Abstract:
A DRAM including an array of storage elements arranged in lines and columns, and for each column: write means adapted to biasing at least a selected one of the elements to a charge level chosen from among a first predetermined high level and a second predetermined low level, combined with read circuitry adapted to determining whether the stored charge level is greater or smaller than a predetermined charge level; and isolation circuitry adapted to isolating the array from the read and/or write means, each column further including refreshment means, distinct from the read and write circuit, for increasing, beyond the first and second predetermined levels, the charge stored in a storage element.

Description:
CROSS REFERENCE TO THE RELATED APPLICATIONS  
       [0001]     This application is a continuation of prior application Ser. No. 10/337,215, filed Jan. 6, 2003, which in turn is a continuation of prior application Ser. No. 10/210,864, filed on Aug. 1, 2002 which in turn is a continuation prior application Ser. No. 10/033,384, filed on Dec. 28, 2001 entitled DRAM REFRESHMENT. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to forming, in integrated form, memory devices. More specifically, the present invention relates to forming DRAMs, that is, memories requiring periodic refreshment.  
         [0004]     2. Discussion of the Related Art  
         [0005]     The increase in DRAM densities leads to forming control elements (generally MOS transistors) of memory points (capacitors) of more and more reduced dimensions (surface areas). This reduction in dimensions goes along with a reduction of the voltage levels that these elements can tolerate.  
         [0006]     Accordingly, the use of DRAM memory point control transistors having reduced surface areas leads to reducing the charge levels stored in these memory points. Such a reduction makes the discrimination more difficult, at the level of sense amplifiers associated with the memory, between the states of an addressed storage element and of an associated reference storage element.  
         [0007]     A first solution for overcoming this disadvantage would consist of increasing the responsiveness of sense amplifiers associated with the memory device to increase their accuracy until they are able to detect a level difference as small as approximately 50 mV. However, implementing such a solution means forming sense amplifiers including a greater and greater number of transistors, the thresholds of which must be set more and more accurately. Independently from the complications of the memory device integration process, the increase in the number of transistors forming such accurate sense amplifiers considerably reduces the advantages provided by the reduction of the characteristic dimensions of the control transistors of the storage elements of a DRAM.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention aims at providing a novel type of DRAM device that can be made in integrated form with minimum dimensions, and in which the stored voltage levels are adapted to being discriminated by a standard sense amplifier.  
         [0009]     To achieve these and other objects, the present invention provides a DRAM including an array of storage elements arranged in lines and columns, and for each column: 
        write means adapted to biasing at least a selected one of the elements to a charge level chosen from among a first predetermined high level and a second predetermined low level, combined with read means adapted to determining whether the stored charge level is greater or smaller than a predetermined charge level; and     an isolation means adapted to isolating the array from the read and/or write means,     each column further including refreshment means, distinct from the read and write means, for increasing, beyond the first and second predetermined levels, the charge stored in a storage element.        
 
         [0013]     According to an embodiment of the present invention, the refreshment means is formed of a P-channel dual-gate MOS transistor, interposed between a high supply rail and a node of interconnection of drains of two P-channel dual-gate MOS transistors, the sources of which form input/output terminals, each of which is connected to the gate of the other transistor.  
         [0014]     According to an embodiment of the present invention, the isolation means includes, interposed between each output of the array and one of the input/output terminals of the read-write means, an N-channel dual-gate MOS transistor.  
         [0015]     According to an embodiment of the present invention, each column also includes a stage for precharging the array.  
         [0016]     According to an embodiment of the present invention, the precharge stage includes, interposed between each output of the network and a precharge supply source, an N-channel dual-gate MOS transistor.  
         [0017]     According to an embodiment of the present invention, each column also includes a stage of precharge and balancing of the read means.  
         [0018]     According to an embodiment of the present invention, the precharge and balancing stage includes, interposed between two read terminals of the read means, three MOS transistors of the same type with a common gate, two first ones of which are series-connected between the read terminals, their common point being connected to a precharge supply source, and the last one of which directly short-circuits the read terminals.  
         [0019]     The present invention also provides a method for writing a datum into a memory according to any of the preceding embodiments, including the step of biasing the isolation means so that it is partially open, and enabling the refreshment means.  
