A four-state EEPROM using floating-gate memory cells

An electrically erasable programmable read-only memory (EEPROM) is used in a novel way as a four-state memory by charging the floating gate to determined values. The memory cell and the complete programming and readout circuit are described. Retention characteristics are investigated and found to confirm a thermionic emission model. Retention time is estimated to be more than 22 years at 125°C. Secondary effects like charge trapping in the oxide are successfully suppressed by a controlled writing procedure. Using such a four-state EEPROM instead of a binary cell, a reduction in chip area of 40% can be expected for a 1-kb memory.