Abstract:
The present invention discloses a magnetic random access memory for reading two or more data, by sensing the current flowing into source and drain regions. The current is regulated by the amount of a current flowing through an MRJ in an MRAM cell according to a word line voltage. In order to accomplish this object of the present invention, the MRAM comprises a data detecting circuit for converting a current flowing through an MTJ in the MRAM cell into a voltage and detecting data resulting in magnetization orientation ge.

Description:
BACKGROUND OF THE IVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to a magnetic random access memory (hereinafter, referred to as ‘MRAM’), and in particular, to an MRAM having magnetic tunnel junctions (hereinafter, referred to as ‘MTJ’) between gate metal electrodes and active regions of semiconductor substrates, the MRAM which can read and write two or more data by controlling current flowing through MTJ and current flowing from drain region to source region.  
           [0003]    2. Description of the Prior Art  
           [0004]    Nonvolatile memories are more required to overcome limitations of volatile memories as the demand for portable or communication appliances increases. The volatile memories may lose data when power is turned off. However, the nonvolatile memories are not restricted in number of write and read. As a result, MRAMs is developed using differences of magnetic resistance according to relative arrangements in electrodes.  
           [0005]    The MRAM store magnetic polarization in a magnetic thin film, and perform read and write operations by changing or sensing magnetization orientation according to magnetic fields generated by combining currents in bit and word lines.  
           [0006]    The MRAM may be embodied by using alternative magnetoresistive effects such as GMR(giant magneto resistance) or spin polarization magneto permeation, which are generated due to influence of spins on transmission of electrons. In general, the MRAMs read and write data by utilizing devices using magnetic phenomena such as GMR or MTJ as memory cell.  
           [0007]    First, the MRAM using giant magneto resistance is embodied by using a phenomenon wherein resistance is more differentiated when spin directions are anti-parallel than when parallel in two magnetic layers having an insulating layer therebetween. Second, the MRAM using spin polarization magnetic permeation is embodied by using a phenomenon wherein the current is better permeated when spin directions are parallel than when anti-parallel in two magnetic layers having an insulating layer therebetween.  
           [0008]    [0008]FIG. 1 is a diagram of a cell array of the conventional MRAM.  
           [0009]    An MRAM cell of FIG. 1 includes a plurality of word lines WL 1 ˜WL 4 , a plurality of bit lines BL 1 ˜BL 2  and sense amplifiers SA 1  and SA 2  coupled with a plurality of bit lines BL 1  and BL 2 . A cell  1  selected by word lines and bit lines includes a switching transistor T and an MTJ.  
           [0010]    First, when a word line WL 4  of a plurality word lines WL 1 ˜WL 4  is selected by a word line selecting signal, a predetermined voltage is applied to an MTJ by turn-on of a switching transistor T. As a result, a current having different values according to the magnetization orientation of an MTJ flows into a selected bit line BL 2 , and then a sense amplifier SA 2  senses the current of the bit line BL 2 .  
           [0011]    [0011]FIGS. 2 a  and  2   b  are diagrams of the above-described MTJs.  
           [0012]    As shown in FIGS. 2 a  and  2   b,  the top portion of an MTJ includes a free ferromagnetic layer  2 , and its bottom of a fixed ferromagnetic layer  4 . The free ferromagnetic layer  2  and the fixed ferromagnetic layer  4  consists of NiFeCo/CoFe.  
           [0013]    The thickness of the free ferromagnetic layer  2  is different from that of the fixed ferromagnetic layer  4 . The fixed ferromagnetic layer  4  changes the magnetization orientation only by a strong magnetic field. On the contrary, the free ferromagnetic layer  2  changes the magnetization orientation only by a weak magnetic field. Here, if a weak magnetic field is used, the magnetization orientation of a free ferromagnetic layer is changed, whilethat of a fixed ferromagnetic layer is fixed at one direction, and then a fixed layer is formed. As a result, a magnetic field is generated to change only the magnetization orientation of the top layer without changing the magnetization orientation of the bottom layer during the write operation.  
           [0014]    A tunnel oxide film  3  is formed between a free ferromagnetic layer  2  and a fixed ferromagnetic layer  4 , the tunnel oxide film  3  consisting of Al 2 O 3 .  
           [0015]    Here, FIG. 2 a  shows an example of parallel magnetization orientations in a free ferromagnetic layer  2  and a fixed ferromagnetic layer  4 . If the magnetization orientation is parallel, a current increases.  
           [0016]    [0016]FIG. 2 b  shows an example of anti-parallel magnetization orientations in a free ferromagnetic layer  2  and a fixed ferromagnetic layer  4 . If the magnetization orientation is anti-parallel, a current decreases.  
