Structures and methods to store information representable by a multiple-bit binary word in electrically erasable, programmable read-only memory (EEPROM)
Innovative structures and methods to store information capable of being represented by an n-bit binary word in electrically erasable Programmable Read-Only memories (EEPROM) are disclosed. To program a state below the highest threshold voltage for an N-type Field Effect Transistor (NFET) based EEPROM, the stored charge in the floating gate for the highest threshold voltage is erased down to the desired threshold voltage level of the EEPROM by applying an appropriate voltage to the control gate and drain of the NFET. The erase-down uses drain-avalanche-hot hole injection (DAHHI) for the NFET memory device to achieve the precise threshold voltage desired for the NFET EEPROM device. The method takes advantage of the self-convergent mechanism from the DAHHI current in the device, when the device reaches a steady state. For a “READ” operation, a read voltage is applied to the control gate and the drain is connected by a current load to the positive voltage supply. Using the distinctive threshold voltage associated with the different stored charges, the output voltage from the drain is distinctively recognized and converted back to the original n-bit word. A similar method for a PFET EEPROM is also disclosed.
1. A non-volatile memory structure for storing in a single nonvolatile memory cell information representable by multiple binary bits, comprising:
a nonvolatile memory cell having a drain electrode, and a source electrode capable of being coupled to a first reference voltage supply; and
a switch circuit coupled to the drain electrode of the non-volatile memory cell, the switch circuit presenting to the drain electrode non-volatile memory cell any one of multiple different voltages each representing a different value representable by the multiple binary bits.
2. A non-volatile memory structure as in claim 1 , wherein the switch circuit comprises a resistive element coupling the drain electrode to a second reference voltage supply.
3. A non-volatile memory structure as in claim 2 , wherein the resistive element comprises a diode-connected MOS transistor.
4. A non-volatile memory structure as in claim 2 , wherein the resistive element comprises an MOS transistor capable of being biased to operate in a linear region.
5. A non-volatile memory structure as in claim 1 , wherein the switch circuit includes a plurality of resistors connected in parallel.
6. A non-volatile memory structure as in claim 5 , wherein the resistances of the plurality of resistors are different.
7. A non-volatile memory structure as in claim 1 , further comprising a control gate voltage switch having one end connected to a control gate and another end capable of being switched between a high voltage source and an erase voltage source.
8. A non-volatile memory structure as in claim 1 , wherein said nonvolatile memory cell comprises an electrically erasable programmable read-only memory (EEPROM).
9. A method for storing information capable of being represented by multiple binary bits in a single nonvolatile memory cell, comprising:
programming the nonvolatile memory cell to a first threshold voltage by applying a first voltage to the control gate and the drain electrode of the nonvolatile memory cell; and
programming the non-volatile memory cell to a second threshold voltage by applying a gate voltage to the control gate and applying at the drain electrode of the non-volatile memory cell any one of a plurality of predetermined voltages each representing a value representable by the multiple binary bits.
10. A method as in claim 9 , wherein the predetermined voltage at the drain electrode is provided by a switch circuit comprising a resistive element coupling the drain electrode to a reference voltage supply.
11. A method as in claim 10 , wherein the resistive element comprises a diode-connected MOS transistor.
12. A method as in claim 10 , wherein the resistive element comprises an MOS transistor capable of being biased to operate in a linear region.
13. A method as in claim 9 , wherein the predetermined voltage at the drain electrode is provided by a switch circuit comprising a plurality of resistors connected in parallel.
14. A method as in claim 13 , wherein the resistance of each resistor in said plurality of resistors differs from the resistances of the other resistors in said plurality of resistors.
15. A method as in claim 9 , wherein said nonvolatile memory cell comprises an electrically erasable programmable read-only memory (EEPROM).
16. A method for reading out information stored in a single nonvolatile memory cell including a control gate, a source, and a drain, said information capable of being represented by an n-bit binary word, where n is a selected integer, comprising:
applying a read voltage to said control gate of said nonvolatile memory; and
connecting said drain to a selected voltage through a current load;
wherein a driving current of said nonvolatile memory cell in response to a threshold voltage of said memory cell being applied to said control gate generates an output voltage corresponding to said information stored in said cell.
17. The method of claim 16 , wherein said output voltage can be compared to voltages in a table corresponding to a series of n-bit words and converted to the original n-bit word by selecting the n-bit word in the table corresponding to a voltage in said table which corresponds most closely to said output voltage.
18. The method of claim 16 , wherein said nonvolatile memory comprises an NFET based electrically erasable programmable read-Only memory (EEPROM).
19. A method for reading out information stored in a single nonvolatile memory cell including a control gate, a source, and a drain, said information capable of being represented by an n-bit binary word, where n is a selected integer, comprising:
applying a read voltage to said control gate of said nonvolatile memory; and
connecting said drain to ground through a current sink;
wherein a driving current of said nonvolatile memory passing through said current sink in response to a threshold voltage of said memory cell being applied to said control gate generates an output voltage corresponding to said information stored in said cell.
20. The method of claim 19 , wherein said output voltage can be compared to voltages in a table corresponding to a series of n-bit words and converted to the original n-bit word by selecting the n-bit word in the table corresponding to the voltage in said table which corresponds most closely to said output voltage.
21. The method of claim 19 , wherein said nonvolatile memory comprises a PFET based electrically erasable programmable read-only memory (EEPROM).