Abstract:
A ferroelectric memory device includes: a plurality of cell groups, wherein each cell group includes a transistor and at least two ferroelectric capacitors commonly coupled to the transistor; at least one word line for selecting the cell groups; at least two plate lines for driving the capacitors contained in a memory cell of a selected cell group; and at least one bit line for transmitting data to the selected memory cell. Therefore, the integrity of device is increased by coupling at least two memory cells to one bit line and one word line through one transistor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a semiconductor memory device; and, more particularly, to a ferroelectric memory device including a plurality of cell groups, each cell group having a transistor and a plurality of ferroelectric capacitors commonly coupled to the transistor. 
     DESCRIPTION OF THE PRIOR ART 
     A memory cell of a conventional ferroelectric random access memory (FeRAM) includes one transistor and one ferroelectric capacitor. It is well known that a ferroelectric capacitor shows a hysteresis characteristic between the charge and the voltage applied to the two terminals of the ferroelectric capacitor. 
     FIG. 1A shows a symbol of a ferroelectric capacitor with the two terminals, and FIG. 1B shows a hysteresis characteristic of a ferroelectric capacitor. 
     A characteristic of a ferroelectric capacitor will be described with reference to FIGS. 1A and 1B. An amount of charge stored in the ferroelectric capacitor varies according to a polarization state S 1  or S 2  even if the voltage difference between the two terminals is zero. Thus, binary logic data can be stored in the ferroelectric capacitor even in the absence of power supply. The ferroelectric capacitor differs from the linear capacitor on that point. The polarization states are varied with the voltage applied to the two terminals of the ferroelectric capacitor, so that the amount of charge stored in the ferroelectric capacitor is varied. 
     As shown in FIG. 1B, the polarization state of the ferroelectric capacitor is changed from the state S 1  to a state S 3  by a large negative voltage applied thereto. And the polarization sate is changed from the state S 3  to the state S 2  when the voltage difference between the two thermals is zero. As described above, the amount of the charge is varied with the applied voltage, so that the ferroelectric capacitor can be used as a storage element of a non-volatile memory device. 
     FIG. 1C is a waveform showing a potential change  10  of a bit line caused by the charge sharing between a ferroelectric capacitor Cf and a bit line parasitic capacitor Cb in case where a voltage is applied to a plate line PL. 
     In FIG. 1C, reference symbols V 1  and V 0  denote a potential of a bit line when data “1” and “0” are stored in the ferroelectric capacitor Cf, respectively. 
     FIG. 2 is a circuit diagram illustrating an array  100  of conventional FeRAM memory cells, each of the memory cells having a folded bit line architecture. For example, a memory cell  20  for storing 1 bit of conventional FeRAM includes one NMOS transistor N 21  and one ferroelectric capacitor C 21 . A gate of the NMOS transistor N 21  is coupled to a word line WLO, a drain of the NMOS transistor N 21  is coupled to a bit line Bit 0  and the other a source of the NMOS transistor N 21  is coupled to the ferroelectric capacitor C 21 . Two electrodes of the ferroelectric capacitor C 21  are coupled to the NMOS transistor N 21  and a cell plate line PL 0 , respectively. 
     First electrodes of each ferroelectric capacitor C 21  to C 28  are coupled to NMOS transistors N 21  to N 28 , respectively, and second electrodes of each ferroelectric capacitor C 21  to C 28  is commonly coupled to the plate line PL 0 . The word lines WL 0  to WL 3  are perpendicular to the bit lines Bit 0  to Bitb 3  and parallel to the plate lines PL 0  and PL 1 . 
     A predetermined voltage must be applied to the two electrodes of the ferroelectric capacitors during a reading or a writing operation. Therefore, a high state voltage Vcc or a low state voltage Vss is applied to the plate line PL 0 . 
     The read operation of the conventional FeRAM device will be described in detail with reference to FIGS. 1B and 2. 
     In order to read data stored in the ferroelectric capacitor C 21 , the word line WL 0  is selected and activated, and the rest word lines WL 1 , WL 2  and WL 3  are remained as inactivated. A high level signal is applied to the plate line PLS 0  coupled to the gate of an NMOS transistor N 20 . A low signal is applied to the global plate line GPL coupled to the NMOS transistor N 20  through an inverter INV 20 . As a result, the high level signal is applied to the plate line PL 0 . 
