Abstract:
The present invention provides a method and apparatus for writing a programmable conductor random access memory (PCRAM) element. After a read operation of the memory element a complement logical state from that read is written back to the memory element. In one embodiment the memory element is then again written back to its original state. In another embodiment logic circuitry keeps track of whether the original logic state or its complement are stored in the memory element so that during the next read the stored logic will be correctly read.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates to integrated memory circuits. More specifically, it relates to a method for writing a programmable conductor random access memory (PCRAM) cell.  
         BACKGROUND OF THE INVENTION  
         [0002]    Dynamic random access memory (DRAM) integrated circuit arrays have existed for more than thirty years and their dramatic increase in storage capacity has been achieved through advances in semiconductor fabrication technology and circuit design technology. The tremendous advances in these two technologies have also achieved higher levels of integration that permit dramatic reductions in memory array size and cost, as well as increased process yield.  
           [0003]    [0003]FIG. 1 is a schematic diagram of a DRAM memory cell  100  comprising an access transistor  101  and a capacitor  102 . The capacitor  102 , which is coupled to a Vcc/2 potential source and the transistor  101 , stores one bit of data in the form of a charge. Typically, a charge of one polarity (e.g., a charge corresponding to a potential difference across the capacitor  102  of +Vcc/2) is stored in the capacitor  102  to represent a binary “1” while a charge of the opposite polarity (e.g., a charge corresponding to a potential difference across the capacitor  102  of −Vcc/2) represents a binary “0.” The gate of the transistor  101  is coupled to a word line  103 , thereby permitting the word line  103  to control whether the capacitor  102  is conductively coupled via the transistor  101  to a bit line  104 . The default state of each word line  103  is at ground potential, which causes the transistor  101  to be switched off, thereby electrically isolating capacitor  102 .  
           [0004]    One of the drawbacks associated with DRAM cells  100  is that the charge on the capacitor  102  may naturally decay over time, even if the capacitor  102  remains electrically isolated. Thus, DRAM cells  100  require periodic refreshing. Additionally, as discussed below, refreshing is also required after a memory cell  100  has been accessed, for example, as part of a read operation.  
           [0005]    Efforts continue to identify other forms of memory elements for us in memory cells, particularly for memory elements which do not required frequent refresh operations. Recent studies have focused on resistive materials that can be programmed to exhibit either high or low stable ohmic states. A programmable resistance element of such material could be programmed (set) to a high resistive state to store, for example, a binary “1” data bit or programmed (set) to a low resistive state to store a binary “0,” data bit. The stored data bit could then be read by detecting the magnitude of a readout current switched through the resistive memory element by an access device, thus indicating its programmed stable resistance state.  
           [0006]    Recently programmable conductor materials, such as chalcogenide glasses, have been investigated as data storage memory cells for use in memory devices. U.S. Pat. Nos. 5,761,115, 5,896,312, 5,914,893, and 6,084,796 all describe chalcogenide glass materials which can be used as programmable conductor memory elements and are incorporated herein by reference. One characteristic of such an element is that it typically includes a chalcogenide glass which is doped with metal ion and a cathode and anode spaced apart on a surface of the glass. Application of a voltage across the cathode and anode causes the glass to achieve a low resistance state. One theory for this is that the applied voltage causes growth of a nearly non-volatile metal dendrite in or on the surface of the glass which changes the resistance and capacitance of the memory element which can then be used to store data.  
           [0007]    One particularly promising programmable conductor material is a chalcogenide glass formed as an alloy system including Ge:Se:Ag for example, a Ge x :Se (1−x)  composition which is doped with silver. A memory element comprised of a chalcogenide glass has a natural stable high resistive state but can be programmed to a low resistance state by passing a current pulse from a voltage of suitable polarity through the cell. A chalcogenide memory element is simply written over by the appropriate current pulse and voltage polarity (reverse of that which writes the cell to a low resistance state) to reprogram it, and thus does not need to be erased. Moreover, a memory element of chalcogenide material is nearly nonvolatile, in that it need only be rarely refreshed in order to retain its programmed low resistance state. Such memory cells, unlike DRAM cells, can be accessed without requiring a refresh.  
