# Patent

## Title
8-transistor SRAM cell design with outer pass-gate diodes

## Abstract

An 8-transistor SRAM cell which includes two pull-up transistors and two pull-down transistors in cross-coupled inverter configuration for storing a single data bit; first and second pass-gate transistors having a gate terminal coupled to a write word line and a source or drain of each of the pass-gate transistors coupled to a write bit line through a series outer diode between the pass-gate and the write bit line oriented to block charge transfer from the write bit line into the cell; and first and second read transistors coupled to the two pull-up and two pull-down transistors, one of the read transistors having a gate terminal coupled to a read word line and a source or a drain coupled to a read bit line. The 8-transistor SRAM cell is adapted to prevent the value of the bit stored in the cell from changing state.

## Background
RELATED APPLICATION
This application is related to U.S. patent application Ser. No. 13/345,619 entitled “8-TRANSISTOR SRAM CELL DESIGN WITH SCHOTTKY DIODES”, filed even date herewith, and U.S. patent application Ser. No. 13/345,629 entitled “8-TRANSISTOR SRAM CELL DESIGN WITH INNER PASS-GATE JUNCTION DIODES”, filed even date herewith, the disclosures of which are incorporated by reference herein.
BACKGROUND
The present invention relates to 8-transistor SRAM cell designs and, more particularly, relates to an 8-transistor SRAM cell design having outer pass-gate diodes to enable column select functionality.
A static random access memory (SRAM) is a significant memory device due to its high speed, low power consumption and simple operation. Unlike a dynamic random access memory (DRAM) cell, the SRAM does not need to regularly refresh the stored data as SRAM uses bistable latching circuitry to store each bit.
As variability concerns mount in future complementary metal oxide semiconductor (CMOS) technologies, SRAM cell stability, which depends on delicately balanced transistor characteristics, becomes a significant concern.
In traditional 6-transistor SRAM, cells must be both stable during a read event and writeable during a write event. Ignoring redundancy, such functionality must be preserved for each cell under worst-case variation. For cell stability during a read, it is desirable to strengthen the storage inverters and weaken the pass-gates. The opposite is desired for cell writeability: a weak storage inverter and strong pass-gates. This delicate balance of transistor strength ratios can be severely impacted by device variations, which dramatically degrade stability and write margins, especially in scaled technologies.
In a 6-transistor SRAM cell, variability tolerance is compromised by the conflicting needs of cell read stability and writeability. Because the same pass-gate devices are used to both read and write the cell, it is inevitable that the two conditions cannot be simultaneously optimized.
In an 8-transistor SRAM cell, two transistors are added to create a disturb-free read mechanism. Since read and write are controlled by separate devices within the cell, the two are entirely decoupled—a level that 6-transistor SRAM cells can never reach. This widens the cell optimization space to achieve sufficient stability and writeability margins.
While the 8-transistor cell solves read stability issues, a similar problem arises during a write operation if column select functionality (also known as half-select, partial write, or masked write) is desired. In such a scenario, the write word line is activated, but it is desired that only some of the bits tied to this write word line are written. This is a common technique used in 6-transistor SRAM arrays to facilitate bit interleaving and array floorplanning and is achieved by appropriately biasing the bit lines in each column. Those bits that are not to be written experience a bias comparable to a read disturb. In 8-transistor SRAM, operating the memory in such a fashion would unintentionally write other bits in other columns due to the strong pass-gates employed. In existing 8-transistor SRAM designs, column select functionality is thus prohibited.
BRIEF SUMMARY
The various advantages and purposes of the exemplary embodiments as described above and hereafter are achieved by providing, according to a first aspect of the exemplary embodiments, an 8-transistor SRAM cell which includes two pull-up transistors and two pull-down transistors in cross-coupled inverter configuration for storing a single data bit; first and second pass-gate transistors having a gate terminal coupled to a write word line and a source or drain of each of the pass-gate transistors coupled to a write bit line, each of the pass-gate transistors having an outer diode between the pass-gate and the write bit line; and first and second read transistors coupled to the two pull-up and two pull-down transistors, one of the read transistors having a gate terminal coupled to a read word line and a source or a drain coupled to a read bit line; wherein the 8-transistor SRAM cell is adapted to prevent the value of the bit stored in the cell from changing state while the first and second read transistors affect the signal asserted during the read operation on the read bit line coupled to the cell.
According to a second aspect of the exemplary embodiments, there is provided a memory device which includes a plurality of 8-transistor SRAM cells arranged in columns and rows. Each 8-transistor SRAM cell includes two pull-up transistors and two pull-down transistors in cross-coupled inverter configuration for storing a single data bit; first and second pass-gate transistors having a gate terminal coupled to a write word line and a source or drain of each of the pass-gate transistors coupled to a write bit line, each of the pass-gate transistors having an outer diode between the pass-gate and the write bit line; and first and second read transistors coupled to the two pull-up and two pull-down transistors, one of the read transistors having a gate terminal coupled to a read word line and a source or a drain coupled to a read bit line. The 8-transistor SRAM cell is adapted to prevent the value of the bit stored in the cell from changing state while the first and second read transistors affect the signal asserted during the read operation on the read bit line coupled to the cell.
According to a third aspect of the exemplary embodiments, there is provided an integrated circuit which includes a memory device. The memory device includes: a plurality of 8-transistor SRAM cells arranged in columns and rows, with each 8-transistor SRAM cell including: two pull-up transistors and two pull-down transistors in cross-coupled inverter configuration for storing a single data bit; first and second pass-gate transistors having a gate terminal coupled to a write word line and a source/drain of each of the pass-gate transistors coupled to a write bit line through a series outer diode between the pass-gate and the write bit line; and first and second read transistors coupled to the two pull-up and two pull-down transistors, one of the read transistors having a gate terminal coupled to a read word line and a source or a drain coupled to a read bit line. The 8-transistor SRAM cell is adapted to prevent the value of the bit stored in the cell from changing state while the first and second read transistors affect the signal asserted during the read operation on the read bit line coupled to the cell.



BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
The features of the exemplary embodiments believed to be novel and the elements characteristic of the exemplary embodiments are set forth with particularity in the appended claims. The Figures are for illustration purposes only and are not drawn to scale. The exemplary embodiments, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:
FIG. 1 is a circuit schematic of an 8-transistor SRAM cell.
FIG. 2 is a circuit schematic of an exemplary embodiment of an 8-transistor SRAM cell.
FIG. 3 is a top view of a layout of an 8-transistor SRAM cell.
FIG. 4 is a cross-sectional view of FIG. 3 in the direction of arrows A-A showing an exemplary embodiment.
FIG. 5 is a cross-sectional view of FIG. 3 in the direction of arrows A-A showing another exemplary embodiment.



DETAILED DESCRIPTION
The present exemplary embodiments are directed to an 8-transistor SRAM cell in which column select writing functionality is enabled without creating cell disturbs during column select writing. In the exemplary embodiments, the pass-gates in the 8-transistor SRAM cell are altered to permit higher performance in the pull-down (write mode) configuration and lower performance in the pull-up mode, thus retaining the write margin advantage of the 8-transistor SRAM cell. By keeping the pull-up mode for the pass-gate weak, column select writing mode is permitted without disturbing adjacent bits.
Referring to the Figures in more detail, and particularly referring to FIG. 1, there is illustrated a block diagram of an 8-transistor SRAM 100 which includes cross-coupled inverters 102 for storing a bit and pass-gates 104 for writing a bit. Bits are read through read transistors 106. As noted previously, the two read transistors 106 are added to create a disturb-free read mechanism. Since read and write are controlled by separate devices within the 8-transistor SRAM cell, the two functions of read and write are entirely decoupled.
However, as also noted previously, column select functionality is prohibited in the 8-transistor SRAM cell. The reason for this is that the 8-transistor SRAM cell is optimized for writing so that any bit on a write word line may write to all cells on the write word line, whether intended or not.
FIG. 2 is a block diagram of an 8-transistor SRAM cell (or just cell) in accordance with an exemplary embodiment. With reference to FIG. 2, the cell 200 may include a plurality of transistors. A first group 204 of the plurality of transistors may be employed for storing data in the cell 200. A second group 206 of the plurality of transistors may be employed for affecting a signal coupled to the second group 206 of the plurality of transistors. The second group 206 may be mutually exclusive from the first group 204.
More specifically, the first group 204 may include a first transistor 208 (such as an N-type field effect transistor (NFET)) coupled (such as via a gate terminal) to a first word line (such as a write word line (WWL)) 210 such that the first word line 210 may serve to activate the first transistor 208. The first transistor 208 may be coupled (such as via a source/drain terminal) to the input of a first logic device 214, such as an inverter and the output of a second logic device 216, such as an inverter. The other source/drain terminal of transistor 208 may be coupled to a first bit line 212 (such as a write bit line (WBL)) through a diode 240. The diode 240, which is oriented to block current flow into the cell from the first bit line 212, may be a separate device or, more preferably, integrated into the existing SRAM cell structure to minimize cell size.
Although, the first and second logic devices 214, 216 are shown as inverters, it should be understood that each such inverter may include a pair of coupled transistors (such as an NFET coupled with a P-type field effect transistor (PFET)). The first group 204 may include a second transistor 218, such as an NFET, coupled (such as via a gate terminal) to the first word line 210 such that the first word line 210 may serve to activate the second transistor 218.
