Abstract:
The present invention provides a ferroelectric semiconductor memory device in which the potential of data read out from a normal cell is compared with the reference level of a reference cell so as to determine whether the readout data is the “H” data or the “L” data, wherein since the reference cell is in the relaxed state when reading out data from the normal cell for the first time, the reference cell is reset before reading out data from the normal cell. Then, data is read out from the normal cell, and then the reference cell is reset. In second and subsequent data read operations of reading out data from a normal cell of another address, the reference cell is in the reset state, whereby the reference level is the same between the first data read operation and the second or subsequent data read operation. Thus, the reference level is always kept at a predetermined constant level when data is read out from normal cells.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2005-160234 filed in Japan on May 31, 2005, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a ferroelectric semiconductor memory device and, more particularly, to a technique for generating the reference level.  
         [0003]     In recent years, as the process rules becomes finer and the capacity increases, there is a shift in the type of memory cells employed in ferroelectric semiconductor memory devices, i.e., from those of a 2-transistor 2-ferroelectric capacitor type to those of a 1-transistor 1-ferroelectric capacitor type with which it is possible to realize a smaller memory size. The transistor 1-ferroelectric capacitor type requires a reference cell, in addition to a normal memory cell (hereinafter referred to as a “normal cell”), and there is increasing importance in the technique for generating the reference level in order to realize a high reliability. One conventional technique for generating the reference level in a ferroelectric semiconductor memory device is disclosed in Japanese Laid-Open Patent Publication No. 2004-55007.  
         [0004]     The conventional ferroelectric semiconductor memory device disclosed in this publication will now be described with reference to the drawings.  
         [0005]      FIGS. 3A  to  3 D show a trace on a hysteresis loop when reading out data from normal cells and reference cells in the reset state and in the relaxed state, and the relationship between the “H” level, the “L” level and the reference level in the reset state and in the relaxed state.  FIG. 8  shows a configuration of a memory array of a conventional ferroelectric semiconductor memory device.  FIG. 9  is a timing diagram showing an operation of the conventional ferroelectric semiconductor memory device.  FIGS. 10A and 10B  each schematically show a physical arrangement of normal cells and reference cells in the conventional ferroelectric semiconductor memory device, and an operation thereof.  
         [0006]     The conventional ferroelectric semiconductor memory device will now be described with reference to  FIGS. 3A  to  3 D,  8 ,  10 A and  10 B.  
         [0007]      FIG. 3A  shows a state where the “H” data and the “L” data are read out from a memory cell and a reference cell in the reset state,  FIG. 3B  shows a state where the “H” data and the “L” data are read out from a memory cell and a reference cell in the relaxed state,  FIG. 3C  shows the relationship between the “H” level, the “L” level and the reference level when the reference level is generated by reading out data from a reference cell in the reset state, and  FIG. 3D  shows that when the reference level is generated by reading out data from a reference cell in the relaxed state.  
         [0008]     In  FIG. 8 , BP denotes a bit line precharge signal, SAE denotes a sense amplifier enable signal, WL 1  to WLn denote first to n-th word lines, CP 1  to CPn denote first to n-th cell plate lines, RWL 1  and RWL 2  denote first and second reference word lines, RCP 1  and RCP 2  denote first and second reference cell plate lines, REQ 1  and REQ 2  denote first and second reference equalize signals, RDIN denotes “H” data reset data, XRDIN denotes “L” data reset data, RST denotes a reference reset signal, BL 1  to BL 8   m  (where m is an integer) denote first to 8m-th bit lines,  11  denotes a cell plate driver circuit,  12  denotes reference cell control circuit,  13  denotes a sense amplifier and bit line precharge control circuit,  14  denotes a sense amplifier and bit line precharge circuit,  15  denotes a peripheral circuit,  16  denotes a row decoder circuit,  17  denotes a an 8-bit normal cell array,  18  denotes reference cells for 8 bit lines, T 1  to T 7  denote first to seventh MOS transistors, and C 1  to C 4  denote first to fourth ferroelectric capacitors.  
