Semiconductor devices may include a plurality of active fins each extending in a first direction on a substrate, a gate structure extending on the active fins in a second direction, and a first source/drain layer on first active fins of the active fins adjacent the gate structure. At least one of two opposing sidewalls of a cross-section of the first source/drain layer taken along the second direction may include a curved portion having a slope with respect to an upper surface of the substrate. The slope may decrease from a bottom toward a top thereof.