A semiconductor device includes a substrate including spaced-apart active regions, and device isolating regions isolating the active regions from each other, and a pillar array pattern including a plurality of pillar patterns overlapping the active regions, the plurality of pillar patterns being spaced apart from each other at an equal distance in a first direction and in a second direction intersecting the first direction, wherein the plurality of pillar patterns include first pillar patterns and second pillar patterns disposed alternatingly in the first direction and in the second direction, a shape of a horizontal cross section of the first pillar patterns being different from a shape of a horizontal cross section of the second pillar patterns.