Abstract:
The present invention is a current-mirror-based bias generator for a load less four transistor SRAM as well as associated methods of controlling or modifying the current conducted by the access transistors of such an SRAM. The present invention may be thought of as an adjustable temperature coefficient, bias generator that references, via a current mirror, a reference bank of SRAM cells. The bank of reference cells provides an indication of the necessary conduction characteristics (e.g., gate to source voltage) of the access transistors under various conditions. By applying a bias voltage to the word line the desired current is sourced from the digit line. The bank of reference SRAM cells inherently compensates for process variations. The adjustable temperature coefficient bias generator allows the current sourced by the digit lines to vary greatly as a result of temperature variations. Thus, the present invention compensates for both process variations and temperature variations.

Description:
This application is a divisional of U.S. patent application Ser. No. 09/338,393 filed Jun. 22, 1999 now U.S. Pat. No. 6,198,670 issued Mar. 6, 2001. 
     BACKGROUND OF THE INVENTION 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to static-random-access-memory (SRAM) devices and, more particularly, to SRAM&#39;s utilizing a four transistor design. 
     DESCRIPTION OF THE BACKGROUND 
     To meet customer demand for smaller and more power efficient integrated circuits (ICs), manufacturers are designing newer ICs that operate with lower supply voltages and that include smaller internal subcircuits such as memory cells. Many ICs, such as memory circuits or other circuits such as microprocessors that include onboard memory, include arrays of SRAM cells for data storage. SRAM cells are popular because they operate at a higher speed than dynamic-random-access-memory (DRAM) cells, which must be periodically refreshed. 
     FIG. 1 is a circuit diagram of a conventional 6-transistor (6-T) SRAM cell  10 , which can operate at a relatively low supply voltage, for example 1.5V-3.3V, but which is relatively large. A pair of NMOS access transistors  12  and  14  allow complementary bit values D and {overscore (D)} on digit lines  16  and  18 , respectively, to be read from and to be written to a storage circuit  20  of the cell  10 . The storage circuit  20  includes NMOS pull-down transistors  22  and  26 , which are coupled in a positive-feedback configuration with PMOS pull-up transistors  24  and  28 , respectively. Nodes A and B are the complementary inputs/outputs of the storage circuit  20 , and the respective complementary logic values at these nodes represent the state of the cell  10 . For example, when the node A is at logic 1 and the node B is at logic 0, then the cell  10  is storing a logic 1. Conversely, when the node A is at logic 0 and the node B is at logic 1, then the cell  10  is storing a logic 0. Thus, the cell  10  is bistable, i.e., the cell  10  can have one of two stable states, logic 1 or logic 0. 
     In operation during a read of the cell  10 , a word-line WL, which is coupled to the gates of the transistors  12  and  14 , is driven to a voltage approximately equal to Vcc to activate the transistors  12  and  14 . For example purposes, assume that Vcc=logic 1=5V and Vss=logic 0=0V, and that at the beginning of the read, the cell  10  is storing a logic 0 such that the voltage level at the node A is 0V and the voltage level at the node B is 5V. Also, assume that before the read cycle, the digit lines  16  and  18  are equilibrated to approximately Vcc−Vt. Therefore, the NMOS transistor  12  couples the node A to the digit line  16 , and the NMOS transistor  14  couples the node B to the digit line  18 . For example, assume that the threshold voltages of the transistors  12  and  14  are both 1V, then the transistor  14  couples a maximum of 4V from the digit line  18  to the node B. The transistor  12 , however, couples the digit line  16  to the node A, which pulls down the voltage on the digit line  16  enough (for example, 100-500 millivolts) to cause a sense amp (not shown) coupled to the lines  16  and  18  to read the cell  10  as storing a logic 0. 
