Abstract:
A semiconductor integrated circuit device comprises a semiconductor substrate; an insulating layer formed on the semiconductor substrate; a semiconductor layer insulated from the semiconductor substrate by the insulating layer; source regions of a first conduction type and drain regions of the first conduction type both formed in the semiconductor layer; body regions of a second conduction type formed in the semiconductor layer between the source regions and the drain regions to store data by accumulating or releasing an electric charge; word lines formed on the body regions in electrical isolation from the body regions to extend in a first direction; bit lines connected to the drain regions and extending in a direction different from the first direction; and buried wirings formed in the insulating layer in electrical isolation from the semiconductor substrate and the semiconductor layer, said buried wirings extending in parallel with the bit lines.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-370696, filed on Oct. 30, 2003, 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 integrated circuit device. 
   2. Related Background Art 
   For years, 1T-1C (1 transistor 1 capacitor) DRAM (dynamic random access memory) devices have been manufactured. 1T-1C DRAMs having memory cells reduced in design rule to smaller than 0.1 μm are difficult to manufacture. 
   On this account, a DRAM having FBCs (floating body cells) as shown in  FIG. 12  has been proposed (Japanese Patent Laid-open Publication No. JP2002-246571). FBC comprises a FET (field effect transistor) formed in SOI. The gate G of the FET is connected to a word line WL, the drain D is connected to b bit line BL, and the source S is connected to GND. The floating body FB functions as the data storage node. 
   FBC changes the number of carriers stored in the floating body FB and thereby changes the potential of the floating body FB. Data is stored by a change of threshold voltage of FET caused by the body effect. 
   For writing data “1” in FBC, both the word line WL and the bit line BL are raised to high potentials to bias FET toward the saturated state. Thereby, impact ionization is induced, and holes are stored in the floating body FB. The state with a larger number of holes stored in the floating body FB is regarded as data “1”. 
   For writing data “0” in FBC, the bit line BL is lowered to a negative potential, and the pn junction between the p-type body and the n-type body is thereby biased in the forward direction. As a result, the holes heretofore stored in the floating body FE are released to the bit line BL. 
     FIG. 13  is a cross-sectional view of another DRAM having FBCs (see “A Capacitorless Double-Gate DRAM Cell Design for High Density Applications” by C. Kuo, Tsu-Jae King and Chenming Hu, IEDM Tech. Digest, pp. 843-846, December 2002). This DRAM includes back gate electrodes BG in addition to front gate electrodes FG. 
   For writing data “0” in a floating body FE of this DRAM, both the floating gate electrode FG and the back gate electrode BG are set to high potentials whereas the potential barrier between the floating body FB and the source S is lowered. As a result, the holes heretofore stored in the floating body FE are released to the source S. 
   According to the DRAM shown in  FIG. 12 , the bit line BL is lowered to a negative potential to write data “0” in a selected cell, data of a non-selected cell connected to the same bit line BL and storing data “1” may be undesirably erased. This is generally called “0” disturbance. 
   In order to prevent “0” disturbance, the junction between the potential of the floating body FD of the non-selected cell and its drain D must be held in the reverse-biased or a weakly forward-biased condition (0.7 V or less). Therefore, it is necessary to lower the potential of the floating body FD of the non-selected cell to a sufficiently low negative potential by lowering the word line WL of the non-selected cell to an amply low negative potential. 
   According to the DRAM shown in  FIG. 13 , since the front gate electrode FG and the back gate electrode BG are parallel to each other, all cells connected to the activated word line WL are undesirably rewritten to data “0”. Therefore, upon refreshing operation and writing operation, the sense amplifier latches data of all cells connected to the word line WL before “0” is written (S 1 ). After that, data “0” is once written in all cells (S 2 ), and data “1” is rewritten only in those cells, having stored “1” just before “0” is written, according to the data latched in the sense amplifier (S 3 ). Thus, the DRAM of this type needs steps S 1  through S 3 . Therefore, the DRAM shown in  FIG. 13  needs a long cycle time for the refreshing and writing operations and needs sense amplifier circuits for individual bit lines BL. 
   The DRAM including sense amplifier circuits for individual bit line BL decreases the cell efficiency and increases in chip size because the sense amplifier circuits occupy a large area. This means that FBC will lose its advantage of having a smaller cell size than 1T-1C type DRAM. 
