Abstract:
A voltage reference circuit is provided in the periphery of a memory array. Each subarray of the memory array is associated with a respective voltage driver circuit responsible for generating the cell plate and equilibrate reference voltage for the memory cells in the subarray. The voltage reference circuit is connected to and controls each voltage driver so that each driver generates the proper reference voltage. The distributed circuitry substantially reduces the amount of space used within the memory array while mitigating the problems of prior art voltage generator circuits.

Description:
This application is a continuation application of U.S. patent application Ser. No. 09/653,539, filed Aug. 31, 2000 now a U.S. Pat. No. 6,496,421. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of semiconductor memory devices and, more particularly to semiconductor memory devices having a distributed cell plate and/or digit line equilibrate voltage generator. 
     2. Description of the Related Art 
     FIG. 1 illustrates a portion of a dynamic random access memory (DRAM) device  300 . The DRAM  300  includes a plurality of dynamic memory cells  312 , a plurality of word lines  314  and a plurality of bit lines  316 . For convenience purposes, only two memory cells  312 , word lines  314  and bit lines  316  are illustrated in FIG.  1 . 
     The memory cells  312  are organized as an array of columns and rows. Each column typically includes numerous memory cell pairs, such as the single pair illustrated in FIG.  1 . Although not illustrated, a typical column may contain 1024 or 2048 pairs of memory cells  312 . Each memory cell  312  comprises a storage cell  320  (e.g., a capacitor) and an access device  322 , which is typically a metal oxide semiconductor field effect transistor (MOSFET). 
     Two supply voltages are usually required to operate and access a DRAM cell  312 . The first supply voltage is typically a ground and the second supply voltage is typically referred to as Vcc. A first side or cell plate of the storage cell  320  is connected to an intermediate cell plate reference voltage DVC2 having a potential between Vcc and ground. This cell plate reference voltage DVC2 is typically equal to Vcc/2, or the average of the first and second memory cell supply voltages. The cell plate reference voltage DVC2 is produced by a cell plate generator circuit (not shown). The first cell plates of all of the storage cells  320  are typically connected to the cell plate reference voltage DVC2. 
     A second side of each storage cell  320  is connected to one active terminal of an access device  322 . One of the bit lines  316  is connected to the other active terminal of the access device  322 . The gate or control terminal of the access device  322  is connected to one of the word lines  314 . Thus, each memory cell  312  is connected to a word line  314  and a bit line  316 . The word lines  314  and bit lines  316  form a two-dimensional array having a plurality of intersections. A single memory cell  312  corresponds to each intersection. At an intersection, a word line  314  is used to selectively activate the corresponding memory cell  312 . Activating the memory cell  312  connects its storage cell  320  to the corresponding bit line  316 , which allows conventional memory access operations (e.g., data read, data write and refresh) to occur. 
     The illustrated DRAM  300  also contains an equilibrate circuit  330 . The equilibrate circuit  330  includes two MOSFET transistors  332 ,  334 . One active terminal of each of each transistor  332 , 334  is connected to receive the cell plate reference voltage DVC2. The other active terminal of each transistor  332 ,  334  is connected to one of the adjacent bit lines  316 . The equilibrate circuit  330  is responsive to an equilibrate signal EQ to simultaneously connect the reference voltage DVC2 to the bit lines  316 . During normal memory access operations, the equilibrate signal EQ is activated to “precharge” the bit lines  316  to the reference voltage DVC2 prior to activating the corresponding access transistor  322  and accessing the memory cells  312 . 
