Abstract:
A sense amplifier places a low positive voltage, such as 0.1 to 0.3 volts, on a bit line instead of ground when a memory cell is read by utilizing a current source circuit to output a reference current that biases a Schottky diode. The current source circuit is implemented with a Schottky diode that utilizes the reverse-biased leakage current of the diode to form the reference current. The current source circuit can also be implemented with a current mirror circuit.

Description:
This is a divisional of application Ser. No. 09/320,413, filed May 26, 1999, now U.S. Pat. No. 6,122,204. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to sense amplifiers and, more particularly, to a sense amplifier that has a bias circuit with a reduced size. 
     2. Description of the Related Art 
     A dynamic random access memory (DRAM) cell is a memory device that retains data stored in the cell for only a short period of time even when power is continuously applied to the cell. As a result, a DRAM cell must be periodically refreshed to maintain the data stored in the cell. 
     FIG. 1 shows a cross-sectional diagram that illustrates a conventional DRAM cell  100 . As shown in FIG. 1, DRAM cell  100  includes an access transistor  102  which is formed in a p-type material  110 , and a capacitor  104  which is connected to transistor  102 . 
     Access transistor  102 , in turn, includes spaced-apart source and drain regions  112  and  114  which are formed in material  110 , and a channel region  116  which is defined between regions  112  and  114 . In addition, transistor  102  also includes an access gate  120  which is insulatively formed over channel region  116 . 
     As further shown in FIG. 1, capacitor  104  includes a lower plate  124  which is connected to drain region  114 , a dielectric layer  126  which is formed over lower plate  124 , and an upper plate  128  which is formed over dielectric layer  126 . 
     In operation, a logic “one” is written to DRAM cell  100  by first placing a programming voltage, such as five volts, on source region  112  while a storage voltage, such as five volts, is applied to the top plate  128  of capacitor  104  and ground is applied to material  110 . The storage voltage (which is continuously applied to top plate  128 ) attracts electrons to the lower plate  124  of capacitor  104  where the electrons begin to accumulate. 
     After placing a programming voltage on source region  112 , access gate  120  is pulsed with an access voltage. This pulse turns on access transistor  102  which causes the electrons on the lower plate  124  of capacitor  104  to flow to source region  112 . 
     The electrons flow from the lower plate  124  of capacitor  104  to source region  112  because the lower plate  124  of capacitor  104  has a potential which is less than five volts (some cf the applied voltage is dropped across dielectric layer  126 ), while source region  112  is at five volts. 
     When the trailing edge of the pulse again turns off access transistor  102 , a positive potential is stored on the lower plate  124  of capacitor  104  due to the decreased number of electrons which are present on the lower plate  124  of capacitor  104 . 
     This positive potential, however, lasts only a short time because electrons from leakage currents are readily attracted to the positive potential. As a result, the positive charge stored on the lower plate  124  of capacitor  104  must be “refreshed” by periodically removing the electrons from the lower plate  124  of capacitor  104 . 
     DRAM cell  100  is erased (a logic “zero” is written to a DRAM cell which already has a logic “one” stored in the cell) by placing ground on source region  112 . Once ground has been applied to source region  112 , access gate  120  is again pulsed with the access voltage. 
     This pulse turns on access transistor  102  which causes the electrons in source region  112  to flow to the lower plate  124  of capacitor  104 . The electrons flow from source region  112  to the lower plate  124  of capacitor  104  because the lower plate  124  of capacitor  104  has a greater potential than source region  112 . 
     When the trailing edge of the pulse again turns off access transistor  102 , the positive potential stored on the lower plate  124  of capacitor  104  is removed due to the increased number of electrons which are again present on the lower plate  124  of capacitor  104 . 
     Due to the overhead required to refresh DRAM cells, large numbers of DRAM cells like cell  100  are typically grouped together to form a memory array. FIG. 2 shows a schematic diagram that illustrates a conventional DRAM array  200 . 
