In a silicon-oxide-nitride-oxide-silicon (SONOS) memory device, and methods of manufacturing and operating the same, the SONOS memory device includes a semiconductor layer including source and drain regions and a channel region, an upper stack structure formed on the semiconductor layer, the upper stack structure and the semiconductor layer forming an upper SONOS memory device, and a lower stack structure formed under the semiconductor layer, the lower stack structure and the semiconductor layer forming a lower SONOS memory device.