     Typically, the first cell plate of each storage cell  320  is maintained at the non-varying cell plate reference voltage DVC2. The second cell plate is charged to either the first memory cell supply voltage or the second memory cell supply voltage, depending on whether a “0” or “1” is being written to the cell  320 . Data is read from the cells  312  of the DRAM  300  by activating a word line  314  (via a row decoder), which couples all of the memory cells  312  corresponding to that word line  314  to respective bit lines  316 , which define the columns of the array. One or more bit lines  316  are also activated. When a particular word line  314  is activated, sense amplifier circuitry connected to a bit line  316  detects and amplifies the data bit transferred from the storage cell  320  to its bit line  316  by measuring the potential difference between the activated bit line  316  and a reference line which may be an inactive bit line. The operation of typical DRAM sense amplifier circuitry is described, for example, in U.S. Pat. Nos. 5,627,785; 5,280,205; and 5,042,011, all assigned to Micron Technology Inc., and incorporated by reference herein. 
     While the DRAM  300  has proven to be a reliable architecture, it is not without its shortcomings. For example, the reference voltage DVC2 is generated by a centralized voltage generator circuit within the array of the DRAM  300 . If the array is divided into subarrays, then the DRAM may contain multiple voltage generator circuits. Reference voltage DVC2 lines are then fanned out to the components of the array/subarrays. The voltage generator circuit is relatively large and consumes precious space within the array. There is a desire and need to reduce the amount of space used by the voltage generator circuitry in the array of the DRAM  300 . 
     In addition, the reference voltage DVC2 generated by the voltage generator circuit may swell or experience dips in different portions of the DRAM  300 . That is, different sections of the memory array will have different voltage levels. This adversely effects the operation of the standard DRAM functions such as reads, writes and precharging. Accordingly, there is a desire and need to reduce the amount of reference voltage swells and dips experienced in today&#39;s DRAM arrays. 
     Another problem experienced by the conventional DRAM  300  is bit line coupling. With the current DRAM configuration, the cell plates of the storage cells  320  move, which couples noise onto the bit lines  316 . If there is too much movement, there will be too much noise on the bit lines  316 . Bit line coupling creates memory cell margin problems, and are a direct result of the current centralized voltage generator techniques. Accordingly, there is a desire and need for a DRAM having a voltage generator circuit that reduces bit line coupling within its arrays. 
     SUMMARY OF THE INVENTION 
     The present invention provides voltage generator circuitry that substantially reduces the amount of reference voltage swells and dips in a DRAM memory array. 
     The present invention further provides voltage generator circuitry that substantially reduces bit line coupling within a DRAM memory array. 
     The above and other features and advantages of the invention are achieved by providing a voltage reference circuit in the periphery of a memory array. Each subarray of the memory array is associated with a respective voltage driver circuit responsible for generating the cell plate and equilibrate reference voltage for the memory cells in the subarray. The voltage reference circuit is connected to and controls each voltage driver so that each driver generates the proper reference voltage. The distributed circuitry substantially reduces the amount of space used within the memory array while mitigating the problems of prior art voltage generator circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages and features of the invention will become more apparent from the detailed description of the preferred embodiments of the invention given below with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic diagram of a portion of a conventional dynamic random access memory device; 
     FIG. 2 illustrates a memory incorporating a distributed voltage generator constructed in accordance with an exemplary embodiment of the invention; 
     FIG. 3 is a schematic diagram illustrating a first exemplary voltage generator circuit constructed in accordance with an exemplary embodiment of the invention; 
     FIG. 4 is a schematic diagram illustrating a second exemplary voltage generator circuit constructed in accordance with another exemplary embodiment of the invention; and 
     FIG. 5 is a block diagram of a processor-based system utilizing a memory circuit constructed in accordance with an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 illustrates an exemplary DRAM device  10  incorporating distributed voltage generator circuitry constructed in accordance with an exemplary embodiment of the invention. The DRAM  10  includes a memory array  92 . In this example, the array  92  is divided into eight subarrays  90   a ,  90   b ,  90   c ,  90   d ,  90   e ,  90   f ,  90   g ,  90   h  (hereinafter collectively referred to as “subarrays  90 ”). It should be noted that the array  92  does not have to be subdivided to practice the present invention. Moreover, the number of subarrays  90  illustrated is but one example and is not important to practice the present invention. 