   It is therefore desired to provide a semiconductor integrated circuit device reduced in refreshing cycle time and writing cycle time and reduced in chip size without influences of “0” disturbance. 
   SUMMARY OF THE INVENTION 
   A semiconductor integrated circuit device comprises a semiconductor substrate; an insulating layer formed on the semiconductor substrate; a semiconductor layer insulated from the semiconductor substrate by the insulating layer; source regions of a first conduction type and drain regions of the first conduction type both formed in the semiconductor layer; body regions of a second conduction type formed in the semiconductor layer between the source regions and the drain regions to store data by accumulating or releasing an electric charge; word lines formed on the body regions in electrical isolation from the body regions to extend in a first direction; bit lines connected to the drain regions and extending in a direction different from the first direction; and buried wirings formed in the insulating layer in electrical isolation from the semiconductor substrate and the semiconductor layer, said buried wirings extending in parallel with the bit lines. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a memory portion of DRAM  100  according to the first embodiment of the invention; 
       FIG. 2  is a circuit diagram of a memory cell array  10  and SA/PD  20  in DRAM  100 ; 
       FIG. 3  is a cross-sectional view of the memory portion of DRAM  100  taken along a bit line BL; 
       FIG. 4  is a cross-sectional view of the memory portion of DRAM  100  taken along a word line WL; 
       FIG. 5  is a graph showing the potential of a body region  160  controlled by potentials of the bit line BL, word line WL and plate line PL; 
       FIG. 6  is a graph showing the relation between the potential Vgs of the word line WL and the drain-to-drain current Ids while data is read out from a memory cell MC; 
       FIG. 7  is a block diagram showing a memory portion of DRAM  200  according to the second embodiment of the invention; 
       FIG. 8  is a circuit diagram of a memory cell array  10 , sense amplifier  26  and plate driver  28  in DRAM  200 ; 
       FIG. 9  is a cross-sectional view of the memory portion of DRAM  300  according to the third embodiment of the invention, taken along a word line WL; 
       FIG. 10  is a diagram showing the layout and connection of sense amplifiers of the DRAM  300 ; 
       FIG. 11  is a diagram showing the layout and connection of sense amplifiers of the DRAM  400  according to the fourth embodiment of the invention; 
       FIG. 12  is a cross-sectional view of conventional DRAM having FBCs; and 
       FIG. 13  is a cross-sectional view of conventional DRAM having FBCs. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Some embodiments of the invention will now be explained below with reference to the drawings. The invention, however, is not limited to these embodiments. In these embodiments, even when using n-type semiconductors instead of p-type semiconductors while using p-type semiconductors instead of n-type semiconductors, the same effects are assured. 
   In these embodiments, a back gate of a double-gate SOI transistor is provided in parallel to the bit line. Thereby, the above-mentioned problems are resolved. 
   (First Embodiment) 
     FIG. 1  is a block diagram showing a memory portion of DRAM  100  according to the first embodiment of the invention. The memory portion includes memory cell arrays  10 , sense amplifier/plate driver portions (herein below simply called SA/PDs)  20 , row decoders combined with WL drivers (herein below simply called row decoders)  30 , and a column decoder combined with CSL (column select line) driver (herein below simply called column decoder)  40 . 
   Each memory cell  10  comprises memory cells having a matrix arrangement of FBCs. The plurality of memory cell arrays  10  are aligned side by side. The SA/PDs  20  are provided in every other spaces between memory cell arrays  10 . One SA/PD  20  is connected to memory cell arrays  10  at both sides thereof and can detect and latch data in these memory cell arrays  10 . In addition, the SA/PD  20  can selectively control the potential of the plate lines PL shown in FIG.  2  and can drive the selected plate line PL. 
   Each row decoder  30  is associated with each memory cell array  10 , and can select a word line in the memory cell array  10 . The column decoder  40  is associated with a group of memory cell arrays  10  in side-by-side arrangement, and can select a bit line in the memory cell arrays  10 . 
     FIG. 2  is a circuit diagram of one memory cell array  10  and one SA/PD  20  in DRAM  100 . At the right of the SA/PD  20 , another memory cell array  10  is connected, although not shown. In the memory cell array  10 , N word lines WL 0 ˜WL N−1 , M bit lines BL 0 ˜BL M−1 , and M plate lines PL 0 ˜PL M−1  are provided. Furthermore, memory cells MC are provided in the memory cell array  10  at the crossing points of the word lines WL 0 ˜WL N−1  and bit lines BL 0 ˜BL M−1 . That is, one memory cell array  10  has N*M memory cells MC. Alternatively, L sets of N*M memory cells may be aligned in the direction of the word lines to include N*M*L memory cells in one memory cell array  10 . 