           [0017]    Here, the magnetization orientation of a free ferromagnetic layer  2  is changed by an external magnetic field. An MRAM cell stores logic values of “o” and “1” according to the magnetization orientation of the free ferromagnetic layer  2 .  
           [0018]    However, since a conventional MRAM cell includes 1T+1MTJ, the cell has the complicated structure and the difficult process. The conventional MRAM also has the problem in the cell size.  
         SUMMARY OF THE INVENTION  
         [0019]    Accordingly, it is the first object of the present invention to provide an MRAM for reading/writing data from/to an MRAM cell by adjusting a current flowing in an MTJ according to the volume of voltages applied to word lines.  
           [0020]    It is the second object of the present invention to provide an MRAM for reading data from an MRAM cell by adjusting the amount of a current flowing from a drain region to a source region, by the amount of a current flowing through an MTJ of an MRAM cell, according to the volume of voltages in word lines.  
           [0021]    It is the third object of the present invention to provide an MRAM for reading/writing two or more data, thereby reducing a cell size of MRAM.  
           [0022]    It is the forth object of the present invention to provide an MRAM for reading/writing two or more data, thereby performing an easy process.  
           [0023]    It is the fifth object of the present invention to provide an MRAM for reading/writing two ore more data, thereby improving a sensing margin.  
           [0024]    According to a first aspect of the present invention, a MRAM comprises: an MRAM cell having source and drain regions formed on an active region of a semiconductor substrate; an insulating layer deposited on a channel region of a semiconductor substrate; and an MTJ stacked on an upper portion of the insulating layer, wherein data is written/read to/from the MRAM cell by adjusting/sensing a current flowing through the MTJ in MRAM cell according to voltages applied to word lines of the MRAM cell.  
           [0025]    According to a second aspect of the present invention, a MRAM comprises: an MRAM cell group consisting of a plurality of MRAM cells connected to each other in series between a bit line and a cell plate and having each gate connected to receive singal of a plurality of word lines; and a sense amplifier sensing data applied to the bit line when receiving a sense amplifier enable signal.  
           [0026]    According to a third aspect of the present invention, a MRAM comprises: a first MRAM cell group consisting of a plurality of MRAM cells connected to each other in series between a bit line and a cell plate and having each gate connected to receive signals of a plurality of word lines; a second MRAM cell group consisting of a plurality of MRAM cells connected to each other in series between a bit line bar and a cell plate, and having each gate connected to receive signals of the plurality of word lines; and a sense amplifier connected between the bit line and the bit line bar, and for sensing data applied to the bit line and the bit line bar when receiving a sense amplifier enable signal.  
           [0027]    According to a fourth aspect of the present invention, a MRAM comprises: a data detecting circuit connected to a bit line, and for converting a current flowing through an MTJ in an MRAM cell into a voltage and then detecting data based on the different magnetization orientation of the MTJ in the MRAM cell.  
           [0028]    According to a fifth aspect of the present invention, a MRAM comprises an MRAM cell group consisting of a plurality of MRAM cells connected to each other in series between a bit line and a cell plate and having each gate connected to receive signals of a plurality of word lines; and a data detecting circuit connected to the bit line and, for converting a current flowing through an MTJ in the MRAM cell group into a voltage and then detecting data based on differences of magnetization orientations of the MTJ in the MRAM cell group.  
           [0029]    According to a sixth aspect of the present invention, a MRAM comprises: a first MRAM cell group consisting of a plurality of MRAM cells connected to each other in series between a bit line and a cell plate, and having each gate connected to receive signals of a plurality of word lines; a second MRAM cell group consisting of a plurality of MRAM cells connected to each other in series between a bit line bar and a cell plate, and having each gate connected to receive signals of a plurality of word lines; and a data detecting circuit connected between the bit line and the bit line bar, and for converting currents flowing an MTJ in the first and the second MRAM cell groups into voltages and then detecting data based on differences of magnetization orientations of the MTJ in the first and the second MRAM cell groups. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The present invention will become better understood with reference to the accompanying drawings which are given only by way of illustration and thus are not limitative of the present invention, wherein:  
         [0031]    [0031]FIG. 1 illustrates a cell array of a conventional MRAM;  
         [0032]    [0032]FIGS. 2 a  and  2   b  illustrate cross sectional side views of a general MTJ, respectivly.  
         [0033]    [0033]FIGS. 3 a  and  3   b  illustrate cross sectional side views of MRAM cell in accordance with the present invention, respectively.  
         [0034]    [0034]FIG. 4 illustrates a cross sectional side view of another MRAM cell in accordance with the present invention.  
         [0035]    [0035]FIG. 5 illustrates a graph of an operating region of an MRAM cell in accordance with the present invention.  
         [0036]    [0036]FIGS. 6 a  through  6   d  illustrate differences of magnetization orientation of MTJ in MRAM cell in accordance with the present invention, respectively.  