         [0020]     According to an embodiment of the present invention, the method includes the step of biasing the isolation means so that it is completely open, and enabling the refreshment means.  
         [0021]     According to an embodiment of the present invention, the method includes providing, for the isolating means, a three-level control signal, a first level completely activating the isolation means, a second low level completely inhibiting the isolation means, and a third level being adapted to activating or inhibiting each N-channel dual-gate MOS transistor according to the state of the input/output terminal of the read-write means associated therewith.  
         [0022]     The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  schematically and partially illustrates a column of a memory device according to the present invention;  
         [0024]      FIG. 2  illustrates the variation in time of control and output signals at certain nodes of the circuit of  FIG. 1  during precharge operations; and  
         [0025]      FIG. 3  illustrates the variation with time of control and output signals at certain nodes of the circuit of  FIG. 1  during the reading of a datum. 
     
    
     DETAILED DESCRIPTION  
       [0026]     The same elements have been designated with the same references in the different drawings. For clarity, the waveforms illustrated in FIGS.  2  and/or  3  have been designated by the name of the lines conveying them in  FIG. 1 . Further, only those elements necessary to the understanding of the present invention have been shown and will be described hereafter. In particular, the structures and operations of the different control circuits of the memory have not been detailed and are no object of the present invention. Similarly, the operation and exploitation of the input/output signals of a DRAM will be considered as known and will not be detailed hereafter.  
         [0027]      FIG. 1  schematically and partially illustrates a column of a DRAM device according to the present invention. The column includes an array MA of storage elements  2 . Storage elements  2  are distributed in array MA at the intersection of lines and columns. Each column includes two bit lines, one direct bit line BLd and one reference bit line BLr, shown vertically. Each element  2  is formed of the series connection, between a bit line BLd and a low supply rail or circuit ground, of a control transistor T, for example with an N channel, and of a capacitor C. The junction point of transistor T and capacitor C forms memory point STORE. The gate of transistor T is connected to a word line WL of array MA, shown horizontally. Each column is associated with at least one reference element (not shown) formed, like element  2 , of a control transistor and of a capacitor connected similarly to those of element  2 , but between reference bit line BLr of the same column and a word line (not shown) distinct from line WL. The reference element is associated with a reference memory point STOREref (not shown in  FIG. 1 ).  
         [0028]     Each column of array MA is associated with a read amplifier SA. More specifically, direct bit line BLd is connected to a first read terminal SA 1  of amplifier SA via an isolation and transfer stage  3 . Similarly, reference bit line BLr is connected to a second read terminal SA 2  of read amplifier SA via isolation and transfer stage  3 . Read amplifier SA is bidirectional and is also used in the writing.  
         [0029]     Each amplifier SA is formed of the series connection, between a first high supply rail Vdd and the circuit ground, of two complementary MOS selection transistors P 4  and N 5 , respectively with a P channel and with an N channel, between which is interposed a parallel association of two identical branches B 1  and B 2 . Each branch B 1  and B 2  includes, connected in series, a P-channel MOS transistor P 6 , respectively P 7 , and an N-channel MOS transistor N 8 , respectively N 9 . The common point of transistors P 6  and N 8  of first branch B 1  forms first read terminal SA 1 . The common point of transistors P 7  and N 9  of second branch B 2  forms second read terminal SA 2 . The gates of complementary transistors P 6  and N 8  of branch B 1  are interconnected and connected to terminal SA 2 . The gates of transistors P 7  and N 9  of branch B 2  are interconnected and connected to terminal SA 1 . The drains of transistors P 6  and P 7  are connected to the source of transistor P 4 . The sources of transistors N 8  and N 9  are interconnected to the drain of transistor N 5 . Transistors P 6 , P 7 , N 8  and N 9  form two inverters in antiparallel between read terminals SA 1  and SA 2 . The gates of selection transistors P 4  and N 5  are driven by respective enable signals SELP and SELN. The structure of amplifier SA and of selection elements P 4  and N 5  is conventional.  
         [0030]     According to the present invention, isolation and transfer stage  3  is formed of dual-gate N-channel MOS transistors N 10 , N 11 . Transistors N 10  and N 11  receive on their respective gate a same data transfer control signal BLPASS. The drains of transistors N 10  and N 11  are respectively connected to the end of one of bit lines BLd and BLr. Their sources are connected to one of read terminals SA 1 , SA 2  of read amplifier SA, respectively.  