     As shown in FIG. 2, DRAM array  200  includes a plurality of DRAM cells  100  which are formed in rows and columns in two segments S 1  and S 2 . As further shown in FIG. 2, array  200  also includes a plurality of first bit lines BL 1 -BLm and a plurality of second bit lines BLC 1 -BLCm. 
     The first bit lines BL 1 -BLm are formed adjacent to the columns of cells in first segment S 1  so that each bit line BL is connected to all of the source regions  112  in a column of cells. Similarly, the second bit lines BLC 1 -BLCm are formed adjacent to the columns of cells in second segment S 2  so that each bit line BLC is connected to all of the source regions  112  in a column of cells. 
     Array  200  further includes a plurality of first word lines WL 1 -WLn and a plurality of second word lines WLC 1 -WLCn. The first word lines WL 1 -WLn are formed adjacent to the rows of cells in first segment S 1  so that each word line WL is connected to all of the access gates  120  in a row of cells. Similarly, the second word lines WL 1 -WLn are formed adjacent to the rows of cells in second segment S 2  so that each word line WLC is connected to all of the access gates  120  in a row of cells. 
     As additionally shown in FIG. 2, array  200  includes a sense circuit  210  which has a plurality of sense amplifiers SA 1 -SAm that are connected to the bit lines BL 1 -BLm and BLC 1 -BLCm so that each sense amplifier SA is connected to a bit line from each segment S 1  and S 2 . 
     Each sense amplifier SA includes a first invertor which is formed from transistors M 1  and M 3 , and a second invertor which is formed from transistors M 2  and M 4 . In addition, each sense amplifier SA also includes a power switch transistor M 5  and a ground switch transistor M 6 . 
     Each power switch transistor M 5  provides power to a sense amplifier SA when a first turn on voltage is applied to a power switch line PSL, while each ground switch transistor M 6  connects ground to a sense amplifier SA when a second turn on voltage is applied to a ground switch line GSL. 
     In operation, a cell is programmed by placing a programming voltage, such as five volts, on the bit line that corresponds with the cell to be programmed, while ground is applied to the remaining bit lines. (A storage voltage, such as five volts, is continuously applied to the top plate  128  of each capacitor  104  and ground is applied to material  110 .) 
     After placing a programming voltage on the bit line, the word line that corresponds with the cell to be programmed is pulsed with an access voltage while ground is applied to the remainder of the word lines. This pulse turns on the access transistor  102  which causes the electrons on the lower plate  124  of capacitor  104  to flow to source region  112 . 
     For example, if cell A in FIG. 2 is to be programmed, the programming voltage is applied to bit line BL 1  while ground is applied to bit lines BL 2 -BLm and BLC 1 -BLCm. In addition, word line WL 1  is pulsed with the access voltage while word lines WL 2 -WLn and WLC 1 -WLCn are connected to ground. 
     To read a row of cells, ground is placed on the bit lines in the segment that contain the row of cells to be read, while a logic high voltage is placed on the bit lines in the remaining segment. (Since the sense amplifiers SA are based on cross-coupled inverters, the logic states on the bit lines in one segment are always the opposite of the logic states on the bit lines in the other segment.) Once the voltages have been placed on the bit lines, the bit lines are isolated so that the bit lines are only connected to the sense amplifiers SA. 
     After this, a read voltage, such as five volts, is applied to the word line that corresponds to the row of cells to be read, while ground is applied to the remainder of the word lines. If a cell in the row is storing a logic zero, nothing happens. 
     On the other hand, if a cell in the row is storing a logic one, the positive potential on the capacitor in the cell raises the voltage on the bit line which, in turn, causes the inverters in the sense amplifier to flip. The logic state stored by the cell is then determined by reading the state of the inverters. Since the read step is similar to the step of erasing a programmed cell, each programmed cell must be refreshed after it is read. 
     For example, if the first row of cells in segment  2  is to be read, ground is placed on bit lines BLC 1 -BLCm, while a logic high voltage is placed on bit lines BL 1 -BLm. Once the bit lines have been isolated, the read voltage is applied to word line WLC 1  while ground is applied to word lines WLC 2 -WLCn and WL 1 -WLn. 
