Write assist cell for static random access memory

1. A memory device comprising:

a memory array comprising:

a plurality of memory cells arranged along a plurality of rows extending in a row direction and a plurality of columns extending in a column direction; and

a plurality of write assist cells connected to the plurality of memory cells,

wherein at least one write assist cell of the plurality of write assist cells is in each of the plurality of columns and connected to respective ones of the plurality of memory cells in a same column;

wherein each of the plurality of write assist cells comprises:

a first transistor;

a second transistor;

a first access transistor; and

a second access transistor; and

wherein the first transistor is connected to a first bit line through the first access transistor and the second transistor is connected to a second bit line through the second access transistor;

wherein a first subset of memory cells of the plurality of memory cells and a first subset of write assist cells of the plurality of write assist cells are located in the same column, and wherein each memory cell in the first subset of memory cells and each write assist cell in the first subset of write assist cells is connected to the first bit line and the second bit line; and

wherein the first access transistor is controlled based upon a voltage level of the second bit line and the second access transistor is controlled based upon the voltage level of the first bit line.

2. The memory device of claim 1 , wherein each of the plurality of memory cells is a Static Random Access Memory cell.

3. The memory device of claim 1 , wherein each write assist cell in the first subset of write assist cells is controlled by a single enable signal line that extends in the column direction.

4. The memory device of claim 1 , wherein each write assist cell in the first subset of write assist cells is controlled by a different enable signal line that extends in the row direction.

5. The memory device of claim 1 , wherein each write assist cell in the first subset of write assist cells comprises:

a first inverter comprising the first transistor; and

a second inverter comprising the second transistor;

wherein an input of the first inverter is connected to the second bit line and an input of second inverter is connected to the first bit line; and

wherein an output of the first inverter is connected to the first bit line through the first access transistor and an output of the second inverter is connected to the second bit line through the second access transistor.

6. The memory device of claim 1 , wherein each write assist cell in the first subset of write assist cells comprises a write assist control cell and at least one write assist driver cell.

7. The memory device of claim 6 , wherein the write assist control cell comprises:

the first transistor connected to the first bit line through the first access transistor; and

the second transistor connected to the second bit line through the second access transistor,

wherein the first access transistor and the second access transistor are controlled by an enable signal.

8. The memory device of claim 7 , wherein each of the at least one write assist driver cell comprises:

a third transistor connected to the first bit line through a third access transistor, wherein the third transistor is further connected to the enable signal; and

a fourth transistor connected to the second bit line through a fourth access transistor, wherein the fourth transistor is further connected to the enable signal.

9. The memory device of claim 1 , wherein each of the plurality of memory cells and each of the plurality of write assist cells has an area footprint of two contacted polysilicon pitch.

10. A write assist cell comprising:

a write assist control cell comprising:

a first transistor comprising a first terminal connected to ground voltage and a second terminal connected to a first bit line of an SRAM cell;

a second transistor comprising a third terminal connected to the ground voltage and a fourth terminal connected to a second bit line of the SRAM cell;

a first access transistor connected between the second terminal and the first bit line; and

a second access transistor connected between the fourth terminal and the second bit line; and

at least one write assist driver cell, each of the at least one write driver cell comprising:

a third transistor comprising a seventh terminal connected to the first bit line and an eighth terminal connected to the ground voltage;

a fourth transistor comprising a ninth terminal connected to the second bit line and a tenth terminal connected to the ground voltage;

a third access transistor between the seventh terminal and the first bit line; and

a fourth access transistor between the ninth terminal and the second bit line

wherein the first access transistor and the second access transistor are controlled by an enable signal;

wherein a fifth terminal of the first transistor is connected to the second bit line and a sixth terminal of the second transistor is connected to the first bit line;

wherein the write assist cell and the SRAM cell are part of an SRAM array;

wherein the SRAM cell is configured to store data during a write operation and the write assist cell is configured to maintain a voltage level of the first bit line during the write operation;

wherein a twelfth terminal of the third transistor and a thirteenth terminal of the fourth transistor are connected to the enable signal;

wherein the third access transistor is controlled based upon a voltage level at the fourth terminal; and

wherein the fourth access transistor is controlled based upon the voltage level at the second terminal.

11. The write assist cell of claim 10 , wherein each of the at least one write assist driver cell further comprises:

a fifth transistor connected to the third transistor; and

a sixth transistor connected to the fourth transistor.

12. The write assist cell of claim 10 , wherein each of the write assist control cell and each of the at least one write assist driver cell has an area footprint of two contacted polysilicon pitch.

13. The write assist cell of claim 10 , further comprising:

a fifth transistor of the write assist control cell connected to the first transistor forming a first inverter; and

a sixth transistor of the write assist control cell connected to the second transistor forming a second inverter.

14. A write assist cell comprising:

a first transistor comprising a first terminal connected to ground voltage and a second terminal connected to a first bit line of an SRAM cell;

a second transistor comprising a third terminal connected to the ground voltage and a fourth terminal connected to a second bit line of the SRAM cell;

a first access transistor connected between the second terminal and the first bit line; and

a second access transistor connected between the fourth terminal and the second bit line,

wherein the first access transistor is controlled based upon a voltage level at the second bit line;

wherein the second access transistor is controlled based upon the voltage level at the first bit line;

wherein a fifth terminal of the first transistor and a sixth terminal of the second transistor are connected to an enable signal;

wherein the write assist cell and the SRAM cell are part of an SRAM array; and

wherein the SRAM cell is configured to store data during a write operation and the write assist cell is configured to maintain a voltage level of the first bit line during the write operation.

15. The write assist cell of claim 14 , wherein an area footprint of the write assist cell and the SRAM cell is substantially same.

16. The write assist cell of claim 14 , comprising a write assist control cell and at least one write assist driver cell, and wherein the first transistor, the second transistor, the first access transistor, and the second access transistor are part of the write assist control cell.

17. The write assist cell of claim 14 , wherein the SRAM cell and the write assist cell each has an area footprint of two contacted polysilicon pitch.

18. The write assist cell of claim 14 , further comprising:

a first inverter comprising the first transistor; and

a second inverter comprising the second transistor.

19. The write assist cell of claim 18 , wherein an input of the first inverter is connected to the second bit line and an input of second inverter is connected to the first bit line.

20. The write assist cell of claim 18 , wherein an output of the first inverter is connected to the first bit line through the first access transistor and an output of the second inverter is connected to the second bit line through the second access transistor.