BACKGROUND OF THE INVENTION 
       [0001]    (1) Field of the Invention 
         [0002]    This invention relates to gate drive voltage for magnetic memory technologies, such as Phase Change RAM and Spin Moment Transfer MRAM, sometimes referred to as Spin Torque Transfer MRAM, cells which require programming currents higher than the minimum cell transistor can provide without degrading the life of the cell transistor. More particularly the invention relates to circuits and methods for programming these cell transistors by applying high gate voltages only to those cell transistors into which a one or zero is being written. 
         [0003]    (2) Description of Related Art 
         [0004]    U.S. Pat. No. 7,046,547 B2 and U.S. Pat. No. 6,961,265 B2 to Witcraft et al. describe methods and apparatus that allow data to be stored in a magnetic memory cell, such as a giant magneto-resistance cell. The inventions describe advantageously winding a word line around a magnetic memory cell to increase the magnetic field induced by the word line. 
         [0005]    U.S. Pat. No. 6,985,382 B2 to Fulkerson et al. describe a technique to read a stored state in a magneto-resistive random access memory device, MRAM, such as a giant magneto-resistance MRAM device or a tunneling magneto-resistance device, TMR. The technique uses a bit line that is segmented into a first portion and a second portion. An interface circuit compares the resistance of a first portion and a second portion of a first bit line to the resistance of a first portion and a second portion of a second bit line to determine the logical state of a cell in the first bit line. 
         [0006]    U.S. Pat. No. 6,754,055 B2 to Ono et al. describes a giant magneto-resistive effect element which includes a laminated layer film having a ferromagnetic film, a non-magnetic film, and an anti-ferromagnetic film. 
         [0007]    U.S. Pat. No. 6,714,390 B2 describes a giant magneto-resistive effect element capable of producing a high output and a high resistance and which can cope with a high recording density and a magneto-resistive effect type head, a thin film magnetic memory, and a thin film magnetic sensor each of which includes this giant magneto-resistive effect element. 
       SUMMARY OF THE INVENTION 
       [0008]    Magnetic memory elements using the Giant magneto-resistive effect, such as Phase Change RAM and Spin Moment Transfer MRAM, sometimes referred to as Spin Torque Transfer MRAM, require high programming currents. Since these currents are controlled by a cell transistor, a field effect transistor, a high voltage between the source and/or drain is required to produce sufficient memory cell current to program the memory cells. This high gate to source/drain voltage and high memory cell current can significantly reduce the life of the cell transistor. 
         [0009]    It is a principal objective of this invention to provide a circuit which can write information into individual memory cells, a one or a zero, while minimizing the gate voltage stress in the cell transistors of the memory cells in which no information is being written. 
         [0010]    It is another principal objective of this invention to provide a method of writing information into individual memory cells, a one or a zero, while minimizing the gate voltage stress in the cell transistors of the memory cells in which no information is being written. 
         [0011]    These objectives are achieved by only applying the high gate to drain voltage, or gate to source voltage to the cells in which a one or a zero is to be written so that only those cells which are to be written see the high gate voltage stress one at a time. The voltage stress is further reduced on the cells in which no information is to be written by the use of a precharge-discharge circuit. 
         [0012]    Different voltage levels are required for read and write operations on the memory cells with the write operations requiring the highest currents. To reduce the stress on the cell transistors different gate voltages are applied for read and write operations with the write operation being the most severe. 
         [0013]    In writing information into one of the memory cells only the voltage on the word line connected to gate of the cell transistor in that memory cell is raised, which also raises the gate voltage of all the cell transistors connected to that word line, that is all the cell transistors in that particular row. A double bit line circuit connects a BLC line and a BLT line across all of the memory cells in a particular column. At the same time the voltage of the word line connected to memory cell to be written is raised the voltage across the BLC and BLT lines for the column which contains the memory cell to be written is also raised which raises the voltage across all of the memory cells in the same column as the memory cell being written, however only the memory cell being written in that particular row of the memory array sees the increase in gate voltage. In addition bit line transistors and a precharge circuit are used to further limit the stress on the cell transistors for the cells which are not being written. 
         [0014]    Writing a one in a memory cell requires greater cell current, and thus a greater gate voltage at the cell transistor, than writing a zero. Reading a memory requires the least cell current and thus the least gate voltage. Writing memory cells one cell at a time, using different gate voltages for writing a one and writing a zero, using a different gate voltage for reading a cell, and a precharge circuit are all used to limit the overall voltage stress on the cell transistors in the array. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows a schematic diagram of the memory array circuit for the circuit and method of this invention. 
