Abstract:
A reference cell produces a reference current that is about half of the current produced by a memory cell. The reference cell is essentially the same as the memory cell with an additional current reduction device that can be a transistor. Adjusting a reference voltage applied to the transistor allows the current to be varied. A control circuit to produce the reference voltage includes dedicated memory and reference cells and a feedback circuit that compares the two cells&#39; currents. The feedback circuit applies the reference voltage to the reference cell of the control circuit and adjusts the reference voltage until the current from the reference cell is about half the current of the memory cell. The reference voltage is then applied to other reference cells in a memory array.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of copending U.S. patent application Ser. No. 10/100,705, filed Mar. 18, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of semiconductor capacitively coupled negative differential resistance (“NDR”) devices for data storage, and more particularly to reference cells to be used therewith. 
     2. Description of the Prior Art 
     U.S. Pat. No. 6,229,161 issued to Nemati et al., incorporated herein by reference in its entirety, discloses capacitively coupled NDR devices for use as SRAM memory cells. The cells disclosed by Nemati et al. are hereinafter referred to as thinly capacitively coupled thyristor (“TCCT”) based memory cells. FIG. 1 shows a pair of representative TCCT based memory cells  10  as disclosed by Nemati et al., and FIG. 2 shows a cross-section through one TCCT based memory cell  10  along the line  2 — 2 . FIG. 3 shows a schematic circuit diagram corresponding to the embodiment illustrated in FIGS. 1 and 2. The TCCT based memory cell  10  includes an NDR device  12  and a pass transistor  14 . A charge-plate or gate-like device  16  is disposed adjacent to, and in the case of the illustrated embodiment, surrounding, the NDR device  12 . A P+ region  18  of the NDR device  12  is connected to a metallization layer  20  so that a first voltage V 1 , such as V ddarray , can be applied to the NDR device  12  through the P+ region  18 . An N+ region of the NDR device  12  forms a storage node  22  that is connected to a source of the pass transistor  14 . Where the pass transistor  14  is a MOSFET, it can be characterized by a channel length, L, and a width, W, where L is the spacing between the source and the drain, and W is the width of the pass transistor  14  in the direction perpendicular to the page of the drawing in FIG.  2 . Assuming a constant applied voltage, a current passed by pass transistor  14  will scale proportionally to a ratio of W/L. 
     Successive TCCT based memory cells  10  are joined by three lines, a bit line  26 , a first word line (WL 1 )  28 , and a second word line (WL 2 )  30 . The bit line  26  connects a drain  32  of pass transistor  14  to successive TCCT based memory cells  10 . In a similar fashion, pass transistor  14  includes a gate  34  that forms a portion of the first word line  28 . Likewise, the gate-like device  16  forms a portion of the second word line  30 . 
     Memory arrays of the prior art typically include a large number of memory cells that are each configurable to be in either of two states, a logical “1” state or a logical “0” state. The memory cells are typically arranged in rows and columns and are connected to a grid of word lines and bit lines. In this way any specific memory cell can be written to by applying a signal to the appropriate word lines. Similarly, the state of a memory cell is typically manifested as a signal on one of the bit lines. In order to correctly interpret the state of the memory cell from the signal on the bit line, memory arrays of the prior art typically rely on some form of a reference signal against which the signal on the bit line is compared. 
     One type of memory array of the prior art uses SRAM cells for the memory cells. A conventional SRAM cell stores a voltage and includes two access ports, data and data-bar, where data-bar is a complementary signal to data and serves as a reference. A sensing circuit for the conventional SRAM cell compares the voltages of data and data-bar to determine whether the SRAM cell is storing a “1” or a “0.” 
     Another type of memory array of the prior art uses DRAM cells for the memory cells. A conventional DRAM cell is a capacitor and stores a charge to represent a logical state. When a DRAM cell is read it produces a voltage on a bit line. A typical reference cell for a DRAM memory array is a modified DRAM cell designed to store about half as much charge as the conventional DRAM cell. Accordingly, in a DRAM memory array the voltage produced by the DRAM cell is compared to the voltage produced by the reference cell to determine whether the DRAM cell is storing a “1” or a “0.” 
