A memory device includes first and second semiconductor layers. The first semiconductor layer includes wordlines and bitlines, an upper substrate, and a memory cell array. The memory cell array includes a memory blocks. The second semiconductor layer includes a lower substrate, and an address decoder. Each memory block includes a core region including a memory cells, a first extension region adjacent to a first side of the core region and including a plurality of wordline contacts, and a second extension region adjacent to a second side of the core region and including an insulating mold structure. The second extension region includes step zones and at least one flat zone. Through-hole vias penetrating the insulating mold structure are in the flat zone. The wordlines and the address decoder are electrically connected with each other by at least the through-hole vias.