         [0009]     The gate of the first MOS transistor T 1  is connected to the first word line WL 1 , the drain thereof is connected to the first bit line BL 1 , the source thereof is connected to the first electrode of the first ferroelectric capacitor C 1 , the second electrode of the first ferroelectric capacitor C 1  is connected to the first cell plate line CP 1 , the gate of the second MOS transistor T 2  is connected to the first word line WL 1 , the drain thereof is connected to the fourth bit line BL 4 , the source thereof is connected to the first electrode of the second ferroelectric capacitor C 2 , and the second electrode of the second ferroelectric capacitor C 2  is connected to the first cell plate line CP 1 . Moreover, the gate of the fifth MOS transistor T 5  is connected to the first reference equalize signal REQ 1 , the drain thereof is connected to the second bit line BL 2 , the source thereof is connected to the third bit line BL 3 , the gate of the sixth MOS transistor T 6  is connected to the reference reset signal RST, the drain thereof is connected to the first electrode of the third ferroelectric capacitor C 3 , the source thereof is connected to the “L” data reset data XRDIN, the second electrode of the third ferroelectric capacitor C 3  is connected to the first reference cell plate line RCP 1 , the gate of the seventh MOS transistor T 7  is connected to the reference reset signal RST, the drain thereof is connected to the first electrode of the fourth ferroelectric capacitor C 4 , the source thereof is connected to the “H” data reset data RDIN, and the second electrode of the fourth ferroelectric capacitor C 4  is connected to the first reference cell plate line RCP 1 .  
         [0010]     In  FIG. 9 , BP denotes the bit line precharge signal, WL 1  denotes the first word line, CP 1  denotes the first cell plate line, REQ 1  denotes the first reference equalize signal, RWL 1  denotes the first reference word line, RCP 1  denotes the first reference cell plate line, SAE denotes the sense amplifier enable signal, and BL 1  to BL 4  denote the first to fourth bit lines.  
         [0011]      FIGS. 10A and 10B  each schematically show a physical arrangement of normal cells and reference cells, where each unit of normal or reference cells that is accessed with one address is encircled with a border line.  FIG. 10A  shows a state where the normal cells and the reference cells are all in the relaxed state (each solid circle represents a cell in the relaxed state).  FIG. 10B  shows a state of normal cells and reference cells after accessing normal cells of a particular address (hatched portion), where each solid circle represents a cell in the relaxed state and each open circle represents an accessed cell in the reset state.  
         [0012]     A case will be described below where the conventional ferroelectric semiconductor memory device is formed by (8×n×m) normal cells and (8×2×m) reference cells, and where the “H” data is stored in the first ferroelectric capacitor C 1  and the fourth ferroelectric capacitor C 4 , the “L” data is stored in the second ferroelectric capacitor C 2  and the third ferroelectric capacitor C 3 , and the normal cells and the reference cells are in the reset state. As shown in  FIG. 3A , in the reset state, the “H” data is at point A and the “L” data is at point E.  
         [0013]     The conventional ferroelectric semiconductor memory device first brings the bit line precharge signal BP to “L” at time t 01  in  FIG. 9 , thereby bringing all of the first to 8m-th bit lines BL 1  to BL 8   m  to the floating state. Then, the conventional device brings the first reference equalize signal REQ 1  to the “H” level at time t 02  in  FIG. 9 , the first word line WL 1  and the first reference word line RWL 1  to the “H” level at time t 03 , the first cell plate line CP 1  and the first reference cell plate line RCP 1  to the “H” level at time t 04 , thereby reading out the “H” data from the first ferroelectric capacitor C 1  and the fourth ferroelectric capacitor C 4  in  FIG. 8  and the “L” data from the second ferroelectric capacitor C 2  and the third ferroelectric capacitor C 3 . Then, the “H” data transitions from point A in  FIG. 3A  to point B, and the “L” data transitions from point E to point D, thereby reading out the “H” data to the first bit line BL 1 , the “L” data to the fourth bit line BL 4  and the reference level to the second bit line BL 2  and the third bit line BL 3 . The gradient of the line between point M and point B and that of the line between point N and point D are equal to the bit line capacitance. The conventional device employs a scheme for generating the reference level, in which the device reads out data from two reference cells (i.e., the ferroelectric capacitors C 3  and C 4 ) while equalizing the reference cells by the fifth MOS transistor T 5  of  FIG. 8 . Because the equalization is done in a portion where the ferroelectric capacitance of the “H” data (the tangent Csh 1  at point B in  FIG. 3A ) and the ferroelectric capacitance of the “L” data (the tangent Csl 1  at point D in  FIG. 3A ) are different from each other (Csh 1 &gt;Csl 1 ), it is necessary, for setting the reference level in the middle between the “H” level and the “L” level as shown in  FIG. 3D , that the number x of reference cells storing the “L” data is larger than the number y of reference cells storing the “H” data.  