     One problem with array  200  is that when ground is applied to a bit line during a read operation of the array, each programmed cell  100  in the same column of cells  100  that has access gate  120  connected to ground also has a small sub-threshold leakage current that flows from drain region  114  to source region  112  which, in turn, undesirably erases the cell. 
     One technique for reducing this sub-threshold leakage current is to place a small positive voltage, e.g., 0.1-0.3 volts, rather than ground on the bit lines that are to be read. One technique for providing this small positive voltage is to use sense amplifiers that are biased by the small positive voltage. 
     FIG. 3 shows a schematic diagram that illustrates a conventional sense circuit  300 . As shown in FIG. 3, sense circuit  300  is similar to sense circuit  210  of FIG. 2 and, as a result, utilizes the same reference numerals to designate the structures which are common to both amplifiers. 
     As shown in FIG. 3, sense circuit  300  differs from sense circuit  210  in that sense circuit  300  includes a bias circuit  310 . Bias circuit  310 , in turn, includes a first current source GEN 1 , a transistor M 7  which has a drain and gate connected to current source GEN 1 , and a resistor R 1  which is connected to the source of transistor M 7  and ground. 
     In addition, bias circuit  310  also includes a second current source GEN 2 , a transistor M 8  which has a source, a drain connected to current source GEN 2 , and a gate connected to the gate of transistor M 7 ; and a transistor M 9  which has a source connected to ground, a drain connected to the source of transistor M 8 , and a gate connected to current source GEN 2 . 
     In operation, the output of generator GEN 1  is set so that a small positive reference voltage is dropped across resistor R 1  in response to a reference current IREF flowing through transistor M 7  and resistor R 1 . 
     The reference voltage and the reference current IREF are mirrored so that a bias voltage VLB equal to the reference voltage is present at a summing node NS (the source of transistor M 8  and the drain of transistor M 9 ), and so that a bias current ILB equal to the reference current IREF flows through transistor M 8 . 
     Summing node NS sums the bias current ILB and a sense amp current IS. When each transistor M 5  and M 6  is turned off, the sense amp current IS, which represents the total current flowing out of the sense amplifiers SA, is substantially zero. In this case, transistor M 9  sinks substantially only the bias current ILB. 
     On the other hand, when each transistor M 5  and M 6  is turned on, the sense amp current IS is large. In this case, the voltage on node B rises in response to the increased current flow from the sense amplifiers SA which, in turn, turns transistor M 9  on harder to sink a larger current that includes both the bias current ILB and the large sense amp current IS. 
     Thus, by forming a bias voltage VLB at the summing node NS, the voltage on a bit line during a read operation is equal to the bias voltage VLB plus the voltage drops across transistors M 6  and transistors M 3  or M 4 , depending on which segment is read. 
     One problem with bias circuit  310 , however, is that bias circuit  310  consumes a significant amount of area. Thus, there is a need for a sense amplifier which has a bias circuit that consumes less silicon real estate. 
     SUMMARY OF THE INVENTION 
     A sense amplifier in accordance with the present invention places a low positive voltage, such as 0.1 to 0.3 volts, on a bit line instead of ground when a memory cell is read by utilizing a current source circuit to output a reference current that biases a Schottky diode. The Schottky diode, in turn, can be formed to consume significantly less silicon real estate than the bias circuits conventionally used. 
     The sense amplifier of the present invention includes a detection circuit which is connected to a bit line and a memory line, a first switch which is connected to the detection circuit, and a second switch which is connected to the detection circuit. 
     In accordance with the present invention, the sense amplifier also includes a bias circuit which is connected to the second switch. The bias circuit has a current source circuit which is connected to the second switch, and a first Schottky diode which has an input connected to the second switch. The current source circuit sources a reference current which is sunk by the first Schottky diode. 
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principals of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional diagram illustrating a conventional DRAM cell  100 . 
     FIG. 2 is a schematic diagram illustrating a conventional DRAM array  200 . 
