Abstract:
A data storage device such as a DRAM memory having a plurality of data storage cells  10  is disclosed. Each data storage cell  10  has a physical parameter which varies with time and represents one of two binary logic states. A selection circuit  16 , writing circuits  18  and a refreshing circuit  22  apply input signals to the data storage cells to reverse the variation of the physical parameter with time of at least those cells representing one of the binary logic states by causing a different variation in the physical parameter of cells in one of said states than in the other.

Description:
This application is a divisional application of application Ser. No. 11/048,387, filed Feb. 1, 2005 (now U.S. Pat. No. 7,170,807, which is a divisional application of application Ser. No. 10/487,162, filed Feb. 17, 2004 (now U.S. Pat. No. 6,982,918), which is the National Stage of International Application No. PCT/EP03/02747, filed Mar. 17, 2003, which claims priority to European Patent Application Ser. No. 02077116, filed May 29, 2002, and European Patent Application Ser. No. 02405314, filed Apr. 18, 2002. 
    
    
     TECHNICAL FIELD OF THE INVENTION  
     The present invention relates to a data storage device, and relates particularly, but not exclusively, to a semiconductor memory device. The invention also relates to a method of refreshing a data storage device. 
     BACKGROUND OF THE INVENTION 
     DRAM (Dynamic Random Access Memory) devices are known in which an array of charge storage cells is provided, each storage cell consisting of a single transistor and a single capacitor. As is well known, each storage cell stores a single binary data bit according to whether the associated capacitor is charged (data state “1”) or discharged (data state “0”). It is also well known that the charge stored in the charged capacitors decays with time, and that it is therefore necessary to rewrite the data to the charged storage cells by periodically recharging the capacitors. A conventional DRAM arrangement of this type is shown in  FIG. 1 . The DRAM device shown in  FIG. 1  is provided with m columns and n rows. A data storage cell  10  consisting of a single transistor and a single capacitor is located at each intersection of a row and a column. 
     For each data storage cell, the source of the associated transistor is connected to one terminal of a capacitor, the other terminal of which is connected to a ground terminal or a given reference voltage (not shown), the gates of the transistors of each row are connected together by a respective conductive track  12 , and the drains of the transistors of each column are connected together by a respective conductive track  14 . Each of the conductive tracks  12  is connected to a selection circuit  16  for sequentially scanning the conductive tracks  12  of the memory device, and the conductive tracks  14  are each connected to respective writing circuits  18   i  and reading circuits  20   i , where i varies from 1 to m. 
     In order to refresh the charge states of the data storage cells  10  to counteract the effect of the charge stored in each capacitor decaying with time, the selection circuit  16  scans lines  1  to n by sequentially applying a signal to each conductive track  12  to successively switch on the transistors of all of the data storage cells  10  connected to the conductive track  12  being addressed. This in turn enables the reading circuits  20   i  to determine the charge state of the associated capacitor by determining the current flowing through each transistor. In response to the determination of the charge state of each capacitor determined by the associated reading circuit  20   i , the associated writing circuit  18   i  causes the capacitor to be recharged or not, depending on its previous charge state. 
     Prior art DRAM devices of the type shown in  FIG. 1  suffer from the drawback that modern memory devices have capacities typically of the order of 1 Gb, such devices typically comprising 1048576 lines. The refreshing process typically requires 10 to 50 ns for each line, as a result of which the refreshing process for a 1 Gb device typically requires 10 to 50 ms. Since the refreshing process must typically be carried out about 10 times per second, the time necessary for the refreshing process is of the same order of magnitude as that remaining for the data reading and writing operations. This results in the time required for refreshing limiting the capacity of the memory devices and requiring that special steps be taken to reduce leakage currents. 
     Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a data storage device comprising: 
     a plurality of data storage cells, each said cell having a physical parameter in use which varies with time and has one of two data representing states, each said data representing state representing a respective binary logic state; and 
     refreshing means for applying input signals to each said data storage cell to at least partially reverse variation of said physical parameter with time of at least those data storage cells in a predetermined one of said states, wherein said input signals cause a different variation in said physical parameter in cells in one of said data representing states than the variation caused in cells in the other of said data representing states. 
     Data refreshing means are provided to at least partially reverse the variation of said physical parameter with time by means of input signals causing a different variation in said physical parameter in cells in one of said data representing states than the variation caused in cells in the other of said data representing states. This provides the advantage that each data storage cell can be re-written without the need to read the state of each cell in order to enable the re-write operation to proceed. In addition, the write signal can be input to all data storage cells simultaneously, thus significantly increasing the speed of the refreshing operation compared with prior art devices. 