         [0037]    [0037]FIGS. 7 a  through  7   c  illustrate an operating region of an MRAM cell respectively shown in FIGS. 6 a  through  6   d.    
         [0038]    [0038]FIG. 8 illustrates a graph of an operating region in an MRAM cell shown in FIGS. 7 a  through  7   c.    
         [0039]    [0039]FIG. 9 illustrates a symbol of an MRAM cell in accordance with the present invention.  
         [0040]    [0040]FIGS. 10 through 13 illustrate MRAM cell array in accordance with the present invention respectively.  
         [0041]    [0041]FIG. 14 illustrates a timing diagram during the write operation of an MRAM cell array in accordance with the present invention.  
         [0042]    [0042]FIG. 15 illustrates a timing diagram during the read operation of an MRAM cell array in accordance with the present invention.  
         [0043]    [0043]FIG. 16 illustrates a data detecting circuit for detecting the level of four data in accordance with the present invention.  
         [0044]    [0044]FIG. 17 illustrates a graph of four data and reference voltages of FIG. 16.  
         [0045]    [0045]FIG. 18 illustrates a table of four data of FIG. 17.  
         [0046]    [0046]FIG. 19 illustrates a circuit diagram of a data encoder for making table values of FIG. 18.  
         [0047]    [0047]FIG. 20 illustrates a data detecting circuit for detecting the level of eight data in accordance with the present invention.  
         [0048]    [0048]FIG. 21 illustrates a graph of eight data and reference voltages.  
         [0049]    [0049]FIG. 22 illustrates a tale of eight data of FIG. 21.  
         [0050]    [0050]FIG. 23 illustrates a data encoder for making table values of FIG. 22.  
         [0051]    [0051]FIGS. 24 through 27 illustrate MRAM cell array and data detecting circuit, respectively.  
         [0052]    [0052]FIG. 28 illustrates a timing diagram during the read operation of an MRAM cell array for detecting the level of four data in accordance with the present invention.  
         [0053]    [0053]FIG. 29 illustrates a timing diagram during the write operation of an MRAM cell array for detecting the level of four data in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0054]    An MRAM in accordance with preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0055]    [0055]FIGS. 3 a  and  3   b  are diagrams of MRAM cells in accordance with the present invention.  
         [0056]    MRAM cells shown in FIGS. 3 a  and  3   b  include source and drain regions  12  and  14 , an insulating layer  16 , an MTJ  24 , and a gate metal electrode  26 . The source and drains regions  12  and  14  are formed in a semiconductor substrate  10 . The insulating layer  16  consisting of Al 2 O 3  is deposited on the semiconductor substrate  10 . The MTJ is deposited on the insulating layer  16 , and the gate metal electrode deposited on the top of the MTJ  24  is coupled with a word line. Here, the MTJ  24  include a fixed ferromagnetic layer  18 , a tunnel oxide film  20  and a free ferromagnetic layer  22 .  
         [0057]    The read operation of MRAM cells of FIGS. 3 a  and  3   b  are explained hereinafter.  
         [0058]    An MRAM cell reads logic data of “1” or “0” according to magnetization orientations of a free ferromagnetic layer  22  of an MTJ  24 . FIG. 3 a  shows an example of magnetization orientation for reading a logic value of “1”, and FIG. 3 b  shows an example for reading a logic value of “0”.  
         [0059]    The read operation of an MRAM cell is performed by sensing the amount of current based on magnetization orientation of a free ferromagnetic layer  18  of an MTJ  24 . If a predetermined trigger voltage is applied to a gate metal electrode  26 , a tunneling current I 1  flows into an MTJ  24 . Here, if magnetization orientations are parallel in the fixed ferromagnetic layer  22  and the free ferromagnetic layer  18 , as shown in FIG. 3 a,  the amount of a current I 1  increases. Then, that of a current  12  flowing from a drain region  14  to a source region  12  also increases. On the contrary, if magnetization orientations are anti-parallel in the fixed ferromagnetic layer  22  and the free ferromagnetic layer  18 , as shown in FIG. 3 b,  the amount of the current I 1  decreases and then, that of the current I 2  also decreases.  
         [0060]    As a result, data stored in an MRAM cell can be sensed by setting magnetization orientation of the free ferromagnetic layer  18  parallel, anti-parallel or random, based on that of the fixed ferromagnetic layer  22 .  
         [0061]    Although the read operation of an MRAM cell are not shown, the read operation is performed by applying a predetermined voltage to the MTJ through a gate metal electrode  26  when a predetermined trigger voltage is applied in a source region  12 . A magnetization orientation of a free ferromagnetic layer in an MTJ  24  is determined by a current flowing according to the volume of a voltage applied to the gate electrode  26 . Logic values of “1” or “0” are written to the MRAM cell by the determined magnetization orientation.  
