A semiconductor device may include a substrate including trenches and contact recesses having a curved surface profile, conductive patterns in the trenches, buried contacts including first portions filling the contact recesses and second portions on the first portions, and spacer structures including first and second spacers. The second portions may have a pillar shape and a smaller width than top surfaces of the first portions. The buried contacts may be spaced apart from the conductive patterns by the spacer structures. The first spacers may be on the first portions of the buried contacts at outermost parts of the spacer structures. The first spacers may extend along the second portions of the buried contacts and contact the buried contacts. The second spacers may extend along the side surfaces of the conductive patterns and the trenches. The second spacers may contact the conductive patterns. The first spacers may include silicon oxide.