           [0008]    Since there is a considerable body of known and proven circuitry for reading, writing and refreshing DRAM memory cells, it would be desirable to use the same or similar circuitry with programmable conductor memory elements. However, while conventional read sense amplifier circuitry, associated with DRAM cells, are capable of use in accessing and sensing programmable element random access memory (PCRAM) cells, the natural refresh operation associated with these sense amplifiers is not required for a programmable conductor memory element. Indeed, frequent rewriting of PCRAM memory elements to the same state is not desirable because it can cause the memory element to wear out faster. Accordingly, there is a need and desire for a circuit and method for writing PCRAM cells without causing premature deterioration.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides an improved method for reading a programmable conductor memory element which reduces premature deterioration due to repeated refresh operations. This is accomplished by first performing a read operation on a memory element and then writing the memory element to the opposite or complement logical state from the state which was read and then writing the memory element back to the original logical state. Hence, if following a high resistance state read operation the memory element is to be written to a high resistance state, then the memory element is first written to the opposite state (i.e., low resistance state) and then written back to the original state (i.e., high resistance state). Alternatively, if following a low resistance state read operation the PCRAM cells are to be written to a low resistance state, then the cells are first written to the opposite state (i.e., high resistance state) and then written back to the original state (i.e., low resistance state).  
           [0010]    In an alternative embodiment, after a read operation the memory element may simply be re-written to a state complementary to the read state and logic circuitry associated with a memory device containing the memory element keeps track during a read operation if the read data should output as read or after being inverted. For example, if a high resistance state represents a “1” data value and a low resistance state represents a “0” data value, and if a memory element is read as a “1” that memory element will be written to a “0” state following the read operation. During a subsequent read of the same memory element, as a “0” the logic circuitry will invert the logic state and output it correctly as a “1.” After the subsequent read, the memory element will then be written to a “1” state and the logic circuit will note that no logical state inversion is required for the next read of the memory element. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments of the invention given below with reference to the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 is a schematic diagram of a conventional DRAM cell;  
         [0013]    [0013]FIG. 2 is a schematic diagram of a PCRAM cell;  
         [0014]    [0014]FIG. 3 is a schematic diagram a PCRAM array;  
         [0015]    [0015]FIGS. 4A and 4B are timing diagrams illustrating the voltages on the word and bit lines when a PCRAM cell is read in high resistance and low resistance states, respectively;  
         [0016]    [0016]FIG. 5 is a schematic diagram of a sense amplifier used in the invention;  
         [0017]    [0017]FIG. 6 depicts a flowchart describing an operational flow, in accordance with an exemplary embodiment of the invention;  
         [0018]    [0018]FIG. 7 depicts a voltage arrangement across the PCRAM memory cell of FIG. 2;  
         [0019]    [0019]FIG. 8 is a schematic diagram of a portion of the FIG. 3 circuit with added refresh circuitry; and  
         [0020]    [0020]FIG. 9 is a block diagram of a processor based system including a PCRAM memory device in accordance with the principles of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring to the drawings, where like reference numerals designate like elements, FIG. 2 illustrates a PCRAM cell  400  and in FIG. 3 a memory device  500  comprised of a plurality of PCRAM cells  400   a - 400   h.  As illustrated in FIG. 2, a PCRAM cell  400  comprises an access transistor  401 , a programmable conductor memory element  402 , and a common cell plate  403  for a plurality of cells. The access transistor  401  has its gate coupled to a word line  405  and one terminal coupled to a bit line  406 . A small portion of an array of such cells is shown in FIG. 3 as including bit lines  406   a,    406   a ′,  406   b,    406   b ′, and word lines  405   a,    405   b,    405   c,  and  405   d.  As shown in FIG. 3, the bit lines  406   a,    406   a ′, and  406   b,    406   b ′ are coupled to respective pre-charge circuits  501   a,    501   b,  which can switchably supply a pre-charge potential to the bit lines  406   a,    406   a ′,  406   b,    406   b ′. The other terminal of each access transistor  401  is coupled to one end of an associated programmable conductor memory element  402 , while the other end of the associated programmable conductor memory element  402  is coupled to the cell plate  403 . The cell plate  403  may span and be coupled to several other PCRAM cells. The cell plate  403  is also coupled to a potential source. In the exemplary embodiment the potential source is at (Vdd/2).  
