Abstract:
A semiconductor memory device is provided that uses a single wordline to access both storage cells of a so-called twin cell. A memory device comprises a plurality of wordlines and a plurality of bitlines in an array, with a plurality of storage cells at certain intersections of wordlines and bitlines. A plurality of sense amplifiers are provided, each of which is connected to at least a first pair of bitlines to detect a voltage difference on the bitlines caused by the charge from a twin storage cell comprised of first and second storage cells at the intersection of a single wordline with said first pair of bitlines, respectively. As a result, each cell of a twin storage cell can be accessed with a single wordline.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention is directed to semiconductor devices, and more particularly to sense amplifier organization for semiconductor memory devices.  
         [0002]     In semiconductor memory devices, such as a dynamic random access memory (DRAM) device, sense amplifiers are used to read data from a memory storage cell by evaluating the charge in the storage cell.  FIG. 1  shows an exemplary prior art sense amplifier organization for a DRAM device. There are a plurality of wordlines (WLs)  10  and a plurality of bitlines (BLs)  20  that form a memory array  25 . At the intersection of certain WLs  10  and certain BLs  20  there are storage cells  30 . Sense amplifiers  40  are connected to the BLs  20  to detect a difference between the voltages on the two bitlines caused by the charge in the storage cells  30 .  FIG. 1  shows a folded bitline configuration, as an example.  
         [0003]     In a one-transistor one-capacitor memory device, each WL  10  activates the access transistor (not shown) of a single storage cell. In a so-called “twin cell” organization, two storage cells  30  are used to store a charge that together represent a single bit, e.g., a “1,0”=logic “0” and “0,1”=logic “1”. One way to realize the twin cell is to combine two standard storage cells. To do this in the memory structure shown in  FIG. 1 , it is necessary to activate two WLs in order to access each storage cell of the twin cell because the cells are on two different WLs, as shown by the exemplary twin cell pair at reference numeral  50 .  
         [0004]     It is desirable to reduce the number of wordlines that need to be activated and to reduce the silicon area required for sense amplifiers in a semiconductor memory device that uses the twin cell architecture.  
       SUMMARY OF THE INVENTION  
       [0005]     A semiconductor memory device is provided that uses a single wordline to access both storage cells of a so-called twin cell. The memory device comprises a plurality of wordlines and a plurality of bitlines in an array, with a plurality of storage cells at certain intersections of wordlines and bitlines. A plurality of sense amplifiers are provided, each of which is connected to at least a first pair of bitlines. The sense amplifiers provide a means to detect the voltage difference on the bitlines caused by the charge from a twin storage cell comprised of first and second storage cells at the intersections of a single wordline with said first pair of bitlines, respectively. As a result, each cell of a twin storage cell can be accessed with a single wordline. Moreover, the bitlines may be separated from other active bitlines by at least one bitline. In operation, inactive bitlines are maintained connected to a precharge potential, thereby shielding the active bitlines from any other active bitlines in the same pair or not, such as in an adjacent pair served by a different sense amplifier. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a block diagram showing a prior art sense amplifier organization.  
         [0007]      FIG. 2  is a block diagram according to a first embodiment of the invention.  
         [0008]      FIG. 3  is a block diagram according to a second embodiment of the invention.  
         [0009]      FIG. 4  is a block diagram according to a third embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0010]     With reference to  FIG. 2 , a first embodiment of the invention will be described. In the memory array  25 , there are wordlines (WLs)  10 ( 1 )- 10 ( 9 ) that intersect with bitlines (BLs)  20 ( 1 )- 20 ( 8 ), and at certain of these intersections there are storage cells  30 . One of the WLs, such as WL  10 ( 9 ) may be a redundant WL used to access redundant storage cells in the event the storage cells on one of the other WLs in the array fails. There is a plurality of sense amplifiers (SAs), four of which are shown as an example, at reference numerals  40 ( 1 ),  40 ( 2 ),  40 ( 3 ) and  40 ( 4 ). The SAs are arranged in first and second columns that are on opposite sides of the array. By reorganizing the way that the BLs  20  connect to the SAs, an SA can detect the voltage difference on the bitlines caused by the charge in first and second storage cells that make up a twin storage cell using a single WL. Examples of twin cells are shown at reference numerals  50 ( 1 ) and  50 ( 2 ) on WL  10 ( 1 ), and  52 ( 1 ) and  52 ( 2 ) on WL  10 ( 2 ).  
