Memory device and program operation thereof

1. A memory device, comprising:

a first memory string, the first memory string comprising a first drain, a first drain select gate (DSG) transistor, a plurality of first memory cells, and a first drain dummy transistor between the first drain and the first DSG transistor;

a first bit line coupled to the first drain;

a first drain dummy line coupled to the first drain dummy transistor;

a first DSG line coupled to the first DSG transistor;

a plurality of word lines respectively coupled to the first memory cells; and

a peripheral circuit configured to perform a program operation on a target memory cell of the first memory cells coupled to a selected word line of the word lines, wherein to perform the program operation, the peripheral circuit comprises a bit line driver coupled to the first bit line and configured to apply a first bit line voltage to select the first bit line, and a word line driver coupled to the first drain dummy line and the first DSG line and configured to:

apply a DSG voltage to the first DSG line to turn on the first DSG transistor; and

apply a drain dummy line voltage to the first drain dummy line to turn on the first drain dummy transistor, the drain dummy line voltage being greater than the DSG voltage and being greater than a threshold voltage of the first drain dummy transistor at a lower bound of an operating temperature range of the memory device.

2. The memory device of claim 1 , wherein the lower bound of the operating temperate range is −40° C.

3. The memory device of claim 1 , wherein

the first memory string is a three-dimensional (3D) NAND memory string; and

the first DSG line is above the word lines, and the first drain dummy line is above the first DSG line.

4. The memory device of claim 1 , wherein

the first memory cells comprise a dummy memory cell between the first DSG transistor and the target memory cell;

the word lines comprise a dummy word line coupled to the dummy memory cell; and

the word line driver is coupled to the dummy word line and further configured to apply a dummy word line voltage to the dummy word line, the dummy word line voltage being different from the drain dummy line voltage.

5. The memory device of claim 1 , wherein to perform the program operation, the word line driver is coupled to the word lines and further configured to:

apply a program voltage to the selected word line; and

apply a channel pass voltage to each of a rest of the word lines, the program voltage being greater than the channel pass voltage.

6. The memory device of claim 1 , further comprising:

a second memory string comprising a second drain, a second DSG transistor, a plurality of second memory cells, and a second drain dummy transistor between the second drain and the second DSG transistor;

a second bit line coupled to the second drain;

a second drain dummy line coupled to the second drain dummy transistor; and

a second DSG line coupled to the second DSG transistor;

wherein the plurality of word lines are respectively coupled to the second memory cells; and

to perform the program operation, the bit line driver is coupled to the second bit line and further configured to apply a second bit line voltage to deselect the second bit line, and the word line driver is coupled to the second DSG line and the second drain dummy line and further configured to apply a 0-V voltage to the second DSG line and the second drain dummy line.

7. The memory device of claim 6 , wherein the first and second DSG lines are electrically separated, and the first and second drain dummy lines are electrically separated.

8. A system, comprising:

a memory device configured to store data, the memory device comprising:

a first memory string comprising a first drain, a first drain select gate (DSG) transistor, a plurality of first memory cells, and a first drain dummy transistor between the first drain and the first DSG transistor, the plurality of first memory cells comprising a dummy memory cell between the first DSG transistor and a target memory cell;

a first bit line coupled to the first drain;

a first drain dummy line coupled to the first drain dummy transistor;

a first DSG line coupled to the first DSG transistor;

a plurality of word lines respectively coupled to the first memory cells and comprising a dummy word line coupled to the dummy memory cell; and

a peripheral circuit configured to perform a program operation on the target memory cell of the first memory cells coupled to a selected word line of the word lines, wherein to perform the program operation, the peripheral circuit comprises a bit line driver coupled to the first bit line and configured to apply a first bit line voltage to select the first bit line, and a word line driver coupled to the first DSG line, the dummy word line, and the first drain dummy line and configured to:

apply a DSG voltage to the first DSG line to turn on the first DSG transistor;

apply a drain dummy line voltage to the first drain dummy line to turn on the first drain dummy transistor, the drain dummy line voltage being greater than the DSG voltage; and

apply a dummy word line voltage to the dummy word line, the dummy word line voltage being different from the drain dummy line voltage; and

a memory controller coupled to the memory device and configured to control the memory device.

9. The system of claim 8 , further comprising a host coupled to the memory controller and configured to send or receive the data.

10. The system of claim 8 , wherein the drain dummy line voltage is greater than a threshold voltage of the first drain dummy transistor at a lower bound of an operating temperate range of the memory device.

11. The system of claim 8 , wherein

the first memory string is a three-dimensional (3D) NAND memory string; and

the first DSG line is above the word lines, and the first drain dummy line is above the first DSG line.

12. The system of claim 8 , wherein to perform the program operation, the word line driver is coupled to the word lines and further configured to:

apply a program voltage to the selected word line; and

apply a channel pass voltage to each of a rest of the word lines, the program voltage being greater than the channel pass voltage.

13. The system of claim 10 , wherein the lower bound of the operating temperate range is −40° C.

14. A method for operating a memory device, the memory device comprising a first memory string comprising a first drain, a first drain select gate (DSG) transistor, a plurality of first memory cells, and a first drain dummy transistor between the first drain and the first DSG transistor; a first bit line coupled to the first drain; a first drain dummy line coupled to the first drain dummy transistor; a first DSG line coupled to the first DSG transistor; and a plurality of word lines respectively coupled to the first memory cells; the method comprising:

performing a program operation on a target memory cell of the first memory cells coupled to a selected word line of the word lines, wherein performing the program operation comprises:

applying a first bit line voltage to select the first bit line;

applying a DSG voltage to the first DSG line to turn on the first DSG transistor; and

applying a drain dummy line voltage to the first drain dummy line to turn on the first drain dummy transistor, the drain dummy line voltage being greater than the DSG voltage and being greater than a threshold voltage of the first drain dummy transistor at a lower bound of an operating temperature range of the memory device.

15. The method of claim 14 , wherein the lower bound of the operating temperate range is −40° C.

16. The method of claim 14 , wherein

the first memory string is a three-dimensional (3D) NAND memory string; and

the first DSG line is above the word lines, and the first drain dummy line is above the first DSG line.

17. The method of claim 14 , wherein

the first memory cells comprise a dummy memory cell between the first DSG transistor and the target memory cell;

the word lines comprise a dummy word line coupled to the dummy memory cell; and

performing the program operation further comprises applying a dummy word line voltage to the dummy word line, the dummy word line voltage being different from the drain dummy line voltage.

18. The method of claim 14 , wherein performing the program operation further comprises:

applying a program voltage to the selected word line; and

applying a channel pass voltage to each of a rest of the word lines, the program voltage being greater than the channel pass voltage.

19. The method of claim 14 , wherein

the memory device further comprises a second memory string comprising a second drain, a second DSG transistor, a plurality of second memory cells, and a second drain dummy transistor between the second drain and the second DSG transistor; a second bit line coupled to the second drain; a second drain dummy line coupled to the second drain dummy transistor; and a second DSG line coupled to the second DSG transistor;

the plurality of word lines are respectively coupled to the second memory cells of the second memory string; and

performing the program operation further comprises:

applying a second bit line voltage to deselect the second bit line; and

applying a 0-V voltage to the second DSG line and the second drain dummy line.

20. The method of claim 19 , wherein the first and second DSG lines are electrically separated, and the first and second drain dummy lines are electrically separated.