     As described above, the bit line BitO and the storage node S 1  are precharged to the ground level, so that the voltage between the two terminals of the ferroelectric capacitor C 21  becomes −Vcc. 
     Referring to FIG. 1B, the polarization state of ferroelectric capacitor is changed from state S 1  or S 2  to the direction of S 3 , when a negative voltage is applied to the ferroelectric capacitor C 21 . Therefore, the voltage of the bit line Bit 0  is changed according to the variation of the charge amount ΔQ 1  or ΔQ 1 . 
     As a result, the capacitance of the parasitic bit line capacitor Cb is changed. That is, data is transferred between the parasitic bit line capacitor Cb and the ferroelectric capacitor C 21  by a charge sharing. 
     In FIG. 1B, S 1  and S 2  denote logic data “1” and “0”, respectively. The polarization state S 1  is changed to the state corresponding to the voltage V 1 , and the polarization state S 2  is changed to a state corresponding to the voltage V 0 , when a large negative voltage is applied to the ferroelectric capacitor C 21 . The capacitance obtained by the change of the polarization state S 1  to the direction of the polarization state S 3  is larger than the capacitance obtained by the change the polarization state S 2  to the direction of the polarization state S 3 . Therefore, the voltage V 1  is lager than the voltage V 0 . This relation is expressed by the following equation:                  Δ                 V1     =       Cf1     Cf1   +   Cb       ×   Δ                 Vp       ,       Δ                 V0     =       Cf0     Cf0   +   Cb       ×   Δ                 Vp               (     Eq   .              1     )                                
     where, ΔVp represents the amount of voltage variation of plate line, and 
     Cf1 and Cf0 represents the equivalent capacitance of data logic “1” and “0” states, respectively. 
     In Eq. 1, the ΔV1 is larger than the ΔV0 since the Cf1 is larger than the Cf0. 
     As described above, the memory cell of the conventional FeRAM includes one switching NMOS transistor and one ferroelectric capacitor. In order to drive a memory cell, a plurality of bit lines, word lines and plate lines coupled to each memory cell are driven, respectively. Therefore, the conventional FeRAM has the limitation in the improvement of the device integration. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention is to provide a ferroelectric memory device capable of increasing the integrity of device by coupling at least two memory cells to one bit line and one word line through one transistor. 
     In accordance with one aspect of the present invention, there is provided a ferroelectric memory device comprising: a plurality of cell groups, wherein each cell group includes a transistor and at least two ferroelectric capacitors commonly coupled to the transistor; at least one word line for selecting the cell groups; at least two plate lines for driving the capacitors contained in a memory cell of a selected cell group; and at least one bit line for transmitting data to the selected memory cell. 
     In accordance with another aspect of the present, there is provided a ferroelectric memory device comprising: a plurality of memory cells, wherein each memory cell includes a ferroelectric capacitor for storing data; a reference voltage generating means for generating a reference voltage; a sense amplifying means for sensing and amplifying signals outputted from the memory cell; a precharging means; a plurality of bit line pairs precharged by the precharging means, wherein each bit line pair includes a first bit line for transmitting data from the memory cell to the sense amplifying means and the second bit line for transmitting the reference voltage to the sense amplifying means; a plurality of cell groups, wherein each cell group includes a plurality of memory cells which are commonly coupled to the same word line and the same bit line pair through one transistor; and a plurality of plate lines for respectively driving the ferroelectric capacitors contained in each memory cell of the same cell group. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which: 
     FIG. 1A presents a symbol of a conventional ferroelectric capacitor; 
     FIG. 1B is a graph illustrating the hysteresis characteristic of a conventional ferroelectric capacitor; 
     FIG. 1C is a schematic circuit diagram illustrating the voltage variation of bit line when a voltage is applied to the plate line and the charge sharing is generated between ferroelectric capacitor and bit line parasitic capacitance; 
     FIG. 2 shows a circuit diagram illustrating conventional cell array of a ferroelectric memory device having folded bit line architecture; 
     FIG. 3 depicts a circuit diagram illustrating an array of ferroelectric memory cell in accordance with a preferred embodiment of the present invention; 
     FIGS. 4A and 4B are schematic circuit diagrams illustrating a charge sharing relation between a ferroelectric capacitor and a parasitic capacitor at reading operation; 
     FIG. 5 presents a circuit diagram illustrating a reference voltage generator, a memory cell array, a sense amplifier and a precharge part of ferroelectric memory device in accordance with the present invention; 
     FIG. 6 shows timing diagram of the memory device shown in FIG. 5; and 
     FIG. 7 presents a circuit diagram illustrating the cell array of a ferroelectric memory device in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, a ferroelectric random access memory (FeRAM)  200  according to an embodiment of the present invention comprises a plurality of cell groups G 31  to G 38  including two adjacent memory cells coupled to the same bit line and the same word line. Each memory cell in the same cell group includes a ferroelectric capacitor commonly coupled the same NMOS transistor and respectively driven by different plate lines. 