         [0014]     Then, the conventional device brings the first cell plate line CP 1  and the first reference cell plate line RCP 1  to “L” at time t 05  in  FIG. 9 , the first reference word line RWL 1  to “L” at time t 06 , and the first reference equalize signal REQ 1  to “L” at time t 07 , and amplifies the readout data at time t 09  by a sense amplifier (not shown).  
         [0015]     At time t 10 , as the first cell plate line CP 1  and the first reference cell plate line RCP 1  are brought to “H”, the normal cells and the reference cells are overwritten (reset) with the “L” data. At time t 11 , as the first reference cell plate line RCP 1  is brought to “L”, the reference cells are overwritten with the “H” data. At time t 12 , as the first cell plate line CP 1  is brought to “L”, the normal cells are overwritten (reset) with the “H” data. Finally, the device brings the bit line precharge signal BP to “H” and the sense amplifier enable signal SAE to “L” at time t 14 , and the first word line WL 1  to “L” at time t 16 , thus completing the operation.  
         [0016]     After data have been written to and stored in the normal cells and the reference cells and the cells have transitioned to the relaxed state, the conventional ferroelectric semiconductor memory device operates as follows.  
         [0017]     The “H” data of the normal cells and the reference cells is at point P in  FIG. 3B , and the “L” data is at point Q in  FIG. 3B . When data is read out from the normal cells and the reference cells, the “H” data transitions from point P to point G and the “L” data transitions from point Q to point J, thereby reading out the charge in accordance with the ferroelectric capacitances at the tangent Csh 2  at point G and the tangent Csl 2  at point J, and the reference level is generated by equalizing all of the first to 8m-th bit lines BL 1  to BL 8   m  of  FIG. 8 . In the relaxed state, the tangent Csh 2  at point G&lt;the tangent Csh 1  at point B, the tangent Csl 2  at point J&gt;the tangent Csl 1  at point D, and the number x of reference cells storing the “L” data is larger than the number y of reference cells storing the “H” data, whereby the reference level is shifted to the “H” data side from the level in the middle between the “H” level and the “L” level in the relaxed state (indicated by a dotted line), as shown in  FIG. 3D . If the number y of the “H” data and the number x of the “L” data are equal to each other, the reference level will be in the middle between the “H” level and the “L” level in the relaxed state (indicated by the dotted line).  
         [0018]     However, the conventional ferroelectric semiconductor memory device has a problem in that the reference level in the first data read operation of reading out data from normal cells is different from that in the second and subsequent data read operations. This will now be discussed in detail.  
         [0019]     Referring to  FIG. 9 , after data have been written to and stored in the normal cells and the reference cells and the cells have transitioned to the relaxed state, the conventional ferroelectric semiconductor memory device operates as follows. When reading out data from an address, the data read operation is performed under the relationship between the “H” level, the “L” level and the relaxed reference level of the normal cells in the relaxed state as shown in  FIG. 3D .  
         [0020]     However, after the data read operation, the normal cells and the reference cells from which data have been read out both return to the reset state. Therefore, when reading out data from normal cells of the next address, the “H” level and the “L” level of the normal cells from which the data is read out will be as shown in  FIG. 3D , whereas the reference level will be as shown in  FIG. 3C , thus failing to achieve the reference level of the positional relationship as shown in  FIG. 3D , and the reference level will be set substantially in the middle between the “H” level and the “L” level in the relaxed state. As a result, the relationship between the “H” level, the “L” level and the reference level of normal cells from which the first data read operation is performed as shown in  FIG. 3D  is different from that of normal cells from which the second and subsequent data read operations are performed.  
       SUMMARY OF THE INVENTION  
       [0021]     It is therefore an object of the present invention to always keep the reference level at the same level in the first data read operation and in the second and subsequent data read operations.  
         [0022]     In order to achieve this object, the present invention sets the reference level always based on reference cells in the reset state, in view of the fact that reference cells in the first data read operation are in the relaxed state and reference cells in the second and subsequent data read operations are in the reset state.  
         [0023]     Specifically, a ferroelectric semiconductor memory device of the present invention includes: a large number of normal cells formed by ferroelectric memory elements; a reference cell; a control circuit for reading out a reference level of the reference cell when reading out data of one of the large number of normal cells; and a sense amplifier for amplifying a potential difference between a potential of the data read out from the normal cell and the reference level of the reference cell, wherein the control circuit sets the reference level to a predetermined potential, the predetermined potential being between a potential read out from the reference cell storing a high-potential data and a potential read out from the reference cell storing a low-potential data when a difference between the potential of the high-potential data and the potential of the low-potential data is at maximum based on conditions of the reference cell, and the predetermined potential being greater than or equal to a sensitivity of the sense amplifier.  