     In a preferred embodiment, said at least partial reversal of said physical parameter occurs to a greater extent for said cells in said predetermined one of said data representing states than in the other of said states, and said input signals are applied sufficiently frequently in use that said states remain distinguishable from each other. 
     The device may further comprise writing means for applying input signals to each said data storage cell to adjust said physical parameter of said cell to select the binary logic state represented by each said data storage cell. 
     The device may further comprise reading means for determining the data representing state of each said data storage cell. 
     Said input signals may at least partially reverse variation of said physical parameter for cells in each of said data representing states. 
     The data storage device may be a semiconductor device. 
     In a preferred embodiment, at least some of said data storage cells each comprise a respective field effect transistor having a first threshold voltage when in said first state and a second threshold voltage when in said second state. 
     In a preferred embodiment, at least one said field effect transistor comprises a respective source, a respective drain, a respective body arranged between the corresponding said source and said drain and adapted to retain an electrical charge generated in said body representing one or the other of two binary data states, and at least one respective gate adjacent the corresponding said body, wherein said refreshing means is adapted to apply voltage signals between at least one said gate and said drain of each said cell lying between said first and second threshold voltages. 
     Said refreshing means may be adapted to also apply signals to at least partially reverse the variation of said physical parameter in the other of said data representing states. 
     Said refreshing means is preferably adapted to apply signals partially reversing the variation of said charge in the other of said data representing states by means of recombination of charge carriers with charge carriers of opposite polarity. 
     The device may be a memory device. 
     According to another aspect of the present invention, there is provided a method of refreshing data in a data storage device comprising a plurality of data storage cells, each said cell having a physical parameter in use which varies with time and has one of two data representing states, each said data representing state representing a respective binary logic state, the method comprising applying input signals to each said data storage cell to at least partially reverse variation of said physical parameter with time of at least those data storage cells in a predetermined one of said states, wherein said input signals cause a different variation in said physical parameter in cells in one of said data representing states than the variation caused in cells in the other of said data representing states. 
     Said input signals may cause said at least partial reversal of said physical parameter occurs to a greater extent for said cells in said predetermined one of said data representing states than in the other of said states. 
     The method may further comprise the step of applying signals to at least partially reverse the variation of said physical parameter in the other of said data representing states. 
     The step of applying signals to at least partially reverse the variation of said physical parameter in the other of said data representing states preferably causes recombination of charge carriers with charge carriers of opposite polarity. 
     Said input signals may at least partially reverse variation of said physical parameter for cells in each of said data representing states. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of a prior art DRAM memory device; 
         FIG. 2  is a schematic representation, corresponding to  FIG. 1 , of a DRAM device embodying the present invention; 
         FIG. 3  shows a pulse pattern to be applied to the gates and drains of the transistors of the device of  FIG. 2  during a refresh operation; and 
         FIG. 4  shows the variation of charge with time in the body of each transistor of the device of  FIG. 2  in each of the two charge states. 
         FIG. 5  is a representation of a prior art memory cell. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 2 , in which parts common to the device of  FIG. 1  are denoted by like reference numerals, a DRAM device has an array of m columns and n lines of memory cells  10 , each of which comprises an NMOS SOI (silicon on insulator) field effect transistor, as described in greater detail in European patent application number 01810587.4 (which corresponds, at least in part, to U.S. Pat. No. 6,969,662). As described in greater detail in that document, it is possible to generate a charge in the body of such a transistor by the application of predetermined voltage pulses between the gate and drain and between the source and drain of the transistor. (See, for example.  FIG. 5 ). In particular, a positive charge is generated and stored in the body of an NMOS transistor by means of the application of a voltage between the source and drain of the transistor when in its conductive state, the voltage difference between the source and drain generating electron-hole pairs by impact ionisation. The electrons are then removed to the source or drain, and the holes are stored in the body of the transistor. The positive charge can be removed by forward biasing the body-drain junction. 
     In the DRAM device of  FIG. 2 , the source of each transistor is connected to a ground terminal, the gates of the transistors of each line are connected to a conductive track  12 , and the drains of the transistors of each column are connected to a conductive track  14 . As in the device of  FIG. 1 , the conductive tracks  12  are all connected to a selection circuit  16 , and a writing circuit  18   i  and reading circuit  20   i  is connected to each conducting track  14 . A refreshing circuit  22  is also provided, the refreshing circuit  22  being connected to each of the conductive tracks  12  via the selection circuit, and to each of the conductive tracks  14  via the corresponding writing circuits  18   i.    
