A semiconductor device includes a lower semiconductor layer with first conductive regions and including at least one dummy first conductive region, an upper semiconductor layer with second conductive regions on the lower semiconductor layer and including at least one dummy second conductive region, a penetration hole in the upper semiconductor layer and penetrating the dummy second conductive region and the upper semiconductor layer under the dummy second conductive region, a lower conductive line on the lower semiconductor layer and electrically connected to the first conductive regions, an upper conductive line on the upper semiconductor layer and electrically connected to the second conductive regions, and a first conductive plug in the penetration hole between the lower conductive line and the upper conductive line, the first conductive plug electrically connecting the lower and upper conductive lines and being spaced apart from sidewalls of the penetration hole.