Abstract:
A first memory cell array includes a first bit line and a second bit line arranged to read data out of a memory cell containing a ferroelectric capacitor. A second memory cell array includes a third bit line and a fourth bit line arranged to read data out of a memory cell containing a ferroelectric capacitor. A sense amp circuit detects and amplifies a potential difference caused between any two of the first through fourth bit lines. A decoupling circuit selectively connects any two of the first through fourth bit lines to the sense amp circuit and decouples the remainder from the sense amp circuit. A bit-line potential control circuit is arranged between the decoupling circuit and the first and second memory cell arrays to fix the bit lines decoupled from the sense amp circuit by the decoupling circuit to a first potential.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from prior Japanese Patent Application No. 2007-272102, filed on Oct. 19, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor memory device, and more particularly to a ferroelectric memory comprising memory cells each containing a ferroelectric capacitor and a transistor. 
     2. Description of the Related Art 
     A ferroelectric memory (FeRAM) is such a semiconductor memory device that utilizes the hysteresis characteristic of a ferroelectric capacitor to store data nonvolatile in accordance with magnitudes of two different polarized states of the ferroelectric. 
     A memory cell in the conventional ferroelectric memory generally uses the architecture similar to that of a DRAM, which comprises a ferroelectric capacitor replaced with a paraelectric capacitor, and a selection transistor serially connected to the ferroelectric capacitor (see JP 2001-250376A, for example). Plural such memory cells are arranged in grid to configure a memory cell array. In data reading, a word line (selection line) associated with a memory cell targeted for read is activated by turning on a selection transistor to connect the memory cell to a bit line. 
     A cell transistor and a ferroelectric memory are connected in parallel to configure one memory cell and such memory cells are serially connected to configure a memory cell block in a ferroelectric memory of the so-called TC parallel unit serial connection type as known (see JP 2005-4811A, for example). In the ferroelectric memory of the TC parallel unit serial connection type, each memory cell block can be connected to a bit line when a block selection transistor having a gate connected to a block selection line (selection line) turns on. 
     In either the DRAM-analogous structure or the structure of the TC parallel unit serial connection type, the ferroelectric memory may adopt a 2-transistor/2-cell system (2T2C system) that uses two memory cells for reading complementary data, and a 1-transistor/1-cell system (1T1C system) that uses one memory cell for reading. 
     One having the DRAM-analogous structure is described by way of example. In the 2T2C system, the word line connected to a read-targeted memory cell and the word line associated with a complementary memory cell that holds complementary data are selected to connect the memory cell to a bit line and the complementary memory cell to a complementary bit line. 
     Thereafter, a plate voltage is applied to a plate line, and a voltage is applied across the ferroelectric capacitors contained in the read-targeted and complementary memory cells. The charge on the ferroelectric capacitor in the memory cell is read out to the bit line while the charge on the ferroelectric capacitor in the complementary memory cell is read out to the complementary bit line. The potentials on the paired bit lines are compared and amplified at a sense amp. 
     In the 1T1C system, on the other hand, the word line connected to a read-targeted cell is selected to connect the memory cell to a bit line. Thereafter, a plate voltage is applied to a plate line connected to the memory cell, and a voltage is applied across the ferroelectric capacitor contained in the memory cell. 
     The charge on the ferroelectric capacitor in the memory cell is read out to the bit line while a reference voltage is generated from a reference voltage generator and applied to a complementary bit line paired with the bit line. The potentials on the paired bit lines are compared and amplified at a sense amp. The TC parallel unit serial connection type is also similar to the above except that a block selection transistor is used to select a memory cell block and a word line to select a memory cell. 
     The 1T1C system and the 2T2C system have respective advantages and disadvantages. Therefore, it is preferable if one ferroelectric memory can be configured to execute the 1T1C system and the 2T2C system selectively. JP2005-4811A proposes such the ferroelectric memory. 
     The selective execution of the 1T1C system and the 2T2C system requires two selection transistors (or block selection transistors) configured independently drivable for selecting a bit line pair. An execution of the 1T1C system requires one of the complementary bit lines used in data reading and the other used as a shield line. 
     Therefore, selection of a pair of bit lines requires the preparation of a set of (two) word lines paired, thereby causing an increase in the area of the memory cell array and thus the chip area. 