         [0062]    [0062]FIG. 4 is a diagram of a stacked-type MRAM cell in accordance with the present invention. The MRAM cell includes source and drain regions  32  and  34 , an insulating layer  36 , an MTJ  44 , a gate metal electrode  46 . The source and drain regions  32  and  34  are formed in a semiconductor substrate  30 . The insulating layer  36  consists of Al 2 O 3 , and is deposited on an active region of the substrate  30 . The MTJ  44  is deposited on the insulating layer  36 . The gate metal electrode  46  formed on the top of the MTJ  44 , and is coupled with a word line. Here, the MTJ has a stacked structure wherein a tunnel oxide film  40  and a free ferromagnetic layer  42  are repeatedly stacked on the fixed ferromagnetic layer  38 . The tunnel oxide film  40  consists of Al 2 O 3 .  
         [0063]    The read/write operation of MRAM cells shown in FIG. 4 will not be described because it is the same as the operations of MRAM cells shown in FIGS. 3 a  and  3   b.    
         [0064]    [0064]FIG. 5 is a graph illustrating changes in the amount of a current  12  according to a word line voltage of FIGS. 3 a  and  3   b.    
         [0065]    Here, a threshold voltage of an MRAM cell will be referred to as Vtn, a word line voltage as V WL , a tunneling voltage, where a current I 1  flows, as Vtunnel.  
         [0066]    The operation voltage region of a word line may be divided into three regions.  
         [0067]    In, Section  4 -A a word line voltage V WL  does not reach a threshold voltage Vtn of an MRAM cell, and thus a perpendicular current I 1  and a horizontal current I 2  are not generated in the channel (a logic value of “0”). A current I 1  is perpendicular to a channel, and a current I 2  is horizontal to a channel. As a result, currents of a word line and a bit line cannot flow into a word line and a bit line in Section  4 -A.  
         [0068]    In Section  4 -B, a horizontal current I 2  is generated in the channel when a word line voltage V WL  exceeds a threshold voltage Vtn of an MRAM, while a perpendicular current I 1  still is not generated(a logic value of “0”) because a word line voltage V WL  does not arrive at a tunneling voltage Vtunnel of a gate oxide film. As a result, a current of an MRAM cell is regulated only by a voltage of a gate electrode regardless of magnetization orientation of an MTJ in this section.  
         [0069]    In Section  4 -C, currents I 1  and I 2  are simultaneously generated in the channel when a word line voltage V WL  exceeds a threshold voltage Vtn and a tunneling voltage Vtunnel. As a result, the amount of a current I 1  is determined according to magnetization orientation of an MTJ  15 , and a current corresponding to data stored in an MRAM cell is applied to a bit line by regulating the amount of a current I 2  according to a current I 1 .  
         [0070]    [0070]FIGS. 6 a  through  6   d  illustrate MRAM cells wherein magnetization orientation of an MTJ is divided into four steps.  
         [0071]    As shown in FIGS. 6 a  through  6   d,  when differences of magnetization orientations of 0°, 60°, 120° and 180° in MTJs are detected by sensing currents I 2   a,  I 2   b,  I 2   c  and I 2   d,  four data are read from an MRAM cell.  
         [0072]    [0072]FIGS. 7 a  through  7   c  illustrate operation regions of MRAM cells in accordance with the present invention; and FIG. 8 is a graph illustrating an operation region according to a voltage applied to a word line WL of an MRAM cell in accordance with the present invention.  
         [0073]    Hereinafter, the change of a current I 2  according to the voltage applied to a word line of an MRAM cell will be explained with reference to FIGS. 7 a  through  8 .  
         [0074]    Here, a threshold voltage of an MRAM cell will be referred to as Vtn, a word line voltage as V WL , and a tunneling voltage, wherein a current I 1  flows, as Vtunnel.  
         [0075]    The operation region according to a word line voltage V WL  may be divided into three regions.  
         [0076]    In Section  5 -A of FIGS. 7 a  through  8 , a word line voltage V WL  does not reach a threshold voltage Vtn of an MRAM cell, and thus a perpendicular current I 1  and a horizontal current I 2  are not generated in a channel(logic value of “0”). As a result, currents of a word line and a bit line cannot flow into a word line and a bit line in Section  4 -A.  
         [0077]    In Section  5 -B of FIGS. 7 a  through  8 , a current  12  is generated when a word line voltage V WL  exceeds a threshold voltage Vtn of an MRAM, while a current I 1  is not generated in the channel(a logic value of “0”) because a word line voltage V WL  does not arrive at a tunneling voltage Vtunnel of a gate oxide film. As a result, a current of an MRAM cell is regulated only by a voltage of a gate electrode regardless of magnetization orientation of an MTJ in this section.  
