Abstract:
A technique for reducing the bitline leakage current while maintaining a level of performance characteristics of low threshold voltage transistors in deep submicron CMOS technology incorporates a reference voltage generator circuit in combination with bias transistor MBIAS. The output of a static logic gate is connected to the input terminal of the pull-down devices. The reduction in leakage current through pull-down devices whenever a read operation is not performed contributes to a significant reduction in overall leakage current in the circuit.

Description:
RELATED APPLICATION  
       [0001]     The present application claims priority of India Patent Application No. 2598/Del/2004 filed Dec. 30, 2004, which is incorporated herein in its entirety by this referenced.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to a memory device with reduced leakage current.  
       BACKGROUND OF THE INVENTION  
       [0003]     The impact of the subthreshold leakage current on the circuit performance should be considered seriously as device dimension have scaled down to deep submicron level in CMOS technology. This is a significant problem in memory structures using precharging circuitry which frequently require discharging of the bitlines to allow bit sensing in memories.  
         [0004]      FIG. 1  illustrates the schematic diagram for a conventional memory cell that operates as a single storage unit in a larger memory structure. One problem with such memory cells is the leakage current through multiple read transistors  30  coupled to a shared bitline  14  which can result in erroneous read operation. Increased leakage current severely affects the performance of the memory circuits (e.g., register files). Further, the problem is increased by noise on the read signal line due to coupling noise.  
         [0005]     One way to reduce the leakage current is utilization of high threshold voltage (V T ) devices. However, such devices exhibit reduced performance in terms of device speed and area. In addition, manufacturing costs are increased in high V T  devices due to the additional silicon layers required in such devices.  
         [0006]     To overcome this problem,  FIG. 2  illustrates memory circuits according to U.S. Pat. No. 6,320,795. The patent discloses a register file cell  40  which is capable of reducing leakage current and is less likely to require a larger keeper transistor  26  to prevent erroneous reads. Memory cell  40  includes a pull-down transistor (MPD)  42 , a static logic gate  44 , and a storage cell  46 . Pull-down transistor  42  is operative for discharging the bitline  14  to the ground  36  when a predetermined control indication is received at the input terminal thereof from the logic gate  44 . In this embodiment, an N-channel IGFET device is used as the pull-down transistor  42  and therefore, the predetermined control indication is a logic high value applied to the gate terminal of pull-down transistor  42 .  
         [0007]     Logic gate  44  acts to buffer the input of the pull-down transistor  42  from the noise commonly associated with the read signal. Logic gate  44  includes two input terminals  48  and  50 .  
         [0008]     When the read signal is logic low and the data stored in the register file cell is logic high, logic gate  44  (NOR gate) outputs a logic high value to the gate terminal of pull-down transistor  42 . As a result, pull-down transistor  42  discharges bitline  42  to ground  36 . When stored data value in cell is logic low and when the read signal is logic low, the output of logic gate  44  is logic low which results in pull-down transistor  42  to be turned off and hence, bitline  14  is not discharged. The output of the logic gate  44  is logic low when the active low read signal is logic high, regardless of the data bit value stored in cell  46 .  
         [0009]     Thus the goal is to isolate pull-down transistor  42  from the read noise associated with the read signal. Although the application of a static logic gate helps to reduce such leakage current to a significant level in such circuits, this approach is effective for reducing the leakage currents generated only due to the noise voltages on the input terminal of pull-down transistor  42 . The technique does not have significant impact on the leakage currents typically associated with low V T  scaled devices.  
         [0010]      FIG. 3  shows a schematic diagram illustrating a register file cell  60  with another embodiment. Memory cell  60  includes a pull-down transistor  62  (MPD), logic gate  64 , a storage cell  66 , a bias device  72  and a read transistor  74  (MREAD). Logic gate  64  of memory cell  60  provides isolation between a possibly noisy read signal and the input terminal of pull-down transistor  62 , in the same way as in previous case. In addition, bias device  72  is operative for applying a bias voltage to pull-down transistor  62  during appropriate periods and thereby reduces the level of the current leakage through the device during those periods. Thus memory cell  60  can be implemented using low V T  transistors to achieve high performance operation while still maintaining high robustness.  
         [0011]     When the read signal is logic high, a read transistor  74  couples the second output terminal of pull-down transistor  62  to ground  36 . Therefore a logic low voltage is present at the second input  70  of logic gate  64 . During a read operation, the output of logic gate  64  is logic high when the data bit stored within cell  66  is logic low. Under this condition, pull-down transistor  62  is turned on and bitline  14  is discharged to the ground  36  through read transistor  74 . When the data bit stored within cell  66  is logic high, the output of logic gate  64  is low and pull-down transistor  62  remains off.  
