A semiconductor device includes a substrate including an active region. A gate structure is disposed in the substrate and extends to traverse the active region in a first horizontal direction. Bit line structures traverse the gate structure and extend in a second horizontal direction, intersecting the first horizontal direction. Fence structures are disposed between the bit line structures. The fence structures are spaced apart from each other in the second horizontal direction. A contact plug is disposed between the bit line structures and between the fence structures. The contact plug includes first and second side surfaces that are spaced apart from each other in the second horizontal direction and third and fourth side surfaces that are spaced apart from each other in the first horizontal direction. A doping concentration of the first side surface is higher than a doping concentration of the third side surface.