         [0022]    The access transistor  401 , as well as the other access transistors, are depicted as N-type CMOS transistors, however, it should be understood that P-type CMOS transistors may be used as long as the corresponding polarities of the other components and voltages are modified accordingly. The programmable conductor memory element  402  is preferably made of chalcogenide glass having a Ge x :Se: (1−x)  composition (e.g., x=0.2 to 0.3) which is doped with silver, however, it should be understood that any other bi-stable programmable conductor material known to those with ordinary skill in the art may also be used. In the exemplary embodiment of a Ge x :Se: (1−x)  glass where x=0.2 to 0.3, the programmable conductor memory element  402  stores a binary “0” state when it has a resistance of approximately 10 K ohm, and a binary “1” when it has a resistance greater than 10 M ohm. The programmable conductor memory element is normally at rest in a high resistive state but can be programmed to a low resistance state, e.g., binary “0” state, by applying a voltage greater than or equal to approximately +0.25 volt across the memory element. A memory element programmed to a low resistance state can be programmed to a high resistance value, e.g., a binary “1” state, by applying a voltage greater than or equal to approximately −0.25 volt across the memory element. The programmable conductor can be nondestructively read by applying a reading voltage having a magnitude of less than 0.25 volt across the memory element. In the exemplary embodiment, the reading voltage of approximately 0.2 volt can be used. However, it should be readily apparent that other alternate voltages may be selected as required for programmable conductor memory elements formed of the indicated or other material compositions.  
         [0023]    [0023]FIG. 3 illustrates a memory device  500  comprising a pair of memory arrays  550   a,    550   b.  Each memory array  550   a,    550   b  includes a plurality of memory cells  400   a - 400   d,    400   e - 400   h  arranged such that the memory cells  400  along any given bit line  406   a,    406   a ′,  406   b,    406   b ′ do not share a common word line  405   a - 405   d.  Conversely, the memory cells  400  along any word line  405   a - 405   d  do not share a common bit line  406   a,    406   a ′,  406   b,    406   b ′. A selected word line is switchably coupled to an associated word line driver  512   a - 512   d  via a transistor  510   a - 510   d  in accordance with the output of a row decoder. A bit line  406   a,    406   a ′,  406   b,    406   b ′ is selected for use in accordance with the output of a column decoder. Each memory array  550   a,    550   b  has its own set of bit lines. For example, memory array  550   a  includes bit lines  406   a,    406   b,  while memory array  550   b  includes bit lines  406   a ′,  406   b ′. The bit lines from each adjacent pair of memory arrays  550   a,    550   b  are coupled to a common sense amplifier  300   a,    300   b.  For example, bit lines  406   a,    406   a ′ are coupled to sense amplifier  300   a,  while bit lines  406   b,    406   b ′ are coupled to sense amplifier  300   b.  For simplicity, FIG. 5 illustrates a memory device having only two arrays  550   a,    550   b,  and eight cells  400   a - 400   h.  However, it should be understood that the illustrated memory device would typically have significantly more cells and arrays.  
         [0024]    The memory device  500  also includes a plurality of pre-charge circuits  501   a - 501   b.  One pre-charge circuit (e.g.,  501   a ) is shown as being provided for each pair of bit lines coupled to a sense amplifier (e.g.,  406   a,    406   a ′), however, other pre-charge arrangements are also possible. Each pre-charge circuit (e.g.,  501   a ) includes two transistors (e.g.,  502   a,    502   b ). One terminal of each transistor is coupled to a potential source Vdd. In the exemplary embodiment, the potential source Vdd is 2.5 volts. Another terminal of each transistor (e.g.,  502   a,    502   b ) is coupled to its corresponding bit line (e.g.,  406   a,    406   a ′, respectively). The gate of the each transistor (e.g.,  502   a,    502   b ) is coupled to a pre-charge control signal. As illustrated, the transistors (e.g.,  502   a,    502   b ) are P-MOS type transistor. Thus, when the pre-charge signal is low, the transistors (e.g.,  502   a,    502   b ) conduct, thereby pre-charging the bit lines (e.g.,  406   a,    406   a ′). When the pre-charge signal is high, the transistors (e.g.,  502   a,    502   b ) are switched off Due to capacitance inherent in the bit lines (e.g.,  406   a,    406   a ′), the bit lines will hold the pre-charge voltage level of 2.5 volts for a period of time.  
         [0025]    Reading a PCRAM cell, for example, cell  400   a,  in the PCRAM device  500  comprises the operations of accessing a memory element and sensing/refreshing the memory element.  