         [0012]     When the read signal is logic high (i. e., a read operation is being performed for cell  60 ), bias device  72  (P-channel IGFET) is off and has substantially no effect on the circuit. When the read signal is logic low (i. e., a read operation is not being performed for cell  60 ), bias device  72  couples the supply terminal  18  to the second output terminal of pull-down transistor  62 . This places a logic high voltage on the second input  70  of logic gate  64 , which forces the output of the logic gate to a logic low value. Therefore a negative voltage exists from the input terminal of pull-down transistor  62  to the second output terminal of pull-down transistor  62 . As transistor  62  is an N-channel IGFET device, the negative voltage from the input terminal (the gate) of pull-down transistor  62  to the second output terminal ( the source) of pull-down transistor  62  reduces the leakage current through pull-down transistor  62  to negligible levels. When the read signal again switches to a logic high value, the bias voltage is removed from pull-down transistor  62  and a read operation takes place.  
         [0013]     As described in the prior art, such an embodiment is capable of reducing leakage current through pull-down transistor  62  due to the effect of read noise associated with read signal on the input of pull-down transistor  62 . This helps to reduce the leakage level through pull-down transistor  62  due to generation of a negative voltage from the input terminal (i.e., the gate terminal) of pull-down transistor  62  to the second output terminal (i. e., the source terminal) of pull-down transistor  62 .  
         [0014]     The drawback of such an arrangement is its inability to check the leakage current through pull-down transistor  74 . Also the presence of bias transistor X raises the potential of intermediate node  80  near to supply voltage whenever read signal is low (i. e., when a read operation is not being performed). This technique appears to be unable to reduce the leakage current to the same order at a very low potential of intermediate node. In such memory circuit arrangement, significant leakage current is produced because of low V T  pull-down devices  107  and  117  used to maintain high performance. Hence, the goal of the bias device is to reduce leakage through pull-down transistor  62  during some or all of the non-read period associated with a register file cell.  
       SUMMARY OF THE INVENTION  
       [0015]     To obviate the aforesaid drawbacks the object of the instant invention is to provide a memory device with reduced leakage current.  
         [0016]     Another object of the instant invention is to lower down the leakage current through pull-down low threshold voltage (V T ) semiconductor device.  
         [0017]     Another object of the instant invention is to provide memory cells using submicron technology with improved performance characteristics in speed, area and cost.  
         [0018]     A memory device of the present invention having reduced leakage current includes at least one bitline and a plurality of memory cells, with each memory cell passing at least one output of a storage cell and each output coupled to each bitline through read access circuitry. The read access circuitry includes a logic device responsive to data value stored in the storage cells and a read signal for generating a control output, a first switching device having its control terminal coupled to the control output of the logic device and its first terminal coupled to the bitline, a second switching device with its control terminal coupled to the control output of the logic device for passing a low voltage to a common terminal of said first and said second switching devices, and a third switching device for passing a node reference voltage to the common terminal which is responsive to the control output of the logic device.  
         [0019]     Preferably, the node reference voltage for the third switching device in all memory cells is passed from a common reference voltage generator. In a preferred embodiment, the common reference voltage generator includes a plurality of diode-connected transistors connected in series, with one end of the series coupled to a high voltage source and other end of the series coupled to the output of the common reference voltage generator. 
     
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS  
       [0020]      FIG. 1  illustrates a commonly used memory circuit.  
         [0021]      FIGS. 2 and 3  show embodiments of U.S. Pat. No. 6,320,795.  
         [0022]      FIG. 4  shows a circuit in accordance with the invention.  
         [0023]      FIG. 5  illustrates another embodiment of the present invention.  
         [0024]      FIG. 6  show the simulation results. 
     
    
     DETAILED DESCRIPTION  
       [0025]      FIG. 4  illustrates a memory cell according to one of the embodiments of the instant invention. A memory cell  100  comprises a storage cell  101 , a static logic gate  102 , a reference voltage generator circuit  104 , pull-down transistors  107  and  117 , and a bias transistor MBIAS  115 . A pull-down transistor  107  is coupled to bitline  108  for conditionally discharging the precharged bitline  108  during a read operation based on a data stored in storage cell  101 . Static logic gate  102 , which is a NOR gate in the present embodiment, drives pull-down transistor  107  depending upon the Read signal and data stored in cell  101 . It also act as a buffer between possibly noisy read signal and the input terminal of pull-down transistor  107 . In present embodiment a reference voltage generator circuit  104  along with a bias transistor MBIAS(P-channel MOSFET) is used for significant reduction of bitline  108  leakage through ground  113 .  