The second transistor 218 may be coupled (such as via a source/drain terminal) to the output of the first logic device 214 and the input of the second logic device 216. The other source/drain terminal of transistor 218 may be coupled to a second bit line 220 (such as a write bit line complement (WBLb)) through a diode 242. The diode 242, which is oriented to block current flow into the cell from the second bit line 220, may be a separate device or, more preferably, integrated into the existing SRAM cell structure to avoid any area penalty. A voltage of a first node 222 at the input of the first logic device 214 and the output of the second logic device 216 may serve as a value (e.g., of a bit) stored in the cell 200. Alternatively, a voltage at another node included in the first group 204 may serve as the value stored in the cell 200. For example, the voltage of a second node 224 at the output of the first logic device 214 and the input of the second logic device 216 may serve as the value stored in the cell 200. Although the first and second transistors 208, 218 are coupled to the same word line 210, in some embodiments, the first and second transistors 208, 218 may be coupled to different word lines. In this manner, in some embodiments, the first group 204 may include six transistors and two integrated diodes.
The second group 206 may include a seventh transistor 226 (e.g., an NFET) of the cell 200 coupled (e.g., via a gate terminal) to a read word line (RWL) 228 such that the second word line 228 may serve to activate the seventh transistor 226. The seventh transistor 226 may be coupled (e.g., via a drain or source terminal) to a third bit line 230 (RBL). During a read operation, the second group 206 may cause the value of a signal on the third bit line 230 to track the value stored by the cell 200 (e.g., the value of the first node 222 of the first group 204). Therefore, the third bit line 230 may serve as an output of the cell 200.
Further, the second group 206 may include an eighth transistor 232 (e.g., NFET) of the cell 200 coupled to the seventh transistor 226 (e.g., a source or drain terminal of the seventh transistor 226) via a drain or source terminal. The eighth transistor 232 may be coupled (e.g., via a source or drain terminal) to a low voltage, such as ground. Additionally, the eighth transistor 232 may be coupled (e.g., via a gate terminal) to the second node 224 such that the voltage of the second node 224, which is related to the voltage of the first node 222 that serves as the value stored in the cell 200, may serve to activate the eighth transistor 232. In this manner, the second group 206 may include two transistors. Although the cell 200 includes NFET transistors and logic devices, such as inverters (each of which may include an NFET coupled to a PFET), different types of transistors and/or logic devices may be employed.
The cell 200 may comprise one or more portions of a memory 234. More specifically, although only one cell is shown, the memory 234 may include a plurality of cells 200 arranged into rows and/or columns. The memory 234 may form part of an integrated circuit.
It would be desirable to perform the column select function in an 8-transistor SRAM cell, but as noted above, it is typically prohibited because of write disturbs. It is the presence of diode 240 adjacent to pass-gate 208 and diode 242 adjacent to pass-gate 218 that allows the enablement of column select in the cell 200. In the exemplary embodiments, the pass-gates 208 and 218 in series combination with the diodes 240 and 242 permit higher performance in the pull-down mode (writing a logical ‘0’ from the bit line into the cell) configuration and lower performance in the pull-up mode (when a logical ‘1’ on the bit line pulls up/disturbs the cell). Such asymmetric behavior retains the write margin advantage of the 8-transistor SRAM cell while providing for a way to disable writing of a cell by appropriately setting the BL voltages (i.e. column select). When a WWL 210 is activated, those bits to be written should have their WBL 212 and WBLb 220 lines set to the true and complement versions of the new data. Those bits tied to this WWL 210 that should not be written should have both WBL 212 and WBLb 220 held high—with the configured diodes blocking any charge transfer that would otherwise disturb the cell.
FIG. 3 presents a top-down view of an exemplary embodiment of an 8-transistor SRAM cell layout 300, which contains active regions, well isolation regions, gate structures, and contact structures. Additional mask levels are implied and should be well understood by those skilled in the art. The exemplary cell 300 may be fabricated in a conventional semiconductor substrate.
