Abstract:
This invention discloses a static random access memory (SRAM) cell comprising a pair of cross-coupled inverters connected between a positive supply voltage (Vcc) and a first node, a first NMOS transistor with a gate and drain connected to the first node and a source connected to a ground, and a second NMOS transistor with a drain and source connected to the first node and the ground, respectively, and a gate connected to a control-line.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to static random access memory (SRAM) cell, and, more particularly, to dual port SRAM cells. 
         [0002]    Semiconductor memory devices include, for example, static random access memory, or SRAM, and dynamic random access memory, or DRAM. DRAM memory cell has only one transistor and one capacitor, so it provides a high degree of integration. But DRAM requires constant refreshing, its power consumption and slow speed limit its use mainly for computer main memories. SRAM cell, on the other hand, is bi-stable, meaning it can maintain its state indefinitely as long as an adequate power is supplied. SRAM can operate at a higher speed and lower power dissipation, so computer cache memories use exclusively SRAMs. Other applications include embedded memories and networking equipment memories. 
         [0003]    One well-known conventional structure of a SRAM cell is a six transistor (6T) cell that comprises six metal-oxide-semiconductor (MOS) transistors. Briefly, a 6T SRAM cell  100 , as shown in  FIG. 1 , comprises two identical cross-coupled inverters  102  and  104  that form a latch circuit, i.e., one inverter&#39;s output connected to the other inverter&#39;s input. The latch circuit is connected between a power and a ground. Each inverter  102  or  104  comprises a NMOS pull-down transistor  115  or  125  and a PMOS pull-up transistor  110  or  120 . The inverter&#39;s outputs serve as two storage nodes C and D, when one is pulled to low voltage, the other is pulled to high voltage. A complementary bit-line pair  150  and  155  is coupled to the pair of storage nodes C and D via a pair of pass-gate transistors  130  and  135 , respectively. The gates of the pass-gate transistors  130  and  135  are commonly connected to a word-line  140 . When the word-line voltage is switched to a system high voltage, or Vcc, the pass-gate transistors  130  and  135  are turned on to allow the storage nodes C and D to be accessible by the bit-line pair  150  and  155 , respectively. When the word-line voltage is switched to a system low voltage, or Vss, the pass-gate transistors  130  and  135  are turned off and the storage nodes C and D are essentially isolated from the bit lines, although some leakage can occur. Nevertheless, as long as Vcc is maintained above a threshold, the state of the storage nodes C and D is maintained indefinitely. 
         [0004]    However, the traditional SRAM cell  100  has leakage current caused by both gate and off-state leakages. Assuming nodes C and D are at logic 0 and 1, respectively, in a static state, the pull-up transistor  110  and the pull-down transistor  125  contribute off-state leakages I_off_PU and I_off_PD, respectively, while the pull-up transistor  120  and the pull-down transistor  115  contribute gate leakages I_gate_PU and I_gate_PD, respectively. The pass-gate transistor  130  also contributes an off-state leakage, I_off_PG. Besides, the pass-gate transistor  135  contributes additional gate leakage, I_gate_PG. Therefore, a total leakage current, Isb, of the traditional SRAM cell  100  can be expressed as: Isb=(I_off_PU+I_off_PD+I_gate_PU+I_gate_PD)+I_off_PG+I_gate_PG. In more advanced process technologies, such as 80 nm and under, especially for high speed applications, thin gate oxide and shallow junction may make the leakage current in the traditional SRAM cell  100  unacceptable. Then the traditional SRAM cell  100  has only limited applications in the more advanced 
         [0005]    Various techniques have been proposed to reduce the SRAM cell leakage current. For example, lowering power supply voltage to stand-by cells may reduce leakage current thereof. But operating SRAM array on multiple supply voltages increases design complexity and may also lowers the SRAM speed. Another example is to provide a virtual ground to each row of SRAM cells. With the boosted ground voltage, the SRAM cell leakage can also be reduced. But by using word-lines to control the rows of the virtual ground, this technique may also suffer slowed cell operation. 
         [0006]    As such, what is desired is a SRAM cell that not only has suppressed leakage current, but also does not sacrifice operation speed. 
       SUMMARY 
       [0007]    This invention discloses a static random access memory (SRAM) cell. According to one embodiment of the present invention, the SRAM cell comprises a pair of cross-coupled inverters connected between a positive supply voltage (Vcc) and a first node, a first NMOS transistor with a gate and drain connected to the first node and a source connected to a ground, and a second NMOS transistor with a drain and source connected to the first node and the ground, respectively, and a gate connected to a control-line. 
         [0008]    According to another embodiment, the SRAM cell further comprises a pair of pass-gate transistors with sources/drains connected to the cross-coupled inverters, and gates connected to a word-line. 
         [0009]    According to yet another embodiment, the control-line and the word-line becomes one line in a memory array formed by a plurality of the SRAM cells. 
         [0010]    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 
         [0011]    The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer conception of the invention, and of the components and operation of systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore non-limiting, embodiments illustrated in the drawings, wherein like reference numbers (if they occur in more than one view) designate the same elements. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. 
           [0012]      FIG. 1  is a schematic diagram illustrating a conventional 6-T SRAM cell. 
           [0013]      FIG. 2  is a schematic diagram illustrating an 8-T low leakage SRAM cell according to one embodiment of the present invention. 
           [0014]      FIG. 3  is a schematic diagram illustrating a SRAM cell array incorporating the 8T low leakage SRAM cell of  FIG. 2 . 
       
