Abstract:
An SRAM cell includes: a first PMOS transistor having a source coupled to a supply voltage; a second PMOS transistor having a source coupled to the supply voltage, a drain coupled to a gate of the first PMOS transistor, and a gate coupled to a drain of the first PMOS transistor; a first write switch module coupled between the first PMOS transistor and a complementary supply voltage; a second write switch module coupled between the second PMOS transistor and the complementary supply voltage; and a read switch module coupled between the gate of the first PMOS transistor and a read bit line, wherein the first write switch module, the second write switch module, and the read switch module are controlled separately to write or read a logic value to or from one or more storage nodes at the drains of the first and second PMOS transistors.

Description:
BACKGROUND 
   The present invention relates generally to integrated circuit designs, and more particularly to a static random access memory (SRAM) device with a low operation voltage. 
   Static random access memory (SRAM) is typically used for the temporary storage of data in a computer system. SRAM retains its memory state without the need of any data refresh operations as long as it is supplied with power. A SRAM device is comprised of an array of “cells,” each of which retains one “bit” of data. A typical SRAM cell may include two cross coupled inverters and two access transistors connecting the inverters to complementary bit-lines. The two access transistors are controlled by word-lines to select the cell for read or write operation. In read operation, the access transistors are switched on to allow the charges retained at storage nodes of the cross coupled inverters to be read via the bit line and its complement. In write operation, the access transistors are switched on and the voltage on the bit line or the complementary bit line is raised to a certain level to flip the memory state of the cell. 
     FIG. 1  schematically illustrates a typical six-transistor SRAM cell  100 . The SRAM cell  100  is comprised of PMOS transistors  102  and  104 , and NMOS transistors  106 ,  108 ,  110  and  112 . The PMOS transistor  102  has its source connected to a supply voltage Vcc, and its drain connected to a drain of the NMOS transistor  106 . The PMOS transistor  104  has its source connected to the supply voltage Vcc, and its drain connected to a drain of the NMOS transistor  108 . The sources of the NMOS transistors  106  and  108  are connected together to a complementary supply voltage, such as ground voltage or Vss. The gates of the PMOS transistor  102  and the NMOS transistor  106  are connected together to a storage node  114 , which is further connected to the drains of the PMOS transistor  104  and the NMOS transistor  108 . The gates of the PMOS transistor  104  and the NMOS transistor  108  are connected together to a storage node  116 , which is further connected to the drains of the PMOS transistor  102  and the NMOS transistor  106 . The NMOS transistor  110  connects the storage node  116  to a bit line BL, and the NMOS transistor  112  connects the storage node  114  to a complementary bit line BLB. The gates of the NMOS transistors  110  and  112  are controlled by a word line WL. When the voltage on the word line WL is a logic “1,” the NMOS transistors  110  and  112  are turned on to allow a bit of data to be read from or written into the storage nodes  114  and  116  via the bit line BL and the complementary bit line BLB. 
   One drawback of the typical six-transistor SRAM cell  100  is that it requires a relatively high operation voltage Vdd, which becomes a bottleneck, for designing new generation SRAMs. As the semiconductor processing technology advances, integrated circuits become smaller in size, and their supply voltage Vcc becomes lower in order to reduce power consumption. However, because the operation voltage Vdd of the conventional SRAM cell  100  has to remain at a certain level, it becomes the bottleneck of the efforts in designing the new generation SRAM with lower supply voltage Vcc. 
     FIG. 2  illustrates a conventional two-port SRAM cell  200  comprised of PMOS transistors  202  and  204 , and NMOS transistors  206 ,  208 ,  210 ,  212 ,  214 , and  216 . In write operation, the NMOS transistors  210  and  212  are turned on for allowing a logic “1” or “0” to be written into the storage nodes  218  and  220 . In read operation, the NMOS transistor  216  is turned on and the read bit line BL is pre-charged to a high voltage. If the voltage at the storage node  218  is high, the NMOS transistor  214  will be turned on and the read bit line BL will be pulled low. If the voltage at the storage node  218  is low, the NMOS transistor  214  will be turned off, and the voltage on the read bit line BL will remain high. 
   It is understood by those skilled in the art of integrated circuit design that although the operation voltage applied to read word line WL can be set lower than that of the conventional six-transistor SRAM cell, the operation voltage applied to write word line WL cannot be lowered significantly. As such, what is needed is to design a new SRAM cell that can operate with low operation voltage in both read and write operation. 
