Semiconductor devices may include a substrate, gate electrodes on the substrate, and source/drain regions at both sides of each of the gate electrodes. Each of the gate electrodes may include a gate insulating pattern on the substrate, a lower work-function electrode pattern that is on the gate insulating pattern and has a recessed upper surface, and an upper work-function electrode pattern that conformally extends on the recessed upper surface of the lower work-function electrode pattern. Topmost surfaces of the lower work-function electrode patterns may be disposed at an equal level, and the upper work-function electrode patterns may have different thicknesses from each other.