     FIG. 3 is a schematic diagram illustrating a conventional sense circuit  300 . 
     FIG. 4 is a schematic diagram illustrating a bias circuit  400  in accordance with the present invention. 
     FIG. 5 is a schematic diagram illustrating a sense amplifier  500  in accordance with the present invention. 
     FIG. 6 is a schematic diagram illustrating a bias circuit  600  in accordance with the present invention. 
     FIG. 7 is a schematic diagram illustrating a sense amplifier  700  in accordance with the present invention. 
     FIG. 8 is a schematic diagram illustrating a bias circuit  800  in accordance with the present invention. 
     FIG. 9 is a schematic diagram illustrating a sense amplifier  900  in accordance with the present invention. 
     FIG. 10 is a schematic diagram illustrating a sense amplifier  1000  in accordance with the present invention. 
     FIGS. 11A-11B are a pair of graphs illustrating the I/V relationship of a Schottky diode which has titanium silicide (TiSi 2 ) formed over a lightly-doped n-type region. 
     FIGS. 12A-12B are a pair of graphs illustrating the I/V relationship of a Schottky diode which has titanium silicide (TiSi 2 ) formed over a lightly-doped p-type region. 
     FIGS. 13A-13B are a pair of graphs illustrating the I/V relationship of a Schottky diode which has cobalt silicide (CoSi 2 ) formed over a lightly-doped n-type region. 
     FIGS. 14A-14B are a pair of graphs illustrating the I/V relationship of a Schottky diode which has cobalt silicide (CoSi 2 ) formed over a lightly-doped p-type region. 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 shows a schematic diagram that illustrates a bias circuit  400  in accordance with the present invention. As shown in FIG. 4, bias circuit  400  includes a current generator GEN that sources a small reference current IREF, and a Schottky diode  410  which has an input connected to generator GEN and an output connected to ground. In operation, diode  410  is biased by the reference current IREF to produce a small positive bias voltage VLB, e.g., 0.1 to 0.3 volts, at the input to diode  410 . 
     FIG. 5 shows a schematic diagram that illustrates a sense amplifier  500  in accordance with the present invention. As shown in FIG. 5, amplifier  500  includes a detection circuit  510  which is connected to a bit line  512  and a memory line  514 , such as a bit line or a reference line. (Sense amplifier  500  can be connected to two bit lines, or to a bit line and a reference line where the reference line is used to set the logic state of the bit line.) 
     Detection circuit  510  includes a first inverter  516  which has an output connected to bit line  512  and an input connected to memory line  514 . Inverter  516  is implemented with a p-channel transistor M 1  and a n-channel transistor M 3 . Transistor M 1  has a source, a drain connected to bit line  512 , and a gate connected to memory line  514 . Transistor M 3  has a source, a drain connected to the drain of transistor M 1 , and a gate connected to memory line  514 . 
     Detection circuit  510  also includes a second inverter  518  which has an output connected to memory line  514  and an input connected to bit line  512 . Inverter  518  is implemented with a p-channel transistor M 2  and a n-channel transistor M 4 . 
     Transistor M 2  has a source connected to the source of transistor M 1 , a drain connected to memory line  514 , and a gate connected to bit line  512 . Transistor M 4  has a source connected to the source of transistor M 3 , a drain connected to the drain of transistor M 3 , and a gate connected to bit line  512 . 
     As further shown in FIG. 5, sense amplifier  500  also includes a first switch which is implemented with a p-channel transistor M 5 , and a second switch which is implemented with a n-channel transistor M 6 . Transistor M 5  has a source connected to a power node, a drain connected to the sources of transistors M 1  and M 2 , and a gate connected to a first control line C 1 . Transistor M 6  has a source, a drain connected to the sources of transistors M 3  and M 4 , and a gate connected to a second control line C 2 . 
     In accordance with the present invention, sense amplifier  500  further includes bias circuit  400  which has the input of Schottky diode  410  connected to the source of switch M 6 . 