     In comparison to the conventional SRAM cell, a TCCT based memory cell  10  has only a single port, namely bit line  26 . In further comparison to both the SRAM and DRAM cells, the TCCT based memory cell  10  does not produce a voltage but instead produces a current. More specifically, TCCT based memory cell  10  has an “on” state wherein it generates a current that is received by bit line  26 . TCCT based memory cell  10  also has an “off” state wherein it produces essentially no current. Accordingly, voltage-based reference cells of the prior art are inadequate for determining the state of a TCCT based memory cell  10  and a new type of reference is needed. 
     A reference cell to be used in a memory array of TCCT based memory cells  10  should produce a reference current with an amount that is somewhere within the range defined by the currents generated by TCCT based memory cell  10  in the “on” and “off” states, and preferably about half the magnitude of the current generated by TCCT based memory cell  10  in the “on” state. It is well known, however, that the amount of current produced by TCCT based memory cell  10  varies as a function of temperature, variations in manufacturing, operating conditions (i.e., voltages), among other things. Therefore, what is desired is a reference cell capable of generating a reference current that will remain at a suitable magnitude such as about half the intensity of the current generated by a TCCT based memory cell  10  in the “on” state despite variations in manufacturing and operating conditions. 
     SUMMARY 
     A reference cell for a TCCT based memory cell includes an NDR device, a switch, and a current reduction element arranged together with a bit line and two word lines. The NDR device includes a doped semiconductor layer between first and second ends, the first end configured to have a first voltage applied thereto. The NDR device also includes a gate-like device disposed adjacent to the doped semiconductor layer. The switch is preferably a pass transistor that includes a source coupled to the second end of the NDR device, a drain, and a gate coupled to the first word line. The second word line is coupled to the gate-like device. The current its reduction clement is coupled between the bit line and the drain of the pass transistor. In some embodiments the current reduction element is a second pass transistor including a gate having a second voltage applied thereto. In these embodiments the reference cell produces an amount of current that is sufficient to be used as a reference. By applying an appropriate voltage to the second pass transistor, the second pass transistor can be made to have an appropriate resistance such that the desired current reduction is obtained. 
     These embodiments are advantageous in that a reference cell can be made to be in every respect the same as a TCCT based memory cell with the additional feature of a current reduction element. This way a reference current produced by the reference cell will be less than the amount of current produced by the TCCT based memory cell in the “on” state. In other embodiments the same advantages are achieved with an NDR device as described coupled to a single pass transistor. In these embodiments a voltage is applied to a gate of the single pass transistor such that it produces a resistance equal to the sum of the resistances of the first and second pass transistors in the previous embodiments. 
     Other embodiments of the invention are directed to a circuit for generating a reference voltage to control a current output of a reference cell. These embodiments allow the current output from a reference cell of the invention to be continuously maintained at any desired value, though preferably at about half of the amount of current produced by a TCCT based memory cell. The circuit to generate a reference voltage includes a TCCT based memory cell to produce a first current, a pair of reference cells as described above, each producing a current, and a feedback circuit. In these embodiments the reference cell produces the reference voltage from the feedback circuit which varies the reference current as a function of the difference between the first current and the sum of the two currents from the reference cells. The generated reference voltage is also applied to the second pass transistors to provide feedback to the two reference cells. 
     In specific embodiments the reference voltage is adjusted so that each reference cell produces a current equal to half of the current produced by the TCCT based memory cell. These embodiments can be advantageously used to apply the same reference voltage to a pass transistor in another reference cell outside of the circuit so that it will also produce a current equal to half of the current produced by the memory cell. 
     Other embodiments of the invention are directed to a memory array including a TCCT based memory cell coupled to a first bit line, a reference cell coupled to a second bit line, and means for determining a state of the TCCT based memory cell by comparing a first current on the first bit line and a second current on the second bit line. Still other embodiments of the memory array further include a circuit to generate a reference voltage to control a current output of a reference cell, as described above. 
     Still other embodiments are directed to a method of producing a reference current against which a current from a TCCT based memory cell can be compared. In these embodiments a reference cell and a circuit to produce a reference voltage are both provided. The reference cell includes an NDR device configured to produce a current and a pass transistor connected to the NDR device. The circuit is configured to produce a reference voltage that is applied to the gate of the pass transistor. In this way a current produced by the NDR device is reduced by the resistance of the pass transistor so that a reference current is obtained. The degree to which the current produced by the NDR device is reduced is determined by the magnitude of the reference voltage applied to the gate of the pass transistor. 