         [0031]     A feature of the present invention is that signal BLPASS is chosen so that, when active (high level), its level is such that one of transistors N 10  or N 11  is off while the other transistor N 11  or N 10  is on, according to the state of the read terminals with which they are respectively associated. This feature will better appear from the description of  FIG. 3 .  
         [0032]     Read amplifier SA is associated with a conventional precharge and balancing stage  12  formed of three P-channel MOS transistors P 13 , P 14 , and P 15  with common gates. Transistors P 13  and P 14  are series-connected between terminals SA 1  and SA 2 . The junction point of transistors P 13  and P 14  is connected to supply rail Vdd. Transistor P 15  directly short-circuits terminals SA 1  and SA 2 . The common gate of transistors P 13 , P 14 , and P 15  receives a precharge control signal PSA.  
         [0033]     Direct bit line BLd and reference bit line BLr are also connected to a precharge device  16 . According to the present invention, device  16  includes a dual-gate N-channel MOS transistor N 17  or N 18 , interposed between high supply rail Vdd and the end of bit line BLd or BLr at the output of array MA. Precharge stage  16  is interposed between array MA and transfer stage  3 . The source of precharge transistor N 17 , N 18  of each bit line BLd, BLr is thus connected to the drain of the corresponding transfer transistor N 10 , N 11 . The gates of transistors N 17 , N 18  are interconnected and receive a same bit line precharge control signal PBL.  
         [0034]     According to the present invention, a column of a DRAM-type memory device further includes a refreshment stage  19 . Stage  19  includes two input/output terminals OUT 20  and OUT 21  respectively connected to one end of one of bit lines BLd and BLr of the considered column. The involved end of the bit lines is that opposite to that connected to bit line precharge stage  16  and transfer stage  3 . Terminal OUT 20 , associated with line BLd, is the source of a dual-gate P-channel MOS transistor P 22 . Terminal OUT 21 , associated with line BLr, is the source of a dual-gate P-channel MOS transistor P 23 . Terminal OUT 20  is also connected to the gate of transistor P 23 . Terminal OUT 21  is also connected to the gate of transistor P 22 . The drains of transistors P 22  and P 23  are interconnected to the source of a dual-gate P-channel transistor P 24  for controlling refreshment stage  19 .  
         [0035]     The drain of transistor P 24  is connected to a second high supply rail Vcc, the voltage level of which is, according to the present invention, greater than the write levels of storage elements  2  and to the precharge level of amplifier SA. In the considered case, the highest write level and the precharge level of amplifier SA are equal to the voltage of first high supply rail Vdd. The gate of transistor P 24  receives a refreshment control signal RESTORE.  
         [0036]     The operation of a memory device according to the present invention will be explained hereafter in relation with  FIG. 2 , which illustrates the variation along time of various control and/or output signals of a dynamic memory according to the present invention, in an operation of precharge of a storage element  2  ( FIG. 1 ) or of read terminals SA 1  and SA 2 .  FIGS. 2A, 2B ,  2 C,  2 D,  2 E, and  2 F respectively show control signals RESTORE, WL, PBL, BLPASS, SELN ({overscore (SELP)}), and PSA.  FIGS. 2G  to  2 J illustrate the voltage variations along time, respectively, on line BLd, at point STORE, and on terminals SA 1  and SA 2 .  