         [0078]    In Section  5 -C, currents I 1  and I 2  are simultaneously generated in the channel when a word line voltage V WL  exceeds a threshold voltage Vtn and a tunneling voltage Vtunnel. The relative differences of magnetization orientation are generated according to the differences of voltage applied to a gate metal electrode in this section. The relative differences are divided into four steps A, B, C and D.  
         [0079]    The relative difference of magnetization orientation is not generated in the step A. Large differences of magnetization orientation are generated in the steps B, C, and D, thereby resulting in the highest value of resistance in the step D and the lowest value of resistance in the step A.  
         [0080]    In Section  5 -C of FIG. 8, the amount of a current I 1  and I 2  are determined according to magnetization orientation of an MTJ. As a result, the amount of a current corresponding to data stored in an MRAM cell is applied to a bit line in this section.  
         [0081]    [0081]FIG. 9 is a diagram representing an MRAM cell by a symbol. Hereinafter, an MRAM cell of the present invention will be showed as a symbol in FIG. 9.  
         [0082]    [0082]FIGS. 10 through 13 illustrate examples of MRAM cell arrays in accordance with the present invention.  
         [0083]    [0083]FIG. 10 illustrate an array of NAND-MRAM cells in accordance with the present invention.  
         [0084]    A cell array of an MRAM shown in FIG. 10 includes an MRAM cell group connected to a bit line and a word line and a sense amplifier connected to the bit line. This sense amplifier outputs a data signal SA_OUT amplified according to input of a sense amplifier enable signal SEN.  
         [0085]    Here, an MRAM cell group includes n MRAM connected in series. The n MRAM cells also have one terminal coupled with bit lines BL (including BL 1  . . . BLn) and other terminal coupled with cell plates CP.  
         [0086]    In other words, n MRAM cell groups  110 - 1 ˜ 100 - 4  include MRAM cells  7 - 1 ,  7 A- 1 ,  7 B- 1  and  7 C- 1  having each drain connected to bit lines BL, and MRAM cells  7 -n,  7 A-n,  7 B-n and  7 C-n having each source connected to cell plates CP, respectively. A bit line BL is then coupled with a plurality of MRAM cells. MRAM cell groups  100 - 1 ˜ 100 - 4  include MRAM cells having each gate connected to word lines WL(WL 1 _ 0 ˜WLn_ 0 , WL 1 _ 1 ˜WLn_ 1 ). Word lines WL 1 _ 0 -WLn 13    0  are connected to MRAM cells  7 - 1 ˜ 7 -n in a MRAM cell group  100 - 1  and MRAM cells  7 A- 1 ˜ 7 A-n in a MRAM cell group  100 - 2 . In the same way, word lines WL 1 - 1 ˜WLn_ 1  are connected in common to MRAM cells  7 B- 1 ˜ 7 B-n in an MRAM cell group  100 - 3  and MRAM cells  7 C- 1 ˜ 7 C-n in a MRAM cell group  100 - 4 .  
         [0087]    [0087]FIG. 11 illustrates an NAND-MRAM folded bit line cell array in accordance with the present invention.  
         [0088]    An MRAM of FIG. 11 includes two MRAM cell groups and a sense amplifier. The MRAM cell groups are connected to word lines, bit lines or bit line bars, and receive switching contro signals. A sense amplifier is connected in common to bit line and bit line bar.  
         [0089]    The MRAM cell groups  200 - 1  and  200 - 2  have MRAM cells connected to each other in forms of an NAND. The MRAM cell groups have also switching transistors N 1  and N 2  connected to a bit line BL and a bit line bar BLB, respectively. MRAM cells  8 - 1 ˜ 8 -n in the MRAM cell group  200 - 1  are connected between one terminal of a switching transistor N 1  and a cell plate CP. In the same way, MRAM cells  8 B- 1 ˜ 8 B-n are connected between one terminal of a switching transistor N 2  and a cell plate CP. Switching control signals CSW 1  and CSW 2  are respectively applied to gates of switching transistors N 1  and N 2 . Word lines WL 1 ˜ 1 WLn are connected in common to gates of MRAM cells connected to the same bit line BL and the Bit line bar BLB.  
         [0090]    [0090]FIG. 12 illustrates an array of 2NAND-MRAM cells in accordance with the present invention.  
         [0091]    An MRAM of FIG. 12 will not be described here because it has the same structure of the MRAM in FIG. 11, except that a switching control signal CSW 3  is applied to each gate of switching transistors N 3  and N 4 .  
         [0092]    [0092]FIG. 13 illustrates an array of switching control NAND-MRAM cells.  
