A semiconductor device including a first insulating interlayer on a substrate; a second insulating interlayer on the first insulating interlayer; a gate structure extending through the first insulating interlayer and the second insulating interlayer on the substrate, a lower portion of the gate structure having a first width, and an upper portion of the gate structure having a second width that is greater than the first width and that gradually increases from a bottom toward a top thereof; and a spacer structure on a sidewall of the gate structure, a width of an upper portion of the spacer structure being less than a width of a lower portion of the spacer structure.