         [0011]     More specifically, the connections between BLs and SAs are made such that a unique pair of two non-adjacent (e.g., every other) BLs is connected to each SA. That is, the BLs in the pair are separated from each other by at least one other bitline. For example, the BL pair consisting of BLs  20 ( 1 ) and  20 ( 3 ) is connected to SA  40 ( 1 ), the BL pair consisting of BLs  20 ( 2 ) and  20 ( 4 ) is connected to SA  40 ( 3 ), the BL pair of BLs  20 ( 5 ) and  20 ( 7 ) is connected to SA  40 ( 2 ), and the BL pair consisting of BLs  20 ( 6 ) and  20 ( 8 ) is connected to SA  40 ( 4 ). Again, as shown in  FIG. 2 , with this connection arrangement, only a single WL needs to be activated to access each storage cell of a twin cell.  
         [0012]     In addition, because every second BL stays precharged rather than adjacent BLs, the active BLs are better shielded from each other thereby reducing coupling effects between them. More specifically, for each SA, there is circuitry provided (though not shown for simplicity), sometimes known as an equalizer circuit in the art, that connects a precharge voltage to a BL served by the SA while that BL is inactive. Thus, between each active BL there is interspersed at least one other BL that is not floating, but rather is maintained connected to the precharge voltage or potential while they are inactive. Consequently, these inactive BLs serve as a shield between the active BLs that are being sensed to prevent coupling between the active BLs so that the active BLs do not work against each other during the charge sensing process.  
         [0013]     Furthermore, this shielding from coupling extends to any other active BL in the same BL pair or not, e.g., between adjacent BL pairs. That is, bitlines which may be active concurrently are separated by at least one inactive bitline which is at the same time connected to a precharge potential. For example, the BL pair consisting of BLs  20 ( 1 ) and  20 ( 3 ) served by SA  40 ( 1 ) are separated by BL  20 ( 4 ) from the BL pair consisting of BLs  20 ( 5 ) and  20 ( 7 ) served by SA  40 ( 2 ), and SA 40 ( 1 ) and SA 40 ( 2 ) are not active concurrently. A similar relationship exists in the other SA column for the BL pair consisting of BLs  20 ( 2 ) and  20 ( 4 ) served by SA  40 ( 3 ) and the BL pair consisting of BLs  20 ( 6 ) and  20 ( 8 ) served by SA 40  ( 4 ).  
         [0014]     In the SA configuration of  FIG. 2 , all of the SAs in one of the columns in the array  25  are activated when a particular WL is activated. A first power supply signal PS 1  is connected by a wire in common to each SA in the left column and a second power supply signal PS 2  is connected by a wire in common to each SA in the right column. The SAs in the left column are connected to half of the BLs in the array and the SAs in the right column are connected to the remaining half of the BLs in the array. For example, when WL  10 ( 1 ),  10 ( 4 ),  10 ( 5 ),  10 ( 8 ) or  10 ( 9 ) is activated, a first power supply signal PS 1  goes active to activate the entire left hand column of SAs, and when WL  10 ( 2 ),  10 ( 3 ),  10 ( 6 ) or  10 ( 7 ) is activated, a second power supply signal PS 2  goes active to activate the entire right hand column of SAs.  
         [0015]     Turning to  FIG. 3 , a sense amplifier organization is shown according to a second embodiment. In this arrangement, the SAs are each connected to a pair of BLs (alternating BLs, similar to the embodiment of  FIG. 2 ), but are connected to the BLs so that when a given WL is activated, an entire column of SAs is not activated. The SAs in the left column are connected to half of the BLs in the array and the SAs in the right column are connected to the remaining half of the BLs in the array. However, for any activated WL, half of the SAs in the left hand column are activated and half of the SAs in the right hand column are activated. For example, when WL  10 ( 1 ),  10 ( 4 ),  10 ( 5 ),  10 ( 8 ) or  10 ( 9 ) is activated, SA  40 ( 3 ) in right column of SAs is activated and SA  40 ( 2 ) is activated in the left column of SAs. Similarly, when WL  10 ( 2 ),  10 ( 3 ),  10 ( 6 ) or  10 ( 7 ) is activated, SA  40 ( 1 ) is activated in the left column of SAs and SA  40 ( 4 ) is activated in the right column of SAs. The configuration shown in  FIG. 3  also has the shielding benefits afforded by interspersing inactive BLs connected to a precharge voltage between active BLs, as described above in conjunction with  FIG. 2 .  