     Yet other embodiments are directed to a method for reading a state of a TCCT based memory cell. In these embodiments the method includes operating the TCCT based memory cell to produce a first current on a first bit line, operating a reference cell to produce a second current on a second bit line, operating a circuit to provide a reference voltage to the reference cell, and comparing the first and second currents. Operating the TCCT based memory cell includes both applying a voltage to one end of the TCCT based memory cell to generate a current, and applying another voltage to a gate of a pass transistor to connect the TCCT based memory cell to the first bit line. The reference cell is similarly operated. The circuit is operated by operating a circuit memory cell and a circuit reference cell. The circuit memory cell is a dedicated TCCT based memory cell that is not used for memory purposes; instead it is used to produce a current that is representative of the current produced by other TCCT based memory cells in an array. The circuit reference cell is also dedicated to the circuit and likewise is used to produce a current that is representative of the current produced by other reference cells in the array. A feedback circuit is configured to receive the currents produced by the circuit memory cell and the circuit reference cell, provide a reference voltage to the circuit reference cell to controls the current output of the circuit reference cell, and to adjust the reference voltage until the current from the circuit reference cell is about half of the current from the circuit memory cell. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings where like reference numerals frequently refer to similar elements and in which: 
     FIG. 1 shows a TCCT based memory cell of the prior art; 
     FIG. 2 shows a cross-section of the TCCT based memory cell of FIG. 1; 
     FIG. 3 shows a schematic circuit diagram of the TCCT based memory cell of FIG. 1; 
     FIG. 4 shows a schematic circuit diagram of an exemplary reference cell of a specific embodiment the invention; 
     FIG. 5 shows a schematic circuit diagram of another example of a reference cell in accordance with another embodiment of the invention; 
     FIG. 6A shows a schematic circuit diagram of an exemplary NDR based reference voltage generator circuit according to an embodiment of the invention; 
     FIG. 6B shows a schematic circuit diagram of an exemplary SRAM based reference voltage generator circuit according to an embodiment of the invention; 
     FIG. 6C shows a schematic circuit diagram of an exemplary MRAM based reference voltage generator circuit according to an embodiment of the invention; 
     FIG. 6D shows a schematic circuit diagram of an exemplary flash memory based reference voltage generator circuit according to an embodiment of the invention; 
     FIG. 7 shows a schematic circuit diagram of another example of a reference voltage generator circuit of another embodiment of the present invention; 
     FIG. 8 shows a block diagram illustrating an example of a feedback circuit according to an embodiment of the invention; 
     FIG. 9 shows a schematic circuit diagram of an example of a current comparator of the invention; 
     FIG. 10 shows a schematic circuit diagram of an example of a ramp output voltage generator of the invention; 
     FIG. 11 shows a memory array including an exemplary reference cell for each bit line in accordance with a specific embodiment; 
     FIG. 12 shows another memory array including another example of a reference cell for each bit line in accordance with another embodiment; 
     FIG. 13 shows a schematic circuit diagram of an exemplary NDR based reference voltage generator circuit according to another embodiment of the invention; and 
     FIG. 14 shows a schematic circuit diagram of another exemplary memory array  220  of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4 shows a schematic circuit diagram of an exemplary reference cell  40  in accordance to a specific embodiment of the invention. As in the TCCT based memory cell  10  (FIG.  1 ), the reference cell  40  includes an NDR device  42  having a first end connected to a source of a pass transistor  44 . A gate-like device  46  is disposed adjacent to the NDR device  42 . A first word line  48  is connected to a gate of the pass transistor  44 , a second word line  50  is connected to the gate-like device  46 , and a first voltage V 1 , such as V ddarray , can be applied to the NDR device  42  at a second end. 
