A semiconductor device includes a first semiconductor structure including circuit devices and first bonding pads; and a second semiconductor structure connected to the first semiconductor structure, the second semiconductor structure including a base layer; a first memory cell structure including first gate electrodes and first channels penetrating through the first gate electrodes; a second memory cell structure including second gate electrodes and second channels penetrating through the second gate electrodes; bit lines between the first and the second memory cell structures, and electrically connected to the first and second channels in common; first and second conductive layers on the second surface of the base layer; a pad insulating layer having an opening exposing a portion of the second conductive layer; and second bonding pads disposed to correspond to the first bonding pads in a lower portion of the second memory cell structure.