         [0024]     In one embodiment of the present invention, a plurality of reference cells are provided; and the control circuit generates the reference level by equalizing two or more of the plurality of reference cells.  
         [0025]     In one embodiment of the present invention, the control circuit varies the reference level by varying a ratio between the number of reference cells storing high-potential data and the number of reference cells storing low-potential data.  
         [0026]     In one embodiment of the present invention, the ratio between the number of reference cells storing the high-potential data and the number of reference cells storing the low-potential data is stored in a non-volatile memory or a latch circuit, other than a ferroelectric memory element, or set by using physical or electrical fuses.  
         [0027]     In one embodiment of the present invention, the control circuit resets all of the reference cells before accessing one or more of the large number of normal cells.  
         [0028]     In one embodiment of the present invention, for the operation of resetting all of the reference cells before accessing the normal cells, the control circuit sets a reset time to be shorter than a data write time for the normal cells.  
         [0029]     In one embodiment of the present invention, the control circuit does not overwrite data to the reference cell after accessing the normal cells.  
         [0030]     In one embodiment of the present invention, the control circuit overwrites data to the reference cell after accessing the normal cells.  
         [0031]     In one embodiment of the present invention, the reference cell is formed by a paraelectric capacitor.  
         [0032]     Thus, with the ferroelectric semiconductor memory device of the present invention, in the first normal cell data read operation and in the second and subsequent normal cell data read operations, the reference level of the reference cell is at the same level, e.g., the reference level of a normal cell (ferroelectric element) in the reset state. Therefore, in the first data read operation and in the second and subsequent data read operations, the readout data is determined to be either the H data or the L data always with respect to the same reference level.  
         [0033]     Particularly, in the present invention, although the voltage application period for which the voltage is applied to a ferroelectric capacitor needs to be set while taking the retention into account for data write operations of writing data to normal cells, the voltage application period for a reference cell can be set to be the minimum voltage application period with which the ferroelectric capacitor of the reference cell can be reset because the reference cell is reset before accessing the normal cells. Thus, the voltage application period can be set to be shorter than that for the ferroelectric capacitors of the normal cells, whereby even in a case where a single reference cell is provided for a plurality of normal cells, the total stress application time for the reference cell can be set to a similar level to that for the normal cells. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]      FIG. 1  shows a configuration of a memory array provided in a ferroelectric semiconductor memory device according to a first embodiment and a second embodiment of the present invention.  
         [0035]      FIG. 2  is a timing diagram showing an operation according to the first embodiment of the present invention.  
         [0036]      FIG. 3A  shows a trace on a hysteresis loop when reading out data from normal cells and reference cells in the reset state, and  FIG. 3B  shows that when reading out data from normal cells and reference cells in the relaxed state.  FIG. 3C  shows the relationship between the “H” level, the “L” level and the reference level when reading out data from normal cells and reference cells in the reset state, and  FIG. 3D  shows that when reading out data from normal cells and reference cells in the relaxed state.  
         [0037]      FIG. 4  is a timing diagram showing an operation according to the second embodiment of the present invention.  
         [0038]      FIG. 5  shows a trace on a hysteresis loop when resetting normal cells and reference cells in the relaxed state according to the first and second embodiments of the present invention.  
         [0039]      FIG. 6  shows a trace on a hysteresis loop when resetting reference cells according to the first and second embodiments of the present invention.  
         [0040]      FIGS. 7A and 7B  each schematically show a physical arrangement of, and an operation of, normal cells and reference cells according to the first and second embodiments of the present invention, wherein  FIG. 7A  shows a case where the normal cells and the reference cells are all in the relaxed state, and  FIG. 7B  shows a case where normal cells are in the relaxed state and the reference cells are in the reset state.  
         [0041]      FIG. 8  shows a configuration of a memory array of a conventional ferroelectric semiconductor memory device.  
         [0042]      FIG. 9  is a timing diagram showing an operation of the conventional ferroelectric semiconductor memory device.  
         [0043]      FIGS. 10A and 10B  each schematically show a physical arrangement of, and an operation of, normal cells and reference cells of the conventional ferroelectric semiconductor memory device, wherein  FIG. 10A  shows a case where the normal cells and the reference cells are all in the relaxed state, and  FIG. 10B  shows a case where accessed normal cells and all the reference cells are in the reset state and the rest of the normal cells are in the relaxed state. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0044]     Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.  