     SUMMARY OF THE INVENTION 
     In an aspect the present invention provides a semiconductor memory device, comprising: a first memory cell array including a plurality of arrayed memory cells each containing a ferroelectric capacitor, a first bit line and a second bit line arranged to read data out of the memory cell, selection lines arranged to selectively connect the memory cell to the first bit line or the second bit line, and a plate line arranged to apply a drive voltage to the ferroelectric capacitor; a second memory cell array including a plurality of arrayed memory cells each containing a ferroelectric capacitor, a third bit line and a fourth bit line arranged to read data out of the memory cell, selection lines arranged to selectively connect the memory cell to the third bit line or the fourth bit line, and a plate line arranged to apply a drive voltage to the ferroelectric capacitor; a sense amp circuit operative to detect and amplify a potential difference caused between any two of the first through fourth bit lines; a decoupling circuit operative to selectively connect any two of the first through fourth bit lines to the sense amp circuit and decouple the remainder from the sense amp circuit; and a bit-line potential control circuit arranged between the decoupling circuit and the first and second memory cell arrays and operative to fix the voltage of the bit lines decoupled from the sense amp circuit by the decoupling circuit to a first potential, the bit lines being among the first through fourth bit lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows circuitry of a ferroelectric memory according to a first embodiment of the present invention. 
         FIG. 2  is a circuit diagram showing a configuration of a memory cell block MCBij, a decoupling circuit  11  and a bit-line potential control circuit  12 . 
         FIG. 3  is a circuit diagram showing a configuration example of a sense amp circuit S/Ai. 
         FIG. 4  illustrates operation in data reading by the 2T2C system in the ferroelectric memory of the first embodiment of the present invention. 
         FIG. 5  illustrates operation in data reading by the 2T2C system in the ferroelectric memory of the first embodiment of the present invention. 
         FIG. 6  illustrates operation in data reading by the 1T1C system in the ferroelectric memory of the first embodiment of the present invention. 
         FIG. 7  illustrates operation in data reading by the 1T1C system in the ferroelectric memory of the first embodiment of the present invention 
         FIG. 8  shows circuitry of a ferroelectric memory according to a second embodiment of the present invention of the present invention. 
         FIG. 9  shows circuitry of a ferroelectric memory according to a third embodiment of the present invention of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The embodiments of the invention will now be described with reference to the drawings. 
     First Embodiment 
       FIG. 1  shows circuitry of a ferroelectric memory of the TC parallel unit serial connection type according to a first embodiment of the present invention.  FIG. 2  shows a configuration of a memory cell block MCB of the TC parallel unit serial connection type contained in a memory cell array of  FIG. 1 . 
     The ferroelectric memory comprises two memory cell arrays MCA 1 , MCA 2  arranged laterally symmetrical as shown in  FIG. 1 . The memory cell array MCA includes an array of plural memory cell blocks MCB of the TC parallel unit serial connection type.  FIG. 1  typically shows only one vertical row of memory cell blocks MCB for simplicity though the array of memory cell blocks is not limited to this example but rather memory cell blocks can be arrayed over plural rows. 
     The memory cell array MCA also includes word lines WL for selecting among memory cells, plate lines PL, /PL, and bit lines BL for reading data out of memory cells. 
     Referring to  FIG. 2 , a configuration of the memory cell block MCB is described. All the memory cell blocks MCB have the similar configuration and accordingly a memory cell block MCB 01  connected to a bit line BL 01  is described herein by way of example. 
     The memory cell block MCB 01  comprises plural (8 in this example) serially connected memory cells each including a cell transistor or n-type MOS transistor Mi (i=0-7) and a ferroelectric capacitor Ci (i=0-7) connected in parallel. The transistor Mi has a gate connected to a word line WLi. 
     A ferroelectric capacitor C 7  located at one end of the memory cell block MCB 01  has a plate electrode, which is connected to the plate line PL 1  for applying a drive voltage. The other end of the memory cell block MCB 01  is connected via a block selection transistor S 1  to the bit line BL 01 . The block selection transistor S 1  is on/off controlled with a block selection signal BS 1 . 
