US Patent 7558108 3-bit NROM flash and method of operating same

Operation of conventional nitride read-only-memory (NROM) cells is modified, such that each charge trapping region of an NROM cell is capable of storing any one of three charge states. For example, each charge trapping region can have an erased state, a first programmed state, or a second programmed state. Each of these states results in a different threshold voltage. During a read operation, the threshold voltages associated with two charge trapping regions are identified and decoded to provide a 3-bit data value. If each NROM cell includes two separate charge trapping regions, two NROM cells can store a total of 6-bits of data. The average storage density is therefore increased from two bits per NROM cell to three bits per NROM cell.