     In operation, once a first logic state has been placed on bit line  512  and a second logic state has been placed on memory line  514 , control line C 1  is lowered to turn on transistor M 5  while control line C 2  is raised to turn on transistor M 6 . 
     The line  512  or  514  which has the logic high state turns on the n-channel transistor M 3  or M 4  which has a drain connected to the opposite line  512  or  514 . As a result, the voltage on the opposite line  512  or  514  is equal to the bias voltage VLB plus the voltage drops associated with transistors M 3  or M 4 , and M 6 . 
     For example, if a logic low is placed on bit line  512  and a logic high is placed on memory line  514 , transistor M 1  is turned off and transistor M 3  is turned on. In addition, transistor M 2  is turned on and transistor M 4  is turned off. Thus, with transistors M 3  and M 6  turned on, the voltage on bit line  512  is equal to the bias voltage VLB plus the voltage drops associated with transistors M 3  and M 6 . 
     FIG. 6 shows a schematic diagram that illustrates a bias circuit  600  in accordance with the present invention. Bias circuit  600  is similar to bias circuit  400  and, as a result, utilizes the same reference numerals to designate the structures which are common to both circuits. 
     As shown in FIG. 6, bias circuit  600  differs from bias circuit  400  in that the current generator GEN in circuit  400  is implemented in circuit  600  with a Schottky diode  610  which has an input connected to a power node and an output connected to the input of Schottky diode  410 . In operation, Schottky diode  610  is formed so that diode  610  has a reverse-bias leakage current IL that functions as the reference current IREF. 
     FIG. 7 shows a schematic diagram that illustrates a sense amplifier  700  in accordance with the present invention. Sense amplifier  700  is similar to sense amplifier  500  and, as a result, utilizes the same reference numerals to designate the structures which are common to both amplifiers. 
     As shown in FIG. 7, sense amplifier  700  differs from sense amplifier  500  in that amplifier  700  utilizes bias circuit  600  rather than bias circuit  400 . Sense amplifier  700  operates the same as sense amplifier  500 . 
     FIG. 8 shows a schematic diagram that illustrates a bias circuit  800  in accordance with the present invention. Bias circuit  800  is similar to bias circuit  400  and, as a result, utilizes the same reference numerals to designate the structures which are common to both circuits. 
     As shown in FIG. 8, bias circuit  800  differs from bias circuit  400  in that the current generator GEN in circuit  400  is implemented in circuit  800  with a current mirror  810 . As further shown in FIG. 8, current mirror  810  includes a first transistor M 1  which has a source connected to a power supply node, a drain, and a gate connected to the drain. 
     In addition, current mirror  810  also includes a resistor R 1  which is connected to the drain of transistor M 1  and ground, and a second transistor M 2  which has a source connected to the power supply node, a drain connected to the input of Schottky diode  410 , and a gate connected to the gate of the first transistor M 1 . 
     In operation, resistor R 1  defines a reference current IREF which flows through diode-connected transistor M 1  and resistor R 1 . The reference current IREF is mirrored by transistor M 2 , and is sufficient to bias Schottky diode  410  to set the low positive bias voltage VLB at the input of diode  410 . 
     FIG. 9 shows a schematic diagram that illustrates a sense amplifier  900  in accordance with the present invention. Sense amplifier  900  is similar to sense amplifier  500  and, as a result, utilizes the same reference numerals to designate the structures which are common to both amplifiers. 
     As shown in FIG. 9, sense amplifier  900  differs from sense amplifier  500  in that amplifier  900  utilizes bias circuit  800  rather than bias circuit  400 . Sense amplifier  900  operates the same as sense amplifier  500 . 
     FIG. 10 shows a schematic diagram that illustrates a sense amplifier  1000  in accordance with the present invention. As shown in FIG. 10, sense amplifier  1000  includes a detection circuit  1010  which is connected to a bit line  1012  and a memory line  1014 , such as a bit line or a reference line. 
     Detection circuit  1010  includes a first inverter  1016  which has an output connected to bit line  1012  and an input connected to memory line  1014 . Inverter  1016  is implemented with a p-channel transistor M 1 , bias circuit  600 , and a n-channel transistor M 3 . Transistor M 1  has a source, a drain connected to bit line  1012 , and a gate connected to memory line  1014 . 