     In the memory cell block MCB 01 , all word lines WLi (i=0-7) are made “H” at the time of standby so that no voltage is applied to every ferroelectric capacitor Ci. In contrast, only a word line associated with a read-targeted memory cell, for example, a word line WL 0  is made “L” for data read. In this case, a voltage is applied across the ferroelectric capacitor C 0  such that the cell data held in the ferroelectric capacitor C 0  causes a corresponding voltage on the bit line BL 01 , which enables data read. Other memory cell blocks MCB also have the similar configuration. 
     Referring again to  FIG. 1  for further description, there are provided plural sense amp circuits S/Ai (i=0, 1, . . . , n) between the memory cell arrays MCA 1  and MCA 2  as in the so-called shared sense amp format. The sense amp circuits S/Ai are connected to four bit lines BLij (j=1-4: first through fourth bit lines). 
     The bit lines BLi 1  and Bli 2  configure a bit line pair for execution of the 2T2C system. The bit lines BLi 3  and BLi 4  configure a bit line pair for execution of the 2T2C system. As described later, the bit lines BLi 1  and BLi 4  in combination or the bit lines BLi 2  and BLi 3  in combination configure a bit line pair for execution of the 1T1C system. 
     The bit line BLij can be connected to the memory cell block MCBij when the selection transistor S 1  is turned on. The sense amp circuit S/Ai may include a comparator and a reference potential generation circuit as described later. 
     In this embodiment, all memory cell blocks MCB present in one memory cell array MCA 1  (or  2 ) are configured selectable with a single block selection signal BS 1  (or  2 ). Therefore, the area of the memory cell array can be reduced. 
     In the case of one memory cell array MCA 1  provided with a single block selection signal (BS 1  or BS 2 ), in order to make not only the 2T2C system but also the 1T1C system executable, the present embodiment comprises a decoupling circuit  11 . The decoupling circuit  11  includes four n-type MOS transistors  111 - 114 , which are connected to the bit lines BLi 1 - 4 , on four bit lines BLi- 4  as shown in  FIG. 2 . 
     The transistors  111 - 114  are supplied with respective control signals Phi 1 - 4  on gates thereof and on/off controlled. Thus, the bit lines BLi 1 - 4  are selectively coupled to or decoupled from the sense amp circuit S/Ai. 
     A bit-line potential control circuit  12  is used to control the bit line decoupled from the sense amp circuit S/Ai by the decoupling circuit  11  such that the potential thereon can be charged up to a certain potential, specifically the same potential as the drive voltage applied to the plate line PL. 
     The bit-line potential control circuit  12  includes n-type MOS transistors  121 ,  122  as shown in  FIG. 2 . The transistors  121 ,  122  each have one end applied with the reference potential Vref, the other end connected to the bit line BLi 1 - 4 , and a gate given a control signal Eq 11 - 4 . The control signals Eq 11 - 4  are independent signals. 
       FIG. 3  shows circuitry of the sense amp circuit S/Ai in detail. The sense amp circuit S/Ai includes a sense amp (comparator)  13 , a reference potential generation circuit  14 , and a discharge circuit  15 . 
     The sense amp  13  comprises a p-type sense amp SAP including p-type MOS transistors QP 1 -QP 3 , and an n-type sense amp SAN including n-type MOS transistors QN 1 -QN 3 . The p-type sense amp SAP and the n-type sense amp SAN are controlled with respective sense amp enable signals /SEP, SEN to operate. 
     The p-type MOS transistors QP 1 , QP 2  contained in the p-type sense amp SAP are serially connected between paired sense lines SL, /SL and connected at gates thereof to the paired sense lines SL, /SL to configure a flip-flop circuit. The paired sense lines SL, /SL are such lines that are selectively connected to the bit lines BLi 1 - 4  by the decoupling circuit  11 . 
     A p-type MOS transistor QP 3  is connected between a common connection point of both p-type MOS transistors QP 1 , QP 2  and a supply voltage VAA. The p-type MOS transistor QP 3  has a gate supplied with the sense amp enable signal /SEP to on/off control the p-type sense amp SAP. 
     The n-type MOS transistors QN 1 , QN 2  contained in the n-type sense amp SAN are serially connected between the paired sense lines SL, /SL and connected at gates thereof to the paired sense lines SL, /SL to configure a flip-flop circuit. 
