A memory device includes a first electrode, a second electrode spaced apart from the first electrode and a nanotube or nanowire network disposed between the first electrode and the second electrode, having a stacked structure of a P-type network and an N-type network, and having a diode characteristic. Since the nanotube or nanowire network has the stacked structure of the P-type network and the N-type network, and has the diode characteristic, it is possible to enhance a degree of integration of the memory device and simplify the fabrication processes without separately requiring a selection device.