As shown in FIG. 3, pass-gate transistor 302 and pull-down transistor 304 are formed within a connected active region 306 with no isolation between them, and pull-down transistor 308 and pass-gate transistor 310 are formed within a connected active region 312. Pull-up transistors 314 and 316 are formed within active regions 318 and 320, respectively. Further, read transistors 322 and 324 are formed within a connected active region 326. The active regions 306, 312, 318, 320 and 326 may be formed within a semiconductor substrate and are separated from one another by dielectric isolation regions. Gate structures 328 and 330 are arranged above active region 306 to form gates of pass-gate transistor 302 and pull-down transistor 304, respectively. Above active region 312, gate structures 332 and 334 are arranged to form gates of pull-down transistor 308 and pass-gate transistor 310, respectively. In a similar manner, gate structure 332 forms gates of pull-up transistor 314 and one of the read transistors 322, gate structure 330 additionally forms the gate of pull-up transistor 316 and gate structure 336 forms the gate of the second read transistor 324. Also shown in FIG. 3 are contact 338 for coupling with a write word line, contact 340 for also coupling with the write word line, contact 342 for coupling with a write bit line, contact 344 for coupling with the write bit line complement, contact 346 for coupling with the read word line and contact 348 for coupling with the read bit line.
FIG. 4 is a cross-section of FIG. 3 in the direction of arrows A-A′. For purposes of illustration and not limitation, semiconductor structure 400 is a semiconductor on insulator (SOI) structure and includes a semiconductor substrate 402, an insulating layer 404 and top semiconductor on insulator (SOI) layer 406. The insulating layer 404 may be an oxide and may be referred to as a buried oxide layer. Further shown are pass-gate transistor 310, gate structure 334 for pass-gate transistor 310, pull-down transistor 308 and gate structure 332 for pull-down transistor 308. Pass-gate transistor 310 and pull-down transistor 308 are connected by active area 312 within SOI layer.
Pass-gate transistor 310 may have a source/drains 408 and 410 while pull-down transistor 308 may have source/drains 412 and 414. Source/drain 410 may be connected to source/drain 412 in a shared diffusion as shown in FIG. 4. Connected to the various sources/drains 408, 410, 412, 414 may be silicide contacts 416 and metal contacts 418.
As noted previously, pass-gate transistors (208 and 218 in FIGS. 2; 302 and 310 in FIG. 3) may further be connected in series with an outer diode (240 and 242 in FIG. 2). As further noted previously, the outer diode may be a separate device with separate isolation and active regions, but may preferably be integrated into the existing SRAM cell structure to minimize cell area. FIG. 4 illustrates a preferred exemplary embodiment where the diode 420 is formed as part of the source/drain 408 for pass-gate transistor 310. The diode 420 may be formed by epitaxial growth of a p-type layer 422 (for example, boron or boron fluoride (BF2)) followed by epitaxial growth of an n-type layer 424 (for example, phosphorus or arsenic). The combination of p-type layer 422 and n-type layer 424 form a pn junction diode 420. In order to make good electrical contact between the pass-gate source/drain region 408 and the p-type terminal 422 of diode 420, it is important to ensure that the pn junction between these two regions be heavily doped on both sides (i.e. p+ and n+ doping) to enable a tunneling junction contact. Such junction engineering can be achieved through known silicon epitaxy techniques. While FIG. 4 focuses on the formation of diode 420 on the source/drain terminal of pass-gate transistor 310, it should be noted that an analogous structure can be used to form a corresponding diode on pass-gate transistor 302, which is connected to the complementary WBL.
An alternative embodiment is illustrated in FIG. 5, in which the pn junction diode 520 may be formed without any additional process steps over that needed for many standard CMOS process flows. In many advanced technologies, n-type and p-type silicon epitaxial steps are often already used to improve the parasitic source/drain resistance of NFETs and PFETs, respectively. In such technologies, the source/drain region 508 may be exposed to both epitaxial process steps through simple adjustment of block-level masks. The p-type epitaxial layer 522 may be formed when the semiconductor wafer undergoes epitaxial growth for PFETs and the n-type epitaxial layer 524 may be formed when the semiconductor wafer undergoes epitaxial growth for NFETs. This n-type epitaxial step would likely result in raised source/drain structures 526 on the n-type source drain regions.
It will be apparent to those skilled in the art having regard to this disclosure that other modifications of the exemplary embodiments beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.