     An n-type MOS transistor QN 3  is connected between a common connection point of both n-type MOS transistors QN 1 , QN 2  and the ground potential GND. The n-type MOS transistor QN 3  has a gate supplied with the sense amp enable signal SEN to on/off control the n-type sense amp SAN. 
     The reference potential generation circuit  14  includes a reset transistor QN 4 , selection transistors QN 5 , QN 6 , and a dummy capacitor DCC. The selection transistor QN 5  is connected between the sense line /SL and one end of the dummy capacitor DCC. The selection transistor QN 6  is connected between the sense line SL and one end of the dummy capacitor DCC. 
     The other end of the dummy capacitor DCC is given a dummy plate potential DPL. Both selection transistors QN 5 , QN 6  have respective gates connected to dummy word lines DWL 0 , DWL 1 . 
     The reset transistor QN 4  has one end connected to one end of the dummy capacitor DCC and the other end given the reference potential VDC. Thus, one end of the dummy capacitor DCC is reset to the reference potential VDC with a reset signal DRST. Of the dummy word lines DWL 0 , DWL 1 , one corresponding to the complementary bit line is made “H” to apply the reference potential to the complementary bit line. 
     The discharge circuit  15  includes n-type MOS transistors QN 7 , QN 8  connected between the sense line SL or /SL and the ground potential Vss, and an n-type MOS transistor QN 9  connected between the paired sense lines SL and /SL. These transistors QN 7 - 9  have respective gates given a control signal BEQL. After sensing by the sense amp  13 , the control signal BEQL turns to “H” to discharge the sense lines SL, /SL to the ground potential. 
     With the above configuration, the ferroelectric memory of the present embodiment can selectively execute either the 2T2C system or the 1T1C system. 
     In the case of data read/write by the 2T2C system, the control signals are made Phi 1 =Phi 2 =“H” and Phi 3 =Phi 4 =“L” or Phi 1 =Phi 2 =“L” and Phi 3 =Phi 4 =“H”. Namely, the bit line only in either the memory cell amp MCA 1  or MCA 2  is connected to the sense amp circuit S/Ai to read cell data and complementary data, as in the case of read/write in a format like the so-called folded structure. 
     In the case of data read/write by the 1T1C system, the control signals are made Phi 1 =Phi 4 =“H” and Phi 2 =Phi 3 =“L” or Phi 1 =Phi 4 =“L” and Phi 2 =Phi 3 =“H”. Namely, the bit lines extending one by one from both the memory cell array MCA 1 , MCA 2  are connected to the sense amp circuit S/Ai to execute read/write in a format like the so-called open line system. 
     Reading data by the 2T2C system is described first with reference to  FIGS. 4 and 5 . In the below described example, the control signals are made Phi 1 =Phi 2 =“H” and Phi 3 =Phi 4 =“L” to read data out of the memory cell amp MCA 1 . 
     First, in order to select a read-targeted memory cell MC 0  from the memory cell block MCB 01 , the word line WL 0  is set at “L” at time t 1  (while retaining other word lines WL 1 - 7  at “H”) and then the block selection signal BS 1  is set at “H”. As a result, the read-targeted memory cell MC 0  is connected to the bit line BL 01 . 
     On the other hand, in the memory cell block MCB 02 , a memory cell MC 0 ′ operative to store complementary data is connected to the bit line BL 02 . The potentials on the bit lines BL 01 , BL 02  set at the reference potential Vref in the initial state are once discharged to the ground potential Vss. 
     Subsequently, the plate lines PL, /PL are brought from the reference potential Vref to the supply voltage VAA at time t 2  to apply the supply voltage VAA across the ferroelectric capacitors contained in the memory cells MC 0 , MC 0 ′. This enables the charge to be read out of the ferroelectric capacitors in the memory cells MC 0 , MC 0 ′ to the bit lines BL 01 , BL 02 . 
     The bit lines BL 01 , BL 02  are given potentials in accordance with the data held in the memory cells MC 0 , MC 0 ′. The potential difference between the bit lines BL 01 , BL 02  can be compared/amplified when the sense amp  13  is activated at time t 3 . 
     Thereafter, the potentials on the plate lines PL, /PL are once fallen to the ground potential Vss at time t 4 , thereby rewriting data in the cell, which has been “1” read, that is, destructively read. The potentials on the plate lines PL, /PL are returned to the reference potential Vref at time t 5 . 
