A semiconductor device includes a 2-D material channel layer, a gate structure, and source/drain electrodes. The gate structure is over a channel region of the 2-D material channel layer. The source/drain electrodes are over source/drain regions of the 2-D material channel layer, respectively. Each of the source/drain electrodes includes a 2-D material electrode and a metal electrode. The 2-D material electrode is below a bottom surface of a corresponding one of the source/drain regions of the 2-D material channel layer. The metal electrode is over a top surface of the corresponding one of the source/drain regions of the 2-D material channel layer.