Abstract:
An SRAM array is disclosed. The SRAM array includes a plurality of SRAM cells. In one embodiment, the SRAM cells are 6-T SRAM cells that further includes a voltage bias device. The voltage bias device raises the voltage level of a low voltage rail V ss  such that the plurality of SRAM cells are connected to a raised low voltage rail.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to integrated circuits, and more particularly, to a SRAM array having reduced bitline leakage current. 
     BACKGROUND OF THE INVENTION 
     Metal oxide semiconductor (MOS) static random access memory (SRAM) arrays are comprised of an array of SRAM cells. The SRAM cells are read, erased, and written to by means of bitlines (BL) and wordlines (WL). In one common design, the SRAM cells consist of load elements in a flip-flop configuration, together with two select transistors. 
     FIG. 1 is a schematic circuit diagram of a six-transistor (6T) SRAM cell  101  that is commonly and widely used in SRAM memory arrays. The SRAM cell  101  is known in the art as a 6T SRAM cell. The SRAM cell  101  includes N-type MOS (NMOS) transistors N 1  and N 2  (hereinafter transistors N 1  and N 2 ) coupled between V SS  (typically ground) and nodes A and B, respectively. Nodes A and B are further coupled to V DD  by pull-up P-type MOS (PMOS) transistors P 1  and P 2  (hereinafter transistors P 1  and P 2 ), respectively. Node A is further coupled to the gates of transistors P 2  and N 2  and node B is similarly coupled to the gates of transistors P 1  and N 1 . 
     Information is stored in SRAM cell  101  in the form of voltage levels in the flip-flop formed by the two cross-coupled inverters formed by transistors P 1 , N 1  and P 2 , N 2 , respectively. In particular, when node A is at a logic low state (the voltage of node A being approximately equal to V SS ), transistor P 2  is on (in a low resistance state or conducting) and transistor N 2  is off (in a high resistance state or non conducting). When transistor P 2  is on and transistor N 2  is off, node B is at a logic high state (the voltage of node B is pulled up to approximately V DD ). Further, when node B is at a logic high state, transistor P 1  is off and transistor N 1  is on. When transistor P 1  is off and transistor N 1  is on, node A is at a logic low state (the voltage of node A is pulled down to approximately V SS ). In this manner, SRAM cell  101  remains in a latched state. 
     Nodes A and B are further coupled to a “bitline” and a “bitline-not” by NMOS select transistors N 3  and N 4  (hereinafter transistors N 3  and N 4 ), respectively. The gates of transistors N 3  and N 4  are coupled to a word line to enable read and write operations, as those skilled in the art will understand. 
     A read operation is performed by turning on the word line and allowing one side of the SRAM cell to start pulling down on one line of the bitline pair. For example, if node A is low and the word line is pulled high, then a current will flow through select transistor N 3  and transistor N 1  to ground or V ss . 
     When node A is low and the word line is low, the SRAM cell  101  has a leakage current  103  that flows from the bitline through the select transistor N 3  and transistor N 1  down to ground or V ss . 
     As the size of the SRAM cells decreases, the amount of read current produced by the SRAM cell also decreases, particularly as the supply voltage V cc  is lowered as technology advances. As the amount of read current decreases, the leakage current becomes larger relative to the read current, thereby making it more difficult to accurately read the SRAM cell. Thus, it is desirable to lower the amount of leakage current from each SRAM cell. 
     One method of solving this problem is to reduce the number of SRAM cells per bitline, thereby reducing the overall amount of leakage current. However, this increases the amount of overhead devices such as sense amplifiers, column multiplexer circuitry, etc. for a given number of SRAM cells. 
     Another approach is to increase the channel length of the transistors in the SRAM cell. However, this increases the SRAM cell size. Furthermore, effect of this approach is limited, as the transistor devices get smaller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the accompanying Figures. 
     FIG. 1 is a schematic diagram of a prior art SRAM cell. 
     FIG. 2 is a schematic diagram of an SRAM cell incorporating the present invention. 
     FIG. 3 is a schematic diagram of an SRAM array incorporating an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Methods and apparatus&#39; for a SRAM memory array having reduced leakage current are disclosed. The subject of the invention will be described with reference to numerous details set forth below, and the accompanying drawings will illustrate the invention. The following description of the drawings are illustrative of the invention and are not to be construed as limiting the invention. Numerous specific details are described to derive a thorough understanding of present invention. However, in certain circumstances, well known, or conventional details are not described in order not to obscure the present invention in detail. 
     In general, the amount of leakage current through a channel of a transistor follows an exponential relationship with the gate to source voltage difference (V gs ), in the sub threshold region of transistor behavior. Empirically, it has been determined to increase about 10 times per 100 millivolt increase in V gs  in the sub threshold region. Moreover, it has an approximately linear relationship with V cc , the supply voltage. 
