Non-volatile memory device, memory system including the same, and method of operating the same

1. A method of operating a non-volatile memory device, the method comprising:

setting a search region defined by a start read voltage and an end read voltage;

determining whether the search region belongs to a reference region;

changing the search region when it is determined that the search region does not belong to the reference region; and

searching for a new read voltage based on the search region when it is determined that the search region belongs to the reference region.

2. The method of claim 1 , wherein the determining whether the search region belongs to the reference region comprises:

counting a number of first memory cells having a threshold voltage lower than the start read voltage and comparing the number of first memory cells with a number of first reference memory cells; and

counting a number of second memory cells having a threshold voltage lower than the end read voltage and comparing the number of second memory cells with a number of second reference memory cells.

3. The method of claim 2 , wherein the changing the search region comprises:

increasing the start read voltage and the end read voltage by a first shift voltage when the number of first memory cells is not greater than the number of first reference memory cells; and

decreasing the start read voltage and the end read voltage by a second shift voltage when the number of second memory cells is greater than the number of second reference memory cells.

4. The method of claim 3 , wherein the number of first memory cells and the number of second reference memory cells are calculated and stored in advance.

5. The method of claim 3 , wherein the number of first reference memory cells is a number of memory cells having a threshold voltage lower than a first reference threshold voltage; the number of second reference memory cells is a number of memory cells having a threshold voltage lower than a second reference threshold voltage; and the first and second reference threshold voltages are threshold voltages that have maximum numbers of memory cells in adjacent threshold voltage distributions, respectively.

6. The method of claim 1 , wherein the determining whether the search region belongs to the reference region comprises:

counting a number of first memory cells having a threshold voltage higher than the start read voltage and comparing the number of first memory cells with a number of first reference memory cells; and

counting a number of second memory cells having a threshold voltage higher than the end read voltage and comparing the number of second memory cells with a number of second reference memory cells.

7. The method of claim 6 , wherein the changing the search region comprises:

increasing the start read voltage and the end read voltage by a first shift voltage when the number of first memory cells is greater than the number of first reference memory cells; and

decreasing the start read voltage and the end read voltage by a second shift voltage when the number of second memory cells is not greater than the number of second reference memory cells.

8. The method of claim 7 , wherein the number of first memory cells and the number of second reference memory cells are calculated and stored in advance.

9. The method of claim 7 , wherein the number of first reference memory cells is a number of memory cells having a threshold voltage higher than a first reference threshold voltage; the number of second reference memory cells is a number of memory cells having a threshold voltage higher than a second reference threshold voltage; and the first and second reference threshold voltages are threshold voltages that have maximum numbers of memory cells in adjacent threshold voltage distributions, respectively.

10. The method of claim 1 , wherein the searching for the new read voltage comprises:

dividing a region between the start read voltage and the end read voltage into at least two ranges and counting a number of memory cells in each of the ranges when it is determined that the search region belongs to the reference region; and

searching for the new read voltage using a result of counting the number of memory cells in each of the ranges.

11. The method of claim 10 , wherein the counting the number of memory cells in each of the ranges comprises:

counting a number of memory cells having a threshold voltage belonging to a first threshold voltage range that has the start read voltage as a start point to obtain a first range cell count;

counting a number of memory cells having a threshold voltage belonging to a second threshold voltage range that has the end read voltage as an end point to obtain a second range cell count; and

searching for an optimal read voltage using the first range cell count and the second range cell count.

12. The method of claim 1 , wherein the non-volatile memory device comprises a three-dimensional memory cell array, the three-dimensional memory cell array comprises first memory cells connected to a first word line and second memory cells which are connected to a second word line and positioned above the first memory cells, and the search region or the searching for the new read voltage is different between the first word line and the second word line.

13. A method of operating a non-volatile memory device, the method comprising:

comparing a first start read voltage with a first reference threshold voltage;

comparing a first end read voltage with a second reference threshold voltage;

changing the first start read voltage and the first end read voltage based on both comparison results to generate a second start read voltage and a second end read voltage; and

searching for a new read voltage based on the second start read voltage and the second end read voltage that have been changed.

14. The method of claim 13 , wherein the changing the first start read voltage and the first end read voltage comprises:

increasing the first start read voltage and the first end read voltage by a first shift voltage when the first start read voltage is not greater than the first reference threshold voltage; and

decreasing the first start read voltage and the first end read voltage by a second shift voltage when the first end read voltage is greater than the second reference threshold voltage.

15. The method of claim 13 , wherein the comparing the first start read voltage with the first reference threshold voltage comprises:

counting a number of first memory cells having a threshold voltage lower than the first start read voltage; and

comparing the number of first memory cells with a number of first reference memory cells,

wherein the number of first reference memory cells is a number of memory cells having a threshold voltage lower than the first reference threshold voltage and is calculated without actual counting.

16. The method of claim 15 , wherein the comparing the first end read voltage with the second reference threshold voltage comprises:

counting a number of second memory cells having a threshold voltage lower than the first end read voltage; and

comparing the number of second memory cells with a number of second reference memory cells,

wherein the number of second reference memory cells is a number of memory cells having a threshold voltage lower than the second reference threshold voltage and is calculated without actual counting.

17. The method of claim 15 , wherein the comparing the first start read voltage with the first reference threshold voltage further comprises comparing the number of first memory cells with a number of third reference memory cells when a difference between the number of first memory cells and the number of first reference memory cells is greater than a threshold value.

18. The method of claim 13 , wherein the comparing the first start read voltage with the first reference threshold voltage comprises:

counting a number of first memory cells having a threshold voltage higher than the first start read voltage; and

comparing the number of first memory cells with a number of first reference memory cells,

wherein the number of first reference memory cells is a number of memory cells having a threshold voltage higher than the first reference threshold voltage and is calculated without actual counting.

19. The method of claim 18 , wherein the comparing the first end read voltage with the second reference threshold voltage comprises:

counting a number of second memory cells having a threshold voltage higher than the first end read voltage; and

comparing the number of second memory cells with a number of second reference memory cells,

wherein the number of second reference memory cells is a number of memory cells having a threshold voltage higher than the second reference threshold voltage and is calculated without actual counting.

20. The method of claim 18 , wherein the first shift voltage changes according to a difference between the number of first memory cells and the number of first reference memory cells.