         [0026]    During a read operation a small potential difference is created between the bit lines (e.g.,  406   a,    406   a ′) coupled to the same sense amplifier (e.g.,  300   a ) for a memory cell e.g., a  400   a,  being read. One of the bit lines, e.g.  406   a  maintains an applied pre-charge voltage as a reference for the sense amplifier  300   a,  while the other bit line e.g.  406   a ′ starts with a voltage slightly higher than the pre-charge voltage due to parasitic capacitance between that bit line and an associated row line of the cell  400  being read, e.g.  400   a.  During a read operation for e.g. cell  400   a,  the voltage on bit line  406   a  is discharged through memory element e.g.  402   a.  This small potential difference between the reference voltage and the discharging voltage on bit line  406   a  can be sensed by sense amplifier  300   a  to determine the resistance and logical state of memory element  400   a.    
         [0027]    Now also referring to FIG. 6, the read operation begins with the pre-charging of the bit lines  406   a,    406   a ′,  406   b,    406   b ′ of the memory device  500  via pre-charge circuits  501   a - 501   b  (step S 1 ). The bit lines may be pre-charged by temporarily bringing the pre-charge signal low, causing transistors  502   a - 502   d  to conduct the pre-charge voltage (Vdd) to the bit lines  406   a,    406   a ′,  406   b,    406   b ′. Once the pre-charge signal returns to a high state, the transistors  502   a - 502   d  stop conducting, but the bit lines  406   a,    406   a ′,  406   b,    406   b ′ will remain at the pre-charge potential for a predetermined period due to the capacitance inherent in the bit lines. The precharge period is illustrated as the time prior to T1 in FIGS. 4A, 4B.  
         [0028]    In the exemplary embodiment, a selected pair of bit lines e.g.  406   a,    406   a ′, are pre-charged to 2.5 volts and the cell plate  403   a,    403   b  is tied to 1.25 volts (Vdd/2). However, the pre-charge voltage in bit line  406   a  is slightly higher as noted, e.g. to 2.6 volts when row line  405   a  is activated. When the row line is activated at time T1 (FIGS. 4A, 4B), e.g. row line  405   a,  there is a voltage drop across the access transistor, e.g.  401   a,  which causes a voltage of approximately 0.2 volts to appear across the memory element  402   a.  The potential difference between the bit line  406   a  and the cell plate  403   a  will cause the bit line to discharge to the cell plate through the conducting access transistor  401  and the programmable conductor memory element  402   a.  The discharge rate is dependent upon the resistive state of the programmable conductor memory element  402 . That is, a low resistive state will cause the voltage on selected bit line  406   a  to discharge faster than a high resistive state. As the bit line discharges, its voltage will fall from the original voltage of approximately 2.6 volts toward the cell plate voltage.  
         [0029]    In the memory device  500 , the word lines  405   a - 405   d  are normally at ground potential. Thus the access transistors  401   a - 401   d  are normally switched off. Referring now to FIGS. 4A and 4B, at time T1, the word line  405   a  associated with a cell  400   a  to be read is activated by bringing its potential from ground to a predetermined level (step S 2  in FIG. 6). The predetermined level is designed to create a reading voltage at the programmable element  402   a,  which as previously explained, must have a magnitude less than the magnitude of a writing voltage. In the exemplary embodiment, the word line  405   a  is brought to approximately 2.25 volts. Since the voltage drop across the transistor  401   a  is approximately 1.15 volts, the potential at the interface between the transistor  401   a  and the programmable element  402   a  is 1.45 volt. This results in a reading voltage of 0.2 volt across the programmable memory element  402   a  since the voltage at the interface between the programmable element  402   a  and the cell plate  403   a  is maintained at 1.25 volt.  
         [0030]    As noted, due to the inherent parasitic capacitance between the word line  401   a  and its associated bit lines  406   a  the potential in the associated bit line  406   a  increase as the word line  401   a  is activated. In the exemplary embodiment, the potential in bit line  406   a  increases by 0.1 volt to 2.6 volt. It should be noted that the word lines  405   c,    405   d  coupled to complementary bit lines  406   a ′,  406   b ′ remain at ground potential. Thus, bit lines  406   a ′,  406   b ′ remain at the pre-charge potential, which is 2.5 volt in the exemplary embodiment.  