     Bias circuit  600  has the inputs of Schottky diodes  410  and  610  connected to the drain of transistor M 1 , while transistor M 3  has a source, a drain connected to the output of Schottky diode  410 , and a gate connected to memory line  1014 . 
     Detection circuit  1010  also includes a second inverter  1018  which has an output connected to memory line  1014  and an input connected to bit line  1012 . Inverter  1018  is implemented with a p-channel transistor M 2 , bias circuit  600 , and a n-channel transistor M 4 . Transistor M 2  has a source connected to the source of transistor M 1 , a drain connected to memory line  1014 , and a gate connected to bit line  1012 . 
     Bias circuit  600  has the inputs of Schottky diodes  410  and  610  connected to the drain of transistor M 2 , while transistor M 4  has a source connected to the source of transistor M 3 , a drain connected to the output of Schottky diode  410 , and a gate connected to bit line  1012 . 
     As further shown in FIG. 10, sense amplifier  1000  also includes a first switch which is implemented with a p-channel transistor M 5 , and a second switch which is implemented with a n-channel transistor M 6 . Transistor M 5  has a source connected to a power node, a drain connected to the sources of transistors M 1  and M 2 , and a gate connected to a first control line C 1 . Transistor M 6  has a source connected to ground, a drain connected to the sources of transistors M 3  and M 4 , and a gate connected to a second control line C 2 . 
     In operation, once a first logic state has been placed on bit line  1012  and a second logic state has been placed on memory line  1014 , control line C 1  is lowered to turn on transistor M 5  while control line C 2  is raised to turn on transistor M 6 . 
     The line  1012  or  1014  which has the logic high state turns on the n-channel transistor M 3  or M 4  which has a drain connected to the opposite line  1012  or  1014 . As a result, the voltage on the opposite line  1012  or  1014  is equal to the bias voltage VLB plus the voltage drops associated with transistors M 3  or M 4 , and M 6 . 
     For example, if a logic low is placed on bit line  1012  and a logic high is placed on memory line  1014 , transistor M 1  is turned off and transistor M 3  is turned on. In addition, transistor M 2  is turned on and transistor M 4  is turned off. Thus, with transistors M 3  and M 6  turned on, the voltage on bit line  1012  is equal to the bias voltage VLB plus the voltage drops associated with transistors M 3  and M 6 . 
     One of the advantages of the present invention is that Schottky diodes consume relatively little silicon real estate. As described in application Ser. No. 09/280,888 for SCHOTTKY DIODE WITH REDUCED SIZE filed on Mar. 29, 1999 by Alexander Kalnitsky et al., which is hereby incorporated by reference, Schottky diodes can be formed through field oxide regions in a manner which requires little if any additional silicon real estate. 
     FIGS. 11A-11B show a pair of graphs that illustrate the I/V relationship of a Schottky diode which has titanium silicide (TiSi 2 ) formed over a lightly-doped n-type region, while FIGS. 12A-12B show a pair of graphs that illustrate the I/V relationship of a Schottky diode which has titanium silicide (TiSi 2 ) formed over a lightly-doped p-type region. 
     FIGS. 13A-13B show a pair of graphs that illustrate the I/V relationship of a Schottky diode which has cobalt silicide (CoSi 2 ) formed over a lightly-doped n-type region, while FIGS. 14A-14B show a pair of graphs that illustrate the I/V relationship of a Schottky diode which has cobalt silicide (CoSi 2 ) formed over a lightly-doped p-type region. 
     As shown in FIGS. 11A-11B,  12 A- 12 B,  13 A- 13 B, and  14 A- 14 B, another advantage of the present invention is that bias circuits  400 ,  600 , and  800  require only a small current to bias Schottky diode  410  to drop approximately 0.1 to 0.3 volts. As a result, power consumption by the bias circuits is very low. 
     It should be understood that various alternatives to the embodiment of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.