## Claims

1. An 8-transistor SRAM cell comprising:
two pull-up transistors and two pull-down transistors in cross-coupled inverter configuration for storing a single data bit;
first and second pass-gate transistors having a gate terminal coupled to a write word line and a source or drain of each of the pass-gate transistors coupled to a write bit line, each of the pass-gate transistors having an outer diode between the pass-gate and the write bit line oriented to block charge transfer from the write bit line into the cell; and
first and second read transistors coupled to the two pull-up and two pull-down transistors, one of the read transistors having a gate terminal coupled to a read word line and a source or a drain coupled to a read bit line;
wherein the 8-transistor SRAM cell is adapted to prevent the value of the bit stored in the cell from changing state while the first and second read transistors affect the signal asserted during the read operation on the read bit line coupled to the cell.

2. The 8-transistor SRAM cell of claim 1 wherein the 8-transistor SRAM cell has column select writing functionality enabled to write a value to the 8-transistor SRAM cell, the write bit lines of the 8-transistor SRAM cell being set to a true and complement version of the value while avoiding inadvertently writing to other 8-transistor SRAM cells tied to the same write word line by setting write bit lines of the other 8-transistor SRAM cells to a logical high value, wherein the other 8-transistor SRAM cells have outer diodes oriented to block charge transfer from the write bit lines into the other 8-transistor SRAM cells.

3. The 8-transistor SRAM cell of claim 1 wherein the 8-transistor SRAM cell has column select writing functionality enabled.

4. The 8-transistor SRAM cell of claim 1 wherein the outer diode is a PN diode connected to the source/drain region of each of the pass-gates and oriented to block charge transfer from the write bit line into the cell.

5. The 8-transistor SRAM cell of claim 1 wherein the outer diode is a PN diode connected to the source/drain region of each of the pass-gates and oriented to block charge transfer from the write bit line into the cell and the 8-transistor SRAM cell has column select writing functionality enabled.

6. The 8-transistor SRAM cell of claim 1 wherein the outer diode is a PN diode formed on top of a source/drain region of each of the pass-gates with a P+/N+ tunneling junction contact.

7. The 8-transistor SRAM cell of claim 1 wherein the outer diode is a PN diode formed on top of a source/drain region of each of the pass-gates with a P+/N+ tunneling junction contact and the 8-transistor SRAM cell has column select writing functionality enabled.