         [0031]    The increased potential of bit line  406   a  is used in combination with the two bi-stable resistive states of the programmable element  402   a  to cause one of the bit lines (e.g.,  406   a ) coupled to a sense amplifier (e.g.,  300   a ) to have either a greater or lesser voltage than the other bit line (e.g.,  406   a ′) coupled to the same sense amplifier  300   a.  The memory is designed and operated so that if the programmable element  402   a  has a high resistive state, bit line  406   a  discharges more slowly, thereby causing it to maintain its relatively higher potential. However, if the programmable element  402   a  has a low resistive state, bit line  406   a  discharges at a faster rate, so that bit line  406  transitions to a lower potential state than bit line  406   a ′. These two effects can be seen by comparing FIG. 4A (illustrating the effects of a programmable element at a high resistive state) and FIG. 4B (illustrating the effects of a programmable element at a low resistive state.)  
         [0032]    [0032]FIG. 5 is detailed illustration of a sense amplifier  300 , which comprises a N-sense amp  310 N and a P-sense amp portion  310 P. The N-sense amp  310 N and the P-sense amp  310 P include nodes NLAT* and ACT, respectively. These nodes are coupled to controllable potential sources (not illustrated) which supply control signals which respectively turn on the N-sense amp  310 N and the P-sense amp  310 P. In an initial state, the transistors,  301 - 304  of the N- and P-sense amps  310 N,  310 P are switched off and no enabling signals are supplied to the NLAT* and ACT modes. The sense operation of sense amplifier  300  is a two phased operation in which the N-sense amp  310 N is triggered before the P-sense amp  310 P.  
         [0033]    The N-sense amp  310 N is triggered by bringing the potential at node NLAT* towards ground potential. As the potential difference between node NLAT* and the bit lines  106 A and  106   a ′ approach the threshold potential of NMOS transistors  301 ,  302 , the transistor with the gate coupled to the higher voltage bit line begins to conduct. This causes the lower voltage bit line to discharge towards the voltage of the NLAT* node. Thus, when node NIAT* reaches ground potential, the lower voltage bit line will also reach ground potential. The other NMOS transistor never conducts since its gate is coupled to the low voltage digit line being discharged towards ground.  
         [0034]    The P-sense amp  310 P is triggered (after the N-sense amp  310 N has been triggered) by bringing the potential at node ACT from ground towards Vdd. As the potential of the lower voltage bit line approaches ground (caused by the earlier triggering of the N-sense amp  310 N), the PMOS transistor with its gate coupled to the lower potential bit line will begin to conduct. This causes the initially higher potential bit line to be charged to a potential Vdd. After both the N- and P-sense amps  310 N,  310 P have been triggered, the higher voltage bit line has its potential elevated to Vdd while the lower potential bit line has it potential reduced to ground. Thus, the process of triggering both sense amps  310 N,  310 P amplifies the potential difference created by the access operation to a level suitable for use in digital circuits. In particular, the bit line  106   a  associated with the memory cell  400   a  being read is driven to ground if the memory cell  400   a  stored a charge corresponding to a binary 0, or to Vdd if the memory cell  400   a  stored a charge corresponding to a binary 1, thereby permitting a compactor (or differential amplifier)  350   a  coupled to bit lines  106   a,    106   a ′ to output a binary 0 or 1 consistent with the data stored in the cell  400   a  on signal line  351 .  
         [0035]    Returning to FIGS. 4A and 4B, at time period T2, the N-sense amplifier  310 N is activated (start of step S 3 ). As previously noted, activating the N-sense amplifier causes the bit line (one of  406   a  and  406   a ′) having the lower potential to be pulled with the NIAT signal toward ground. In the exemplary embodiment, T2 is approximately 30 nanosecond after the world line activation at T1. However, it should be noted that the timing of T2 may be varied without departing from spirit or scope of the invention.  
         [0036]    At time period T3, the P-sense amplifier  310 P is activated. As previously noted, activating the P-sense amplifier causes the bit line (one of  406   a  and  406   a ′) having the higher potential to be pulled towards Vdd. In the exemplary embodiment, T3 is approximately 35 nanosecond after T1 (end of step S 3 ). However, it should be noted that the timing of T4 may also be varied without departing from spirit or scope of the invention.  
         [0037]    As shown in both FIGS. 4A and 4B at time T4 after both the N- and P-sense amplifies have been fired, one of the bit lines  406   a,    406   a ′ is at ground and the other is at Vdd, depending on the resistance of cell  400   a.  Since one bit line coupled to sense amplifier  300   a  is now at ground potential while the other bit line is now at Vdd potential, a comparator (or differential amplifier)  350  can be used to output a value corresponding to the contents of the cell  400   a  on signal line  351   a.    