         [0093]    A MRAM of FIG. 13 comprises MRAM cell groups and a sense amplifier. The MRAM cell groups are connected to, word lines and bit lines, and receive switching control signal. The sense amplifier is connected to a bit line.  
         [0094]    The MRAM cell groups  400 - 1  and  400 - 2  have MRAM cells connected to each other in forms of an NAND. The MRAM cell groups have also switching transistors N 5  and N 6  connected to a bit line BL and a bit line bar BLB, respectively. NRAM cells  10 - 1 ˜ 10 -n in the MRAM cell group  200 - 1  are connected between one terminal of a switching transistor N 5  and a cell plate CP. In the same way, MRAM cells  10 B- 1 ˜ 10 B-n are connected between one terminal of a switching transistor N 6  and a cell plate CP. A switching control signal CSW 4  is respectively applied to gates of switching transistors N 5  and N 6 . Word lines WL 1 ˜WLn are connected in common to gates of MRAM cells connected to bit lines BL˜BLn.  
         [0095]    [0095]FIG. 14 is a timing diagram during the write operation of an MRAM cell array as described above.  
         [0096]    The write operation of an MRAM cell array will be explained, based on the operation of an MRAM cell shown in FIG. 10.  
         [0097]    This read operation is divided into an initial section t 0 , a memory cell selecting section t 1 , a sense amplifier enable section t 2  and a read terminating section t 3 .  
         [0098]    In the initial section t 0 , bit lines and word lines maintain low level voltage which does not read data, and a sense amplifier is disabled.  
         [0099]    In the memory selecting section t 1 , a large word line voltage is applied to a word line WL selected to read data stored in a MRAM cell so that a read operation can be performed in Section  4 -C of FIG. 5. A small word line voltage is applied to a non-selected word line so that a read operation can be performed in Section  4 -B of FIG. 5. Here, currents I 1  and I 2  are both generated in a selected word line because a large word line voltage is applied to a selected word line. As a result, data stored in an MRAM cell can be read according to magnetization orientation of an MTJ. Data in an MRAM connected to a selected word line is then applied to a bit line, and data in a selected MRAM cell is outputted to a sense amplifier connected to a bit line BL. The current corresponding to data in an MRAM cell is outputted to a bit line BL. When a current enough to sense is outputted to a bit line BL, the sense amplifier enable section t 2  is entered.  
         [0100]    In the sense amplifier enable section t 2 , when a sense amplifier enable signal SEN is applied to a sense amplifier SA at a predetermined level, the sense amplifier SA senses data applied to a bit line BL and then outputs sensed data SA_OUT. The sense amplifier senses a plurality of data according to the current supplied to a bit line. A sense amplifier enable signal SEN is applied to the sense amplifier SA. When the predetermined output time is exceeded, a terminating section t 3  is entered.  
         [0101]    In the terminating section t 3 , a signal for selecting a word line WL and a bit line BL, and a signal SEN for enabling a sense amplifier return to the initial section t 0 . As a result, a current corresponding to data stored in an MRAM cell is not outputted to a bit line BL, and a sensed data SA_OUT is not outputted, either.  
         [0102]    Here, data contrary to data applied to a bit line is applied to an MRAM cell connected to a bit line bar BLB shown in FIGS. 11 and 12. A current having a value contrary to logic data applied to a bit line flows in a bit line bar BLB. A sense amplifier senses data according to a current flowing in a bit line bar BLB.  
         [0103]    [0103]FIG. 15 is a timing diagram during the read operation of an MRAM cell array in accordance with the present invention.  
         [0104]    This write operation is divided into an initial section t 0 , a write section t 1  and a write terminating section t 2 .  
         [0105]    In the initial section t 0 , a ground voltage is simultaneously applied to a selected word line WL and an non-selected word line WL. While the write section t 1  is entered, a voltage for generating a predetermined current is applied to a selected word line WL.  
         [0106]    In the write section t 1 , a large word line voltage is applied to a selected word line WL so that a bit line current and a word line current enough to write can flow therein. A large bit line current is applied to an non-selected word line WL so that a word line current required to write cannot flow therein. When a write voltage is applied to a bit line BL, a magnetization orientation of a free ferromagnetic layer is determined by the direction of a current flowing between a cell plate CP and a bit line BL. Data is written in an MRAM cell according to the magnetization orientation of an MTJ. In this way, time for writing data in an MRAM cell is secured in the write section t 1 , and thereafter, a ground voltage is applied to a word line in a write terminating section t 2 .  
         [0107]    [0107]FIG. 16 illustrates a data detecting circuit for detecting the level of four data in accordance with the present invention.  
         [0108]    An MRAM of FIG. 16 has a plurality of MRAM cells connected in series to each other between a bit line BL 1  and a cell plate CP, and having each gate connected to receive signals of word lines WL 1 -WLn. The MRAM includes data detecting circuit  100  connected to a bit line BL 1 .  