     In operation during a write, for example, of a logic 1 to the cell  10 , and making the same assumptions as discussed above for the read, the transistors  12  and  14  are activated as discussed above, and logic 1 is driven onto the digit line  16  and a logic 0 is driven onto the digit line  18 . Thus, the transistor  12  couples 4V (the 5V on the digit line  16  minus the 1V threshold of the transistor  12 ) to the node A, and the transistor  14  couples 0V from the digit line  18  to the node B. The low voltage on the node B turns off the NMOS transistor  26 , and turns on the PMOS transistor  28 . Thus, the inactive NMOS transistor  26  allows the PMOS transistor  28  to pull the node A up to 5V. This high voltage on the node A turns on the NMOS transistor  22  and turns off the PMOS transistor  24 , thus allowing the NMOS transistor  22  to reinforce the logic 0 on the node B. Likewise, if the voltage written to the node B is 4V and that written to the node A is 0V, the positive-feedback configuration ensures that the cell  10  will store a logic 0. 
     Because the PMOS transistors  24  and  28  have low on resistances (typically on the order of a few kilohms), they can pull the respective nodes A and B virtually all the way up to Vcc often in less than 10 nanoseconds (ns), and thus render the cell  10  relatively stable and allow the cell  10  to operate at a low supply voltage as discussed above. But unfortunately, the transistors  26  and  28  cause the cell  10  to be relatively stable and allow the cell  10  to operate at a low supply voltage as discussed above. But unfortunately, the transistors  26  and  28  cause the cell  10  to be approximately 30%-40% larger tan a 4-transistor (4-T) SRAM cell, which is discussed next. 
     FIG. 2 is a circuit diagram of a conventional 4-T SRAM cell  30 , where elements common to FIGS. 1 and 2 are referenced wit like numerals. A major difference between the 6-T cell  10  and the 4-T cell  30  is that the PMOS pull-up transistors  24  and  28  of the 6-T cell  10  are replaced with conventional passive loads  32  and  34 , respectively. For example, the loads  32  and  34  are often polysilicon resistors. Otherwise the topologies of the 6-T cell  10  and the 4-T cell  30  are the same. Furthermore, the 4-T cell  30  operates similarly to the 6-T cell  10 . Because the loads  32  and  34  are usually built in another level above the access transistors  12  and  14  and the NMOS pull-down transistors  22  and  26 , the 4-T cell  30  usually occupies much less area an the 6-T cell  10 . 
     Additional, complex steps are required to form the load elements  32  and  34  such that 4-T cells present the usual complexity versus cost tradeoff. The high resistance values of the loads  32  and  34  can substantially lower the stability margin of the cell  30  as compared with the cell  10 . Thus, under certain conditions, the cell  30  can inadvertently become monostable or read unstable instead of bistable Also, the cell  30  consumes more power than the cell  20  because there is always current flowing from Vcc to Vss through either the load  32  and the NMOS transistor  26  or the load  34  and the NMOS transistor  22 . In contrast, current flow from Vcc to Vss in the cell  20  is always blocked by one of the NMOS/PMOS transistor pairs  22 / 24  and  26 / 28 . Efforts to eliminate load elements  32  and  34  have lead to the development of a load-less four transistor SRAM cell as shown in FIG.  3 . 
     FIG. 3 is a circuit diagram of a conventional load-less 4-T SRAM cell, where elements common to FIGS. 2 and 3 are referenced with like numerals. The difference between the load-less cell  36  and the cell  30  is the elimination of load elements  32  and  34  and the replacement of NMOS transistors  12  and  14  with PMOS transistors  38  and  40 , respectively. 
     With the load-less 4-T SRAM cell of FIG. 3, like all SRAM cells, leakage currents and/or subthreshold currents are generate by transistors  22  and  26  when in the off state, and one will always be in the off state. To prevent the cell  36  from spontaneously changing state, the transistors  38  and  40  must source sufficient load current from the digit lines  16  and  18 , respectively, to offset the leakage and subthreshold currents. The needed load current can vary over many orders of magnitude due to temperature and process variations. However, the load current cannot be too large because the cumulative (along the digit lines  16  and  18 ) load current needs to be significantly less than the cell current for proper noise margin for proper operation of the sense amps. 
