Abstract:
According to one embodiment, a memory cell is disclosed. The memory cell includes a first PMOS transistor, a first NMOS transistor coupled to the first PMOS transistor, a second PMOS transistor and a second NMOS transistor coupled to the first PMOS transistor. The first and second PMOS transistors receiving a bias control signal.

Description:
COPYRIGHT NOTICE  
         [0001]    Contained herein is material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction of the patent disclosure by any person as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all rights to the copyright whatsoever.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to memory devices; more particularly, the present invention relates to static random access memories (SRAMs).  
         BACKGROUND  
         [0003]    Since the dawn of the electronic revolution in the  1970 &#39;s, continuous technological advances in the computer industry have depended upon the ability to store and retrieve an ever-increasing amount of data quickly and inexpensively. Thus, the development of the semiconductor memory has played a major role in the advancement of the computer industry over the past few decades.  
           [0004]    In particular, with the growing demand for large-scale on-chip cache memory for high performance microprocessors, a high-density static random access memories (SRAM) design becomes more significant. Traditionally six transistor (6T) SRAM cells have been implemented for cache memory devices. However, the size of 6T SRAM cells have become undesirable. As a result, four transistor (4T) SRAM cells have become more desirable because of smaller cell areas. Nonetheless, there is a problem with the design of 4T SRAM cells since it is typically difficult to meet read stability requirements.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention. The drawings, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.  
         [0006]    [0006]FIG. 1 is a block diagram of one embodiment of a computer system;  
         [0007]    [0007]FIG. 2 illustrates an exemplary four transistor memory;  
         [0008]    [0008]FIG. 3 illustrates one embodiment of a four transistor memory cell with forward bias; and  
         [0009]    [0009]FIG. 4 illustrates one embodiment of a memory device.  
     
    
     DETAILED DESCRIPTION  
       [0010]    A biasing technique for static random access memories (SRAMs) implementing four transistor memory cells is described. According to one embodiment, the delivery of a forward bias voltage during a memory cell standby state enables an access and load transistor to maintain a storage value within the memory cell by helping to provide a leakage current from the access and load transistor. Moreover, the delivery of a reverse bias voltage during a memory cell read state enables an access and load transistor to prevent the memory cell from switching its value during the read.  
         [0011]    In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.  
         [0012]    Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.  
         [0013]    [0013]FIG. 1 is a block diagram of one embodiment of a computer system  100 . Computer  100  includes a processor  101  that processes data signals. Processor  101  may be a complex instruction set computer (CISC) microprocessor, a reduced instruction set computing (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, a processor implementing a combination of instruction sets, or other processor device.  
         [0014]    In one embodiment, processor  101  is a processor in the Pentium® family of processors including the Pentium® II family and mobile Pentium® and Pentium® II processors available from Intel Corporation of Santa Clara, Calif. Alternatively, other processors may be used. FIG. 1 shows an example of a computer system  100  employing a single processor computer. However, one of ordinary skill in the art will appreciate that computer system  100  may be implemented using having multiple processors.  
         [0015]    Processor  101  is coupled to a processor bus  110 . Processor bus  110  transmits data signals between processor  101  and other components in computer system  100 . Computer system  100  also includes a memory  113 . In one embodiment, memory  113  is a dynamic random access memory (DRAM) device. However, in other embodiments, memory  113  may be a static random access memory (SRAM) device, or other memory device.  
         [0016]    Memory  113  may store instructions and code represented by data signals that may be executed by processor  101 . According to one embodiment, a cache memory  102  resides within processor  101  and stores data signals that are also stored in memory  113 . Cache  102  speeds up memory accesses by processor  101  by taking advantage of its locality of access.  
         [0017]    In another embodiment, cache  102  resides external to processor  101 . Computer system  100  further comprises a bridge memory controller  111  coupled to processor bus  110  and memory  113 . Bridge/memory controller  111  directs data signals between processor  101 , memory  113 , and other components in computer system  100  and bridges the data signals between processor bus  110  and memory  113 .  
         [0018]    Typically six transistor (6T) SRAM cells have been implemented for cache memory devices. However, the size of 6T SRAM cells have become undesirable. As a result, four transistor (4T) SRAM cells have become more desirable because of smaller cell areas. FIG. 2 illustrates an exemplary memory cell. The memory cell includes two PMOS transistors (P 1  and P 2 ) and two NMOS transistors (N 1  and N 2 ). In addition, the memory cell includes storage node  1  and storage node  2 .  
         [0019]    The memory cell typically operates in three modes, STANDBY, READ AND WRITE. While in the STANDBY mode, substantial off-state leakage currents are provided from the PMOS transistors to the respective NMOS transistors in order to maintain data storage at storage node  1  and storage node  2 . Thus, the PMOS transistors are designed to be strong enough to provide the necessary leakage current.  
         [0020]    However, if the PMOS transistors are too strong, the current flowing through transistor P 1  or transistor P 2  during the READ mode can pull nodes  1  and  2  up, thus flipping the memory cell. Consequently, the loss of the memory state occurs. In order to prevent such an occurrence, transistor N 1  typically has to be as least 1.5 times stronger than transistor P 1  to ensure cell stability while in the READ mode. However, the increased size of transistor N 1  results in a larger area for the memory cell.  