   SUMMARY 
   The present invention discloses a SRAM cell with a relatively low operation voltage. In one embodiment of the present invention, the SRAM cell includes a first PMOS transistor having a source coupled to a supply voltage; a second PMOS transistor having a source coupled to the supply voltage, a drain coupled to a gate of the first PMOS transistor, and a gate coupled to a drain of the first PMOS transistor; a first write switch module coupled between the first PMOS transistor and a complementary supply voltage; a second write switch module coupled between the second PMOS transistor and the complementary supply voltage; and a read switch module coupled between the gate of the first PMOS transistor and a read bit line, wherein the first write switch module, the second write switch module, and the read switch module are controlled separately to write or read a logic value to or from one or more storage nodes at the drains of the first and second PMOS transistors. 
   The construction and method of operation of the invention, however, together with additional objectives and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically illustrates a conventional six-transistor SRAM cell. 
       FIG. 2  schematically illustrates a conventional two-port SRAM cell. 
       FIG. 3  schematically illustrates an eight-transistor SRAM cell in accordance with one embodiment of the present invention. 
       FIG. 4  schematically illustrates a ten-transistor SRAM cell in accordance with one embodiment of the present invention. 
       FIG. 5  illustrates a diagram showing a layout view of various bit lines and word lines of the eight-transistor SRAM cell in accordance with one embodiment of the present invention. 
       FIG. 6  illustrates a diagram showing a layout view of various bit lines and word lines of the eight-transistor SRAM cell in accordance with another embodiment of the present invention. 
       FIG. 7  illustrates a diagram showing a layout view of various bit lines and word lines of the ten-transistor SRAM cell in accordance with one embodiment of the present invention. 
       FIG. 8  illustrates a diagram showing a layout view of various bit lines and word lines of the ten-transistor SRAM cell in accordance with another embodiment of the present invention. 
   

   DESCRIPTION 
   This invention is related to a SRAM device with a relatively low operation voltage. The following merely illustrates various embodiments of the present invention for purposes of explaining the principles thereof. It is understood that those skilled in the art of integrated circuit design will be able to devise various equivalents that, although not explicitly described herein, embody the principles of this invention. 
   References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure or characteristic, but every embodiment may not necessarily include the particular feature, structure or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one of ordinary skill in the art to implement such feature, structure or characteristic in connection with other embodiments whether or not explicitly described. 
     FIG. 3  schematically illustrates an eight-transistor SRAM cell  300  in accordance with one embodiment of the present invention. The SRAM cell  300  is comprised of PMOS transistors  302  and  304 , write switch modules  306  and  308 , and a read switch module  310 . The PMOS transistor  302  has a source coupled to a supply voltage Vcc, and a drain coupled to the write switch module  306 , which is further connected to a complementary supply voltage, such as ground or Vss. The PMOS transistor  304  has a source coupled to the supply voltage Vcc, and a drain coupled to the write switch module  308 , which is further connected to the complementary supply voltage. The gate of the PMOS transistor  302  is coupled to the drain of the PMOS transistor  304 , forming a storage node  310 . The gate of the PMOS transistor  304  is coupled to the drain of the PMOS transistor  302 , forming a storage node  312 . 
   The write switch module  306  includes NMOS transistors  314  and  316  serially coupled between the node  312  and the complementary supply voltage. The NMOS transistor  314  has a drain coupled to the node  312 , a source coupled to the drain of the NMOS transistor  316 , and a gate coupled to a write bit line BL. The NMOS transistor  316  has a source coupled to the complementary supply voltage, and a gate coupled to a write word line WL. Similarly, the write switch module  308  includes NMOS transistors  318  and  320  serially coupled between the node  310  and the complementary supply voltage. The NMOS transistor  318  has a drain coupled to the node  310 , a source coupled to the drain of the NMOS transistor  320 , and a gate coupled to a complementary write bit line BLB. The NMOS transistor  320  has a source coupled to the complementary supply voltage, and a gate coupled to the write word line WL. The read switch module  310  includes NMOS transistors  322  and  324  serially coupled between the node  310  and the complementary supply voltage. The NMOS transistor  322  has a source coupled to the complementary supply voltage, and a gate coupled to the node  310 . The NMOS transistor  324  has a source coupled to the drain of the NMOS transistor  322 , a drain coupled to a read bit line BL, and a gate coupled to a read word line WL. 
   In write operation, the NMOS transistor  324  is turned off and the voltage on the write word line WL are raised above a predetermined level to turn on the NMOS transistors  316  and  320 . Depending on whether the node  310  or the node  312  is selected for programming a predetermined logic value, one and only one of the write bit line BL and complementary write bit line BLB is asserted to turn on one and only one of the NMOS transistors  314  and  318 . Supposing the NMOS transistor  314  is turned on and the NMOS transistor  318  is turned off, the node  312  is pulled to the complementary supply voltage, thereby turning the PMOS transistor  304  on. As a result, the node  310  is charged and the node  312  is discharged. At the end of each write cycle, the NMOS transistors  316  and  320  will be turned off, such that the nodes  310  and  312  will remain at their memory states. 
