material layer, source/drain contacts, and a gate electrode. The 2-D material layer is over the substrate, the 2-D material layer includes source/drain regions and a channel region between the source/drain regions, in which the 2-D material layer is made of a transition metal dichalcogenide (TMD). The source/drain contacts are in contact with source/drain regions of the 2-D material layer, in which a binding energy of transition metal atoms at the channel region of the 2-D material layer is different from a binding energy of the transition metal atoms at the source/drain regions of the 2-D material layer. The gate electrode is over the substrate.