   Each memory cell MC is a double-gate SOI transistor formed on SOI (silicon-on-insulator) to include a forward gate FG and a back gate BG as shown in FIG.  3 . 
   Each of the word lines WL 0 ˜WL N−1  is connected to individual forward gates FG of memory cells MC of each row in the memory cell. Each of the bit lines BL 0 ˜BL M−1  is connected to individual drains D of memory cells MC of each column in the memory cell array  10 . Each of the plate lines PL 0 ˜PL M−1  is connected to individual back gates BG of memory cells of each column in the memory cell array  10 . Individual plate lines PL 0 ˜PL M−1  are associated with individual bit lines BL 0 ˜BL M−1 . Preferably, the plate lines PL 0 ˜PL M−1  extend in parallel with the bit lines BL 0 ˜BL M−1.    
   The word lines WL 0 ˜WL N−1  are connected to a row decoder  30  (see FIG.  1 ), respectively. The bit lines BL 0 ˜BL M−1  and the plate lines PL 0 ˜PL M−1  are connected to the SA/PD  20 , respectively. 
   The SA/PD  20  includes sense amplifier/plate driver circuits  21  and BL (bit line)/PL (plate line) selectors  22 . The BP/PL selectors  22  select a pair of bit line and plate line, and the pair of bit line and plate line selected by the BL/PL selectors  22  can be exclusively connected to the sense amplifier/plate driver circuits  21 . On the other hand, the WL driver in the row decoder  30  selects one of word lines WL 0 ˜WL N−1  and can drive the word line. Thus, a memory cell MC at the crossing point of the selected pair of bit line and plate line with the selected word line can be selected. 
   The memory cell array  10  further includes dummy memory cells DMC. Forward gates FG of the dummy memory cells DMC are connected to word lines, and dummy bit lines DBL 0  or DBL 1  are connected to the drains of the dummy memory cells DMC. Back gates BG of the dummy memory cells DMC are connected to dummy plate lines DPL 0  or DPL 1 . 
   The SA/PD  20  further includes DBL/DPL controllers  23  connected to the dummy bit lines DBL 0 , DBL 1 , and dummy plate lines DPL 0 , DPL 1 . 
   Dummy memory cells DMC are used when associated sense amplifiers SA detect data of memory cells. For example, dummy memory cells DMC connected to the dummy bit line DBL 0  store data “0”, and dummy memory cells DMC connected to the dummy bit line DBL 1  store data “1”. Upon detection of the data, the sense amplifier SA adds currents of these dummy memory cells DMC and reduces the current to a half by means of a current mirror circuit (not shown). The sense amplifier SA compares the half current value with the current in each memory cell MC and thereby detects data “1” or data “0” of the memory cells MC. 
   As such, the sense amplifier/plate driver circuit  21  can detect data of memory cells MC by means of the bit lines BL and the word lines WL. Regarding the data detecting method, the invention is not limited to the above-explained method, but may employ any appropriate one of known methods. 
     FIG. 3  is a cross-sectional view of the memory portion of DRAM  100  taken along a bit line BL. The DRAM  100  includes a p-type semiconductor substrate  110 , silicon oxide film  120 , SOI layer  130 , n-type drain regions  140 , n-type source regions  150 , p-type body regions  160 , word lines WL, bit lines BL, n-type plate lines PL and source lines SL. 
   The silicon oxide film  120  is formed on the semiconductor substrate  110 . The plate lines PL are formed in the silicon oxide film  120  and isolated from the semiconductor substrate  110  and the SOI layer  130 . The plate lines PL extend in parallel to the bit lines BL. The SOI layer  130  overlies the silicon oxide film  120  and is isolated from the semiconductor substrate  110  and the silicon oxide film  120 . 
   The drain regions  140  and the source regions  150  are formed in the SOI layer  130 . The body regions  160  are formed between the drain regions  140  and the source regions  150  in the SOI layer  130 . 
   A gate insulating film  170  lies on the body regions  160 , and the word lines WL lie on the gate insulating film  170 . Thereby, the word lines WL are insulated from the body regions  160 . The word lines WL extend in the direction vertical to the sheet plane of FIG.  3 . The bit lines BL are electrically connected to the drain regions  140 , and extend across the word lines WL. 
