A semiconductor device having a field effect transistor and a method of fabricating the same. In-situ doped epitaxial patterns are respectively formed at both sidewalls of a protruded channel pattern from a substrate by performing an in-situ doped epitaxial growth process. The in-situ doped epitaxial pattern has a conformal impurity concentration throughout. Accordingly, source/drain regions with a conformal impurity concentration are connected throughout a channel width of a channel region including both sidewalls of a protruded channel pattern. As a result, it is possible to maximize a driving current of the filed effect transistor, and an on-off characteristic can be highly stabilized.