Abstract:
A differential sensing circuit and sensing method for use in a low voltage memory device. The sensing circuit includes a cross-coupled sensing circuit for coupling with a memory element, a pullup circuit and a multistage pulldown circuit. The multistage pulldown circuit accelerates the latching process of the cross-coupled sensing circuit by briefly pulling the cross-coupled sensing circuit to a potential below ground in order to increase the gate potential differential on at least a portion of the transistors within the cross-coupled sensing circuit. Once the latching transitions have commenced at an acceptable rate, the below-ground potential is removed and the traditional logic level pullup and ground-potential pulldown circuits are activated.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to semiconductor integrated circuits and, in particular, to a sense amplifier device used in integrated circuit memories.  
           [0003]    2. State of the Art  
           [0004]    With the advent of electronic devices such as laptop and hand-held computers as well as other electronic devices that utilize memory components, the need has increased for circuitry that operates efficiently at lower power supply voltages. Additionally, in order to increase performance speeds in memory components, the physical dimensions of the operational circuits have also been decreased which, in turn, requires lower voltages. Older memories and the sense amplifiers used to read stored information from those memories were developed for use with 5.0 volt and 3.0 volt supply voltages and have generally proven inadequate for use with lower voltage memory devices.  
           [0005]    In conventional sense amplifier designs, such as in the case of a DRAM sense amplifier, a so-called “precharging” of the sense amplifier occurs prior to a read operation of the stored memory information. As the voltage levels to such memory devices are reduced, the precharge potential, which is generally one-half the power supply potential, is also reduced. In the case of an MOS memory configuration, when the absolute value of the threshold voltage of a MOS transistor of a sense amplifier is increased, generally due to a “body affect,” and the precharge potential is reduced due to a lowered operating voltage, the difference between the precharge potential and the threshold voltage becomes smaller. Such a small potential difference results in a reduction in the sensing speed and may even result in the failure of a conventional sense amplifier to transition into a sensed state when presented with the data potential in the memory element. Therefore, a need exists for providing an improved circuit for improving the sense operation speed of a sense amplifier in a reduced or low voltage memory device configuration.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    The present invention recognizes the latency and even the inability of prior sense circuit configurations to transition into a latched configuration when applied to low voltage semiconductor memory devices. Accordingly, the present invention includes a differential sensing circuit having a pair of data line inputs (e.g., bit lines) that couple at least indirectly to the memory element. The data line pair further couples to a cross-coupled sensing circuit, configured in one embodiment as cross-coupled CMOS inverters. The cross-coupled sensing circuit provides latching to a specific state in the presence of a differential input signal as supplied by the data line pair. In order to amplify the sensed signal to full logic voltage levels, the sensing circuit further includes a pullup circuit and a pulldown circuit oppositely coupled to the cross-coupled sensing circuit.  
           [0007]    In order to enhance the latching process of the cross-coupled sensing circuit, the pulldown circuit is implemented as an at least two-stage pulldown circuit. The first stage of the pulldown circuit is initially activated prior to the assertion of the pullup circuit. In the first stage of the pulldown circuit, the cross-coupled sensing circuit is briefly pulled down to a voltage potential below the ground reference, which increases the driving abilities of the transistors within the cross-coupled sensing circuit. Increased drive potentials cause the transistors to cascade into their latched state at an improved rate. In order to prevent any deleterious effects resulting from operating logic level circuitry outside the data logic level potentials, the below-ground potential is coupled to the cross-coupled sensing circuit for only a brief time interval.  
           [0008]    Following the activation of the first stage of the pulldown circuit, the pullup circuit pulling the cross-coupled sensing circuit to a pullup potential and at least one other stage of the pulldown circuit which pulls the cross-coupled sensing circuit to a ground potential may be activated, either in a simultaneous manner or in an overlapped but staggered manner. Various embodiments for timing the activation of the respective activation signals is presented with circuitry for preventing the simultaneous activation of other pulling signals with the below-ground pulldown activation.  