     FIG. 4  is a cross-sectional view of the memory portion of DRAM  100  taken along a word line WL (along the X—X line of FIG.  3 ). It will be understood from  FIGS. 3 and 4  that the bit lines BL and plate lines PL are associated with each other and extend in parallel. As best shown in  FIG. 4 , the bit line BL are aligned substantially in equal intervals. The plate lines PL are aligned in the same intervals as those of the bit lines BL. 
   Next referring to  FIGS. 5 and 6 , operations and effects of the DRAM  100  will be explained. Graphs shown in  FIGS. 5 and 6  are results of a simulation of writing “0” or “1” in the DRAM  100 . The simulation was conducted under the conditions: channel length of memory cells MC: L gate =0.175 μm; thickness of the gate insulating film  170 : T oxf =80 angstrom; thickness of the insulating film  175  between the body regions  160  and the plate lines PL: T box =120 angstrom, thickness of the silicon of the body regions  160 : T si =330 angstrom. The acceptor impurity concentration in the body regions  160  is constantly 1.0*10 16  cm −3 . Both the word lines WL and the plate lines PL are made of n-type polysilicon having a sufficiently high impurity concentration. The word lines WL and the plate lines PL function as front gates FG and back gates BG, respectively. 
   The graph of  FIG. 5  shows the potential of the body regions  160  controlled by respective potentials in the bit lines BL, word lines WL and plate lines PL. The abscissa indicates time (in nanosecond) and the ordinate indicates those potentials (in Volt). Potentials of the bit lines BL, word lines WL and plate lines PL are denoted by “V BL ”, “V WL ” and “V PL ”, respectively. Potential of the body regions  160  is denoted by “B BODY ”. 
   The graph of  FIG. 6  shows relations between the potential Vgs of the word line WL and the drain-to-source current Ids while data is read out from a memory cell MC. 
   First referring to  FIG. 5 , let data “1” be written in a memory cell MC. For the period from 0 nm to 42 ns, V WL  is held in 1.5 V and the V BL  is held in 2.0 V to bias the memory cell MC to the saturated sate. As a result, impact ionization occurs in the body region  160 , and the potential of the body region  160  gradually rises. Once the potential of the body region  160  reaches approximately 0.7 V, the current generated by holes becomes substantially equal to the forward current flowing into the pn junction between the body region  160  and the source region  150 , and the potential of the body region  160  becomes substantially stationary. At that time, writing of data “1” in the memory cell MC is completed. 
   Next let the data “1” be maintained in the memory cell MC. After the data “1” is written in the memory cell MC, V BL  is set to  0V  and V WL  to −1.5 V at the point of time, 46 ns. Since V WL  is a negative potential, holes in the body region  160  are maintained. Therefore, the memory cell MC holds data “1”. 
   Next let V WL  be raised to read out the data from the memory cell MC in order to examine whether or not the holes leak from the body region  160 . Leakage of holes from the body region  160  is called “disturbance” herein below. For the period from about 50 ns to 70 ns, setting V BL  to 0.2 V, and raising V WL  from −1.5 V to 1.5 V, V PL  is maintained in −2 V. In this sate, let the data in the body region  160  be monitored. Thus, the potential of the body region  160  is confirmed to maintain approximately 0.6V unchanged. This suggests that no disturbance has occurred. In  FIG. 6 , curve I 1  shows the relation between the word line potential Vgs and the drain current IDs at the time of reading out the data. 
   Again referring to  FIG. 5 , let the data of the memory cell MC be read out by again holding the data “1” and thereafter raising V WL  while keeping V PL  in −2V, for the purpose of confirming any disturbance to data “1”. Referring to  FIG. 6 , the drain current Ids of the memory cell MC then observed overlapped the curve I 1 . It has been confirmed from it that, even when the data is read out from the memory cell MC, the relation between the word line potential Vgs and the drain current IDs is maintained, and it has been confirmed that no disturbance occurred. 
   Next let it examined whether any disturbance occurs by raising the potential of the plate line PL and reading the data of the memory cell MC. For this purpose, for the period from about 84 ns to about 104 ns in  FIG. 5 , the potential V PL  of the plate line PL is raised from −2 V to −5 V. V WL  is maintained in −1.5V. In this case, the potential of the body region  160  is maintained approximately in 0.6 V unchanged. This suggests that no disturbance has occurred. 
