Electrically erasable and programmable non-volatile memory system with write-verify controller using two reference levels

摘要

An EEPROM includes an array of memory cell transistors, which is divided into cell blocks each including NAND cell units of series-connected cell transistors. A sense amplifier is connected to bit lines and a comparator. A data-latch circuit is connected to the comparator, for latching a write-data supplied from a data input buffer. After desired cell transistors selected for programming in a selected block are once programmed, a write-verify operation is performed. The comparator compares the actual data read from one of the programmed cell transistors with the write-data, to verify its written state. The write-verify process checks the resulting threshold voltage for variations using first and second reference voltages defining the lower-limit and upper-limit of an allowable variation range. If the comparison results under employment of the first voltage shows that an irregularly written cell transistor remains with an insufficient threshold voltage which is so low as to fail to fall within the range, the write operation continues for the same cell transistor. If the comparison results under employment of the second voltage shows that an excess-written cell transistor remains, the block is rendered "protected" at least partially.