     The magnitude of the read current of an SRAM cell  101 , shown in FIG. 1, is essentially the saturation current of the pull-down transistors N 3  and N 4 . This is proportional to the square of the difference between V cc  and V t , the threshold voltage of the pull-down transistors. Therefore, with respect to V cc , the read current declines faster than the leakage current as the supply voltage (V cc ) is lowered. 
     With these principles in mind, FIG. 2 shows an SRAM cell  201  formed in accordance with the present invention. In many respects, the SRAM cell  201  is substantially similar to the SRAM cell  101  of FIG.  1 . However, as seen in FIG. 2, the sources of the pull-down transistors N 1  and N 2  are not connected directly to V ss  (ground). Instead, the sources of the pull-down transistors N 1  and N 2  are connected to V ss  (ground) through a bias device  203 . The bias device  203  can be a transistor that produces a small positive voltage on its&#39; source node, equivalent to the voltage drop across the channel of the bias transistor  203 . Typically, this voltage drop is on the order of 0.1 volts. Consequently, the pull-down transistors N 1  and N 2  transfer the positive biasing voltage to the source nodes of the access transistors N 3  and N 4 . 
     By raising the voltage on the sources of the NMOS transistors N 1  and N 2  in the SRAM  201  slightly above V cc , the voltage between gate to source (V gs ) of transistors N 1  and N 2  is now negative. The threshold voltage V t  is raised as the reverse-biased source junction depletes part of the channel. These effects combine to reduce leakage current exponentially, while reducing the read current only slightly. 
     Further, although a biasing transistor  203  is used to raise the level of the source nodes of the transistors N 1  and N 2 , any device that can raise the voltage on the source nodes of transistors N 1  and N 2  above V ss  may be used. As one example, a controllable voltage source, or a simple resistor, may be used. In one embodiment, the amount of bias voltage used to raise V ss  is on the order of 0.1 volts. However, higher or lower bias voltages may be implemented to accommodate design variations. For example, by reducing the size of transistors  203 , a higher bias voltage may be applied to the sources of the transistors N 1  and N 2 . This approach further decreases the leakage current, however at the expense of a lower read current signal. As another example, by a “larger” bias transistor having a low voltage drop across its&#39; channel may be used to apply a lower bias voltage to the sources of the transistors N 1  and N 2 . This approach results in a lower decrease in the leakage current, however with the benefit of a higher read current signal. Thus, the present invention may be modified to fit the requirements of a particular integrated circuit. 
     Moreover, although only a single SRAM cell  201  is shown in FIG. 2, the biasing transistor  203  is connected to all of the pull-down transistors N 1  and N 2  of all of the SRAM cells  201  in a single column in the SRAM memory array. Thus, only a single bias transistor  203  is required for each column of the memory array. Further, in one embodiment, the bias transistor  203  is always in the “on” state by biasing the gate of the bias transistor  203  to the supply voltage V cc . 
     As noted above, the use of the bias transistor  203 , while decreasing the leakage current, also has the undesirable effect of lowering the read current. Thus, turning to FIG. 3, in an alternative embodiment, a two level bias transistor scheme may be used. In particular, the bias transistor may comprise a small bias transistor  301  and a large bias transistor  303 . 
     FIG. 3 shows two portions of an SRAM array. The first portion  305  is being accessed, while the second portion  307  is not being accessed. In the second portion of  307  of the SRAM array that is not being accessed (read), only the small bias transistor  301   b  is turned “on” to maintain a relatively high raised V ss  level. This has the effect of significantly reducing leakage current. 
     In the first portion  305  of the SRAM array that is being accessed (read), both the small bias transistor  301   a  and the large bias transistor  303   a  are turned on. By turning on the large bias transistor  303   a , this will tend to bring the source node of the transistors of the SRAM cells  201   a  closer to ground (V ss ). This has the effect of reducing leakage current while maintaining good read current. 
     Thus, the selective scheme of FIG. 3 is beneficial at low V cc  supply voltage conditions where the read current is small. The precise block size should be determined by trade offs between acceptable read current/leakage current ratios and cost and increased area required by the two biasing transistors for each portion of the SRAM array. For blocks of the memory array that are “on,” both the small biasing transistor  301  and the large biasing transistor  303  are turned “on.” This results in the virtual “V ss ” node near ground and allows the memory array to sink read current. 
     By lowering the leakage current, the present invention allows a higher number of SRAM cells per bitline, thereby reducing the amount of overhead for an array of given size. Further, the present invention requires relatively little overhead, using only a few transistor devices for a large number of SRAM cells and little additional routing overhead. Finally, the present invention can maintain channel leakage current small relative to read current even at low supply voltages. 
     While specific embodiments of applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise configuration and components disclosed herein. Various modifications, changes, and variations, which will be apparent to those skilled in the art, may be made in the arrangement, operation, in details of the methods and systems of the present invention disclosed herein without departing from the spirit and scope of the invention. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be used to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established canons of claim interpretation.