           [0016]      FIG. 2A  shows a schematic representation of storing a 1 in the memory cell used in this invention. 
           [0017]      FIG. 2B  shows a schematic representation of storing a 0 in the memory cell used in this invention. 
           [0018]      FIG. 3  shows a timing diagram for circuit and method of this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Refer now to the Drawings for a description of the preferred embodiments of this invention. In these descriptions all transistors are field effect transistors and will be described herein simply as transistors. 
         [0020]      FIG. 1  shows a schematic drawing of the memory cell array used in this invention.  FIG. 1  shows an array of memory cells arranged in rows and columns  100 ,  101 , . . .  10   n ;  110 ,  110 , . . . ,  11   n ; . . . ;  1   m   0 ,  1   m   1 , . . . ,  1   mn . Each of the memory cells has a magnetic memory element  700 ,  701 , . . . ,  70   n ;  710 ,  711 , . . . ,  71   n ;  7   m   0 ,  7   m   1 , . . . ,  7   mn  in series with a cell transistor  800 ,  801 , . . . ,  80   n ;  810 ,  811 , . . . ,  81   n ;  8   m   0 ,  8   m   1 , . . . ,  8   mn . These magnetic memory elements  700 ,  701 , . . . ,  70   n ;  710 ,  711 , . . . ,  71   n ;  7   m   0 ,  7   m   1 , . . . ,  7   mn  are two terminal devices and can be represented by a resistor, as shown in  FIG. 1 . Each column of the array has a BLC line  40 ,  41 , . . . ,  4   n  connected to one terminal of each magnetic memory element  7   x   0 ,  7   x   1 , . . . ,  7   xm  in that column (x takes on values from 0 to n) and a BLT line  50 ,  51 , . . . ,  5   n  connected to the drains of the cell transistors  8   x    0 ,  8   x   1 , . . . ,  8   xm  in that column (x takes on values from 0 to n). The drains of the cell transistors in each cell are connected to the other terminal of the magnetic memory element in that cell, as shown in  FIG. 1 . 
         [0021]    A first voltage source, VD 1 , is connected through a BLT transistor  60 ,  61 , . . . ,  6   n  to each BLT line  50 ,  51 , . . . , Sn. A second voltage, VD 2 , is connected through a BLC transistor  70 ,  71 , . . . ,  7   n  to each BLC line  40 ,  41 , . . . ,  4   n  as shown in  FIG. 1 . The gates of the BLT and BLC transistors for each column are connected together and connected to a gate signal source. As shown in  FIG. 1  the gates to transistors  60  and  70  are connected to gate signal y 0 , the gates of transistors  61  and  71  are connected to gate signal y 1 , . . . , and the gates of transistors  6   n  and  7   n  are connected to gate signal y n  so that the first voltage source, V D1 , and the second voltage source, V D2 , can be applied to the BLT and BLC lines for each column of the array separately and individually. The gates of the cell transistors in each row of the array are connected to a word line  200 ,  201 , . . . ,  20   m  for that row of the array. As shown in  FIG. 1  the word lines  200 ,  201 , . . . ,  20   m  are connected to row decoding circuitry  500 . 
         [0022]    In order to further reduce the stress on the cell transistors in the non selected cells a precharge circuit  504  is used. The precharge circuit  504  is also shown in  FIG. 1 . The precharge circuit has a first transistor  510  and a second transistor  511  for each BLT line and each BLC line. The sources of the first transistors  510  are connected to a precharge voltage supply, V P ,  505 . The drains of the second transistors  511  are connected to a reference voltage supply, V REF ,  506 . The drains of each first transistor  510  are connected to the source of one of the second transistors and to one of the BLT lines or one of the BLC lines. The gates of the first transistors  510  are connected together and to a charge gate signal, V CH ,  507 . The gates of the second transistors  511  are connected together and to a discharge gate signal, V DC ,  508 . For the example being considered for this example it is desired to keep the voltage between the source or drain and gate of the unselected cells at no more than 1.2 volts the voltage between the source or drain of the selected cells at no more than 1.8 volts. 