     Wide temperature variations resulting from cold-data retention testing and burn-in testing are also causes of wide variations in leakage and subthreshold currents, thereby causing wide variations in the load current that must be sourced by transistors  38  and  40 . Such testing, coupled with normal process variations, sense amp margin requirements, as well as yield requirements (e.g., read/write stability requirements, power consumption requirements, etc.) have made the manufacturing of load-less 4-T SRAM&#39;s a difficult matter. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is directed generally to a bias generator used in conjunction with one of the word line or digit line to set the desired level of load current as a function of temperature (or test being performed) to satisfy the simultaneous constraints of yield, sense amp margin, and load current even during cold-data retention testing or burn-in. 
     The present invention is also directed to a method of modifying the level of current conducted by the access transistors of a load-less, four transistor memory cell when the access transistors are in an off state. The method is comprised of the step of generating a temperature dependent bias voltage and connecting that bias voltage to the gate terminals of the access transistors. 
     The present invention is also directed to a current-mirror-based bias generator for a load-less four transistor SRAM as well as associated methods of controlling or modifying the current conducted by the access transistors of such an SRAM. The present invention may be thought of as an adjustable temperature coefficient, bias generator that references, via a current mirror, a reference bank of SRAM cells. The bank of reference cells provides an indication of the necessary conduction characteristics (e.g., gate to source voltage) of the access transistors under various conditions. By applying a bias voltage to the word line the desired current is sourced from the digit line. The bank of reference SRAM cells inherently compensates for process variations. The adjustable temperature coefficient bias generator allows the current sourced by the digit lines to vary greatly as a result of temperature variations. Thus, the present invention compensates for both process variations and temperature variations. Those benefits, and others, will become apparent from the description of the preferred embodiment hereinbelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the present invention to be easily understood and readily practiced, the invention will now be described for purposes of illustration and not limitation, in conjuction with the following figures wherein: 
     FIG. 1 is a circuit diagram of a conventional 6-T SRAM cell; 
     FIG. 2 is a circuit diagram of a conventional 4-T SRAM cell; 
     FIG. 3 is a circuit diagram of a conventional load-less 4-T SRAM cell; 
     FIG. 4 is a circuit diagram of a load-less 4-T SRAM cell in conjunction with a bias generator constructed according to the teachings of the present invention; 
     FIG. 5 is a block diagram of a portion of an array of 4-T SRAM cells incorporating the bias generator of the present invention; 
     FIG. 6 is a block diagram of a load-less 4-T SRAM incorporating the bias generator of the present invention; 
     FIG. 7 is a block diagram of a computer system that includes the 4-T SRAM of FIG. 6; 
     FIG. 8 illustrates another embodiment of the present invention; and 
     FIG. 9 is a block diagram illustrating how the bias voltage on the global bus can be forced to any value. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4 is a circuit diagram of a load-less  4  transistor SRAM cell  36  in conjunction with a bias generator  42  constructed according to the teachings of the present invention. The cell  36  illustrated in FIG. 4 is identical to the cell  36  illustrated in FIG.  3 . The bias generator  42  is comprised of a bank of transistors  44  connected in parallel with each other and connected in series with a temperature dependent constant current source  46 . The bank of transistors  44  is also connected in series to a voltage source through a trimmable resistor  47 . The bank of transistors  44  is fabricated at the same time, and in the same manner, as access transistors  38  and  40 . In that manner, the voltage drop from the gate terminal to the source terminal of each of the transistors  44  should be substantially the same as the gate to source terminal drop of access transistors  38  and  40 . Thus, the voltage drop across the gate and source terminals of each of the transistors  44  is representative of the voltage drop across the gate and source terminals of the transistors  38  and  40 . As an alternative to separately fabricating the bank of transistors  44 , a bank of cells  36  could carry additional wiring so that the gate to source voltage of the access transistors  38  and  40  can be sensed. 