     Coupled to and associated with the subarrays  90  are eight reference voltage drivers  70   a ,  70   b ,  70   c ,  70   d ,  70   e ,  70   f ,  70   g ,  70   h  (hereinafter collectively referred to as “drivers  70 ”). The voltage drivers  70  output the cell plate and equilibrate reference voltage DVC2 and are connected to the cell plates and equilibrate circuits of the corresponding subarrays  90  in a manner illustrated in FIG.  1 . 
     A voltage reference circuit  20  is located in the periphery of the array  92 . The voltage reference circuit  20  is coupled to the drivers  70 . The voltage reference circuit  20  and the distributed voltage drivers  70  define the voltage generator of this exemplary embodiment of the invention. As will become apparent from the descriptions of FIGS. 3 and 4, the drivers  70  consist of only two transistors. By having a single voltage reference circuit  20  located in the periphery of the array  92  and very small drivers  70  within the array  92 , the voltage generator of the present invention substantially reduces the amount of space utilized within the array  92 . Only the voltage drivers  70  require space within the array  92 . This frees up some valuable die space that can be used for other applications or components of the DRAM  10 . Other advantages of the present invention will become apparent from the following description. 
     FIG. 3 is a schematic diagram illustrating first exemplary voltage generator circuitry  22  constructed in accordance with an exemplary embodiment of the invention. The voltage generator circuitry  22  includes a voltage reference circuit  20  and at least one voltage driver circuit  70 . As noted above, if the memory array is divided into subarrays, then the voltage generator circuitry  22  will contain at least one voltage driver circuit  70  for each subarray. The voltage reference circuit  20  contains a first voltage divider  30 , second voltage divider  40  and two diode-connected transistor  50 ,  52 . 
     The first voltage divider  30  contains four series connected divider transistors  32 ,  34 , 36 ,  38 . It is desired that the four transistors  32 ,  34 ,  36 ,  38  are p-channel MOSFETs  32 ,  34 ,  36 ,  38 . The four transistors  32 ,  34 ,  36 ,  38  of the first voltage divider  30  are connected between Vcc and a first node  54 . It is desirable that the four transistors  32 ,  34 ,  36 ,  38  of the first voltage divider  30  are long L MOSFET devices to reduce the amount of current flow in the voltage reference circuit  20 . 
     The second voltage divider  40  contains four series connected divider transistors  42 ,  44 ,  46 ,  48 . It is desired that the four transistors  42 ,  44 ,  46 ,  48  are n-channel MOSFETs  42 ,  44 ,  46 ,  48 . The four transistors  42 ,  44 ,  46 ,  48  of the second voltage divider  40  are connected between a ground potential and a second node  56 . It is desirable that the four transistors  42 ,  44 ,  46 ,  48  of the second voltage divider  40  are long L MOSFET devices to help reduce the amount of current flow in the voltage reference circuit  20 . 
     The first diode-connected transistor  50  is an n-channel MOSFET with its gate connected to its drain at the first node  54 . The second diode-connected transistor  52  is also an n-channel MOSFET with its gate connected to its drain and its source connected at the second node  56 . The first and second diode-connected transistors  50 ,  52  are sized to ensure that the first node  54  is always at a somewhat higher voltage than the second node  56 . 
     The voltage driver  70  contains two driver transistors  72 ,  74 . It is desirable that the first driver transistor  72  be an n-channel MOSFET and the second driver transistor  74  be a p-channel MOSFET. The first driver transistor  72  has its gate connected to the first node  54  of the voltage reference circuit  20  and the second driver transistor  74  has its gate connected to the second node  56  of the voltage reference circuit  20 . The drain of the first driver transistor  72  is connected to Vcc and its source is connected to the source of the second driver transistor  74  at a driver node  76 . The drain of the second driver transistor  74  is connected to a ground potential and its source is connected to the source of the first driver transistor  72  at the driver node  76 . The output at the node is the reference voltage DVC2. In operation, the two voltage dividers  30 ,  40  and their outputs at the first and second nodes  54 ,  56  control the generation of the reference voltage DVC2 by the driver circuit  70 . 