    
    
     DESCRIPTION 
       [0015]    The present invention discloses an 8-T SRAM cell that can reduce cell stand-by leakage current without sacrificing operating speed. 
         [0016]      FIG. 2  is a schematic diagram illustrating an 8-T low leakage SRAM cell  200  according to one embodiment of the present invention. The SRAM cell  200  is formed by adding two NMOS transistors  215  and  225  to the conventional 6T-SRAM cell  100  of  FIG. 1 . Sources and drains of both the NMOS transistors  215  and  225  are connected to the ground and a node V, respectively. The node V becomes a virtual ground for the cell  100  in the 8T-SRAM cell  200 . Apparently, the functional, i.e., data storage, element of the SRAM cell  200  are still performed by the cell  100  included in the cell  200 . 
         [0017]    Referring again to  FIG. 2 , a gate of the NMOS transistor  215  is connected to the drain thereof, therefore, the NMOS transistor  215  functions as a forward biased transistor diode, a voltage drop between the drain and source is maintained at one threshold voltage of the NMOS transistor  215 . The threshold voltage of the NMOS transistor  215  can be controlled by channel implant. For a circuit having 1.2V power supply voltage, the threshold voltage of the NMOS transistor  215 , for instance, may be adjusted at around 0.3V. 
         [0018]    A gate (node E) of the NMOS transistor  225  is controlled by an external signal. During an accessing, i.e., read or write, of the SRAM cell  200 , the external signal is switched to the logic “1”, which turns on the NMOS transistor  225 , which in turn pulls the node V down to the ground. Therefore the SRAM cell  200  functions just the same as the conventional SRAM cell  100 . 
         [0019]    During none access time, the external signal is switched to the logic “0”, which turns off the NMOS transistor  225 , leaving the node V being conducted to the ground only through the NMOS transistor  215 . As aforementioned the NMOS transistor  215  has a threshold voltage drop across its drain and source, therefore, the node V is maintained at one threshold voltage above the ground. Then the total voltage across the SRAM cell  100  will be reduced by the one threshold voltage, which can reduce the leakage current in the SRAM cell  100  during this none access time. 
         [0020]    In such a way, the voltage of node V, or the virtual ground of cell  100 , is automatically controlled during different operation modes. During accessing mode, the node V is conducted to the ground, allows the SRAM cell  200  to operate at full speed. During none accessing mode, the virtual ground voltage is raised to cut down the leakage current in the SRAM cell  200 . A skilled artisan may also realize that the NMOS transistor  225  does not need to be turned on during write operation of the SRAM cell  200 . In fact, higher node V voltage will make writing the SRAM cell speedier. 
         [0021]      FIG. 3  is a schematic diagram illustrating a SRAM cell array  300  incorporating the 8T low leakage SRAM cell  200  of  FIG. 2 . The memory cells C[0:n, 0:m] are identical SRAM cells  200 . A plurality of word-lines WL[0:n] runs in row direction, and a plurality of control-lines CL[0:m] runs in column direction. The row and column directions are substantially perpendicular to each other. Each control-line CL[i], 0≦i≦m, is connected to every node E of the SRAM cells C[0:n, i]. The control-line CL[0:m] carries the external control signal to selectively turn on or off the NMOS transistor  225  in a column of the cell array  300 . For example, if the cell C[ 1 , 1 ] needs to be accessed, the WL[ 1 ] and CL[ 1 ] will be switched to the logic “1”. When CL[ 1 ] is on the logic “1”, the (n+1) number of memory cells C[0:n, 1] are all turned to the real ground, while the rest cells of the array  300  maintain the higher virtual ground voltage to reduce leakage current thereof. 
         [0022]    An array having 512 cells in 125° C. has been simulated to confirm the leakage current reduction effect. Following Table 1 is a simulation result, which shows that the leakage current of the new 8T SRAM cell is more than two fold of reduction from that of the conventional 6T SRAM cell. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Vcc 
               
             
          
           
               
                   
                 0.8 V 
                 0.9 V 
                 1.0 V 
                 1.1 V 
                 1.2 V 
               
               
                   
                   
               
             
          
           
               
                   
                 6T cells 
                   64 nA 
                   71 nA 
                   80 nA 
                   88 nA 
                   97 nA 
               
               
                   
                 8T cells 
                 30.8 nA 
                 33.9 nA 
                 37.2 nA 
                 40.7 nA 
                 44.3 nA 
               
               
                   
                   
               
             
          
         
       
     
         [0023]      FIG. 3  illustrates just one exemplary array arrangement using the SRAM cell  200  of  FIG. 2 , a skilled artisan would realize that control-lines can also travel in the row direction. In fact, the control-lines and the word-lines can be combined into a single line, as reading a cell needs both the word-line and the control-line to be switched to the logic “1” at the same time. 
         [0024]    The above illustration provides many different embodiments or 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. 
         [0025]    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.