     In addition, the block selection signal BS 1  returns to “L” to terminate read operation. The bit lines BL 03 , BL 04  in the memory cell array MCA 2  decoupled from the sense amp circuit S/Ai by the decoupling circuit  11  are both retained at the potential Vref after the control signals Eq 13 ,  14  are made “H” at the bit-line potential control circuit  12 . The control signals Eq 11 , Eq 12  from the bit-line potential control circuit  12  close to the bit lines BL 01 , BL 02  are both made “L”. 
     Reading data by the 1T1C system is described first with reference to  FIGS. 6 and 7 . In the below described example, the control signals are made Phi 1 =Phi 4 =“H” and Phi 2 =Phi 3 =“L” to read data out of the memory cell in the memory cell block MCB 01  to the bit line BL 01 . The data is compared with the reference potential, which is generated from the reference potential circuit  14  and given to the bit line BL 04  for data read. 
     First, in order to select a read-targeted memory cell MC 0 , the word line WL 0  is set at “L” at time t 1  (while retaining other word lines WL 1 - 7  at “H”) and then the block selection signal BS 1  is set at “H”. As a result, the read-targeted memory cell MC 0  is connected to the bit line BL 01 . 
     On the other hand, in the memory cell block MCB 02 , a memory cell MC 0 ′ operative to store complementary data is connected to the bit line BL 02 . In this case, though, the transistor  112  in the decoupling circuit  11  is turned off with the control signal Phi 2 =“L” and accordingly decoupled from the sense amp circuit S/Ai. Further, in the bit-line potential control circuit  12 , the transistor  122  is turned on with the control signal Eq 12 =“H” to charge the bit line BL 02  up to the same potential on the plate line PL or the reference potential Vref. Therefore, no voltage is placed across the memory cell MC 0 ′, thereby preventing data read out of the memory cell MC 0 ′ and destruction of data. 
     Thereafter, the voltage on the plate line /PL connected close to the memory cell block MCB 01  is elevated to the supply voltage VAA at time t 2 . As a result, the supply voltage VAA is applied across the ferroelectric capacitor contained in the memory cell MC 0  and the charge flows from the ferroelectric capacitor C 0  in the memory cell MC 0  to the bit line BL 01 . 
     On the other hand, the voltage on the plate line PL remains at Vref after the time T 2  and the bit line BL 02  is also fixed to the reference potential Vref by the bit-line potential control circuit  12 . Therefore, the potential difference between both ends of the ferroelectric capacitor in the memory cell MC 0 ′ is equal to zero. Accordingly, no data is read out of the memory cell MC 0 ′ and data in the memory cell MC 0 ′ can not be destructed. 
     The potential on the bit line BL 01  is compared with the reference potential given to the bit line BL 04  from the reference potential generation circuit  14  and amplified at the sense amp  13 . The bit line BL 04  is connected to the sense amp circuit S/Ai and accordingly the balance between the bit line capacities on the paired complementary bit lines (BL 01 , BL 04 ) can be retained, thereby enhancing the read margin. 
     The potential on the bit line BL 03  decoupled from the sense amp circuit S/A by the decoupling circuit  11  is fixed to the reference potential Vref by the bit-line potential control circuit  12  (the control signal is made Eq 13 =“H”) to serve as a shield line. 
     As described above, the present embodiment makes it possible to execute not only the 2T2C system but also 1T1C system even if one memory cell array MCA includes only a single block selection line. With this configuration, the memory cell array can reduce the area of the memory cell array in accordance with the lowered number of the block selection line. 
     Second Embodiment 
       FIG. 8  shows circuitry of a ferroelectric memory according to a second embodiment of the present invention. The same elements as those in the first embodiment are denoted with the same reference numerals and omitted from the following detailed description.  FIG. 8  typically shows only one sense amp circuit S/A 0  and bit lines BL 01 - 04  connected thereto, of the configuration of the ferroelectric memory. 
     In this ferroelectric memory, each memory cell includes a ferroelectric capacitor C, and a selection transistor SG 1  serially connected thereto and having a gate connected to the word line WL. The memory cell is connected to one bit line BL 01 - 04 , in place of the connection of the memory cell block of the TC parallel units as in the first embodiment. 