     The reference voltage DVC2 that is output at the driver node  76  is fed back to a third node  60  of the voltage reference circuit  20 . This feedback from the driver node  76  is used to regulate the reference voltage DVC2 in response to changing current conditions. The feedback of the reference voltage DVC2 is implemented by connecting the gates of the transistors  32 ,  34 ,  36 ,  38  of the first voltage divider  30  and the gates of the transistors  42 ,  44 ,  46 ,  48  of the second voltage divider  40  to the reference voltage DVC2 at the third node  60 . With this connection, a decrease in the voltage level of the reference voltage DVC2 decreases the effective resistance of the transistor  32 ,  34 ,  36 ,  38  of the first voltage divider  30  while increasing the effective resistance of the transistor  42 ,  44 ,  46 ,  48  of the second voltage divider  40 . This in turn increases the current supplying ability of the voltage reference  20  and driver  70  and raises the level of the reference voltage DVC2. Conversely, an increase in the voltage level of the reference voltage DVC2 increases the effective resistance of the transistor  32 ,  34 ,  36 ,  38  of the first voltage divider  30  while decreasing the effective resistance of the transistor  42 ,  44 ,  46 ,  48  of the second voltage divider  40 . This in turn decreases the current supplying ability of the voltage reference  20  and driver  70  and lowers the level of the reference voltage DVC2. Thus, the reference voltage DVC2 is regulated to compensate for changing current demands placed on the voltage generator circuitry  22 . 
     As noted above, the use of a single voltage reference circuit  20  located in the periphery of the array  92  (FIG. 2) and distributed drivers  70  within the array  92  reduces the amount of space used within the array  92  by the voltage generator  22  of the present embodiment. Moreover, by having only small transistors  72 ,  74  distributed within the drivers  70 , bit line coupling caused by cell plate movement is also substantially reduced. In addition, by providing distributed driver circuits  70  at each array/subarray, the amount of voltage swelling and dips experienced throughout the DRAM array should also be substantially reduced. Thus, the voltage generator  22  of the present embodiment exhibits several advantages over the prior art. 
     FIG. 4 is a schematic diagram illustrating a second exemplary voltage generator circuit  122  constructed in accordance with another exemplary embodiment of the invention. Wherever possible, reference numerals used to describe components and nodes of the voltage generator circuit  22  in FIG. 3 will be used to describe similar components and nodes in the voltage generator circuit  122  in FIG.  4 . 
     The voltage generator circuitry  122  includes a voltage reference circuit  120  and at least one voltage driver circuit  170 . As noted above, if the memory array is divided into subarrays, then the voltage generator circuitry  122  will contain at least one voltage driver circuit  170  for each subarray. The voltage reference circuit  120  contains a first voltage divider  130 , second voltage divider  140  and two diode-connected transistor  50 ,  52 . 
     The first voltage divider  130  contains four series connected divider transistors  32 ,  34 ,  36 ,  38 . It is desired that the four transistors  32 ,  34 ,  36 ,  38  are p-channel MOSFETs  32 ,  34 ,  36 ,  38 . The four transistors  32 ,  34 ,  36 ,  38  of the first voltage divider  130  are connected between Vcc and a first node  54 . It is desirable that the four transistors  32 ,  34 ,  36 ,  38  of the first voltage divider  130  are long L MOSFET devices to reduce the amount of current flow in the voltage reference circuit  120 . 
     The second voltage divider  140  contains four series connected divider transistors  42 ,  44 ,  46 ,  48 . It is desired that the four transistors  42 ,  44 ,  46 ,  48  are n-channel MOSFETs  42 ,  44 ,  46 ,  48 . The four transistors  42 ,  44 ,  46 ,  48  of the second voltage divider  140  are connected between a ground potential and a second node  56 . It is desirable that the four transistors  42 ,  44 ,  46 ,  48  of the second voltage divider  140  are long L MOSFET devices to help reduce the amount of current flow in the voltage reference circuit  120 . 
