METHOD FOR DETECTING THRESHOLD VOLTAGE OF ARRAY CELL, AND MEMORY

摘要

PROBLEM TO BE SOLVED: To provide a programmable reference used to identify a state of an array cell in a multi-density or low voltage supply flash EEPROM memory array.

SOLUTION: The programmable reference includes one or more reference cells. The array cells are read by applying an identical voltage to gates of a reference cell and the array cell and comparing outputs to determine the array cell state. During reading the array cell, the programmable reference cell is biased the same as the array cell, so that the difference in threshold values between reference cells and array cells remain constant with a change in VCC. A circuitry is included for programming the reference cells utilizing a simple resistor ratio. Programming is performed at test time, preferably by the manufacturer, in order to keep VCC within strict tolerances. The array cells are programmed and read without resistor biasing and under looser tolerances using the reference cells at a later time.

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