     For example, a cell group G 31  includes two memory cells commonly coupled to the same bit line Bit 0  and the same word line WL 0 . Each ferroelectric capacitor C 31  and C 32  in the two memory cells of the cell group G 31  is coupled to the same NMOS transistor N 31  and different plate lines PL 0  and PL 1 , respectively. 
     A gate of the NMOS transistor N 31  is coupled to a word line WL 0 . One source/drain junction of the NMOS transistor N 31  is coupled to a bit line Bit 0  and the other source/drain junction of the NMOS transistor N 31  is coupled to the ferroelectric capacitors C 31  and C 32 . Each first electrode of the ferroelectric capacitors C 31  and C 32  is coupled to the NMOS transistor N 31 , and each second electrode of the ferroelectric capacitors C 31  and C 32  is coupled to the different plate lines PL 0  and PL 1 , respectively. 
     The word lines WL 0  to WL 3  are perpendicular to the bit lines Bit 0  to Bitb 3  and parallel to the plate lines PL 0  to PL 3 . Each plate line is commonly coupled to ferroelectric capacitors in the different cell groups. 
     The conventional FeRAM device shown in FIG. 2 needs eight switching NMOS transistors, eight bit lines, two word lines and one plate line for eight memory cells. The FeRAM according to present invention shown in FIG. 3 only needs four switching NMOS transistors, four bit lines, two word lines and two plate lines for eight memory cells. Therefore, the physical size of the FeRAM device according to the present invention is smaller than that of the conventional FeRAM device. 
     The read operation of the FeRAM device having memory cells according to the present invention will be described in detail with reference to FIG.  3 . 
     In order to read data stored in the ferroelectric capacitor C 31  in the cell group G 31 , the word line WL 0  coupled to the NMOS transistor N 31  is selected and activated, and the rest word lines WL 1  to WL 3  are remained as inactivated. A high level signal is applied to the plate line PLS 0  coupled to the gate of the NMOS transistor N 1 . Additionally, low signal is applied to the global plate line GPL coupled to the NMOS transistor N 1  through an inverter INV 30 . Then, the high level signal, Vcc, is applied to the plate line PL 0 . Therefore, the voltage of the bit line Bit 0  is changed according to the variation of the polarization state of the ferroelectric capacitor C 31  and the logic data “1” or “0” stored in the ferroelectric capacitor C 31  can be transmitted to the bit line Bit 0 . The data transmitted to the bit line Bit 0  can be read out by the sensing and amplifying operation. 
     In order to read data stored from one ferroelectric capacitor C 32  in the cell group G 31 , the word line WL 0  coupled to the NMOS transistor N 31  is selected and activated, and the rest word lines WL 1  to WL 3  are remained as inactivated. A high level signal is applied to the plate line PLS 1  connected to the gate of the NMOS transistor N 2 . A low signal is applied to the global plate line GPL coupled to the NMOS transistor N 2  through an inverter INV 30 . Then, the high level signal, Vcc, is applied to the plate line PL 1 . Therefore, the voltage of the bit line Bit 0  is changed according to the variation of the polarization state of the ferroelectric capacitor C 32  and the logic data “1” or “0” stored in the ferroelectric capacitor C 32  can be transmitted to the bit line Bit 0. 
     FIGS. 4A and 4B are schematic diagrams illustrating a charge sharing relation between a ferroelectric capacitor and a parasitic capacitor during reading operation. 