           [0009]    Various other embodiments are also presented including a sense amplifier comprising the sensing circuit and precharge circuitry for precharging the data line pair to a precharge potential prior to the memory element state sensing process. Further embodiments include a memory device comprising the sensing circuitry as well as memory elements and their associated buffering and decoding.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0010]    In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention:  
         [0011]    [0011]FIG. 1 is a circuit schematic of a sensing circuit, in accordance with an embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a circuit schematic of a sensing circuit, in accordance with another embodiment of the present invention;  
         [0013]    [0013]FIG. 3 is a waveform diagram of the sensing circuit, in accordance with the present invention;  
         [0014]    [0014]FIG. 4 is circuit diagram for controlling the activation of the pulldown circuit, in accordance with an embodiment of the present invention;  
         [0015]    [0015]FIG. 5 is a waveform diagram illustrating the pulldown potential presented to the cross-coupled sensing circuit, in accordance with the pulldown control circuit of FIG. 4;  
         [0016]    [0016]FIG. 6 is circuit diagram for controlling the activation of the pulldown circuit, in accordance with another embodiment of the present invention;  
         [0017]    [0017]FIG. 7 is a waveform diagram illustrating the pulldown potential presented to the cross-coupled sensing circuit, in accordance with the pulldown control circuit of FIG. 6;  
         [0018]    [0018]FIG. 8 is a block diagram of a semiconductor memory including the sensing circuit, in accordance with the present invention;  
         [0019]    [0019]FIG. 9 is a block diagram of an electronic device including the sensing circuit, in accordance with the present invention; and  
         [0020]    [0020]FIG. 10 is diagram of a semiconductor wafer on which is fabricated the memory device including the sensing circuit of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    [0021]FIG. 1 is a diagram illustrating a data line sensing circuit  10  in accordance with an embodiment of the present invention. Sensing circuit  10  includes a PMOS sensing circuit  12  and an NMOS sensing circuit  14  respectively connected between bit or data lines D  16  and D*  18 . The PMOS sensing circuit  12  includes a first PMOS transistor  20  including a source electrode connected to a sensing node  22 , a drain electrode connected to the data line D  16  and a gate electrode connected to the data line D*  18 , and a second PMOS transistor  24  including a source electrode connected to the sensing node  22 , a drain electrode connected to the data line D*  18  and a gate electrode connected to the data line D  16 .  
         [0022]    The NMOS sensing circuit  14  includes a first NMOS transistor  26  with a drain electrode connected to a sensing node  28 , a source electrode connected to the data line D  16  and a gate electrode connected to the data line D*  18 , and a second NMOS transistor  30  with a drain electrode connected to the sensing node  28 , a source electrode connected to the data line D*  18  and a gate electrode connected to the data line D  16 .  
         [0023]    A third PMOS transistor  32  for supplying a power voltage VCC is connected to the sensing node  22 . A third NMOS transistor  34  for supplying a ground voltage VSS is connected to the sensing node  28 . A fourth NMOS transistor  36  for supplying a negative bias voltage—VBB is connected to the sensing node  28 . P-sense control signal  38  is connected to the gate electrode of PMOS transistor  32  and N-sense ground control signal  40  is connected to the gate electrode of the NMOS transistor  34  while the N-sense below-ground control signal  42  is connected to the gate electrode of NMOS transistor  36 . If the third PMOS transistor  32  and either the NMOS transistor  36  or the NMOS transistor  34  are turned on, the power voltage VCC and either the below-ground voltage—VBB or the ground voltage VSS are supplied to the sensing nodes  22  and  28 , respectively, thereby operating the PMOS sensing circuit  12  and the NMOS sensing circuit  14 .  
         [0024]    In the present embodiment of the present invention, sensing circuit pullup circuit  44 , including the pullup PMOS transistor  32  and pullup circuit control logic  46 , cooperatively function as the pullup circuit to a cross-coupled sensing circuit  48 . Similarly, NMOS transistors  34  and  36  cooperatively function as a multistage pulldown circuit  50  including the pulldown NMOS transistors  36  and  34  as well as multistage pulldown circuit control logic  52 .  