     Returning to the bias generator  42 , the current source  46  may be constructed using any known techniques which provide a temperature dependent constant current source. The constant current source will produce one value of current under, for example, cold data-retention test conditions, and another value of current under burn-in test conditions. Thus, for each value of current produced by the temperature dependent constant current source  46 , a different voltage drop across the gate and source terminals of the transistors  44  is produced. That voltage drop is averaged and sensed by an operational amplifier  48 . Although the bias generator  42  would operate if only one transistor for the bank of transistor  44  was provided, by providing a plurality of transistors within bank  44 , a voltage drop which is more representative of the voltage drop experienced in the cells is produced. The voltage drop sensed by the operational amplifier  48  may then be applied to the word line which, as seen in the figure, is connected to the gate terminals of the access transistors  38  and  40 . Thus, when the cell  36  is in the off-state, i.e., transistors  38  and  40  are nonconductive, the bias voltage applied by the operational amplifier  48  may be used to control the conduction characteristics of the access transistors  38  and  40  so as to enable the transistors  38  and  40  to source current from the digit lines  16  and  18 . Because the bias voltage is directly related to the current which is produced by the constant current source  46 , and the current is temperature dependent, the bias voltage is also temperature dependent. Thus, the conduction characteristics of the access transistors  38  and  40  are controlled according to the temperature such that the current required by the cell  36 , for a given temperature, may be properly sourced. 
     The temperature dependent constant current source may receive inputs from a programmable device  45 . The programmable device  45  may contain laser trimmable devices, fuses, or antifuses, which allow manipulation of a value adjust signal (VA) and a temperature coefficient adjust signal (TCA) to provide some degree of control over the bias voltage post fabrication. 
     FIG. 5 is a block diagram of a portion of an array  50  of four transistor SRAM cells  36  incorporating the bias generator  42  of the present invention. In FIG. 5, a plurality of digit lines D, D, and a plurality of word lines WLI-WL 4  are used to interconnect individual memory cells  36 . The bias generator  42 , constructed as shown in FIG. 4, globally provides the bias voltage to the array via global bus  54 . The bias generator  42  may be coupled to each of the word lines WL 1 -WL 4  through a transistor pair  52 . Each transistor pair  52  is comprised of a PMOS and an NMOS transistor. The PMOS transistor may be connected between the bias generator  42  and a word line, e.g., WL 1 . The NMOS transistor may be connected between the word line, e.g. WL 1 , and ground. Each transistor is responsive to a word line select signal, e.g. Sel WL 1 . 
     In operation, only one word line will be active at a time. For word lines not selected, the NMOS transistor of the transistor pair  52  will be off while the PMOS transistor will be on thereby coupling the bias voltage to each of the non-selected word lines. When a word line is selected, e.g., WL 1 , the word line select signal, e.g., Sel WL 1 , will cause the transistors to change state. Specifically, the NMOS transistor will turn on connecting the word line to ground thereby rendering the word line active while the PMOS transistor will turn off thereby ending the application of the bias voltage to the active word line. 
     To provide a particular voltage for a test mode, a voltage source  56  may be coupled to the global bus  54  trough a transistor  58 . The voltage source may be capable of outputting different voltages depending upon one or more control signals  60 . Upon assertion of the signal Tm, the bias generator  42  is disabled and the output of the voltage source  56  is applied to the global bus  54 . Voltage source  56  may include a constant current source as well as a laser trimmable device, fuses, or antifuses as discussed above for the purpose of giving the manufacturer some degree of control over the voltage(s) produced by the voltage source  56  post fabrication. 
     Another way to implement the functionality described in the previous paragraph is through the use of more than one constant current source in the bias generator  42  as shown in FIG. 8 For example, a se constant current source  46 ′ could be operatively connected through a switch  66  to the remainder of the circuit for producing a voltage input to op amp  48 . The constant current source  46 ′ is responsive to a particular test mode instead of being responsive to the temperature. 
     Another embodiment of the present invention is illustrated in FIG.  9 . In FIG. 9, a pad  62  is connected to the global bus  54  through a transistor  64 . Whenever the signal Tm-force is asserted, the bias generator  42  is disabled and the voltage available at the pad  62  is placed on the global bus  54 . Thus, the voltage on global bus  54  may be forced to any value. When the signal Tm-measure is asserted, the voltage on bus  54  can be measured at pad  62 . This functionality is useful for characterization purposes as well as yield and reliability screen. 