     Referring to FIGS. 3 and 4A, it is assumed that the word line WL 0  and the plate line PL 0  are selected to read out data stored in the ferroelectric capacitor C 31  coupled to the bit line capacitor Cb through a switching NMOS transistor N 31 . The ferroelectric capacitor C 31  is also coupled to the two ferroelectric capacitors C 32  and C 34  in series and a parasitic junction capacitance Cj of a switching NMOS transistor N 34 . The ferroelectric capacitor C 31  is also coupled to a ferroelectric capacitor C 33  through the two ferroelectric capacitors C 32  and C 34 . FIG. 4A is an equivalent circuit of all capacitors coupled to the ferroelectric capacitors C 31 . In FIG. 4A, the voltage of storage node S 1  varying with the voltage of the plate line PL 0  is equal to the voltage induced to the bit line Bit 0 . FIG. 4B is an equivalent circuit of FIG.  4 A. 
     In FIGS. 4A and 4B, Cf, Cb, and Cj denote an equivalent capacitance of ferroelectric capacitors, a parasitic capacitance of bit lines, and a parasitic junction capacitance of NMOS transistor at a turn-off state, respectively. One capacitance Cej is caused by the two ferroelectric capacitors C 32  and C 34  coupled in series and the parasitic junction capacitance Cj of the NMOS transistor N 34  at a turn-off state. The other capacitance Cej is also caused by the two ferroelectric capacitors C 32  and C 34  and the parasitic junction capacitance Cj of the NMOS transistor N 35  at turn-off state. 
     When it is assumed that each capacitance of the two ferroelectric capacitors C 32  and C 34  is equal to the capacitance Cf, the capacitance Cej is expressed by the following equation.              Cef   =       1       1   Cf     +     1   Cf     +     1   Cj         ≅   Cj             (     Eq   .              2     )                                
     where, Cf&gt;&gt;Cj 
     In Eq.  2 , the capacitance Cf amounts to hundreds of femto-farad and the capacitance Cj amounts to several femto-farad, therefore the capacitance Cej is approximately equal to the Cj. 
     In FIG. 4B, Ceb denotes the equivalent capacitance between the two ferroelectric capacitors C 32  and C 36  and the parasitic capacitance Cb of the bit line coupled in series as shown in FIG.  4 A. The Ceb can be expressed by the following equation.              Ceb   =       1       1   Cf     +     1   Cf     +     1   Cb         ≅   Cj             (     Eq   .              3     )                                
     where, Cb&gt;&gt;Cj. 
     In Eq.  3 , the capacitance Cb amounts to hundreds of femto-farad and assuming that the capacitance Cb is larger than the capacitance Cf, then the capacitance Ceb is approximately equal to the capacitance Cf. 
     The equivalent capacitance of ferroelectric capacitors C 32 , C 34  and C 33  coupled in series is Cf/3, the equivalent capacitance of the ferroelectric capacitors C 32 , C 36  and C 35  coupled in series is also Cf/3, and the equivalent capacitance of the ferroelectric capacitors C 32 , C 38  and C 37  coupled in series is also Cf/3. Therefore, FIG. 4A can be schematically represented as FIG.  4 B. 
     In FIG. 4B, the amount of voltage variation ΔV s  of storage node S 1  according to the amount of voltage variation ΔV p  of plate line can be expressed by the following equation.                Δ                   V   s       =           Cf   +       Cf   3     ×   3         Cf   +   Cb   +   Cf       ×   Δ                   V   p       =         2      Cf         2      Cf     +   Cb       ×   Δ                   V   p                 (     Eq   .              4     )                                
     Therefore, when the plate line is driven for the reading operation in the FeRAM device shown in FIG. 3, the voltage induced to the bit line can be obtained by the equation 3. 
     FIG. 5 is a circuit diagram illustrating a core part of the FeRAM in accordance with the present invention. As shown in FIG. 5, the FeRAM device according to the present invention includes a memory cell array  200 , a reference voltage generator  100 , a sense amplifier  300 , and a precharge part  400 . The reference voltage generator  100  generates a reference voltage, which is lager than the voltage corresponding to a logic data “0” and smaller than the voltage corresponding to a logic data “1”, to bit lines in the memory cell array  200 . The sense amplifier  300  senses and amplifies the data from the memory cell array  200  based on the reference voltage. The bit lines are precharged to the ground level by the precharge part  400  before the reading operation. 
     FIG. 6 is a timing diagram of the memory device shown in FIG.  5 . 
     Referring to FIGS. 5 and 6, the operation for reading data stored in the ferroelectric capacitor C 51  in a cell group G 51  including two memory cells will be described in detail. 
