A field effect transistor and a semiconductor device including the same are provided. The semiconductor device may include a channel layer, which is provided on a substrate and includes a two-dimensional atomic layer made of a first material, and a source/drain layer, which is provided on the substrate and includes a second material. The first material may be one of phosphorus allotropes, the second material may be one of carbon allotropes, and the channel layer and the source/drain layer may be connected to each other by covalent bonds between the first and second materials.