     Reference cell  40  also includes a current reduction element  52  connected between a drain of the pass transistor  44  and a bit line  54 . The current reduction element  52  prevents a certain amount of a current produced by the NDR device  42  from reaching the bit line  54 . In a specific embodiment, the current reduction element  52  reduces the current reaching the bit line  54  by a predetermined amount such as about ½. Current reduction element  52  can take many forms, the simplest of which is a resistor having an appropriate resistance. In other embodiments, current reduction element  52  is a transistor and the appropriate resistance is produced by adjusting a gate length. In, a similar fashion, instead of adding a separate element as the current reduction element  52 , the function is added to pass transistor  44  by providing it with a longer gate length than a pass transistor  14  (FIG.  1 ). Another method for reducing the current reaching the bit line  54  is to vary aspects of the NDR device  42  in such a way as to decrease its current output when in a low resistance (“on”) state, for example by providing the NDR device  42  with a narrower gate width. Each of these reference cell  40  embodiments is capable of producing a reference current, however, none effectively produce a reference current that varies proportionally with a current from a TCCT based memory cell  10  (FIG. 1) as temperature is varied so that the desired ½ ratio is maintained. Manufacturing variability over each process corner can also make it difficult to produce the desired ½ ratio in these embodiments. In another example, current reduction element  52  has a variable resistance so that the desired current can be maintained on the bit line  54  by increasing as well as decreasing the resistance of current, reduction element  52 . 
     FIG. 5 shows a second pass transistor  56  serving to reduce the current from the NDR device  42 . The second pass transistor  56  is controlled by a variable reference voltage V REF    58 . A feedback loop monitoring the current on the bit line  54  can be used to continuously adjust the reference voltage  58  to adjust the resistance of the second pass transistor  56 . 
     FIG. 6A shows a schematic circuit diagram of an exemplary reference voltage generator circuit  60  including a TCCT based memory cell  62  and two reference cells  64  and  66 . All three cells  62 ,  64 , and  66  are connected to a common line  68  carrying a first voltage V 1  and to common first and second word lines  70  and  72 , as shown. Accordingly, all three cells  62 ,  64 , and  66  operate in parallel such that all three produce current at the same time. The TCCT based memory cell  62  produces a first current I 1  and the two reference cells  64  and  66  produce second and third currents I 2  and I 3 , respectively. 
     The reference voltage generator circuit  60  also includes a feedback circuit  74 . The feedback circuit  74  is configured to receive two inputs, I 1  from the TCCT based memory cell  62  and the summed currents I 2  and I 3  from reference cells  64  and  66 . Ideally, I 2  and I 3  should always be the same as reference cells  64  and  66  are fabricated to be the same and are operated by the same voltages. The feedback circuit  74  is also configured to output a variable reference voltage V REF    76 . The variable reference voltage V REF    76  is configured to be applied to the second pass transistors  78  and  80 . It can be seen that as variable reference voltage V REF    76  is varied the resistances of second pass transistors  78  and  80  also vary and that the currents I 2  and I 3  also vary. It can further be seen that the feedback circuit  74  can therefore continually adjust the variable reference voltage V REF    76  so that I 2 +I 3  is maintained to be equal to I 1 . Provided that I 2  equals I 3 , when I 2 +I 3 =I 1  then each of I 2  and I 3  is equal to ½I 1 . 
     It will be understood that the embodiment shown in FIG. 6A is but one specific embodiment. In another embodiment, two or more TCCT based memory cells  62  are employed and their output currents are summed before entering the feedback circuit  74 . In this embodiment, for each additional TCCT based memory cell  62  two more reference cells  64  and  66  are also added. For example, where 3 TCCT based memory cells  62  are employed, the outputs of  6  reference cells would be summed as the second input to the feedback circuit  74 . While this embodiment requires more devices and uses more space on a die, it has the advantage that the variable reference voltage V REF    76  is the product of an averaging over many cells and is therefore less sensitive to minor variations between the cells. In still other embodiments different ratios of reference cells to TCCT based memory cells  62  are employed. For example, 4 reference cells to one TCCT based memory cell  62  would yield a variable reference voltage V REF    76  that when applied to a reference cell would cause the reference cell to produce a current equal to ¼I 1 . Other examples can be readily envisioned by one having ordinary skill in the art. 
     It will also be understood that although the embodiments shown in the various drawings such as FIG. 6A are specific to NDR devices and TCCT based memory cells, the invention is more broadly applicable to any memory device that produces a variable current depending on a stored state. As an example, FIG. 6B illustrates another embodiment of a reference voltage generator circuit  81  in which the NDR devices have been replaced with SRAM cells  83 . Similarly, FIGS. 6C and 6D illustrate additional embodiments of a reference voltage generator circuit  85 ,  89  in which the NDR devices have been replaced either with MRAM cells  87  or memory cells with floating gates such as flash memory cells  91 . It will be further apparent that in the present invention it is possible to use a combination of different current-producing memory devices. For example, in FIG. 6A the reference cells  64  and  66  can be made with SRAM cells  83  as in FIG. 6B, while the memory cell  62  can include an NDR device as shown. 
