Field effect transistors, methods of fabricating a carbon insulating layer using molecular beam epitaxy and methods of fabricating a field effect transistor using the same are provided, the methods of fabricating the carbon insulating layer include maintaining a substrate disposed in a molecular beam epitaxy chamber at a temperature in a range of about 300° C. to about 500° C. and maintaining the chamber in vacuum of 10−11 Torr or less prior to performing an epitaxy process, and supplying a carbon source to the chamber to form a carbon insulating layer on the substrate. The carbon insulating layer is formed of diamond-like carbon and tetrahedral amorphous carbon.