     The first diode-connected transistor  50  is an n-channel MOSFET with its gate connected to its drain at the first node  54 . The second diode-connected transistor  52  is also an n-channel MOSFET with its gate connected to its drain and its source connected at the second node  56 . The first and second diode-connected transistors  50 ,  52  are sized to ensure that the first node  54  is always at a somewhat higher voltage than the second node  56 . 
     The voltage driver  170  contains two driver transistors  72 ,  74 . It is desirable that the first driver transistor  72  be an n-channel MOSFET and the second driver transistor  74  be a p-channel MOSFET. The first driver transistor  72  has its gate connected to the first node  54  of the voltage reference circuit  120  and the second driver transistor  74  has its gate connected to the second node  56  of the voltage reference circuit  120 . The drain of the first driver transistor  72  is connected to Vcc and its source is connected to the source of the second driver transistor  74  at a driver node  76 . The drain of the second driver transistor  74  is connected to a ground potential and its source is connected to the source of the first driver transistor  72  at the driver node  76 . The output at the node is the reference voltage DVC2. In operation, the two voltage dividers  130 ,  140  and their outputs at the first and second nodes  54 ,  56  control the generation of the reference voltage DVC2 by the driver circuit  170 . 
     Unlike the voltage generator  22  illustrated in FIG. 1, the reference voltage DVC2 that is output from the voltage driver  170  is not fed back into the voltage reference circuit  120 . Instead, the gates of the transistors  32 ,  34 ,  36 ,  38  of the first voltage divider  130  are tied together and connected to a ground potential and the gates of the transistors  42 ,  44 ,  46 ,  48  of the second voltage divider  140  are tied together and connected to Vcc. With this configuration, the voltage generator  122  reduces the number of wires used to connect the voltage reference  120  to the voltage driver  170 . Thus, the amount of area used within the array by the voltage generator  122  is even further reduced. The tradeoff, however, is that the voltage generator  122  will not have a DVC2 feedback that can be used to compensate for changing current. 
     As noted above, the use of a single voltage reference circuit  120  located in the periphery of the array  92  (FIG. 2) and distributed drivers  170  within the array  92  reduces the amount of space used within the array  92  by the voltage generator  122  of the present embodiment. Moreover, by having only small transistors  72 ,  74  distributed within the drivers  170 , bit line coupling caused by cell plate movement is also substantially reduced. In addition, by providing distributed driver circuits  170  at each array/subarray, the amount of voltage swelling and dips experienced throughout the DRAM array should also be substantially reduced. Thus, the voltage generator  122  of the present embodiment exhibits several advantages over the prior art. 
     FIG. 5 is a block diagram of a processor-based system  200  utilizing a DRAM  212  constructed in accordance with one of the embodiments of the present invention. That is, the DRAM  212  utilizes the distributed voltage generators  22 ,  122  illustrated in FIGS. 3 and 4. The processor-based system  200  may be a computer system, a process control system or any other system employing a processor and associated memory. The system  200  includes a central processing unit (CPU)  202 , e.g., a microprocessor, that communicates with the DRAM  212  and an I/O device  208  over a bus  220 . It must be noted that the bus  220  may be a series of buses and bridges commonly used in a processor-based system, but for convenience purposes only, the bus  220  has been illustrated as a single bus. A second I/O device  210  is illustrated, but is not necessary to practice the invention. The processor-based system  200  also includes read-only memory (ROM)  214  and may include peripheral devices such as a floppy disk drive  204  and a compact disk (CD) ROM drive  206  that also communicates with the CPU  202  over the bus  220  as is well known in the art. 
     It should be appreciated that the distributed voltage generator circuits of the present invention can be used to supply the reference voltage DVC2 to individual arrays of the memory device, as well as the individual subarrays of each array, using a single voltage reference circuit. That is, a single voltage reference circuit can be located in the periphery of multiple arrays with connections to the distributed drivers associated with the arrays (and subarrays). 
     While the invention has been described in detail in connection with the exemplary embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.