Structure and method for compensating for programming threshold shift

Abstract

A structure and method are provided for accurately compensating for the neighbor effect in reading the state of a memory cell in a virtual ground memory array. When the supply voltage Vcc of the memory array increases above a selected maximum value Vccmax, a programmed memory cell reads as an unprogrammed cell. Due to the neighbor effect, a neighboring cell, which was initially unprogrammed during the programming of the memory cell but was subsequently also programmed, causes Vccmax of the memory cell to be lower in a read cycle than in the previous program/verify cycle, resulting in discarding the memory device. In accordance with the present invention, circuitry activated during the read cycle compensates for the neighbor effect by including an unprogrammed replica EPROM transistor identical to the memory cell in parallel with the neighbor of the memory cell being read. This unprogrammed replica EPROM transistor essentially replaces the effect of the actual neighbor memory cell on the memory cell being read and thus ensures that the Vccmax of the memory cell in the read cycle is equal to or greater than the corresponding Vccmax of the memory cell in the program/verify mode.