         [0025]    [0025]FIG. 2 illustrates a data line sensing circuit in accordance with another embodiment of the presenting invention. In the present embodiment, a data line sensing circuit  60  includes the cross-coupled sensing circuit  48  for sensing the state of data lines D  16  and D*  18 . To aid in the sensing process, the data line sensing circuit  60  further includes the pullup circuit  44  which includes the PMOS transistor  32  controlled by the P-sense control signal  38  as generated by pullup circuit control logic  46 . Additionally, data line sensing circuit  60  further includes a multistage pulldown circuit  62  for providing a lower potential or voltage reference for pulling the sensing node  28  toward the provided lower voltage. In the present embodiment, the multistage pulldown circuit  62  includes a low Vt NMOS transistor  64  for reducing current leakage in the multistage pulldown circuit of the present embodiment. Furthermore, the multistage pulldown circuit further includes the NMOS transistors  34  and  36  as well as a multistage pulldown circuit control logic  66  for generating N-sense latch control signal  68 , N-sense ground control signal  40  and N-sense below-ground control signal  42 . While a multistage pulldown circuit has been illustrated as including only two stages within the pulldown circuit, the present invention contemplates applications where waveforming of the pulldown potential profile would be desirable and, therefore, multiple stages beyond two pulldown transistors is contemplated within the scope of the present invention.  
         [0026]    [0026]FIG. 3 is a waveform diagram illustrating the related signaling, in accordance with an embodiment of the present invention. In accordance with conventional memory device bit sensing, in an initial state, the data lines D  16 , D*  18  (FIGS. 1 and 2) are precharged to an intermediate voltage of approximately ½ the level of VCC. The data lines are generally pass-gate coupled to the cross-coupled sensing circuit  48  (FIGS. 1 and 2) with present data signals illustrated on data line D  16  and data line D*  18 . Prior to the activation of either the pullup circuit  44  or the multistage pulldown circuit, a small differential signal is present on the respective data lines D  16 , D*  18  as illustrated in FIG. 3. Once a sense enablement phase* control signal  70  becomes active low, the multistage pulldown circuit becomes activated. In the embodiment of FIG. 2, an N-sense latch control signal  68  is activated by the multistage pulldown control logic  66  (FIG. 2) which provides current leakage protection to the cross-coupled sensing circuit  48  (FIG. 2). Subsequently, the N-sense below-ground control signal  42  activates NMOS transistor  36  (FIGS. 1 and 2) and pulls the cross-coupled sensing circuit  48  toward a voltage that is less than ground potential, thereby increasing the VGS across the transistors of the cross-coupled sensing circuit  48  to an amount greater than the Vt of the transistors. By increasing the VGS across these transistors, the cross-coupled sensing circuit  48  is able to latch at a much faster rate. Alternatively, it is also desirable to “precharge” the source electrode of transistor  64  by activating the N-sense below-ground control signal just prior to the activation of transistor  64 .  
         [0027]    As illustrated in FIG. 3, once the N-sense below-ground control signal  42  becomes active, the data lines D  16  and D*  18  diverge at a much greater rate than occurs without utilization of a below-ground voltage, illustrated as the dashed waveform  18 ′. In order to prevent over negative charging of the cross-coupled sensing circuit  48  and presenting deleterious conditions in a subsequent sense operation, once the separation has been accelerated, the cross-coupled sensing circuit  48  is coupled to a ground reference through activation of the N-sense ground control signal  40 .  
         [0028]    Two separate control circuits are presented in FIGS. 4 and 6 for preventing the simultaneous activation of both the N-sense below-ground control signal  42  and the P-sense control signal  38  which would result in an undesirable reference voltage shift. FIG. 4 illustrates a circuit having input signals of the sense enablement phase* control signal  70  and a P-sense activate status signal  72  which are combined through logic gates to prevent a simultaneous activation. FIG. 5 illustrates the multistage pulldown waveform as generated by the circuit of FIG. 4 and as observed at an N-sense activation status signal  74 . A time period  76  identifies the duration for activation of the N-sense below-ground control signal  42  and a time period  77  identifies a duration wherein both the N-sense ground control signal  40  and the P-sense control signal  38  may be activated. It should be appreciated that this duration is a function of the relative speeds and threshold voltages of the transistors of the cross-coupled sensing circuit  48 . It is desirable that the duration be adjusted to facilitate a more rapid separation of the differential signals while not retaining a negative potential for an extended duration after the pullup circuit  44  becomes active. Methods and circuits for implementing duty cycle adjustments to timing circuits are appreciated by those of ordinary skill in the art.  