         [0026]     When the read signal is logic low (i. e., a read operation is to be performed) and data stored in bit cell  101  is also logic low, output of the static logic gate  102  is logic high. This turns on pull-down transistors  107  and  117  resulting in discharging of bitline  108  through the ground  113 . Under this condition a logic high is present at the gate terminal  116  of bias transistor MBIAS and hence turning it off. Therefore no effect of reference voltage generator  104  is seen at node  118  of the circuit. For the opposite case when read signal is logic high, the output of the static logic gate  102  is logic low regardless of data bit value stored in cell  101 . As the output of the static logic gate  102  is connected to gate terminal of the both pull-down transistors  107  and  117 , both pull-down devices are off and therefore bitline  108  is decoupled from the ground  113 .  
         [0027]     When the read signal is high (i. e., no read operation is being performed) a low logic value at the output of the static logic gate  102 , turns on the bias transistor MBIAS. Therefore the voltage reference generator circuit  104  raises the potential level of the intermediate node  118  just above the threshold voltage (depending on the nature of the reference voltage generator circuit) of pull-down devices  107  and  117 . The leakage current through pull-down transistor  107  approaches a lower level as the potential at intermediate node  118  is just above the threshold voltage of the pull-down device. Also, as intermediate node  118  is charged to a lower potential (just above the V T  of pull-down device), unlike to the supply level in U.S. Pat. No. 6,320,795, hence circuit performance is improved due to reduction in charging/discharging time.  
         [0028]      FIG. 5  illustrates a simplest embodiment of the voltage reference generator of the present invention. According to this embodiment, diode connected transistors  51  and  52  are connected in series to high voltage supply  109  to produce voltage drop and produce desired voltage at the output. The goal behind the adding of a voltage reference generator circuit is to provide a lower voltage just above V T  of pull-down device to the intermediate node  118 . The area overhead due to the addition of the circuit  104  is negligible because the reference voltage generator circuit  104  is shared among a number of memory cell circuit  100 .  
         [0029]      FIG. 6  illustrates that the goal of lowering down the leakage current through pull-down device  107  is achieved at a very low potential Oust above the V T  of pull-down device) of intermediate node  118 , in comparison to a significantly high potential (near to supply) of U.S. Pat. No. 6,320,795. As the intermediate node  118  potential reaches below the V T  of pull-down device, the bit-Line leakage current increases abruptly. Hence, the best case scenario is when the intermediate node potential is just above the V T  of the pull-down device  107 .  
         [0030]     Table 1, shows the simulation results for the set-up of the prior art as shown in  FIG. 2 .  
         [0031]     Simulation results for the set-up of the present invention shown in  FIG. 4  are given in Table 2. Simulation is performed under the following conditions: Pull-down devices  107  and  117  are 4 microns in size. The size of static logic gate  102  and keeper device  110  are kept to a minimum and the load of the bitline  108  is taken as 100 fF.  
         [0032]     Table 2 shows the variation in leakage current with potential at intermediate node  118 . Bitline  108  is pulled down only when both read enable and data value stored in cell  101  are logic low.  
         [0033]     As the intermediate node potential approaches above V T  of the pull-down device, the total leakage current reduces significantly. Simulation result shows ˜57% reduction in total leakage current at node potential 305 mv in comparison to the prior art.  
                                                             TABLE 2                           PRIOR ART SIMULATIONS.                READ   BIT-LINE   PMOS   TOTAL       DATA   ENABLE   LEAKAGE   LEAKAGE   LEAKAGE               LOGIC HIGH   LOGIC     1E−12   7.89E−09   7.891E−09           LOW       LOGIC HIGH   LOGIC   7.89E−09    0.5E−09    8.39E−09           HIGH                    PRESENT INVENTION SIMULATIONS            NODE   BIT-LINE   Vref. Gen.           POTENTIAL   LEAKAGE   LEAKAGE   TOTAL LEAKAGE               778 mv   1.22E−12   6.15E−09   6.151E−09       545 mv   1.45E−12   4.65E−09   4.651E−09       *305 mv    1.87E−12   3.36E−09   3.361E−09       126 mv    0.1E−09   2.40E−09    2.5E−09                  
 
         [0034]     Although the present invention is described in reference to register file memories with a single bitline, it can applied to all types of memories in CMOS ICs requiring precharge/discharge mechanism. According to yet another embodiment, the circuitry can be extended to memories with multiple bitline for memories producing stored data value and its complementary value. According to yet another embodiment, the logic gates or transistor used in the embodiment may be changed for the memory to be in active phase for high read signal. Those of ordinary skill in the art will appreciate that various combinations and arrangements may be employed without departing from the scope of the invention  
         [0035]     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an exemplary embodiment thereof, it is the intention of the following claims to encompass and include such changes.