       First Embodiment  
       [0045]     A ferroelectric semiconductor memory device according to a first embodiment of the present invention will now be described.  
         [0046]      FIG. 1  shows a configuration of a memory array according to the first embodiment of the present invention.  FIG. 2  is a timing diagram showing an operation according to the first embodiment of the present invention.  FIGS. 3A  to  3 D show a trace on a hysteresis loop when reading out data from normal cells (ferroelectric memory elements) and reference cells in the reset state and in the relaxed state, and the relationship between the “H” level, the “L” level and the reference level in the reset state and in the relaxed state.  FIG. 5  shows a trace on a hysteresis loop when resetting normal cells and reference cells in the relaxed state according to the first and second embodiments of the present invention.  FIG. 6  shows a trace on a hysteresis loop when resetting reference cells according to the present embodiment.  FIGS. 7A and 7B  each schematically show a physical arrangement of, and an operation of, normal cells and reference cells according to the present embodiment.  
         [0047]     First, the ferroelectric semiconductor memory device of the present embodiment will be described with reference to FIGS.  1  to  3 D and  5  to  7 B.  
         [0048]     In  FIG. 1 , BP denotes a bit line precharge signal, SAE denotes a sense amplifier enable signal, WL 1  to WLn denote first to n-th word lines, CP 1  to CPn denote first to n-th cell plate lines, RWL 1  and RWL 2  denote first and second reference word lines, RCP 1  and RCP 2  denote first and second reference cell plate lines, REQ 1  and REQ 2  denote first and second reference equalize signals, RDIN denotes “H” data reset data, XRDIN denotes “L” data reset data, RST denotes a reference reset signal, BL 1  to BL 8   m  (where m is an integer) denote first to 8m-th bit lines,  11  denotes a cell plate driver circuit,  12  denotes reference cell control circuit,  13  denotes a sense amplifier and bit line precharge control circuit,  14  denotes a sense amplifier and bit line precharge circuit,  15  denotes a peripheral circuit,  16  denotes a row decoder circuit,  17  denotes a an 8-bit normal cell array, and  18  denotes reference cells for 8 bit lines. The reference cell control circuit  12  and the row decoder circuit  16  together form a control circuit for reading out the reference level of a large number of reference cells  17 .  
         [0049]     Moreover, T 1  to T 7  denote first to seventh MOS transistors, and C 1  to C 4  denote first to fourth ferroelectric capacitors, wherein the gate of the first MOS transistor T 1  is connected to the first word line WL 1 , the drain thereof is connected to the first bit line BL 1 , the source thereof is connected to the first electrode of the first ferroelectric capacitor C 1 , the second electrode of the first ferroelectric capacitor C 1  is connected to the first cell plate line CP 1 , the gate of the second MOS transistor T 2  is connected to the first word line WL 1 , the drain thereof is connected to the fourth bit line BL 4 , the source thereof is connected to the first electrode of the second ferroelectric capacitor C 2 , and the second electrode of the second ferroelectric capacitor C 2  is connected to the first cell plate line CP 1 .  
         [0050]     Moreover, the gate of the fifth MOS transistor T 5  is connected to the first reference equalize signal REQ 1 , the drain thereof is connected to the second bit line BL 2 , the source thereof is connected to the third bit line BL 3 , the gate of the sixth MOS transistor T 6  is connected to the reference reset signal RST, the drain thereof is connected to the first electrode of the third ferroelectric capacitor C 3 , the source thereof is connected to “L” data reset data, the second electrode of the third ferroelectric capacitor C 3  is connected to the first reference cell plate line RCP 1 , the gate of the seventh MOS transistor T 7  is connected to the reference reset signal RST, the drain thereof is connected to the first electrode of the fourth ferroelectric capacitor C 4 , the source thereof is connected to “H” data reset data, and the second electrode of the fourth ferroelectric capacitor C 4  is connected to the first reference cell plate line RCP 1 . Moreover, the gate of the eighth MOS transistor T 8  is connected to the first reference equalize signal REQ 1 , the drain thereof is connected to the source of the fifth MOS transistor T 5  (i.e., the third bit line BL 3 ), and the source thereof is connected to the drain of the other fifth MOS transistor T 5  (i.e., the sixth bit line BL 6 ). Similarly, along the line of the second reference equalize signal REQ 2 , there are provided the ninth MOS transistor T 9 , which is similar to the fifth MOS transistor T 5 , and the tenth MOS transistor T 10 , which is similar to the eighth MOS transistor T 8 .  