   For the purpose of confirming that no disturbance occurs, the data of the memory cell MC is read out by raising the potential of the word line WL for the period from about 108 ns to about 110 ns. With reference to  FIG. 6 , the drain current Ids of the memory cell MC observed here also overlapped the curve I 1 . It has been confirmed from it that, even when the potential of the plate line PL is raised while V WL  of the word line WL is maintained in −1.5 V, the relation between the word line potential Vgs and the drain current Ids is maintained, and disturbance did not occur. 
   Finally, for the period from about 116 ns to about 156 ns, the potential V WL  and the potential V PL  are raised to 1.5 V and −0.5 V, respectively. Thereby, data “0” is written in the memory cell MC. As a result, the potential of the body region  160  decreases. The duration of time for this writing was approximately 40 nm. After the data “0” is written in the memory cell MC, it is held at the point of time, 158 ns, approximately. Thereafter, by reading out the data “0” from the memory cell MC in the period from about 160 ns to about 162 ns, the curve I 0  shown in  FIG. 6  was obtained. It is appreciated from this result that the drain current Ids reliably decreases, and the data “0” is certainly written in the memory cell MC. 
   As such, if the potential of only one of the word line WL and the plate line PL is raised, the potential barrier between the body region  160  and the source region  150  does not decrease sufficiently. Therefore, holes in the body region  160  are not released to the source region  150 , and the data “1” is maintained. On the other hand, if both the word line WL and the plate line PL are raised in potential, the potential barrier between the body region  160  and the source region  150  decreases sufficiently. As a results holes in the body region  160  are released to the source region  150 , and the data “0” is written in the memory cell MC. 
   As such, when both the word line WL and the plate line PL are raised in potential, the data “0” is written in the memory cell MC (see the points of time 116 ns ˜156 ns in FIG.  5 ). This means that the data “0” is written in the memory cell MC selected by a word line WL and a plate line PL. On the other hand, when only one of the word line WL and the plate line PL is raised in potential, the data “1” stored in the memory cell MC does not change (see the point of time 46 ns ˜108 ns in FIG.  5 ). This means that no disturbance occurs against non-selected memory cells MC storing data “1”. 
   In this manner, upon a refreshing operation, it is possible to select a memory cell MC having stored data “0” by means of a word line WL and a plate line PL and to write the data “0” only in that memory cell MC once again. 
   In the prior art shown in  FIG. 13 , since the forward gates FG (word lines) and the back gates BG (plate lines) are parallel, it is not possible to select a particular memory cell MC alone and write the data “0” only in the memory cell MC, and it is therefore necessary to execute three steps S 1 , S 2  and S 3  for refreshing old data and writing new data. 
   In contrast, in the instant embodiment of the invention, since the plate lines PL extend across the word lines WL and substantially in parallel to the bit lines BL. Therefore, it is possible to select a memory cell MC at a crossing point of a word line WL and a plate line PL to write “0” and simultaneously select a memory cell MC at a crossing point of a word line WL and a bit line BL to write the data “1”. Thus, the embodiment need only one step of writing data “0” or “1” in the memory cell MC for refreshing old data and writing new data. As a result, the instant embodiment reduces the cycle time for refreshing and writing operations than the prior art. 
   In addition, the DRAM  100  according to the instant embodiment need not read data from all memory cells MC and latch them in the process of refreshing and writing operations. Therefore, the sense amplifier need not be provided in one-to-one association with each bit line BL, but one sense amplifier is sufficient for one memory cell array  10 . As a result, in the semiconductor chip, the proportion of the area occupied by the sense amplifiers is reduced, the ratio of cell area increases, and the chip size is reduced. 
   Further, since this embodiment includes the word lines WL and the plate lines PL as forward gates and back gates, respectively, the problem of GIDL does not occur in this embodiment. 
   The duration of time for writing the data “0”, that is, the duration of time where potentials of the word line WL and the plate line PL are kept high, is important. As shown in  FIG. 6 , at the point of time, 156 ns, where the writing of the data “0” is completed, the potential of the body region  160  is in the course of decreasing. On the other hand, continuing the writing of data “0” until the potential of the body region  160  stabilizes results in elongating the cycle time of the refreshing operation. Therefore, writing of data “0” is continued only until the potential becomes sufficiently distinctive from the potential for data “1”, but not continued beyond it up to a balanced condition. Therefore, in order to prevent fluctuation of data, it is important to manage the duration of time for writing data “0”. 