         [0037]     A precharge of a direct bit line BLd ( FIG. 1 ) of a given column and of the associated reference bit line may occur, for example, before any write operation of a datum into this element  2 . The precharge starts by isolating network MA from amplifier SA. For this purpose, isolation control signal BLPASS is, according to its initial state, brought to or maintained at zero at a time to ( FIG. 2D ). Transistors N 10  and N 11  are then both off, provided that the levels of points SA 1  and SA 2  are higher than −Vt, where Vt represents the threshold voltage of transistors N 10  and N 11 . This condition can always be respected by properly choosing the precharge levels. Cells  2  are isolated from the direct and indirect bit lines by maintaining their control transistor open. At a time t 1 , transistors N 17  and N 18  of precharge stage  16  are turned on by bringing to its highest voltage level VNon precharge signal PBL ( FIG. 2C ). Then, direct bit line BLd ( FIG. 2G ) and point STORE ( FIG. 2H ) bias to the voltage level of precharge transistor N 17 , for example, Vdd, which is reached at a time t 2 . Similarly, reference bit line BLr is precharged to the level set by the supply of precharge transistor N 18 , preferably identical to the supply level of transistor N 17 , for example, Vdd. In such a precharge, refreshment stage  19  is disabled. Indeed, refreshment control signal RESTORE ( FIG. 2A ) is maintained at the high state VPoff, thus guaranteeing the off state of transistor P 24 . On the other hand, the relatively high precharge voltage levels of bit lines BLd and BLr are transmitted to terminals OUT 20  and OUT 21 , that is, onto the gates of P-channel transistors P 22  and P 23 . These transistors are then controlled to be turned off. After time t 2  of level stabilization on bit line BLd (and BLr), precharge signal PBL is switched at time t 3  to disable precharge stage  16 .  
         [0038]     A precharge of read terminals SA 1  and SA 2  of read amplifier SA is necessary before a read operation of data stored in any of elements  2  of network MA. In such a precharge, network MA must be isolated from amplifier SA. This condition being fulfilled in  FIG. 2 , the operation of precharge of amplifier SA will be described hereafter in relation with  FIGS. 2D, 2F ,  21 , and  2 J. The isolation between network MA and amplifier SA being ensured at time t 0  by a low state of signal BLPASS, transistors P 13 , P 14 , and P 15  of stage  12  are controlled to be turned on by a switching of signal PSA ( FIG. 2F ) at a time t 4 . Transistors P 13  and P 14  then copy on terminals SA 1  ( FIG. 21 ) and SA 2  ( FIG. 2J ) the voltage level set at their common point, for example, level Vdd. The voltage levels of points SA 1  and SA 2  are balanced by transistor P 15 . The precharge of terminals SA 1  and SA 2  ends after stabilization at a subsequent time t 5  by the disabling of stage  12 .  
         [0039]     Those skilled in the art should understand that the foregoing description of operations of precharge of the bit lines and of the amplifier in relation with the same  FIG. 2  implies no synchronization between the two operations. Such a description has been made as a non-limiting example only, since the precharges are both performed while network MA is isolated from amplifier SA, that is, when signal BLPASS is low. Any other operation could however be performed on array MA during a precharge of read amplifier SA, such as, for example, a refreshment, requiring isolation between network MA and amplifier SA. Conversely, during a precharge of one or several of storage elements  2  of array MA, any operation such as, for example, the loading on terminals SA 1  and SA 2  of data to be transferred to network MA, imposing an isolation between network MA and amplifier SA, could be performed.  
         [0040]     Successive read and refresh operations of a datum contained in a storage element  2  will be detailed hereafter in relation with  FIG. 3 , which illustrates the variation along time of various control and/or output signals of a dynamic memory according to the present invention.  FIG. 3A  shows signal RESTORE.  FIG. 3B  shows signal WL.  FIG. 3C  shows signal PBL.  FIG. 3D  shows signal BLPASS.  FIG. 3E  shows signal SELN ({overscore (SELP)}).  FIG. 3F  shows signal PSA.  FIGS. 3G  ad  3 H illustrate the variations along time, respectively, on direct bit line BLd and reference bit lines BLr.  FIGS. 31 and 3 J illustrate the voltage variation along time, respectively, on storage nodes STORE and STOREref.  FIGS. 3K and 3L  illustrate the voltage variations along time, respectively, on terminals SA 1  and SA 2 .  