         [0051]     Moreover, in  FIG. 2 , BP denotes a bit line precharge signal, SAE denotes a sense amplifier enable signal, WL 1  denotes a first word line, CP 1  denotes a first cell plate line, RWL 1  denotes a first reference word line, RCP 1  denotes a first reference cell plate line, REQ 1  and REQ 2  denote first and second reference equalize signals, RDIN denotes “H” data reset data, XRDIN denotes “L” data reset data, RST denotes a reference reset signal, and BL 1  to BL 4  denote first to fourth bit lines.  
         [0052]      FIG. 3A  shows a trace on a hysteresis loop when reading out the “H” data (high-potential data) and the “L” data (low-potential data) from normal cells and reference cells in the reset state, and  FIG. 3B  shows a trace on a hysteresis loop when reading out the “H” data and the “L” data from normal cells and reference cells in the relaxed state. In these figures, the horizontal axis represents the voltage, and the vertical axis represents the amount of polarized charge.  FIG. 3C  shows the relationship between the “H” level, the “L” level and the reference level when reading out the “H” data and the “L” data from normal cells and reference cells when they are in the reset state, and  FIG. 3D  shows the relationship between the “H” level, the “L” level and the reference level when reading out the “H” data and the “L” data from normal cells and reference cells when they are in the relaxed state. In these figures, the horizontal axis represents the time and the vertical axis represents the voltage.  
         [0053]     Where the ferroelectric semiconductor memory device of the present embodiment is formed by (8×n×m) normal cells and (8×2×m) reference cells, the ferroelectric semiconductor memory device operates as follows, with the “H” data stored in the first ferroelectric capacitor C 1  and the fourth ferroelectric capacitor C 4  and the “L” data stored in the second ferroelectric capacitor C 2  and the third ferroelectric capacitor C 3 , after data have been written to and stored in the normal cells and the reference cells and the cells have transitioned to the relaxed state. The “H” data, which is at point A when in the reset state, is at point P and the “L” data, which is at point E when in the reset state, is at point Q, as shown in  FIG. 3B , and normal cells and reference cells are all in the relaxed state as shown in  FIG. 7A .  
         [0054]     The ferroelectric semiconductor memory device of the present embodiment first brings the bit line precharge signal BP to “L” at time t 01  in  FIG. 2 , thereby bringing all of the first to 8m-th bit lines BL 1  to BL 8   m  of  FIG. 1  to the floating state. Then, the device brings the “H” data reset data RDIN to “H” at time t 02  in  FIG. 1 , the reference reset signal RST to “H” at time t 03 , and the first reference cell plate line RCP 1  to “H” at time t 04 , thereby resetting the “L” data of the reference cells. The device brings the first reference cell plate line RCP 1  to “L” at time t 05 , thereby resetting the “H” data of the reference cells. Thus, the “H” data at point P transitions to point A via point F, and the “L” data is reset to point E via point J and point C as shown in  FIG. 5 , whereby the normal cells are in the relaxed state (represented by solid circles) and the reference cells are in the reset state (represented by open circles) as shown in  FIG. 7B .  
         [0055]     Then, when accessing the normal cells in the hatched portion of  FIG. 7B , the device brings the first reference equalize signal REQ 1  to the “H” level at time t 08 , the first word line WL 1  and the first reference word line RWL 1  to the “H” level at time t 09 , and the first cell plate line CP 1  and the first reference cell plate line RCP 1  to the “H” level at time t 10 , thereby reading out the “H” data from the first ferroelectric capacitor C 1  and the fourth ferroelectric capacitor C 4  of  FIG. 1  and the “L” data from the second ferroelectric capacitor C 2  and the third ferroelectric capacitor C 3 . Then, the “H” data transitions from point P in  FIG. 3B  to point G, and the “L” data transitions from point Q to point J, thereby reading out the “H” data to the first bit line BL 1 , the “L” data to the fourth bit line BL 4 , and the reference level to the second bit line BL 2  and the third bit line BL 3 . The gradient of the line between point R and point G and that of the line between point S and point J are equal to the bit line capacitance.  
         [0056]     The device employs a scheme for generating the reference level, in which the device reads out data from four reference cells (the ferroelectric capacitors C 3  and C 4 ) while equalizing the reference cells by the fifth and eighth MOS transistors T 5  and T 8  in one reference cell  18  whose internal configuration is shown in  FIG. 1 , and outputs one reference level commonly to four bit lines BL 2 , BL 3 , BL 6  and BL 7 . Thus, for a total of m reference cells  18 , 4m reference cells (the ferroelectric capacitors C 3  and C 4 ) are equalized, and one reference level is output commonly to 4m bit lines.  