   (Second Embodiment) 
     FIG. 7  is a block diagram showing the memory portion of DRAM  200  according to the second embodiment of the invention. In this embodiment, the sense amplifier portion  26  and the plate driver portion  28  are provided in separate locations. The other structural features of this embodiment are identical to those of the first embodiment. They are, therefore, not explained here. 
   The sense amplifier portion  26  is located near one side of the memory cell array  10 . The plate driver portion  28  is located near the opposite side of the memory cell array  10  to be opposed to the sense amplifier portion  26  via the memory cell array  10 . As such, the sense amplifier portion  26  and the plate driver portion  28  are provided to appear alternately in spaces between every two adjacent memory cell arrays  10 , and they each are commonly used for two memory cell arrays  10  at both sides thereof. 
     FIG. 8  is a circuit diagram showing one memory cell array  10 , one sense amplifier  26  and one plate driver  28 . In harmony with the circuit arrangement including the sense amplifier portion  26  and the plate driver portion  28  as separate portions, the sense amplifier/plate driver circuit  21  shown in  FIG. 2  are separated to a sense amplifier circuit  221  and a plate driver circuit  224  in this embodiment. Similarly, the BL/PL selector  22  shown in  FIG. 2  is separated to a BL selector  222  and a PL selector  225  in this embodiment. Furthermore, the DBL/DPL selector  23  shown in  FIG. 2  is separated to a DBL selector  223  and a DPL selector  226  in this embodiment. The sense amplifier circuit  221 , BL selector  222  and DBL selector  223  are involved in the sense amplifier  26  whereas the plate driver circuit  224 , PL selector  225  and DPL selector  226  are involved in the plate driver  28 . 
   The sense amplifier circuit  221  and the plate driver circuit  224  are connected by a plate drive line PDL. A plate drive signal is transferred from the sense amplifier circuit  221  to the plate driver circuit  224  via the plate drive line PDL. 
   If data detected by the sense amplifier portion  26  is “0” in a refreshing or writing operation, the plate drive signal transfers the information to the plate driver portion  28 . Thus, the plate driver portion  28  can selectively drive the plate line PL upon writing data “0”. 
   In case of writing data “0” from outside in a writing operation, a peripheral data bus may directly transfer the information to the plate driver portion  28 . In case the sense amplifier portion  26  should process both the data “1” and data “0” in a writing operation, the sense amplifier portion  26  may transfer the information of data “0” alone to the late driver portion  28  via the plate drive line PDL. Thus, the sense amplifier portion  26  can control the timing for driving the plate driver portion  28 . Wiring of the plate driver line PDL can be formed from the same metal wiring layer as that of the column select line CSL, which is the top layer on the memory cell array. 
   A simulation of operation of the DRAM  200  results identical to the first embodiment under the same conditions. Therefore, the second embodiment has the same effects as these of the first embodiment. In addition, the second embodiment makes it easier to design the sense amplifier portion  26  and the plate driver portion  28  because they are separately positioned. Especially when the bit lines BL and the plate lines PL are arranged in a fine pitch, it is difficult from the standpoint of the circuit design to drive the bit lines BL and the plate lines PL independently from the same direction. Therefore, the second embodiment is particularly effective for a design having a fine pitch arrangement of bit lines BL and plate lines PL. 
   (Third Embodiment) 
     FIG. 9  is a cross-sectional view of the memory portion of DRAM  300  according to the third embodiment of the invention, taken along a word line WL. When the DRAM  300  is cut along a BL line, it will appear identical to FIG.  3 . The cross-sectional view of  FIG. 9  may be same as the cross-sectional view taken along the X—X line of FIG.  3 . 
   As shown in  FIG. 9 , each plate line PL in this embodiment is associated with four bit lines BL and four body regions  160 . A simulation of operation of the DRAM  300  results identical to that of the first embodiment under the same conditions. 
   According to the instant embodiment, even when there is a difficulty in forming the plate lines PL in a fine pitch arrangement, or in forming the plate lines PL in precise alignment with the bit line BL, the plate line PL can be formed in parallel with the bit lines BL. 
   In this embodiment, however, if data “0” is written in a memory cell in the same manner as the first and second embodiments, a plurality of memory cells connected to bit lines BL associated with a particular plate line PL are rewritten to data “0” simultaneously. 