         [0041]     A read operation occurs after an operation of precharge of bit lines BLd and BLr and of terminals SA 1  and SA 2 . For simplification, as illustrated in  FIGS. 3G, 3H ,  3 K, and  3 L, it is considered, as previously, that the different precharge levels are equal and equal to a first high write supply Vdd. These precharge states being stabilized, a read operation is performed while stages  12  of precharge of terminals SA 1  and SA 2 , and  16  of precharge of lines BLd, BLr, respectively, are disabled by the maintaining of signals PSA ( FIG. 3F ) and PBL ( FIG. 3C ) at respective high (for example, Vdd) and low (for example, 0) states. These conditions being fulfilled, a read operation starts at a time t 30  by the selection of storage element  2  and of the associated reference element by the turning-on of their respective control transistors T (that is, the switching to a relatively high state, for example Vdd, of their respective control signals WL and WLref,  FIG. 3B ). Simultaneously, transfer stage  3  is made completely conductive by the switching to its highest state VRS of signal BLPASS ( FIG. 3D ). Then, the voltage levels on terminals SA 1  and SA 2  tend to balance with the levels of the storage elements. In practice, it is assumed that storage element  2  contains a datum “1”. Point STORE then is at its highest voltage level VRS ( FIG. 31 ). Anti-symmetrically, it is assumed that the reference storage element contains a datum “0”. Storage point STOREref then initially is at a null voltage level. At time t 30 , the voltage levels on bit line BLd and at point STORE tend towards a relatively high voltage level V 1 , between VRS and Vdd. Anti-symmetrically, reference bit line BLr, terminal SA 2  and reference storage point STOREref tend to balance at a voltage level V 2  at most equal to Vdd. Indeed, on the direct bit line side, the charges must balance between a point at voltage VRS and a point at voltage Vdd. However, on reference bit line BLr, they need to balance between a point at a null voltage and a point at a voltage Vdd. There then appears between terminals SA 1  and SA 2  a voltage difference ΔV equal to the difference existing between voltages V 1  and V 2 . At a time t 31 , read amplifier SA is enabled by an appropriate switching of its selection signals SELN and SELP ( FIG. 3E ). Voltage difference ΔV is then amplified by amplifier SA. The relatively high and null voltage states thus reached by terminals SA 1  and SA 2  then correspond to the levels initially stored respectively in the storage and reference elements.  
         [0042]     According to the present invention, such a read operation is followed by an operation of refreshment of datum “1”. For this purpose, the following is performed. Once the read levels have been reached, isolation and transfer stage  3  is partially unbalanced at a time t 33 , by a control signal BLPASS at a level (for example, Vcc) lower than the high turn-on level VNon of this stage.  
         [0043]     In fact, voltage Vcc is chosen to be lower than the high level (for example, Vdd) of a read terminal, plus threshold voltage Vt of transistors N 10  and N 11 . Advantage is then taken from the fact the transistors N 10  and N 11  having an N channel, they let through the levels which are lower than the level of their gate voltage minus their threshold voltage. If the read terminal (for example, SA 1 ) is high, the corresponding transistor (for example, N 10 ) of isolation stage  3  is off. If the read terminal (for example, SA 2 ) is low, the corresponding transistor (for example, N 11 ) is on. Amplifier SA is thus partially isolated from network MA.  
         [0044]     Refreshment stage  19  is then enabled by a transition from a high state to a low state of its control signal RESTORE (time t 33 ). Refresh voltage Vcc for supplying refresh control transistor P 24  is then transmitted over direct bit line BLd. The charge stored on memory node STORE is then increased, as illustrated in  FIG. 31 . Indeed, as discussed previously, level Vcc is higher than write level Vdd. Since transistor N 11  is on, isolation stage  3  enables, from as soon as time t 33  of switching of refresh control signal RESTORE, reference bit line BLr to be biased to the low level by the second terminal SA 2  of read amplifier SA. Line BLr then transmits this low level onto the gate of P-channel transistor P 22  of refreshment stage  19  connected to direct bit line BLd. Transistor P 22  is then controlled to be turned on and output OUT 20  of refreshment stage  19  copies refreshment voltage Vcc. The initial high state of direct bit line BLd is transmitted to the gate of transistor P 23 , which remains off and prevents transmission of refresh level VRS over reference bit line BLr. Such a transmission would cause a conflict of values at the level of read terminals SA 1  and SA 2 , which would result in an undesirable oscillation of the levels in the circuit. Due to the turning-off of transistor N 10 , stage  3  isolates terminal SA 1  from line BLd and thus avoids any conflict between levels VRS and Vdd. Accordingly, the sole datum “1” stored in element  2  has its voltage raised. Once this raised voltage level has stabilized at VRS, the refreshment of element  2  can be interrupted at a time t 34  which also corresponds to the end of the reading, that is, to the deselection of the storage element and to the total closing of transfer stage  3 , signals WL, WLref and BLPASS being brought down to 0.  
