A non-volatile semiconductor memory device is provided comprising a memory area and a circuitry area. The memory area includes a plurality of memory cells and a set of array reference cells that are programmable to have a threshold voltage corresponding to an erased or a programmed state of a memory...http://www.google.de/patents/US7259993?utm_source=gb-gplus-sharePatent US7259993 - Reference scheme for a non-volatile semiconductor memory device