     The ferroelectric capacitor C has one end connected to plate lines PL 1 , /PL 1 . In one memory cell array MCA, memory cells aligned in the same row are configured to share a word line WL and simultaneously selectable by selecting the word line WL. Therefore, the area of the memory cell array can be reduced in accordance with the lowered number of the word lines. 
     Even if memory cells aligned in the same row include only a single word line, it is possible to execute not only the 2T2C system but also 1T1C system if the decoupling circuit  11  similar to that in the first embodiment is provided as above. 
     Operation is executed almost in the same manner as that in the first embodiment. Namely, in the case of data read/write by the 2T2C system, the control signals are made Phi 1 =Phi 2 =“H” and Phi 3 =Phi 4 =“L” or Phi 1 =Phi 2 =“L” and Phi 3 =Phi 4 =“H”. On the other hand, in the case of data read/write by the 1T1C system, the control signals are made Phi 1 =Phi 4 =“H” and Phi 2 =Phi 3 =“L” or Phi 1 =Phi 4 =“L” and Phi 2 =Phi 3 =“H”. The plate lines PL 1 , /PL 1  are also driven in the same manner as those shown in  FIGS. 5 and 7 . 
     Third Embodiment 
       FIG. 9  shows circuitry of a ferroelectric memory according to a third embodiment of the present invention. The same elements as those in the first embodiment are denoted with the same reference numerals and omitted from the following detailed description.  FIG. 9  typically shows only one sense amp circuit S/A 0  and bit lines BL 01 - 04  connected thereto, of the configuration of the ferroelectric memory. 
     In this embodiment, two sub-bit lines SBLij, SBLij′ extend from each of four bit lines BLij (j=1-4) connected to one sense amp circuit S/Ai, different from the above-described embodiments. Namely, any two of eight sub-bit lines can be connected to one sense amp circuit S/Ai via the bit line BLij. The sub-bit lines SBLij, SBLij′ are connected to the memory cell blocks MCBij, MCBij′ similar to those in the first embodiment (the memory cell block close to the memory cell array MCA 2  is omitted from  FIG. 9 ). 
     A sub-decoupling circuit  16  is provided between the sub-bit lines SBLij, SBLij′ and the bit line SBLij. The sub-decoupling circuit  16  includes an n-type MOS transistor  161  and an n-type MOS transistor  162  for one bit line BLij. The n-type MOS transistor  161  is connected between the sub-bit line SBLij and the bit line BLij and turned on when a control signal Trs 0  is given to the gate thereof. The n-type MOS transistor  162  is connected between the sub-bit line SBLij′ and the bit line BLij and turned on when a control signal Trs 1  is given to the gate thereof. 
     When either of the control signals Trs 0 , Trs 1  is made “H”, one of the sub-bit lines SBLij, SBLij′ is selectively connected to the bit line BLij and the other is decoupled from the bit line BLij and the sense amp S/Ai. 
     The sub-bit lines SBLij, SBLij′ are provided with a sub-bit line potential control circuit  17 . The sub-bit line potential control circuit  17  includes n-type MOS transistors  171 ,  172  as shown in  FIG. 9 . The transistors  171 ,  172  each have one end supplied with the reference voltage Vref, the other end connected to the sub-bit line SBLij, SBLij′, and a gate given a control signal SEq 1   j . The control signals SEq 1   j  are independent signals. 
     The present embodiment is thus provided with the decoupling circuit  11  and the sub-decoupling circuit  16  so that one bit line BLij can be connected selectively to either of two memory cell blocks MCBij, MCBij′. Namely, one sense amp circuit S/Ai can be connected to eight memory cell blocks. When the decoupling circuit  11  and the sub-decoupling circuit  16  are switched, either of the 2T2C system and the 1T1C system can be executed selectively like the above embodiments. 
     The embodiments of the invention have been described above though the present invention is not limited to these embodiments but rather can be given various modifications, additions, replacements and deletions without departing from the scope and spirit of the invention. 
     For example, the above embodiments employ the plate line drive system for varying the plate line voltage to execute data read/write to memory cells. A plate line fixation system for retaining the plate line voltage unchanged while discharging the bit line voltage to execute data read/write may also be applicable to the present invention.