     In yet another embodiment, the first pass transistors of the reference cells are removed, as shown in FIG.  7 . Instead, second pass transistors  82  and  84  are made to each have a resistance greater than the resistances of second pass transistors  78  and  80  (FIG. 6A) by the additional resistance of the pass transistor  44  (FIG.  4 ). Second pass transistors  82  and  84  can be made to have the additional resistance, for example, by operating at a variable reference voltage V REF    86  that is higher than the variable reference voltage V REF    76  (FIG.  6 A). The additional resistance can also be obtained by adjusting a gate length of each of the second pass transistors  82  and  84 . In another embodiment, the pass transistor associated with WL 1  and I 1  is optional and is absent from the circuit depicted in FIG.  7 . 
     Referring back to FIG. 5, it will be apparent that the variable reference voltage V REF    76  can also be applied to the second pass transistor  56  of a reference cell outside of the reference voltage generator circuit  60  to generate a current on bit line  54  equal to ½I 1 . Because the variable reference voltage V REF    76  of FIG. 7 is variable, as conditions such as temperature change causing the current I 1  to change, the feedback circuit  74  can continually adjust the variable reference voltage V REF    76  so that the currents I 2  and I 3  each remain equal to ½I 1 . Similarly, the current on bit line  54  of FIG. 5 will also be adjusted to remain equal to ½I 1  as the conditions vary, provided that the conditions vary uniformly over the reference voltage generator circuit  60  and the outside reference cell which could be, for example, on a different part of the same die. In some embodiments, to increase the ratio of memory cells to reference cells in order to increase the overall density of memory cells on a die, a single reference cell will be located in a central location such as next to a sense amplifier configured to compare an output current from the reference cell to an output current from any of the memory cells. 
     FIG. 8 is a block diagram illustrating one possible feedback circuit  88  including a current comparator  90  and a ramp output voltage generator  92  in accordance with a specific embodiment of the invention. The current comparator  90  continuously monitors the first current I 1  and the sum of currents I 2  and I 3 . If the sum of currents I 2  and I 3  is greater than I 1  the current comparator  90  signals the ramp output voltage generator  92  to be in an active state in which it progressively decreases the voltage of variable reference voltage V REF    76 . Decreasing the variable reference voltage V REF    76  will, in turn, decrease the summation of currents I 2  and I 3 . Once the sum of currents I 2  and I 3  equals or falls just slightly below the first current I 1  the current comparator  90  signals the ramp output voltage generator  92  to be in an inactive state in which the voltage of variable reference voltage V REF    76  is held constant. In another embodiment, once the sum of currents I 2  and I 3  equals or falls just slightly below the first current I 1  the current comparator  90  signals the ramp output voltage generator  92  to be in an active state in which it progressively increases the voltage of variable reference voltage V REF    76 . Increasing the variable reference voltage V REF    76  will, in turn, increase the summation of currents I 2  and I 3  until the summed currents equal the first current I 1 . One having ordinary skill in the art should appreciate that the feedback circuit can operate to ramp up or down the reference voltage to properly set the reference current. 
     FIG. 9 shows a schematic circuit diagram of an exemplary current comparator  90 . Although the particular embodiment shown in FIG. 9 operates on an appropriate duty cycle to periodically compare the first current I 1  with the sum of currents I 2  and I 3 , it will be understood that a current comparator  90  can also operate with continuous sampling. In the exemplary current comparator depicted in FIG. 9, MOSFET devices M 1 , M 2 , M 3  and M 4  form a CMOS cross-coupled latch operating as a high gain positive feedback amplifier where such configuration is well known in the art. MOSFET devices M 5  and M 6  are biased in their linear regions and provide for a low-impedance clamp between the input currents and a common potential, such as ground. The current comparator operates in two phases: (1) a pre-charge phase and (2) a sensing phase. In the pre-charge phase, the pre-charge signal is high and the sense signal is low. Device M 7  and M 8  are activated and thus equalize the potentials of devices M 2  and M 4  (i.e., logic low or ground). Therefore, the voltage at node A is driven to be equal to node B (i.e., V A  equals V B ). In an alternate embodiment, the geometric ratios and sizes of devices M 3  and M 4  are designed to be different than devices M 1  and M 2  so that the point at which a current difference triggers a difference in voltages at nodes A and B is optimized. One having ordinary skill in the art should appreciate how to implement such design considerations by configuring the appropriate device size. 