         [0038]    Next, FIG. 7 shows a voltage chart describing a re-write/refresh operation for a read memory cell  400   a  in accordance with an exemplary embodiment of the invention. In this exemplary process flow, the following parameters of the programmable conductor memory cells are presumed: i) that the voltage across an element  402  required to write from a high resistance state to a low resistance state is 0.25V; and ii) that the voltage across elements  402  required to write from a low resistance state to a high resistance state is −0.25V. It should be readily apparent that alternative voltages may be used depending on the material composition, size and construction of the programmable conductor memory element  402 .  
         [0039]    Referring back to FIG. 6 after the read operation occurs, the write process begins at step S 4  by first writing to the opposite data that was sensed at step S 3 . Hence, if memory cell  400   a  is to be written back to a “1,” state (from step S 3 ) then the cell is first written to the opposite or complement data (i.e., “0” state), as shown in step S 4 , and then written back to the correct read state (i.e., “1”), as shown in step S 5 . Alternatively, if memory cell  400   a  is to be written back to a “0” state, then the cell is first written to the opposite data state (i.e., “1”) and then written back to the correct data state (i.e., “0”).  
         [0040]    As shown in FIG. 7, and assuming Vdd=2.5 volts and Vdd/2=1.25 volts, to write a programmable memory element  402   a  to a low resistance state requires a voltage of greater than or equal to +0.25 across the memory element  402   a.  Thus, if the bit line voltage V1 is set to Vdd and the access transistor  401   a  is turned on to 2.5 volts, and if there is approximately a V2=1 volt drop or less across the transistor, the voltage V3 drop across the cell is +0.25 or higher which is sufficient to program it to a low resistance state.  
         [0041]    If memory element  402   a  is to be written to a high resistance state, then the voltage across the memory element  402   a  must be less than −0.25 volts. To obtain this the bit line voltage V1 can be set to ground and the word line is activated. If the voltage drop V2 across the transistor is again 1 volt or less, then the voltage drop across the memory element V2=−0.25, or less which is sufficient to program a memory element to a high resistance state.  
         [0042]    Thus, by controlling the voltage on the bit line and word line following the memory element read operation, a read memory element can be set to a particular logic state. In the invention, once a memory element  402   a  is read, it is refreshed by first writing the logic state of the memory element to its complementary state and then writing it back again to its original logic state. Thus, if a read memory element  402   a  holds a high resistance state e.g. logic “1,” it is refreshed by first writing it to a low resistance state, e.g. logic “0.” And then writing it again to a high resistance state e.g. logic “1.” 
         [0043]    [0043]FIG. 8 illustrates one exemplary refresh circuit which can be used to refresh a read memory element. FIG. 8 illustrates a portion of the FIG. 3 memory structure including the memory cell  400   a  including access transistor  401   a  and memory element  402   a.  Also shown is a bit line charge circuit  911  which is used following a read operation to set bit line  406   a  to voltage of either Vdd or ground.  
         [0044]    [0044]FIG. 8 also illustrates a refresh signal generating circuit  921  for generating the refresh signal applied to control the bit line charge circuit  911 . The refresh signal generating circuit  921  includes an inverter  903  which receives an output signal from bit line  406   a  and a delay circuit  905 .  
         [0045]    The operation of the refresh circuit illustrated in FIG. 8 will now be explained with reference to the timing diagram shown in FIGS. 4A and 4B. The timing diagram of FIG. 4A shows the bit line  406   a  as having a value of Vdd after a read operation at time T4. Because the voltage on bit line  406   a  is Vdd and the word line  405   a  has been turned to 2.5V, the high resistance state of memory element  402   a  is read, and the Vdd on the bit line will cause the memory element to see +0.25 volts across it which will automatically program it to a low resistance state before time T4 when a read operation is completed. In order to reprogram the memory element back to a high resistance state, the bit line  406   a  is brought to ground by an inversion of the Vdd value in inverter  903  and application of the inverted bit line  406   a  voltage as an input to bit line charge circuit  911  and drop the bit line voltage to ground at time T5. Then, if word line  405   a  is then enabled, as shown at time T6 in FIG. 4A, there will be a negative 0.25 volt across the memory element  402   a  which is sufficient to reprogram it back to a high resistance state. Although FIG. 4A shows word line  405   a  disabled at time T4 and re-enabled at time T6, it is also possible to keep word line  405   a  enabled through the time period T4 to T7. Likewise, as shown in FIG. 4B if the memory element  402   a  originally held a low resistance value, prior to time T4, the ground voltage on the bit line  406   a  produced by the sense amplifier  300   a  will produce a negative 0.25 volts across the memory element  402   a  automatically programming it to a high resistance state. The ground voltage on the bit line  406   a  is converted to a Vdd voltage by inverter  903  and the delayed inverted signal is applied to bit line charge circuit  911  to apply a voltage of Vdd to bit line  406   a  at T5. Then word line  405   a  is enabled at T6 to program the memory element  402   a  back to a low resistance state. Again, word line  405   a  in FIG. 4B can also remain enabled during the period T4 to T7 if desired.  