         [0016]     The configuration shown in  FIG. 3  has the advantage that the power supply wire to the SA columns is split since only half of a column of SAs is activated for any activated WL. This is shown in  FIG. 3  where the first power supply signal PS 1  is routed by a wire in common to half of the SAs in each column, e.g., SA  40 ( 2 ) and SA  40 ( 3 ) and the second power supply signal PS 2  is routed by a wire to the remaining half of the SAs in each column, e.g., SA  40 ( 1 ) and  40 ( 4 ). This makes routing the power supply wires to the SAs easier and also reduces voltage drop in the wires.  
         [0017]      FIG. 4  shows still another configuration where the number of SAs in a column can be reduced by one-half. Since only half of the BLs are being used at any one time, the SAs can be multiplexed to pairs of BLs so that only half as many SAs are required, which leads to significant reduction in the size of a memory chip.  
         [0018]     Specifically, in the example shown in  FIG. 4 , there is a multiplexer (MUX)  60 ( 1 ) associated with SA  40 ( 1 ) and a MUX  60 ( 2 ) associated with SA  40 ( 2 ). MUX  60 ( 1 ) is connected to BLs  20 ( 1 )- 20 ( 4 ), and depending on which of the WLs is activated, is controlled to select either BL pair  20 ( 1 )/ 20 ( 3 ) or the BL pair  20 ( 2 )/ 20 ( 4 ) for connection to the SA  40 ( 1 ). Similarly, MUX  60 ( 2 ) is connected to BLs  20 ( 5 )- 20 ( 8 ), and depending on which of the WLs is activated, is controlled to select either BL pair  20 ( 5 )/ 20 ( 7 ) or BL pair  20 ( 6 )/ 20 ( 8 ) for connection to the SA  40 ( 2 ). A MUX control signal is connected to each MUX  60 ( 1 ) and  60 ( 2 ) to control which BL pair it connects to its associated SA. When WL  10 ( 1 ),  10 ( 4 ),  10 ( 5 ),  10 ( 8 ) or  10 ( 9 ) is activated, MUX  60 ( 1 ) is controlled to select BL pair  20 ( 1 )/ 20 ( 3 ) for connection to SA  40 ( 1 ) and MUX  60 ( 2 ) is controlled to select BL pair  20 ( 5 )/ 20 ( 7 ) for connection to SA  40 ( 2 ). When WL  10 ( 2 ),  10 ( 3 ),  10 ( 6 ) or  10 ( 7 ) is activated, MUX  60 ( 1 ) is controlled to select BL pair  20 ( 2 )/ 20 ( 4 ) for connection to SA  40 ( 1 ) and MUX  60 ( 2 ) is controlled to select BL pair  20 ( 6 )/ 20 ( 8 ) to SA  40 ( 2 ). The shielding benefits described above are also provided by the configuration shown in  FIG. 4 . In the case of the embodiment of  FIG. 4 , the equalization circuit that supplies the precharge voltage or potential to the inactive BLs would be included in the MUX&#39;s  60 ( 1 ) and  60 ( 2 ) or otherwise configured such that they are always connected to the inactive BLs regardless of which BL pair is connected to the associated SA.  
         [0019]     The foregoing description is made with respect to an array of eight WLs (plus one redundant WL) and eight BLs only for simplicity and only as an example. It should be understood that the concepts described herein are applicable to a memory array of any size (larger or smaller) with or without column (bitline) or row (wordline) redundancy. Moreover, while the foregoing discloses that the bitline pair which is connected to the sense amplifier is separated by one other bitline, it is possible that the bitline pair may be separated by more than one other bitline if the storage cell array configuration is adapted to suit such an arrangement. Thus, in general, each sense amplifier is connected to a unique pair of bitlines that are separated by at least one other bitline.  
         [0020]     The system and methods described herein may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative and not meant to be limiting.