         [0023]    Refer now to  FIGS. 2A and 2B  for a description of storing a 1 or a 0 in the memory cell. As shown in  FIG. 2A  a 1 is stored in the magnetic memory element  4  by causing a current to flow from the cell transistor  3  into the magnetic memory element  4 . This current is caused to flow by applying voltages to the BLC line  6 , the BLT line  7 , and the gate  8  of the cell transistor  3 . As shown in  FIG. 2B  a 0 is stored in the magnetic memory element  4  by causing a current to flow from the magnetic memory element  4  into the cell transistor  3 . This current also is caused to flow by applying voltages to the BLC line  6 , the BLT line  7 , and the gate  8  of the cell transistor  3 . The stress conditions on the cell transistor  3  are different when the magnetic memory element is being read or being written. Also, the stress conditions on the cell transistor when a 1 is written into the magnetic memory element are different from the stress conditions when a 0 is being written into the magnetic memory element. As can be seen from  FIGS. 2A and 2B  the voltage at the inter connecting point between the cell transistor  3  and the magnetic memory element  5  and the gate  8  of the cell transistor  3  will be higher when a 1 is being written into the magnetic memory element than when a 0 is being written into the magnetic memory element. 
         [0024]    Refer now to  FIGS. 1 and 3 . The circuit and methods of this invention provide means to limit the stress on the cell transistor to acceptable levels for reading the magnetic memory element or writing either a 1 or a 0 in the magnetic memory element. This is accomplished by writing either a 1 or a 0 into a single magnetic memory element at a time and by using a precharge circuit  540 . Writing a 1 into cell  111  followed by writing a 0 into cell  100 , see  FIG. 1 , will now be described with reference to the timing diagram shown in  FIG. 3 . Although not shown in  FIG. 3 , during the writing a 1 into cell  111  and a 0 into cell  100  the gate signals y 2 , y 3 , . . . , y n  to the BLT transistors  62 ,  63 , . . . ,  6   n  for the remaining BLT lines and to the BLC transistors  72 ,  73 , . . . ,  7   n  for the remaining BLC lines remain low so that the these transistors remain off and neither the first voltage source V D1  nor the second voltage source V D2  are connected to these BLT and BLC lines. 
         [0025]    In this description of the operation of the memory the reference voltage or low voltage will be ground or zero volts. Those skilled in the art will recognize that it is the difference between the applied voltages and the reference voltage that is important and the reference voltage could be different that zero volts as long as the voltage differences remain the same. During the first time interval from T 0  to T 1  the gate signals y 0  and y 1  applied to the BLT and BLC transistors  60 ,  70 ,  61 , and  71  remain low so that the BLT and BLC transistors  60 ,  70 ,  61 , and  71  remain off. During this time interval the V D2  voltage supply remains low, in this example zero volts, and the V D1  voltage supply is raised from zero to about 0.8 volts, however the V D1  and V D2  voltage supplies are not connected to the BLC line  40  or the and BLT line  50  for the 100 memory cell nor to the BLC line  41  or the BLT line  51  for the 111 memory cell. During this time interval the charge gate signal, V CH ,  507  becomes high while the discharge gate signal remains low turning the first transistors  510  on and the second transistors  511  off so that the precharge voltage supply, V P ,  505  is connected to all of the BLT  50 ,  51 , . . . ,  5   n  and BLC lines  40 ,  41 , . . . ,  4   n . In this example the precharge voltage supply, V P ,  505  is about 1.0 volts so that all of the BLT and BLC lines are precharged to 1.0 volts during this first time interval. 
         [0026]    Also during this first time interval the signal applied to word line zero, WL 0 ,  200  remains at zero and the signal applied to word line  1 , WL 1 ,  201  is increased to 2.2 volts. The signal applied to the remaining word lines  202 ,  203 , . . . ,  20   m  remains at zero throughout this example. These conditions place a maximum of 1.2 volts between the source or drain and the gate of the cell transistor  811  for cell  111  and a maximum of 1.0 volts between the gate and source or drain of the remaining cell transistors during this first time interval. 
