Memory device with negative thresholds
A method for data storage in a memory that includes a plurality of analog memory cells includes storing data in the memory by writing first storage values to the cells. One or more read reference levels are defined for reading the cells, such that at least one of the read reference levels is negative. After storing the data, second storage values are read from the cells using the read reference levels, so as to reconstruct the stored data. In another disclosed method, data is stored in the memory by mapping the data to first storage values selected from a set of the nominal storage values, and writing the first storage values to the cells. The set of nominal storage values is defined such that at least one of the nominal storage values is negative.
1. A method for data storage in a memory that includes a plurality of analog memory cells, the method comprising:
defining a set of nominal storage values, at least one of which is negative;
storing data in the memory by mapping the data to first storage values selected from the set of the nominal storage values, and writing the first storage values to the cells; and
after storing the data, reading second storage values from the cells using one or more read reference levels, so as to reconstruct the stored data.
2. The method according to claim 1 , wherein writing the first storage values comprises applying respective gate voltages and source voltages to the cells and varying the gate voltages from first gate voltages to second gate voltages so as to add electrical charges to the cells, and wherein writing the at least one negative nominal storage value to a given cell comprises setting the first and second gate voltages applied to the given cell to be lower than the source voltage applied to the given cell.
3. The method according to claim 1 , wherein storing the data comprises writing the negative at least one of the nominal storage values to some of the cells.
4. The method according to claim 1 , wherein at least two of the nominal storage values are negative.
5. The method according to claim 1 , wherein at least one of the read reference levels is negative.
6. The method according to claim 1 , wherein storing the data comprises verifying the first storage values written to the cells using one or more verification reference levels, at least one of which is negative.
7. The method according to claim 6 , wherein verifying the first storage values comprises applying respective gate voltages and source voltages to the cells and measuring source-drain currents flowing though the cells responsively to the applied source and gate voltages, and wherein verifying the first storage value using the at least one negative verification reference level comprises setting the gate voltages to be lower than the respective source voltages.
8. The method according to claim 7 , wherein verifying the first storage values comprises applying multiple Programming and Verification (P&V) iterations to the cells.
9. The method according to claim 8 , wherein applying the multiple P&V iterations comprises modifying parameters of a subsequent P&V iteration based on the source-drain currents measured in a previous P&V iteration.
10. The method according to claim 9 , wherein modifying the parameters comprises selecting a subset of the cells based on the source-drain currents measured in the previous P&V iteration, and applying the subsequent P&V iteration only to the selected subset.
11. The method according to claim 1 , wherein the cells comprise NAND Flash cells.
12. The method according to claim 1 , wherein defining the nominal storage values comprises evaluating a condition and including the at least one negative nominal storage value in the set of the nominal storage values responsively to the evaluated condition.
13. Apparatus for data storage in a memory that includes a plurality of analog memory cells, the apparatus comprising:
programming circuitry, which is coupled to define a set of nominal storage values, at least one of which is negative, and to store data in the memory by mapping the data to first storage values selected from the set of the nominal storage values and writing the first storage values to the cells; and
reading circuitry, which is coupled, after the data was stored, to read second storage values from the cells using one or more read reference levels, so as to reconstruct the stored data.
14. The apparatus according to claim 13 , wherein the programming circuitry is coupled to write the first storage values by applying respective gate voltages and source voltages to the cells and varying the gate voltages from first gate voltages to second gate voltages so as to add electrical charges to the cells, and to set the first and second gate voltages applied to a given cell to be lower than the source voltage applied to the given cell so as to write the at least one negative nominal storage value to the given cell.
15. The apparatus according to claim 13 , wherein the programming circuitry is coupled to write the negative at least one of the nominal storage values to some of the cells.
16. The apparatus according to claim 13 , wherein at least two of the nominal storage values are negative.
17. The apparatus according to claim 13 , wherein at least one of the read reference levels is negative.
18. The apparatus according to claim 13 , wherein the programming circuitry is coupled to verify the first storage values written to the cells using one or more verification reference levels, at least one of which is negative.
19. The apparatus according to claim 18 , wherein the programming circuitry is coupled to verify the first storage values by applying respective gate voltages and source voltages to the cells and measuring source-drain currents flowing though the cells responsively to the applied source and gate voltages, and to set the gate voltages to be lower than the respective source voltages so as to verify the first storage value using the at least one negative verification reference level.
20. The apparatus according to claim 19 , wherein the programming circuitry is coupled to apply multiple Programming and Verification (P&V) iterations to the cells.
21. The apparatus according to claim 20 , wherein the programming circuitry is coupled to modify parameters of a subsequent P&V iteration based on the source-drain currents measured in a previous P&V iteration.
22. The apparatus according to claim 20 , wherein the programming circuitry is coupled to select a subset of the cells based on the source-drain currents measured in the previous P&V iteration, and to apply the subsequent P&V iteration only to the selected subset.
23. The apparatus according to claim 13 , wherein the cells comprise NAND Flash cells.
24. The apparatus according to claim 13 , wherein the programming circuitry is coupled to evaluate a condition and to include the at least one negative nominal storage value in the set of the nominal storage values responsively to the evaluated condition.
25. Apparatus for data storage, comprising:
a memory, which comprises a plurality of analog memory cells;
programming circuitry, which is coupled to define a set of nominal storage values, at least one of which is negative, and to store data in the memory by mapping the data to first storage values selected from the set of the nominal storage values and writing the first storage values to the cells; and
reading circuitry, which is coupled, after the data was stored, to read second storage values from the cells using one or more read reference levels, so as to reconstruct the stored data.