     During the sensing phase, the pre-charge signal is low and the sense signal is high. Currents I 1  and the sum of currents I 2  and I 3  flow into devices M 5  and M 6 , respectively. Differences between currents I 1  and the sum of currents I 2  and I 3  generates a difference in between currents I A  and I B , which in turn leads to a difference in voltages between nodes A and B. For example, if the sum of currents I 2  and I 3  is greater than current I 1 , then the capacitor C ref  will contain more charge over time (i.e., discharges slower) than C 1 . With C ref  having more charge over time than C 1 , the voltage at node B is shifted to a higher potential than node A. 
     As the voltage at node B increases and approaches a higher potential (e.g., V dd ), the degree in which device M 2  is turned on also increases. When M 2  is turned on, node A reaches a potential of about zero volts while conversely node B increases to high potential, such as V dd , as device M 3  increasingly turns on. Therefore, if the sum of currents I 2  and I 3  is greater than current I 1 , node B will be driven high and that state will be latched into the latch as V cnt . Otherwise, if the sum of currents I 2  and I 3  is less than current I 1 , node B will be driven low and that state will be latched into the latch as V cnt . 
     FIG. 10 shows a schematic circuit diagram of but one possible embodiment of a ramp output voltage generator  92  according to the present invention. Devices M 11  and M 13  and devices M 12  and M 16  form current mirrors designed so that I 13  mirrors I 11  and I 16  Mirrors I 12 . Currents I 12  and I 11  are generated by constant current sources as are known in the art. Devices M 14  and M 15  operate as switches to either charge or discharge the capacitor at the positive input of the amplifier. For example, if node B latches V cnt  at a high level, device M 15  will be turned on, thus discharging the capacitor. In turn, the voltage difference between the amplifier inputs will be as such as to decrease the variable reference voltage V ref . In particular, when the sum of currents I 2  and I 3  is greater than current I 1 , V ref  will decrease to reduce the sum of currents I 2  and I 3  until the sum is substantially equivalent to current I 1 . The opposite actions occur when node B latches V cnt  at a low level and turns on device M 14  to charge the capacitor. One having ordinary skill in the art should appreciate how to adapt and to modify the exemplary circuits shown in FIGS. 9 and 10 to practice the present invention. 
     FIG. 11 shows a memory array  110  including a plurality of TCCT based memory cells arranged by rows and columns. Each row includes a series of TCCT based memory cells and a reference cell sharing a common bit line. During a read operation a TCCT based memory cell in a first row produces a current on a first bit line while a reference cell in another row produces a reference current on another bit line. A controller (not shown) contains logic required to select individual TCCT based memory cells and to select a reference cell on a different bit line. The two currents on the two bit lines are then compared, for example, at a sense amplifier to determine the state of the TCCT based memory cell. In other embodiments, the memory array includes a single reference cell near the sense amplifier instead of devoting space to a reference cell on each row in the memory array  110 . In other embodiments a reference cell is placed on every n th  row. Many other variations will be readily apparent to one having ordinary skill in the art. 
     FIG. 12 shows a memory array  120  that is similar to memory array  110 . Memory array  120  differs from memory array  110  only in that the reference cells are of the embodiment used in the circuit shown in FIG.  7 . It will be appreciated that the exemplary reference cell circuit shown in FIG. 7 is compatible in use with memory array  120  shown in FIG.  12 . Similarly, the exemplary reference cell circuit shown in FIG. 6A is likewise compatible in use with memory array  110  shown in FIG.  11 . 
     Referring again to FIG. 6A, it will be appreciated that although the invention has been described in terms of NDR devices, the reference voltage generator circuit  60  would still work if the NDR device and its nearest pass transistor in the TCCT based memory cell  62  were replaced with some other current-producing memory device. Likewise, the NDR device and its nearest pass transistor in the two reference cells  64  and  66  can also be replaced with some other current-producing memory device. A reference voltage generator circuit  60  is also useable with a memory array  110  where the TCCT based memory cells are replaced with another current-producing memory device. 