   In read operation, the NMOS transistors  316  and  320  are turned off, the read BL is pre-charged to a high state, and the voltage on the read word line WL is raised above a predetermined level to turn on the NMOS transistor  324 . If the node  310  is at a high state, the NMOS transistor  322  will be turned on, thereby pulling the read bit line BL to the complementary supply voltage. If the node  310  is at a low state, the NMOS transistor  322  will be turned off, and the voltage on the read bit line BL remains high. By sensing the signals on the read bit line BL, the memory state at node  310  can be determined. 
   One of the advantages of the proposed SRAM cell structure is that its operation voltage can be significantly lower than that of the conventional SRAM cell. The threshold voltage of the NMOS transistors  314 ,  316 ,  318 ,  320 ,  322  and  324  can be designed to be much lower than that of the PMOS transistors  302  and  304 . In this embodiment, the absolute value of the threshold voltage of the NMOS transistor is lower than that of the PMOS transistor by at least 100 mV. As a result, the operation voltage on the write word line WL, write bit line BL, complementary write bit line BLB, and read word line WL can be set at a very low level for both read and write cycles. Thus, the proposed SRAM cell structure can operate with a low operation voltage, thereby reducing its power consumption. 
   Another advantage of the proposed SRAM cell structure is that the charges retained at the storage nodes  312  and  310  will not be destabilized during a read cycle. As shown in the drawing, the charges are trapped among the gate of the PMOS transistor  302 , the drain of the PMOS transistor  304 , the drain of the NMOS transistor  318 , and the gate of the NMOS transistor  322 . In other words, the charges at the node  310  will not be discharged through the read bit line BL. Thus, they will not be destabilized during a read cycle. 
     FIG. 4  schematically illustrates an SRAM cell  400  in accordance with one embodiment of the present invention. The major difference between the cell  300  shown in  FIG. 3  and the cell  400  is that the cell  400  includes two read switch modules  402  and  404 . The read switch module  402  includes NMOS transistors  406  and  408  serially coupled between the node  410  and the complementary supply voltage. The NMOS transistor  406  has a source coupled to the complementary supply voltage, and a gate coupled to the node  410 . The NMOS transistor  408  has a source coupled to the drain of the NMOS transistor  406 , a drain coupled to a read bit line BL, and a gate coupled to a read word line WL. Similarly, the read switch module  402  includes NMOS transistors  412  and  414  serially coupled between the node  416  and the complementary supply voltage. The NMOS transistor  412  has a source coupled to the complementary supply voltage, and a gate coupled to the node  416 . The NMOS transistor  414  has a source coupled to the drain of the NMOS transistor  412 , a drain coupled to a read bit line BL, and a gate coupled to a read word line WL. Since the memory states at the nodes  410  and  416  are complementary, the signal readings on read bit line BL and complementary read bit line BLB are also complementary. 
     FIG. 5  illustrates a diagram  500  showing a layout view of various bit lines and word lines of the SRAM cell  300  in  FIG. 3  in accordance with one embodiment of the present invention. The write bit line, read bit line, supply voltage line, and complementary write bit line are arranged along the same direction across the pitch of the cell on the same metallization layer. The write word line and read word line are arranged along another direction on another metallization layer. This arrangement can reduce the coupling effect, due to the shortened bit lines, and the shielding effect among those conductive lines. 
     FIG. 6  illustrates a diagram  600  showing a layout view of various bit lines and word lines of the SRAM cell  300  in  FIG. 3  in accordance with another embodiment of the present invention. The write bit line, read bit line, supply voltage line, and complementary write bit line are arranged along the same direction across the pitch of the cell on the same metallization layer. The write word line and read word line are combined as a single conductive line along another direction on another metallization layer. This arrangement can reduce the coupling effect, due to the shortened bit lines, and the shielding effect among those conductive lines. 
     FIG. 7  illustrates a diagram  700  showing a layout view of various bit lines and word lines of the SRAM cell  400  in  FIG. 4  in accordance with one embodiment of the present invention. The write bit line, read bit line, supply voltage line, complementary read bit line, and complementary write bit line are arranged along the same direction across the pitch of the cell on the same metallization layer. The write word line and read word line are arranged along another direction on another metallization layer. This arrangement can reduce the coupling effect, due to the shortened bit lines, and the shielding effect among those conductive lines. 
     FIG. 8  illustrates a diagram  800  showing a layout view of various bit lines and word lines of the SRAM cell  400  in  FIG. 4  in accordance with another embodiment of the present invention. The write bit line, read bit line, supply voltage line, complementary read bit line and complementary write bit line are arranged along the same direction across the pitch of the cell on the same metallization layer. The write word line and read word line are combined as a single conductive line along another direction on another metallization layer. This arrangement can reduce the coupling effect, due to the shortened bit lines, and the shielding effect among those conductive lines. 
   The above illustration provides many different embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.