         [0027]    As shown in  FIG. 3  during the second time interval, between T 1  and T 2 , the gate signal y 1  for the BLC  41  and BLT  51  lines for the column containing memory cell  111  is raised from low to high while the gate charge signal, V CH , is reduced to zero and the discharge gate signal, V DC , remains at zero turning off the first transistors  510  and second transistors. The first voltage supply, V D1 , remains at 0.8 volts and the second voltage supply, V D2 , remains at zero volts during the second time interval. The voltage on the BLT line  51  for memory cell  111  is the same as the first voltage supply, V D1 , of 0.8 volts, the voltage on the BLC line  41  for memory cell  111  is the same as the second voltage supply of 0 volts. The voltage on the remaining BLT and BLC lines remain at 1.0 volts due to the capacitance of the lines. The voltage on the word line one, WL 1 ,  201  remains at 2.2 volts and the voltage on word line zero, WL 0 ,  200  remains at 0.0 volts. These conditions provide 0.8 volts between the BLT  51  and BLC  41  lines connected to memory cell  111  and 2.2 volts to the gate of the cell transistor for writing a 1 into memory cell  111 . The voltage drop across the magnetic memory element  711  in memory cell  111  when a 1 is written in about 0.4 volts so that the maximum voltage seen from the gate to the drain of the cell transistor  811  is 1.8 volts and from the gate to the source of the cell transistor  811  is 1.4 volts. The maximum voltage seen between the gate and source or drain of the unselected cell transistors is 1.2 volts because of the remaining 1.0 volt precharge on the remaining BLT and BLC lines. 
         [0028]    In the next time interval between T 2  and T 3  the gate signals to the BLT and BLC transistors are returned to zero, the voltage of the first voltage supply, V D1 , is returned to zero, the charge voltage signal, V CH , remains at zero, the discharge voltage signal, V DC , is raised so that the first transistors  510  are turned off and the second transistors  511  are turned on connecting all the BLT and BLC lines to the V REF  voltage supply or ground. The writing a 1 in memory cell  111  is then completed. 
         [0029]    The writing a 0 into memory cell  100  begins with the next time interval from T 3  to T 4  where the charge voltage signal, V CH ,  507  is raised while the discharge voltage signal, V DC ,  508  remains low turning on the first transistors  510 , turning off the second transistors  511  and connecting all the BLT and BLC lines to the precharge voltage supply, V P , of 1.0 volts. During this time interval the second voltage supply, V D2 , is raised to 0.4 volts and the first voltage supply, V D1 , remains at zero through the write a 0 in cell  100  operation. During the tine interval T 3  to T 4  the signal applied to word line zero, WL 0 , is raised to 1.8 volts and the signal applied to word line one, WL 1 , remains at 0.0 volts. 
         [0030]    In the next time interval, T 4  to T 5 , the gate signal y 0  for the BLC  40  and BLT  50  lines for the column containing memory cell  100  is raised from low to high while the gate charge signal, V CH , is reduced to zero and the discharge gate signal, V DC , remains at zero turning off the first transistors  510  and second transistors  511 . The first voltage supply, V D1 , remains at 0.0 volts and the second voltage supply, V D2 , remains at 0.4 volts during this time interval. The voltage on the BLT line  50  for memory cell  100  is the same as the first voltage supply, V D1 , of 0.0 volts, the voltage on the BLC line  40  for memory cell  100  is the same as the second voltage supply of 0.4 volts. The voltage on the remaining BLT and BLC lines remain at 1.0 volts due to the capacitance of the lines. The voltage on the word line zero, WL 0 ,  200  remains at 1.8 volts and the voltage on word line one, WL 1 ,  201  remains at 0.0 volts. These conditions provide 0.4 volts between the BLC  40  and BLT  50  lines connected to memory cell  100  and 1.8 volts to the gate of the cell transistor for writing a 0 into memory cell  100 . The maximum voltage seen from the gate to the drain or source of the cell transistor  800  is 1.8 volts. The maximum voltage seen between the gate and source or drain of the unselected cell transistors is 0.8 volts because of the remaining 1.0 volt precharge on the remaining BLT and BLC lines. 
         [0031]    After T 5  the gate signals y 0  and y 1  are returned to zero, the gate discharge signal, V DC , is raised turning on transistors  511  and the gate charge signal, V CH , remains low keeping transistors  510  turned off so that all the BLC lines and BLT lines are returned to the potential of the reference voltage supply, V REF , or ground potential. At this point another write operation or a read operation can begin. During a read operation smaller voltages between the gate and source or drain of the cell transistors are required, so that the maximum voltage between the gate and source or drain of the cell transistors is 1.8 volts for a selected cell and 1.2 volts for unselected cells for a write 1 operation and 1.8 volts for a selected cell and 0.8 volts for unselected cell for a write 0 operation. 
         [0032]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.
This invention describes a circuit and method to limit the stress caused by gate voltages required to write a one or zero in magnetic memory elements using the Giant magneto-resistive effect, such as Phase Change RAM and Spin Moment Transfer MRAM, sometimes referred to as Spin Torque Transfer MRAM, which require high programming currents. The circuit and method selects one cell at a time for writing a one or a zero, different voltages to write a one or a zero, and a precharge circuit to limit the stress on non selected cells.