         [0057]     Because the equalization is done in a portion where the ferroelectric capacitance of the “H” data (the tangent Csh 2  at point G in  FIG. 3B ) and the ferroelectric capacitance of the “L” data (the tangent Csl 2  at point J in  FIG. 3B ) are different from each other (Csh 2 &gt;Csl 2 ), and because the ferroelectric capacitance of the “H” data of the reference cells is smaller than that in the reset state (the tangent Csh 1  at point B in  FIG. 3A ) (Csh 2 &lt;Csh 1 ), and the ferroelectric capacitance of the “L” data of the reference cells is larger than that in the reset state (the tangent Csl 1  at point D in  FIG. 3A ) (Csl 2 &gt;Csl 1 ), it is necessary, for setting the reference level in the middle between the “H” level and the “L” level as shown in  FIG. 3D  (indicated by the dotted line), that the number x of reference cells storing the “L” data is larger than that when the reference cells are in the reset state. The number y of reference cells storing the “H” data and the number x of reference cells storing the “L” data are set to optimal numbers such that there is obtained a predetermined potential, wherein the predetermined potential is between the “H” data and the “L” data when the potential difference between the “H” data and the “L” data is at the worst (largest) level, and is greater than or equal to the sensitivity of the sense amplifier.  
         [0058]     Then, the device brings the first cell plate line CP 1  and the first reference cell plate line RCP 1  to “L” at time t 12  in  FIG. 2 , the first reference word line RWL 1  to “L” at time t 13 , and the first reference equalize signal REQ 1  to “L” at time t 14 , and amplifies the readout data at time t 16  by a sense amplifier (not shown). At time t 17 , as the first cell plate line CP 1  is brought to “H”, the normal cells are overwritten (reset) with the “L” data. At time t 19 , as the first cell plate line CP 1  is brought to “L”, the normal cells are overwritten with the “H” data.  
         [0059]     Finally, the device brings the sense amplifier enable signal SAE to “L” and the bit line precharge signal BP to “H” at time t 21 , and the first word line WL 1  to “L” at time t 23 , thus completing the operation.  
         [0060]     In the ferroelectric semiconductor memory device of the present embodiment, when data is written to a normal cell, the voltage application period for which the voltage is applied to the ferroelectric capacitor should be set while taking the retention into account. However, for a reference cell, which is reset before accessing a normal cell, the voltage application period can be set to any period as long as the ferroelectric capacitor of the reference cell can be reset. Thus, the period can be set to be shorter than the voltage application period for which the voltage is applied to the ferroelectric capacitor of a normal cell. This will be discussed in detail below with reference to the timing diagram of  FIG. 2  and the hysteresis loop of  FIG. 6 .  
         [0061]     When a normal cell storing the “H” data is overwritten with the “L” data while taking the retention into account, the state transitions from point A of  FIG. 6  to point E via point C in the period t 17 -t 19  of  FIG. 2 . When a normal cell storing the “L” data is overwritten with the “H” data, the state transitions from point E of  FIG. 6  to point A via point F in the period t 19 -t 21  of  FIG. 2 .  
         [0062]     On the other hand, the state of a reference cell when it is reset at time t 04  in  FIG. 2  depends on the data read out from a normal cell in the previous data read cycle. Where the data read out from the normal cell is the “H” data, the voltage applied to the ferroelectric capacitor of the reference cell becomes 0 V after the reference cell is amplified to the “L” data, whereby the cell is at point E of  FIG. 6 . Where the data read out from the normal cell is the “L” data, the voltage applied to the ferroelectric capacitor of the reference cell becomes 0 V after the reference cell is amplified to the “H” data, whereby the cell is at point A of  FIG. 6 . The reset time for a reference cell takes the maximum value when the reference cell to be reset to the “H” data is being at “L” and when the reference cell to be reset to the “L” data is being at “H”. In that state, if the overwriting time for a reference cell is made shorter than that for a normal cell as in the period t 04 -t 05  in  FIG. 2 , the state transitions from point A of  FIG. 6  to point E′ via point C′ when resetting from “H” to “L”, and the state transitions from point E of  FIG. 6  to point A′ via point F′ when resetting from “L” to “H”. A data read operation from a reference cell is performed, starting from point A′ and point E′, and the ferroelectric capacitance value of the reference cell at the time of equalization is the tangent Csh 3 ′ at point B′ and the tangent Csl 3 ′ at point D′ of  FIG. 6 . As compared with the tangent Csh 3  at point B and the tangent Csl 3  at point D in a case where the reset time for a reference cell is equal to that for a normal cell, the tangent Csh 3 ′ and the tangent Csl 3 ′ are such that Csh 3 ′&lt;Csh 3  and Csl 3 ′&gt;Csl 3 . Since the capacitance value is larger for the “L” data, it is necessary that the number x of reference cells storing the “L” data is larger than that in a case where the reset time is equal to that for a normal cell.  