         [0109]    The data detecting circuit  100  includes a current-voltage converter  110  connected to a bit line BL 1 , sense amplifiers  120 ,  130  and  140  connected to the current-voltage converter  110 , and a data encoder  150  connected to the sense amplifiers  120 ,  130  and  140 .  
         [0110]    An MRAM cell  5 - 1  has its drain connected to a bit line BL 1  and its source connected to a drain of an MRAM cell  5 - 2 . In this way, n MRAM cells  5 - 1 ˜ 5 -n are connected in series to each other. A final MRAM cell  5 -n has its source connected to a cell plate CP. MRAM cells  5 - 1 ˜ 5 -n have each gate connected to receive signals of word lines WL 1 ˜WLn, respectively.  
         [0111]    A current-voltage converter  110  connected to a bit line BL 1  converts a current flowing in MRAM cells  5 - 1 ˜ 5 -n into a voltage, and then detects data according to the differences of magnetization orientation of an MTJ in the MRAM cell. The current-voltage converter  100  transmits the detected data to sense amplifiers  120 ,  130  and  140  having different reference levels Ref_a, Ref_b and Ref_c.  
         [0112]    The sense amplifiers  120 ,  130  and  140  generate data D 1 , D 2  and D 3  according to reference levels Ref_a, Ref_b and Ref_c, using data according to magnetization orientation, and then transmit the data D 1 , D 2  and D 3  to a data encoder  150 .  
         [0113]    The data encoder  150  encodes the data D 1 , D 2  and D 3  transmitted from the sense amplifiers  120 ,  130  and  140 , and then outputs 2 bit data.  
         [0114]    Hereinafter, the process of generating 2 bit data in the data encoder  150  will be explained with reference to FIGS. 17 through 19.  
         [0115]    [0115]FIG. 17 is a graph illustrating the relations between reference voltages Ref_a, Ref_b and Ref_c, and four data A, B, C and D according to the difference of magnetization orientation of an MTJ.  
         [0116]    [0116]FIG. 18 is a table illustrating values of three data D 1 , D 2  and D 3  according to reference voltage Ref_a, Ref_b, Ref_c, and values of 2 bit data of X and Y generated from encoding data D 1 , D 2  and D 3 .  
         [0117]    [0117]FIG. 19 is a circuit diagram of a data encoder  150  for encoding data D 1 , D 2  and D 3  and generating 2 bit data X and Y.  
         [0118]    The data encoder  150  includes an AND gate AND 1  and a logic circuit  152 . The AND gate AND 1  logically combines data D 1  and D 2 , and outputs data X. The logic circuit  152  logically combines data D 1 , D 2  and D 3 , and outputs data Y.  
         [0119]    The logic circuit  152  includes an AND gate AND 2 , inverters I 1  and I 2 , an AND gate AND 3  and an OR gate OR 1 . The AND gate AND 2  AND-combines data D 1 , D 2  and D 3 . The inverters I 1  and I 2  inverts data D 2  and D 3 . The AND gate AND 3  AND-combines output signals of inverters I 1  and I 2 . The OR gate OR 1  OR-combines output signals of AND gates AND 2  and AND 3 , and outputs data Y.  
         [0120]    A table of FIG. 18 exactly shows the values of 2 bit data X and Y outputted from the data encoder  150  shown in FIG. 19.  
         [0121]    A data detecting circuit for detecting an MRAM cell array and eight data levels will be explained with reference to FIG. 20.  
         [0122]    [0122]FIG. 20 has the same structure of FIG. 16, except that a data detecting circuit  200  detects eight data levels.  
         [0123]    The data detecting circuit  200  includes a current-voltage converter  210  connected to a bit line BL 1 , seven sense amplifiers  220 - 280  connected to the current-voltage converter  210 , and a data encoder  290  connected to the seven sense amplifiers  220 ˜ 280 .  
         [0124]    The current-voltage converter  210  converts a current flowing in an MRAM cell into a voltage, and detects data A, B, C, D, E, F and G according to magnetization orientation of an MTJ in the MRAM cell. The current-voltage converter  210  then transmits the detected data to sense amplifiers  220 ˜ 280  having different reference voltages Ref_a˜Ref_g.  
         [0125]    The sense amplifiers  220 ˜ 280  generate data D 1 , D 2 , D 3 , D 4 , D 5 , D 6  and D 7  according to reference levels Ref_a˜Ref_g, using data resulted from magnetization orientation transmitted from the current-voltage converter  210 . The sense amplifiers  220 ˜ 280  then transmits the data D 1 , D 2 , D 3 , D 4 , D 5 , D 6  and D 7  to a data encoder  290 .  