         [0021]    [0021]FIG. 3 illustrates one embodiment of a memory cell  300 . Memory cell  300  includes PMOS transistors  310  and  320 , and NMOS transistors  330  and  340 . Transistors  310  and  320  serve as access and load transistors. The gate of transistor  310  is coupled to WORDLINE. WORDLINE is used to activate a row of cells  300  within a SRAM device (e.g., cache  102 ).  
         [0022]    The source of transistor  310  is coupled to one of two bit lines (BITLINE), while the drain is coupled to the drain of transistor  330  at storage node  1 . The BITLINE is used to activate a column of cells within the SRAM device. The gate of transistor  320  is also coupled to WORDLINE. The source of transistor  320  is coupled to the other bit line (BITLINE#), and the drain is coupled to the drain of transistor  340  at storage node  2 .  
         [0023]    Transistors  330  and  340  serve as the body of the SRAM device. The gate of transistor  330  is coupled to the drain of transistor  320  at storage node  2 . As described above, the drain of transistor  330  is coupled to storage node  1 , and the source is coupled to ground. The gate of transistor  340  is coupled to the drain of transistor  310  at storage node  1 . Also, the drain of transistor  340  is coupled to node  2 , and the source is coupled to ground.  
         [0024]    According to one embodiment, transistors  310  and  320  include a body bias (Vb) that is received from a body control signal. In one embodiment, the body control signal is received from processor  101 . However, in other embodiments, the body control signal may be received from memory controller  111 . In a further embodiment, transistors  310  and  320  receive a forward body bias during the STANDBY mode and a reverse body bias during the READ mode, as described in further detail below.  
         [0025]    While operating in the STANDBY mode, both bit lines and WORDLINE are at a high logic level (e.g., logic 1). Assuming that storage node  1  starts at a high logic level (e.g., logic 0) and storage node  2  starts at a low logic level, node  1  is driven to a low logic level and node  2  is driven high. As a result, a data value is being stored at node  2 . The off-state leakage current from transistor  320  helps maintain a logic high value at storage node  2 . During the STANDBY mode, the body control signal is received at transistors  310  and  320  as a forward bias.  
         [0026]    Forward bias is a voltage supplied to transistors  310  and  320  that is less than Vcc. The forward bias lowers the threshold voltage of transistor  310  and transistor  320 , and increases the off state current of transistor  310  and  320 . Thus, additional leakage current from transistor  320  can compensate for the current loss at storage node  2  in order to maintain the logic high state.  
         [0027]    While operating in the READ mode, both bit lines are at a high logic level, while WORDLINE is at a low logic level. Assuming that storage node  1  starts at a low logic level and storage node  2  starts at a high logic level, storage node  1  is driven high and storage node  2  is driven low. Consequently, current will flow through transistor  310  from BITLINE to storage node  1 . As described above, the body control signal is received at transistors  310  and  320  as a reverse bias during the READ mode.  
         [0028]    Reverse bias is a voltage supplied to transistors  310  and  320  that is greater than Vcc. The reverse bias makes transistors  310  and  320  increases the threshold voltage of transistor  310  and the ratio of the drive-current of NMOS over PMOS transistors increases. As a result, storage node  1  is prevented from being pulled up during the read by current from transistor  310 , causing the memory cell to flip. Thus, the read stability of memory cell  300  is improved.  
         [0029]    [0029]FIG. 4 illustrates one embodiment of a memory  400 . According to one embodiment, memory  400  is implemented as cache  102 . However, memory  400  may be implemented as any type of SRAM device used in computer system  100 . Memory  400  includes memory cells  410 , N-well  415 , gap cell  420 , N-well contact  430  and a control signal  440 .  
         [0030]    Memory cells  410  include a PMOS component (e.g., transistors  310  and  320 ) and a NMOS component (e.g., transistors  330  and  340 ). N-well  415  includes the network of p-channel transistors formed within. Thus, all PMOS components in each memory cell  410  of memory  400  share N-well  415 . Gap cell  420  in memory  400  that is used for wordline strapping.  
         [0031]    N-well contact  430  is located within gap cell  420 . A control signal  440  is coupled to contact  430  in gap  420 . Control signal  440  is a body control signal that is delivered to contact  430 . Thus, each PMOS component within memory  400  receives body control  440 . As described above, control signal  440  delivers bias voltages to the PMOS component of memory cells  410 . By routing control signal  440  from N-well contact  430 , the area impact from transistors  330  and  340  is minimized.  
         [0032]    The delivery of bias signals to 4T memory cells, enable smaller transistors to be implemented within the cells. As a result, the 4T memory cells are operable with an even smaller area, resulting in smaller SRAMs.  
         [0033]    Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims which in themselves recite only those features regarded as the invention.  
         [0034]    Thus, a biasing technique for SRAMs implementing four transistor memory cells has been described.