         [0046]    An enable transistor  917  controlled by an applied enable signal can be used to enable or disable operation of the bit line charge circuit  911  by a refresh operation.  
         [0047]    The invention may also be used to only write a complementary resistance value a back to a memory element after a read operation. This can be implemented by eliminating the bit line charge circuit  911  and refresh signal generating circuit  921  in FIG. 8 and only performing the first write operation illustrated in FIGS. 4A, 4B, that is, the refresh operation up to time T4. In this embodiment, each time a read operation occurs, the memory element is refreshed to its complementary logic state. With this embodiment a flip flop  951  or other logic element receiving a delayed output from the sense amplifier  300   a,  is used to control an output data inverter  953  to enable/disable the inverter to ensure a proper logic state is output from a memory element  402   a.  For example, if the original logic value stored in memory element  402   a  is a “1,” when this value is read out the first time the memory element  402   a  will be refreshed with a logic “0” value. The “1” data output from sense amplifier will be sensed, after a delay by delay element  953 , and flip flop  951  will be toggled from a “0” to a “1” value. The next time memory element  402   a  is read, the stored logic value of “0” needs to be output as a “1” and flip flop  951  which now holds a “1” value enables inverter  953  so that the read “0” value from memory element  402  is correctly output as a logic “1” value. The output of the sense amplifier is again delayed and used to toggle flip flop  951  back to a “0” state which will not enable inverter  953  on a subsequent read operation. Since the “0” read by sense amplifier  300   a  will be rewritten in memory element  402   a  as a “1,” the next time the memory element  402   a  is read the stored “1” value will be read out and not inverted. This process repeats for each read of memory element  402   a.    
         [0048]    [0048]FIG. 9 is a block diagram of a processor based system  800 , such as a computer system, containing a PCRAM semiconductor memory  802  as described in connection with the other figures. The memory  802  may be constituted as one or more memory chips or memory integrated circuits mounted on a memory module, for example, a plug-in memory module such as a SIMM, DIMM, or other plug-in memory module. The processor based system  800  includes a processor  801 , a memory  802 , a mass storage  803 , and an I/O device  804 , each coupled to a bus  805 . While a single processor  801  is illustrated, it should be understood that processor  801  could be any type of processor and may include multiple processor and/or processors and co-processors. Memory  802  is illustrated in FIG. 9 as having a plurality of PCRAM devise  500 . However, memory  802  may only include a single PCRAM device  500 , or a larger plurality of PCRAM devices  500  than illustrated, and/or may include additional forms of memories, such as non-volatile memory or cache memories. While one mass storage  803  device is illustrated, the processor based system  800  may include a plurality of mass storage devices, possibly of varying types such as, but not limited to, floppy disks, CDROMs, CD-R, CD-RW, DVD, hard disks, and disk arrays. I/O device  804  may likewise comprise a plurality of I/O devices of varying tepes, including, but not limited to keyboard, mouse, graphic cards, monitors, and network interfaces. Bus  805 , while illustrated as a single bus may comprise a plurality of buses and/or bridges, which may be coupled to each other or bridged by other components. Some of the devices  801 - 804  may be coupled to only a single bus  805 , others may be coupled to a plurality of buses  805 .  
         [0049]    While the invention has been described in detail in connection with the exemplary embodiment, it should be understood that the invention is not limited to the above disclosed embodiment. Rather, the invention can be modified to incorporate any number of variations, alternations, substitutions, or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, many different types of equivalent circuits can be used to supply the appropriate read and write voltages to the memory cells, e.g.  400   a.  Accordingly, the invention is not limited by the foregoing description or drawings of specific exemplary embodiments, but is only limited by the scope of the appended claims.