A semiconductor device includes a substrate, semiconductor 2-D material layer, a conductive 2-D material layer, a gate dielectric layer, and a gate electrode. The semiconductor 2-D material layer is over the substrate. The conductive 2-D material layer extends along a source/drain region of the semiconductor 2-D material layer, in which the conductive 2-D material layer comprises a group-IV element. The gate dielectric layer extends along a channel region of the semiconductor 2-D material layer. The gate electrode is over the gate dielectric layer.