         [0029]    [0029]FIG. 4 further illustrates a logic gate  79  for coupling within the multistage pulldown control logic  66  for eliminating the multistage ability of the multistage pulldown circuit  62  during testing, including probe testing. When probe test disable signal  81  is asserted, the sensing circuit  60  functions as a single stage pulldown sensing circuit. Such a sensing mode is desirable for lower-speed testing and segregation. Disable signal  81  may also be used to configure sensing circuit  60  as a slower performing device.  
         [0030]    [0030]FIG. 6 is a circuit diagram of a circuit for generating the N-sense below-ground control signal  42 , in accordance with another embodiment of the present invention. In FIG. 6, the sense enablement phase* control signal  70  couples to logic gates  78 ,  80 ,  82  and delay element  84  to form the N-sense ground control signal  40  and the N-sense below-ground control signal  42 . Additionally, the delay element  84  is adjusted in conjunction with the logic to prevent the simultaneous assertion of both the N-sense below-ground control signal  42  and the P-sense control signal  38 . While the circuit of FIG. 6 is illustrated as one embodiment of a pulse generator, other pulse generator embodiments are also contemplated which may form the desired waveform for the N-sense activation status signal  74  as illustrated in FIG. 7.  
         [0031]    [0031]FIG. 7 illustrates the multistage pulldown waveform as generated by the circuit of FIG. 6 and as observed at the N-sense activation status signal  74 . It should be noted that both the circuits of FIGS. 4 and 6 generate comparable waveforms, namely a time period  76  wherein the N-sense below-ground control signal  42  is activated and a time period  77  wherein both the N-sense ground control signal  40  and the P-sense control signal  38  may be activated, either simultaneously or in a staggered manner, which may be preferable for a particular semiconductor fabrication.  
         [0032]    As shown in FIG. 8, a semiconductor memory  90  includes an array  92  of memory cells  94  activated by wordlines  96  fired by row driving circuitry  98  coupled to address buffers  100  and address decoders  102 . A plurality of data lines  104  communicate logic bits between the memory cells  94  and column selection and sensing circuitry  106  that includes sense amplifiers  108 . Sense amplifiers  108  each include the sensing circuit  10  of the present invention and may further include precharge circuitry  110  and output circuitry  112 .  
         [0033]    In memory operations of the semiconductor memory  90 , the process of reading a logic bit from one of the memory cells  94  begins with the row driving circuitry  98  firing one of the active wordlines  96 . While the wordline  96  is propagating, the memory cells  94  connected to the active wordline  96  fired by the circuitry  98  activate and begin generating differential voltages on the data lines  104  representative of their stored logic bits. Once these differential voltages are of sufficient magnitude to be sensed by the sense amplifiers  108 , the column selection and sensing circuitry  106  fires the sense amplifiers  108 , thus allowing the sense amplifiers  108  to sense the differential voltages on the data lines  104 . A selected one of the logic bits represented by one of the sensed differential voltages is subsequently provided to the output circuitry  112  for use by external circuitry (not shown). Precharging of nodes within the sensing circuit may also be performed by precharge circuitry  110  prior to firing the sensing circuit  10 .  
         [0034]    As shown in FIG. 9, an electronic system  114  includes an input device  116 , an output device  118 , a processor device  120 , and a memory device  122  incorporating the semiconductor memory  90  of FIG. 8. As shown in FIG. 10, a semiconductor wafer  124  incorporates the semiconductor memory  90  of FIG. 8.  
         [0035]    Although the present invention has been described with reference to particular embodiments, the invention is not limited to these described embodiments. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices or methods that operate according to the principles of the invention as described.