Non-volatile memory devices and methods of operating the same

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

selecting a first select transistor from among a plurality of select transistors included in a NAND string; and

performing a first check operation on a first threshold voltage of the first select transistor,

wherein performing the first check operation comprises:

comparing the first threshold voltage with a first lower-limit reference voltage level; and

performing a program operation on the first select transistor when the first threshold voltage is lower than the first lower-limit reference voltage level,

wherein when the first threshold voltage is equal to or higher than the first lower-limit reference voltage level, the first check operation on the first threshold voltage is ended.

2. The method of claim 1 , further comprising performing a second check operation on a second threshold voltage of a second select transistor from among the plurality of select transistors, when the first threshold voltage is equal to or higher than the first lower-limit reference voltage level.

3. The method of claim 1 , wherein performing the first check operation further comprises comparing the first threshold voltage with a first upper-limit reference voltage level after performing the program operation.

4. The method of claim 3 , wherein when a suspend mode is activated during the first check operation, the first check operation is ended only after comparing the first threshold voltage with the first upper-limit reference voltage level.

5. The method of claim 3 , wherein performing the check operation further comprises processing the NAND string or a memory block including the NAND string as a bad NAND string or a bad block when the first threshold voltage is higher than the first upper-limit reference voltage level.

6. The method of claim 5 , wherein threshold voltages of remaining select transistors from among the plurality of select transistors included in the bad NAND string or the bad block are not checked when the NAND string including the first select transistor is processed as a bad NAND string or when the memory block including the NAND string is processed as a bad block.

7. The method of claim 3 , wherein comparing the first threshold voltage with the first upper-limit reference voltage level is performed before a second check operation on a second threshold voltage of a second select transistor from among the plurality of select transistors.

8. The method of claim 3 , further comprising performing a second check operation on a second threshold voltage of a second select transistor from among the plurality of select transistors, when the first threshold voltage is equal to or lower than the first upper-limit reference voltage level.

9. The method of claim 1 , wherein the plurality of select transistors includes at least one string select transistor and at least one ground select transistor.

10. The method of claim 9 , wherein the at least one string select transistor includes an upper string select transistor and a lower string select transistor adjacent to a plurality of memory cells included in the NAND string, and

the first select transistor is selected from among the upper string select transistor, the lower string select transistor, and the at least one ground select transistor.

11. The method of claim 1 , wherein the first select transistor is a programmable cell-type transistor.