     European patent application no. 01810587.4 describes how data can be written to the individual memory cells  10  of the DRAM device of  FIG. 2 , and how the charge state, representing the binary data state, of each memory cell  10  can be determined. The “zero” and “one” binary states of each stored bit are represented by the presence or absence of a net electric charge of a predetermined polarity in the body of the transistor. In order to refresh the data written to the memory cells  10 , the refreshing circuit  22  causes the selection circuit  16  and writing circuit  18   i  to apply pulse I 1  to all of the lines, and pulse I 2 , as shown in  FIG. 3 , to all of the columns. 
     Pulse I 1  beginning at time t 1  and ending at time t 2  is applied to the gates of all of the transistors for a duration of some nanoseconds, and comprises a pulse of +0.6V applied to the gates, and +1.2V applied to the drains of the transistors. Pulse I 2 , which begins at time t 3  and ends at time t 4 , also having a duration of some nanoseconds, consists of the application of a pulse of −2V to the gates alone. The times t 2  and t 3  may be coincident. 
     Referring now to  FIG. 4 , which shows in detail how the charge C stored in the transistor body of each memory cell  10  varies with time for each charge state (representing binary “0” and binary “1” states respectively), the line between the two curves indicates charge level C n  towards which both charge levels tend if no refreshing operation is carried out. It will be appreciated that the two charge levels chosen to represent the respective data states must be sufficiently far apart as to remain distinguishable from each other throughout the range of allowed variation of charge of each level. 
     The cells  10  initially have a net positive charge represented by holes stored in the body of the transistor (representing state “1”) or a much lower negative or substantially zero charge (representing state “0”), i.e. the two charged states being higher than or lower than charge level C n  respectively. Without a refresh operation, the difference between these two charge levels decays generally logarithmically with time. 
     As disclosed in more detail in earlier European patent application no. 01810587.4, the electrical properties of the SOI MOSFET transistors of each data storage cell  10  vary according to the amount of charge stored in the body of the respective transistor. In particular, the threshold voltage of transistors in the higher charge state is lower than that of the transistors in the lower charge state. 
     When pulse I 1  is applied at time t 1 , the voltage applied to the gates is arranged to be between the respective threshold voltages of the transistors in the higher and lower charge states. As a result, the transistor in the higher charge state is switched to its conductive state, i.e. a conductive channel between the source and drain forms in the body of the transistor in the vicinity of the gate, and a current can flow in the channel between the source and the drain at the interface of the body and the insulating layer adjacent the gate. This current flowing in the channel creates electron and hole pairs in the vicinity of the drain by impact ionisation, the holes being stored in the body, while the electrons are removed by the drain. In this way, the positive charge stored in the body of the transistors in the higher charge state increases by an amount Δ1, while the charge of the transistors in the lower charge state increases by a much smaller amount Δ2, since no conductive channel is formed in the transistors of lower charge state. Δ1 is generally 2 to 3 orders of magnitude greater than Δ2. It is found that electrons are trapped in structural defects in the body at its interface with the insulating film between the body and the gate generally to the same extent, regardless of whether the transistors are in the higher or lower charge state. 
     At time t 3 , which is shown in  FIG. 4  for the sake of clarity as being coincident with time t 2 , the second pulse I 2  begins and causes the charge in the transistors of both charge states to be reduced. The pulse I 2  consists of a voltage of −2V applied to the gates, which attracts holes stored in the body of each transistor to the corresponding interface of the body and dielectric film, with the result that the holes recombine with electrons trapped in structural defects at the interface, reducing the net positive charge stored in the body. As can be seen from  FIG. 4 , in this way, the charge in the higher state is reduced back to its initial level, and the charge in the lower state is restored to its previous level. The charge in the transistor in the upper state is reduced by Δ4, and is reduced by Δ3 in the transistor in the lower state, Δ4 and Δ3 being approximately equal to each other. 
     It can therefore be seen that because pulse I 1  has a much more significant effect on transistors in the higher state than in the lower state, this pulse can be applied to all of the memory cells  10  simultaneously, with the effect that the “1” states are refreshed, without the transistors in the “0” state being converted to the “1” state and without the necessity of reading the charge state of each memory cell  10 . This permits a refresh process to be made by whole memory blocks allowing, for example in the case of a 1 Gigabit memory, a refresh process approximately 1000 times faster than in the prior art. It is also possible to use transistors having technical characteristics less difficult to achieve than in the prior art, in particular, transistors having a lower charge retention time, for which the cost is consequently lower. 
     It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. In particular, the order of application of pulses I 1  and I 2  can be reversed, and the above process described with reference to NMOS transistors can also be applied to PMOS transistors, the polarity of the voltages applied to the gates in that case being reversed. Also, JFET type transistors can be used as well as MOSFET type transistors. Furthermore, as well as being applicable to DRAM memory type devices, it will be appreciated by persons skilled in the art that the refreshing process can be applied to other types of data storage device, such as optical imaging devices and memory devices other than DRAM memories.