A semiconductor device including a substrate, a semiconductor layer, a gate, a dielectric structure, and a source/drain structure is provided. The semiconductor layer is disposed on the substrate, and is made of a first low dimensional material. The gate is disposed on the substrate and overlaps the semiconductor layer. The dielectric structure is disposed on the semiconductor layer and includes a trench structure reaching a portion of the semiconductor layer. The source/drain structure includes a barrier layer made of a second low dimensional material continuously extending along the trench structure and a metal fill filling a volume surrounded by the barrier layer.