Method for erasing memory cells in a flash memory device using a positive well bias voltage and a negative word line voltage
View Patent ↗A memory device of the non-volatile type including a memory array having a plurality of memory cells organized as sectors, each sector having a main word line associated with a plurality of local word lines, each local word line coupled to the main word line by a respective local word line driver circuit, each of the local word line driver circuits consisting of a first MOS transistor coupled between the respective main word line and a respective local word line and a second MOS transistor coupled between the respective local word line and a first biasing terminal.
1. A flash memory device comprising:
a plurality of flash memory cells, each flash memory cell comprising a single transistor, said single transistor being a floating gate transistor;
a plurality of main word lines, each main word line of the plurality of main word lines coupled to a corresponding group of local word lines, each local word line coupled to gate terminals of its corresponding floating gate transistors of the plurality of flash memory cells;
a voltage circuit configured to provide a positive well bias voltage, a negative word line voltage, and a positive word line voltage, the voltage circuit coupled to a p-well to apply the positive well bias voltage to the plurality of flash memory cells for an erase operation; and
driver circuit configured to:
pass the negative word line voltage on a selected main word line of the plurality of main word lines to its corresponding group of selected local word lines for the erase operation, the corresponding group of selected local word lines being coupled to gate terminals of corresponding group of selected flash memory cells to be erased, the difference between the positive well bias voltage and the negative word line voltage being sufficient to erase flash memory cells; and
pass the positive word line voltage on unselected main word lines of the plurality of main word lines to their corresponding groups of unselected local word lines, the corresponding group of unselected local word lines being coupled to gate terminals of corresponding groups of unselected flash memory cells, the difference between the positive well bias voltage and the positive word line voltage being insufficient to erase flash memory cells.
2. The flash memory device of claim 1 , wherein the positive well bias voltage is higher than VCC.
3. The flash memory device of claim 2 , wherein the positive well bias voltage is approximately 7.5 volts.
4. The flash memory device of claim 1 , wherein the voltage circuit comprises a pump circuit.
5. The flash memory device of claim 1 , wherein the negative word line voltage is higher in magnitude than VCC.
6. The flash memory device of claim 5 , wherein the negative word line voltage is approximately −7.5 volts.
7. The flash memory device of claim 1 , wherein the positive word line voltage is lower than VCC.
8. The flash memory device of claim 7 , wherein the positive word line voltage is approximately 2.5 volts.
9. The flash memory device of claim 1 , wherein each group of local word lines comprises a sector.
10. The flash memory device of claim 1 , wherein a plurality of groups of local word lines comprises a block.
11. The flash memory device of claim 1 , further comprising a plurality of bit lines, and each of the plurality of flash memory cells is connected to a corresponding one of the plurality of bit lines.
12. The flash memory device of claim 11 , wherein each of the plurality of bit lines is configured to be floating during the erase operation.
13. The flash memory device of claim 1 , wherein the plurality of flash memory cells are NOR flash memory cell transistors.
14. The flash memory device of claim 1 , wherein the plurality of flash memory cells are organized in a parallel flash array.
15. The flash memory device of claim 1 , wherein the plurality of flash memory cells are organized in a serial flash array.