A memory device is provided. The memory device includes a first structure and a second structure stacked on the first structure in a vertical direction. The first structure includes a first substrate, peripheral circuitry, an auxiliary memory cell array, a first insulating layer, and a plurality of first bonding pads. The second structure includes a second substrate, a main memory cell array, a second insulating layer, and a plurality of second bonding pads. The plurality of first bonding pads are in contact with the plurality of second bonding pads, respectively.