     In a standby state, a bit line precharge signal ISO of bit line precharge part  400  is activated in a high voltage level, so that the bit line Bit 0  and bit bar line Bitb 0  are precharged to a ground level. Next, a bit line precharge signal ISO is activated in a low voltage level, so that the bit line Bit 0  and bit bar line Bitb 0  are precharged to 0V. A word line WL 0  is activated to turn on a switching NMOS transistor N 51 , so that a voltage is applied to both terminal of the ferroelectric capacitor C 51 . In case where a low signal is applied to the global plate line GPL and a high level signal is applied to the plate line PL 0  to select the capacitor C 51 , the charge stored in the ferroelectric capacitor C 51  moves to the bit line Bit 0 , and the voltage of the bit line Bit 0  is varied. 
     Next, the switching NMOS transistors N 1  and N 2  are activated to read out a logic data “1” or “0” stored in each ferroelectric capacitor C 1  and C 2 . Then, a plate line control signal is applied to the plate line PL commonly coupled to the ferroelectric capacitors C 1  and C 2 . 
     That is, the ferroelectric capacitors C 1  and C 2  contained in reference voltage generator  100  store a logic data “0” or “1”, and the transistors N 101  and N 102  are turned on under the control of the activated word line RWL 1 . The reference bit bar line RBL and the reference bit bar line RBBL have charges induced by a ferroelectric capacitor C 1  and a ferroelectric capacitor C 2 , respectively. After equalizing a reference bit line RBL with reference bit bar line RBBL, the equalized reference voltage is transferred to one bit line pair through transfer transistor N 103  controlled by a control signal ref_add. At this time, the switching NMOS transistor N 104  controlled by a control signal ref_even in the reference voltage generator  100  is turned on in order to select a bit bar line Bitb 0  used as the reference voltage supply means. After changing the voltage of the bit line, a high voltage and a low voltage are respectively applied to control signal lines SAP and SNP in the sense amplifier  300 . Data stored in the ferroelectric capacitor C 51  can be read out by sensing and amplifying the voltage induced by the ferroelectric capacitor C 51  and the reference voltage from the voltage generator  100 . 
     Referring to FIG. 7, a FeRAM according to another embodiment of the present invention includes a plurality of cell groups G 71  to G 78  having adjacent four memory cells coupled to the same bit line and the same word line. Each of ferroelectric capacitors C 71  to C 74  in the cell group G 71  is commonly coupled to the same switching NMOS transistor N 71  and respectively to the different plate lines PL 0  to PL 4 . 
     A gate of the NMOS transistor N 71  is coupled to a word line WL 0 . One source/drain junction of the NMOS transistor N 71  is coupled to a bit line Bit 0  and the other source/drain junction of the NMOS transistor N 71  is coupled to the ferroelectric capacitors C 71  to C 74 . A first electrode of the each ferroelectric capacitors C 71  to C 74  is commonly coupled to the NMOS transistor N 71 , and a second electrode of the each ferroelectric capacitors C 71  to C 74  is coupled to the different plate lines PL 0  to PL 4 , respectively. Therefore, four memory cells in one cell group G 71  includes one switching NMOS transistor N 71  and four ferroelectric capacitors C 71  to C 74 . 
     The word lines WL 0  to WL 3  are perpendicular to the plate lines PL 0  to PL 3  and parallel to the bit lines Bit 0  to Bitb 3 . Each plate line is commonly coupled to ferroelectric capacitors in the different cell groups. 
     A read operation of the FeRAM device will be described in detail with reference to FIG.  7 . 
     In order to read data, stored in the ferroelectric capacitor C 71  of the cell group G 71 , the word line WLO connected to the NMOS transistor N 71  is selected and activated and the rest word lines WL 1 -WL 3  are remained as inactivated. And a high level signal, Vcc, is applied to the plate line PL 0  coupled to the ferroelectric capacitor C 71 . Therefore, the voltage of the bit line Bit 0  is changed by the variation of the polarization state of the ferroelectric capacitor C 71  and the logic data “1” or “0” can be transmitted to the bit line Bit 0 . The data transmitted to the bit line Bit 0  can be read out by the sensing and amplifying operation. 
     In order to read data stored in the ferroelectric capacitors C 72  to C 74 , the world line and plate lines coupled to the ferroelectric capacitors C 72  to C 74  are respectively selected. 
     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.