Nonvolatile memory device and a method of adjusting a threshold voltage of a ground selection transistor thereof

1. A method of adjusting a threshold voltage of a ground selection transistor in a nonvolatile memory device, the method comprising:

providing a first voltage to a gate of a first ground selection transistor in a read operation; and

providing a second voltage to a gate of a second ground selection transistor in the read operation, wherein the nonvolatile memory device includes at least one string, the string having string selection transistors, memory cells and the first and second ground selection transistors connected in series and stacked on a substrate.

2. The method of claim 1 , wherein the second ground selection transistor is formed on the substrate and the first ground selection transistor is formed on the second ground selection transistor.

3. The method of claim 2 , wherein the first and second ground selection transistors are formed by ion implantation.

4. The method of claim 2 , wherein the first voltage is higher than a read pass voltage.

5. The method of claim 4 , wherein the first voltage is lower than a maximum value of a program voltage.

6. The method of claim 4 , wherein a read operation of a selected memory cell is performed by providing a pre-charge voltage to a bit line connected to the string, the read pass voltage to a string selection line connected with a gate of a selected one of the string selection transistors, a read voltage to a word line connected with the selected memory cell, the read pass voltage to word lines connected with unselected word lines of the word lines, the first voltage to a first ground selection line connected with the gate of the first ground selection transistor, and the second voltage to a second ground selection line connected with the gate of the second ground selection transistor.

7. The method of claim 6 , wherein the second voltage is higher than the read pass voltage.

8. The method of claim 6 , wherein the second voltage is the read pass voltage.

9. The method of claim 6 , wherein the read operation of the selected memory cell is repeated.

10. The method of claim 4 , wherein voltages applied to a bit line connected to the string, string selection lines connected with gates of the string selection lines, and word lines connected with the memory cells have a don't care state, and the second voltage is higher than a read pass voltage and lower than a program voltage.

11. The method of claim 4 , wherein voltages applied to a bit line connected to the string, string selection lines connected with gates of the string selection lines, and word lines connected with the memory cells have a don't care state, and the second voltage is a read pass voltage.

12. The method of claim 1 , further comprising:

determining whether a threshold voltage of the first ground selection transistor is adjusted.

13. A nonvolatile memory device, comprising:

a memory cell array having a plurality of memory blocks, at least one of the memory blocks including at least one string having string selection transistors, memory cells, and first and second ground selection transistors connected in series and formed on a substrate;

a read/write circuit configured to read data from or write data to the memory cell array;

an address decoder configured to generate a block selection signal in response to an address;

a block gating circuit configured to select one of the plurality of memory blocks in response to the block selection signal; and

control logic configured to control the read/write circuit, the address decoder, and the block gating circuit,

wherein the control logic adjusts a threshold voltage of at least one of the first and second ground selection transistors by providing a first voltage to a gate of the first ground selection transistor and a second voltage to a gate of the second ground selection transistor in a read operation.

14. The nonvolatile memory device of claim 13 , further comprising:

at least one first dummy cell connected between the string selection transistors and the memory cells; and

at least one second dummy cell connected between the memory cells and the first and second ground selection transistors.

15. The nonvolatile memory device of claim 13 , wherein the first ground selection transistor has the same structure as at least one of the memory cells and the second ground selection transistor has a different structure than the first ground selection transistor.