     FIG. 6 is a block diagram of a memory circuit  70  which can include cells  36  and the bias generator  42  as previously described. In one embodiment, the memory circuit  70  may be a synchronous SRAM. 
     The memory circuit  70  includes an address register  72 , which receives an address from an ADDRESS bus (not shown). A control logic circuit  74  receives a clock (CLK) signal, and receives enable and write signals on a COMMAND bus (not shown), and communicates with the other circuits of the memory circuit  70 . A burst counter  75  causes the memory circuit  70  to operate in a burst address mode in response to a MODE signal. 
     During a write cycle, write driver circuitry  76  writes date to a memory array  78 . The array  78  is the component of the memory circuit  70  that can include the cells  36  and bias generator  42 . The array  78  also includes an address decoder  80  for decoding the address from the address register  72 . Alternately, the address decoder  80  may be separate from the array  78 . 
     During a read cycle, sense amplifiers  82  amplify and provide the data read from the array  78  to a data input/output (I/O) circuit  84 . The I/O circuit  84  includes output circuits  86 , which provide data from the sense amplifiers  82  to a DATA bus (not shown) during a read cycle The I/O circuit  84  also includes input circuits  88 , which provide data from the DATA bus to the write drivers  76  during a write cycle. The input and output circuits  88  and  86 , respectively, may include conventional registers and buffers. Furthermore, the combination of the write driver circuitry  76  and the sense amplifiers  82  can be referred to as read/write circuitry. The various components shown in FIG. 6, with the exception of the array  78 , constitute a plurality of components for reading information out of, and writing information into, the array  80 . 
     FIG. 7 is a block diagram of an electronic system  90 , such as a computer system, that incorporates the memory circuit  70  of FIG.  6 . The system  90  includes computer circuitry  92  for performing computer functions, such as executing software to perform desired calculations and tasks. The circuitry  92  typically includes a processor  94  and the memory circuit  70 , which is coupled to the processor  94 . One or more input devices  96 , such as a keyboard or a mouse, are coupled to the computer circuitry  92  and allow an operator (not shown) to manually input data thereto. One or more output devices  98  are coupled to the computer circuitry  92  to provide to the operator data generated by the computer circuitry  92 . Examples of such output devices  98  include a printer and a video display unit. One or more data-storage devices  100  are coupled to the computer circuitry  92  to store data on or retrieve data form external storage media (not shown). Examples of the storage devices  100  and the corresponding storage media include drives that accept hard and floppy disks, tape cassettes, and compact disk read-only memories (CD-ROMs). Typically, the computer circuitry  92  includes address, data, and command buses and a clock line that are respectively coupled to the ADDRESS, DATA and COMMAND buses, and the CLK line of the memory circuit  70 . 
     The present invention is also directed to a method of controlling the load current in a load-less four transistor memory cell. The method is comprised of the step of providing a temperature dependent bias voltage to one of the word line or the digit line. The providing step may be comprised of the steps of generating a temperature dependent constant current, generating a voltage drop across two terminals of a transistor representative of the transistors in the memory cell with the temperature dependent constant current, and sensing the voltage drop to produce the bias voltage. The voltage drop may be generated across a plurality of transistors to provide an average value for the voltage drop. By connecting or applying the bias voltage to one of the word line or digit line, the conduction of the access transistors of the memory cell may be controlled. However, because of the different function which the digit line performs in the context of a memory cell, it is considered preferable to apply the bias voltage to the word line. The present invention is also directed to a method of regulating a voltage difference between the word line and the digit line in a load-less four transistor memory cell by applying a temperature dependent bias voltage to one of the word line or the digit line. 
     While the present invention has been described in conjunction with preferred embodiments thereof, those of ordinary skill in the art will recognize that many modifications and variations are possible. For example, the type of transistors used to construct the cell may be varied such that the terminals in issue need not be the gate and source terminals. As previously mentioned, the same result can be achieved by varying the voltage on the digit line or, alternatively, controlling the voltage differential between the word line and the digit line. The foregoing disclosure and the following claims are intended to encompass all such modifications and variations.