         [0045]     A write operation is performed as follows. A write operation starts with the locking on input/output terminals SA 1  and SA 2  of read amplifier SA of the data to be written.  
         [0046]     According to a first embodiment, a power write amplifier (not shown in  FIG. 1 ) which imposes desired states on terminals SA 1  and SA 2  may be used for this locking.  
         [0047]     According to a second embodiment, the amplifier terminals are precharged, as previously described in relation with  FIG. 2 , to a level Vdd. Then, a voltage imbalance between terminals SA 1  and SA 2  is created in the desired direction by means of an appropriate means not shown in  FIG. 1 . Read amplifier SA is then selected. Then, as described previously between times t 33  and t 34  ( FIG. 3 ), the imbalance between terminals SA 1  and SA 2  is amplified.  
         [0048]     Once these states have been locked, the data transfer is possible. For this purpose, as for a reading, storage element  2  and its corresponding reference element are simultaneously selected, while enabling transfer of the data by a switching to its highest state of signal BLPASS. Read amplifier SA being enabled, it controls the voltage levels at points STORE and STOREref. Once these levels have been stabilized, it is desirable to raise datum “1”. For this purpose, as previously discussed, signal BLPASS is brought to a lower level, while stage RESTORE is enabled by a switching to the low state of signal RESTORE. Then, in the way previously described in relation with  FIG. 3 , high level Vdd of the cell containing datum “1”, for example, cell  2 , is raised to level VRS.  
         [0049]     According to an alternative, the refresh operations can only be performed before a reading.  
         [0050]     The control signals are then modified in any appropriate manner to allow for a next operation, for example, of deleting of element  2 , that is, its return to a precharge state as previously described in relation with  FIG. 2 , or the preparation of a subsequent operation of refreshment of or rewriting into element  2 , or its reading.  
         [0051]     An advantage of the present invention is to replace relatively complex and bulky read amplifier devices by the combination of a standard read stage and of a refresh stage only formed of three additional transistors per column. According to the present invention, the difficulty for an amplifier with transistors of reduced surface area in discriminating the value of a high level stored in the read-addressed storage element or in the associated reference element is solved by a raising of this high level. In other words, while the writing is performed in a range at most limited to the voltage level of the first high supply rail Vdd, the reading is performed in a range ranging to the voltage level of the second high supply rail Vcc. Since the amplifier precharge is performed at most at the voltage level of the first high supply rail Vdd, by choosing a voltage level of second supply rail Vcc sufficiently greater than said first value, the discrimination is ensured.  
         [0052]     The space gain and the simplification of memory device manufacturing methods widely compensates for the use of high-voltage dual-gate transistors, especially for transfer stage  3  and precharge stage  16 , which must be able to stand in the off state a relatively high refreshment voltage Vcc.  
         [0053]     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the writing and the reading of a datum “1” has been previously described as a non-limiting example. However, those skilled in the art will be able to adapt the previously described control signals to the writing and/or the reading of a datum “0”. More generally, based on the foregoing description of a few simple operations, those skilled in the art will know how to appropriately control each column of the memory device to obtain a desired behavior.  
         [0054]     Further, those skilled in the art will know how to adapt the different control levels of the different stages to the principles and structures discussed in the foregoing description and to the desired functions. Those skilled in the art will also know how to minimize the number of necessary power supplies, especially by forming complementary MOS transistors, the highest turn-on or turn-off control levels VNon and VPoff of which are equal. Similarly, the different precharge levels of the bit lines or of the amplifier have been considered as being equal to the high write level set by first high supply rail Vdd. However, the precharge levels could be different, that is, lower, from this high write level.  
         [0055]     Finally, the control levels will be adapted according to the principles discussed in the foregoing description and to the desired functions, according to the type of MOS switches used. For example, instead of being P-channel MOS transistors, the three transistors P 13 , P 14 , and P 15  forming the precharge and balancing stage  12  of read amplifier SA may be three N-channel transistors, the corresponding control signal PSA then being the complementary of the previously described signal. More generally, transistors complementary to those used in the foregoing description may be used by correspondingly modifying their control signals.  
         [0056]     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.