     FIG. 13 is a schematic circuit diagram of another exemplary reference voltage generator circuit  200  of the invention. The reference voltage generator circuit  200  including a memory cell  202  and two reference cells  204  and  206 . The memory cell  202  is coupled between a first line  208  at a first voltage V 1  and a feedback circuit  210 , as shown. The memory cell  202  is configured to produce a first current I 1  that is received by the feedback circuit  210 . In FIG. 13 the memory cell  202  is shown as a TCCT based memory cell including an INDR device  212  coupled to a pass gate  214 , however, just as in FIGS. 6B,  6 C, and  6 D, the memory cell  202  can be of another type such as SRAM or MRAM, or can be a memory cell with a floating gate such as a flash memory cell. 
     Reference cells  204  and  206  are coupled in parallel between feedback circuit  210  and a second line  216  coupled to an output node  218  of feedback circuit  210 . Reference cell  204  is configured to produce a second current I 2  and reference cell  206  is configured to produce a third current I 3 , where both currents are received at the feedback circuit  210 . The second and third currents can either be combined on a common line  220  as shown, or can be summed (i.e. combined) at the feedback circuit  210 . In FIG. 13 the reference cells  204 ,  206  are shown as TCCT based reference cells, however, the invention will also work with other types of reference cells such as SRAM or SAM, or a memory cell with a floating gate such as a flash memory cell. 
     The feedback circuit  210  operates as described above with reference to FIG. 6A to produce a reference voltage V ref  at output node  218  by comparing the first current I 1  against the sum of the second current I 2  and the third current I 3 . When the sum of the second current I 2  and the third current I 3  is less than the first current I 1  the reference voltage V ref  is increased. By increasing the reference voltage V ref , the voltage applied to the reference cells  204  and  206  is also increased. By increasing the voltage applied to the reference cells  204  and  206  both will produce more current until the sun of the second current I 2  and the third current I 3  is approximately equal to the first current I 1 . Similarly, if the sum of the second current I 2  and the third current I 3  is more than the first current I 1  the reference voltage V ref  is decreased by the feedback circuit  210  until the sum of the second current I 2  and the third current I 3  is approximately equal to the first current I 1 . 
     Another exemplary reference voltage generator circuit includes reference cell  204  but omits reference cell  206 . In this embodiment the feedback circuit  210  compares the second current I 2  to the first current I 1  and generates a reference voltage in response thereto. Here, the feedback circuit increases the reference voltage V ref  when the second current I 2  is less than about half of the first current Is and decreases the reference voltage V ref  when the second current I 2  is greater than about half of the first current I 1 . Alternatingly, reference cells  204 ,  206  can be accompanied by one or more additional reference cells in parallel similar to the reference cells described in connection with FIG.  6 A. 
     FIG. 14 is a schematic circuit diagram of a representation of a portion of another exemplary memory array  220  of the invention. Memory array  220  includes a memory cell  222  coupled to a first bit line  224 , and a reference voltage generator circuit  226  coupled to a reference cell  228  that is in turn coupled to a second bit line  230 . A suitable reference voltage generator  226  for practicing the present invention is generator  200  shown in FIG.  13 . The first and second bit lines  224  and  230  are each coupled to a sense amplifier  232 . 
     In operation, a common voltage is applied to both the memory cell  222  and the reference voltage generator circuit  226 , and the reference voltage generator circuit  226  outputs a reference voltage V ref  that is applied to the reference cell  228 . The reference cell  228  produces a reference current I ref  that is supplied to the sense amplifier  232  by the second bit line  230 . The memory cell  222  produces a memory current I mem  that is supplied to the sense amplifier  232  by the first bit line  230 . The memory current I mem  is variable (i.e., has different current magnitudes to represent different logical states) and will be either higher or lower that the reference current I ref  depending on a logical state stored in the memory cell  222 . Accordingly, the sense amplifier  232  determines the logical state stored in the memory cell  222  by determining whether the memory current I mem  is higher or lower than the reference current I ref  and outputs the result as a data signal on line  234 . 
     In the foregoing specification, the invention is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the invention is not limited thereto. For example, the pass gates described above to generate a reference current can include a PMOS gate using a TCCT based memory cell with its cathode coupled to a V dd  array. As another example, although the preceding discussion describes generating a reference current at one-half the current to be read, it is also within the scope of the present invention to generate a reference at any level proportionate to the TCCT based memory cell current. Various features and aspects of the above-described invention may be used individually or jointly. Further, the invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive.