A semicondcutor device, and a method of fabricating the semiconductor device including forming on a substrate a device isolation layer defining a plurality of active regions; and forming a plurality of gate lines intersecting the active regions and buried in the substrate. The forming of the gate lines includes forming on the substrate a trench that intersects the active regions; forming a work-function control layer on a sidewall and a bottom surface of the trench; forming a conductive layer on the work-function control layer; sequentially forming a barrier layer and a source layer on the work-function control layer and the conductive layer, the source layer including a work-function control element; and diffusing the work-function control element from the source layer into an upper portion of the work-function control layer.