8. A memory device comprising:
a plurality of 8-transistor SRAM cells arranged in columns and rows, with each 8-transistor SRAM cell comprising:
two pull-up transistors and two pull-down transistors in cross-coupled inverter configuration for storing a single data bit;
first and second pass-gate transistors having a gate terminal coupled to a write word line and a source/drain of each of the pass-gate transistors coupled to a write bit line through a series outer diode between the pass-gate and the write bit line; and
first and second read transistors coupled to the two pull-up and two pull-down transistors, one of the read transistors having a gate terminal coupled to a read word line and a source or a drain coupled to a read bit line;
wherein the 8-transistor SRAM cell is adapted to prevent the value of the bit stored in the cell from changing state while the first and second read transistors affect the signal asserted during the read operation on the read bit line coupled to the cell.

9. The memory device of claim 8 wherein the memory device has column select functionality enabled to write a value to one 8-transistor SRAM cell of the plurality of 8-transistor SRAM cells, the write bit lines of the one 8-transistor SRAM cell being set to a true and complement version of the value, while avoiding inadvertently writing to other 8-transistor SRAM cells of the plurality of 8-transistor SRAM cells tied to the same write word line by setting write bit lines of the other 8-transistor SRAM cell to a logical high value, wherein the other 8-transistor SRAM cells have outer diodes oriented to block charge transfer from the write bit lines into the other 8-transistor SRAM cells.

10. The memory device of claim 8 wherein the memory device has column select functionality enabled.

11. The memory device of claim 8 wherein the plurality of 8-transistor SRAM cells are arranged in columns and rows and wherein the memory device has column select writing enabled to write a value to an 8-transistor SRAM cell without inadvertently also writing a value to an 8-transistor SRAM cell in the same column or an adjacent column.

12. The memory device of claim 8 wherein the outer diode is a PN diode connected to the source/drain region of each of the pass-gates and oriented to block charge transfer from the write bit line into the cell.

13. The memory device of claim 8 wherein the outer diode is a PN diode connected to the source/drain region of each of the pass-gates and oriented to block charge transfer from the write bit line into the cell and the memory device has column select functionality enabled.

14. The memory device of claim 8 wherein the outer diode is PN diode formed on top of a source/drain region of each of the pass-gates with a P+/N+ tunneling junction contact.

15. The memory device of claim 8 wherein the outer diode is PN diode formed on top of a source/drain region of each of the pass-gates with a P+/N+ tunneling junction contact and the memory device has column select functionality enabled.

16. An integrated circuit comprising:
a memory device comprising:
a plurality of 8-transistor SRAM cells arranged in columns and rows, with each 8-transistor SRAM cell comprising:
two pull-up transistors and two pull-down transistors in cross-coupled inverter configuration for storing a single data bit;
first and second pass-gate transistors having a gate terminal coupled to a write word line and a source/drain of each of the pass-gate transistors coupled to a write bit line through a series outer diode between the pass-gate and the write bit line; and
first and second read transistors coupled to the two pull-up and two pull-down transistors, one of the read transistors having a gate terminal coupled to a read word line and a source or a drain coupled to a read bit line;
wherein the 8-transistor SRAM cell is adapted to prevent the value of the bit stored in the cell from changing state while the first and second read transistors affect the signal asserted during the read operation on the read bit line coupled to the cell.

17. The integrated circuit of claim 16 wherein the memory device has column select functionality enabled to write a value to one 8-transistor SRAM cell of the plurality of 8-transistor SRAM cells, the write bit lines of the one 8-transistor SRAM cell being set to a true and complement version of the value, while avoiding inadvertently writing to other 8-transistor SRAM cells of the plurality of 8-transistor SRAM cells tied to the same write word line by setting write bit lines of the other 8-transistor SRAM cell to a logical high value, wherein the other 8-transistor SRAM cells have outer diodes oriented to block charge transfer from the write bit lines into the other 8-transistor SRAM cells.

18. The integrated circuit of claim 16 wherein the memory device has column select functionality enabled.

19. The memory device of claim 16 wherein the plurality of 8-transistor SRAM cells are arranged in columns and rows and wherein the memory device has column select writing enabled to write a value to an 8-transistor SRAM cell without inadvertently also writing a value to an 8-transistor SRAM cell in the same column or an adjacent column.

20. The memory device of claim 16 wherein the outer diode is a PN diode connected to the source/drain region of each of the pass-gates and oriented to block charge transfer from the write bit line into the cell.