   Therefore, this embodiment needs sense amplifiers in the same number of the bit lines BL associated with one plate line PL. Then, the sense amplifiers can read and latch the data of all memory cells before writing data “0” in all memory cells and thereafter rewrite data “1” only in memory cells having stored data “1” before. 
   As such, the third embodiment needs a reduced number of sense amplifiers equal in number to the bit lines BL associated with one plate line PL. Therefore, the third embodiment increases the proportion of the cell area and reduces the chip size than conventional devices. 
     FIG. 10  is a diagram showing the layout and connection of sense amplifiers of the DRAM  300 . The DRAM  300  includes memory cell arrays  301  and SA/PD  302 . Word lines and memory cells are omitted from illustration of FIG.  10 . 
   The plate lines PL 1 ˜PL 4  are connected to sense amplifier circuit  321 ˜ 324 , respectively. Four bit lines BL associated with one plate line PL 1  are connected to the sense amplifier circuits  321 ˜ 324 , respectively. Similarly, four bit lines associated with another plate line PL 2  are connected to the sense amplifiers  321 ˜ 324 , four bit lines associated with still another plate line PL 3  are connected to the sense amplifier circuit  321 ˜ 324 , and four bit lines associated with yet another plate lines PL 4  are connected to the sense amplifier circuits  321 ˜ 324 . 
   Inside the DRAM  300 , memory cells arrays  301  each including a number of memory cells are aligned side-by-side. The SA/PD  302  involves the sense amplifiers  321 ˜ 324  and a BL selector  332 . The sense amplifier circuit  321 ˜ 324  include plate drivers. Positional relation of the memory cell arrays  301  and SA/PD  302  is the same as that of the memory cell arrays  10  and SA/PD  20  shown in FIG.  1 . That is, SA/PDs circuits  302  are provided in every other spaces between adjacent memory cell arrays  301 . Therefore, one SA/PD  302  is connected to two memory cell arrays  301  at both sides thereof. 
   The BL selector  332  randomly selects four bit lines associated with the plate lines PL 1 ˜PL 4  respectively. Thereby, the sense amplifiers  321 ˜ 324  can read and latch data in all memory cells. The plate lines PL 1 ˜PL 4  are driven by one of the sense amplifier circuits  321 ˜ 324  corresponding to the selected group of bit lines BL when data “0” is written in a memory cell. Therefore, the embodiment doe not need a circuit for selecting plate lines PL (PL selector). 
   (Fourth Embodiment) 
     FIG. 11  is a diagram showing the layout and connection of sense amplifiers of the DRAM  400  according to the fourth embodiment of the invention. The DRAM  400  includes memory cell arrays  301 , sense amplifier portions  303  and plate driver portions  350 . This embodiment is different from the third embodiment in including the plate driver portions  350  independently from the sense amplifier portions  303 . Each sense amplifier portions  303  includes sense amplifiers  325 ˜ 328  and a BL selector  332 . The sense amplifiers  325 ˜ 328  are those excluding plate drivers from the sense amplifiers  321 ˜ 325  shown in FIG.  10 . This embodiment includes plate drivers in form of the plate driver portion  350 . 
   Positional relation of the memory cell arrays  301 , sense amplifier portions  303  and plate driver portions  350  is the same as that of the sense amplifier portions  26  and plate driver portions  28  shown in FIG.  7 . That is, each sense amplifier portion  303  is located near one side of a particular memory cell array  301 . Each plate driver portion  350  is located near the opposite side of the memory cell array  301  to be opposed to the sense amplifier portion  303  via the same memory cell array  301 . As such, the sense amplifier portions  303  and the plate driver portions  350  are provided to appear alternately in spaces between every two adjacent memory cell arrays  10 , and they each are commonly used for two memory cell arrays  301  at both sides thereof. 
   This embodiment operates in the same manner as the third embodiment and ensures the same effects. Additionally, this embodiment makes it easier to design the sense amplifier portion  303  and the plate driver portion  350  because they are separately positioned. Especially when the bit lines BL and the plate lines PL are arranged in a fine pitch, it is difficult from the standpoint of the circuit design to drive the bit lines BL and the plate lines PL independently from the same direction. Therefore, the second embodiment is particularly effective for a design having a fine pitch arrangement of bit lines BL and plate lines PL.