         [0063]     As described above, with the ferroelectric semiconductor memory device of the present embodiment, when generating a reference level by equalizing a plurality of reference cells, data can be read out from all the normal cells in the relaxed state by using the same reference level, which is always reset, and the voltage application period for which the voltage is applied to the ferroelectric capacitor of a reference cell can be made shorter than that for a normal cell by about two orders of magnitude, whereby the stress on the ferroelectric capacitor of a reference cell can be set to a similar level to that on the ferroelectric capacitor of a normal cell.  
       Second Embodiment  
       [0064]     A ferroelectric semiconductor memory device according to a second embodiment of the present invention will now be described with reference to the drawings.  
         [0065]      FIG. 4  is a timing diagram showing an operation according to the second embodiment of the present invention, and  FIG. 6  shows a trace on a hysteresis loop when resetting reference cells according to the present embodiment.  
         [0066]     The ferroelectric semiconductor memory device of the present embodiment will be described with reference to  FIGS. 4 and 6 . The present embodiment differs from the first embodiment in that the first reference cell plate line RCP 1  is at “H” in the period t 17 -t 18  in  FIG. 4 . The operation until time t 14  in  FIG. 4  is similar to that of the first embodiment, and will not be further described below.  
         [0067]     Part of the operation of the present embodiment that differs from the first embodiment will now be described below. The device brings the “H” data reset data RDIN to “H” at time t 15  in  FIG. 4 , the reference reset signal RST to “H” at time t 16 , and the first reference cell plate line RCP 1  to “H” at time t 17 , thus resetting the “H” data of reference cells. Then, the device brings the first reference cell plate line RCP 1  to “L” at time t 18 , thus resetting the “L” data. Then, the device brings the “H” data reset data RDIN to “L” at time t 19  and the reference reset signal RST to “L” at time t 20 , thus completing the reset operation for the reference cells.  
         [0068]     At this time, since the reset time for a reference cell is shorter than that for a normal cell, as in the first embodiment, the “H” data of a reference cell is at point A′ of  FIG. 6  and the “L” data is at point E′ of  FIG. 6  when the reset operation is completed. In the reset operation for reference cells in the period of t 02 -t 07  of  FIG. 4  in the next data read cycle, which is performed starting from the above state, the “H” data being at point A′ of  FIG. 6  transitions to point A via point F, and the “L” data being at point E′ transitions to point E via point C.  
         [0069]     As described above, with the ferroelectric semiconductor memory device of the present embodiment, the “H” data and the “L” data of the reference cells after being reset are at point A and point E of  FIG. 6 , respectively, as are the data of the normal cells, whereby the reference level can be more easily controlled to be in the middle between the “H” data and the “L” data of the normal cells than when the reset state of a reference cell is different from that of a normal cell as in the first embodiment. The number y of reference cells storing the “H” data and the number x of reference cells storing the “L” data are set to optimal numbers under the condition where the potential difference between the “H” data and the “L” data is at the worst level.  
         [0070]     In the ferroelectric semiconductor memory devices of the first and second embodiments, the number y of reference cells storing the “H” data and the number x of reference cells storing the “L” data such that the reference level takes an optimal value when the readout potential difference between the “H” data and the “L” data is at the worst level are stored in some of the normal cells  17  of  FIG. 1 , and the optimal value can be changed based on the distribution of the normal cells and the reference cells.  
         [0071]     Alternatively, the numbers y and x of reference cells for optimizing the reference level may be stored in a non-volatile memory or a latch circuit, other than a ferroelectric element, or may be set by using physical or electrical fuses. Then, the data reliability can be improved as compared with the case where these numbers y and x are stored in ferroelectric capacitors of normal cells.  
         [0072]     It is understood that the present invention is applicable to cases where the reference level is generated by using paraelectric capacitors instead of using ferroelectric capacitors, and similar effects to those described above can be obtained also in such cases.