         [0126]    The data encoder  290  encodes the data D 1 , D 2 , D 3 , D 5 , D 6  and D 7  transmitted from the sense amplifiers  220 ˜ 280 , and outputs 3 bit data.  
         [0127]    Hereinafter, the process of generating 3 bit data in the data encoder  290  will be explained with reference to FIGS. 21 through 23.  
         [0128]    [0128]FIG. 21 is a graph illustrating the relation between eight data A, B, C, D, E, F, G and H according to magnetization orientation of an MTJ and reference voltages Ref_a, Ref_b, Ref_c, Ref_d, Ref_e, Ref_f and Ref_g for detecting the data.  
         [0129]    [0129]FIG. 22 is a table illustrating values of data D 1 , D 2 , D 3 , D 4 , D 5 , D 6  and D 7  according to reference voltages Ref_a, Ref_b, Ref_c, Ref_d, Ref_e, Ref_f and Ref_g, and values of 3 bit data X, Y and Z resulted from encoding data D 1 , D 2 , D 3 , D 4 , D 5 , D 6  and D 7 .  
         [0130]    [0130]FIG. 23 is a logic circuit diagram of a data encoder for encoding data D 1 , D 2 , D 3 , D 4 , D 5 , D 6  and D 7  and generating 3 bit data X, Y and Z.  
         [0131]    The data encoder  290  includes a first logic circuit  292 , a second logic circuit  294  and a third logic circuit  296 . The first logic circuit  292  encodes data D 1 , D 2 , D 3 , D 4 , D 5 , D 6  and D 7 , and then generates data X. The second logic circuit  294  encodes data D 1 , D 2 , D 3 , D 4 , D 5 , D 6  and D 7 , and then generates data Y. The third logic circuit  296  encodes D 1 , D 2 , D 3 , D 4 , D 5 , D 6  and D 7 , and then generates data Z.  
         [0132]    In the first logic circuit  292 , an AND gate AND 11  AND-combines data D 1 , D 2 , D 3  and D 4 . An AND gate AND  12  AND-combines data D 5  and D 6 . Inverters I 11  and I 12  respectively inverts data D 6  and D 7 . An AND gate AND  13  AND-combines output signals of inverters I 11  and I 12 . An OR gate OR 11  OR-combines output signals of AND gates AND 12  and AND  13 . An AND gate AND  14  AND-combines output signals of the And gate AND 11  and the OR gate OR 11 , and then outputs data X.  
         [0133]    In the second logic circuit  294 , an AND gate AND 16  AND-combines data D 1  and D 2 . An AND gate AND  16  AND-combines data D 3 , D 4 , D 5  and D 6 . Inverters I 13 , I 14 , I 15  and I 16  respectively inverts data D 4 , D 5 , D 6  and D 7 . An AND gate AND 17  AND-combines output signals of inverters I 13 , I 14 , I 15  and I 16 . An OR-gate OR 12  OR-combines output signals of AND gates AND 16  and AND 17 . An AND gate AND 18  logically combines output signals of the OR gate OR 12  and the AND gate And 15 , and then ouputs data Y.  
         [0134]    In the third logic circuit  296 , an AND gate AND 19  AND-combines data D 1 , D 2 , D 3 , D 4  and D 5 . Inverters I 17  and I 18  respectively inverts data D 6  and D 7 . An AND gate AND 20  AND-combines output signals of inverters I 17  and I 18 . An AND gate AND 21  AND-combines data D 6  and D 7 . An OR gate OR 13  OR-combines output signals of AND gates AND 20  and AND 21 . An AND gate AND 22  AND-combines output signals of the OR gate OR 13  and the AND gate AND 19 . Inverters I 19 , I 20 , I 21  and I 22  invert data D 4 , D 5 , D 6  and D 7 . An AND gate AND 23  AND-combines output signals of inverters I 19 , I 20 , I 21  and I 22 . Inverters I 23  and I 24  respectively inverts data D 2  and D 3 . An AND gate AND 24  AND-combines output signals of inverters I 23  and I 24 . An AND gate AND 25  AND-combines data D 2  and D 3 . An OR gate OR 14  OR-combines output signals of AND gates AND 24  and AND 25 . An AND gate AND 26  logically combines output signals of the AND gate AND 23  and the OR gate OR 26 . An OR gate OR 15  logically combines output signals of AND gates AND 22  and AND 26 , and then output data Z.  
         [0135]    the values of data X, Y and Z outputted from the data encoder  290  are exactly shown in FIG. 22.  
         [0136]    [0136]FIGS. 24 through 27 illustrate MRAMs having different MRAM cell arrays.  
         [0137]    MRAMs shown in FIGS. 24 through 27 have the same structure of MRAMs shown in FIGS. 10 through 13, except that data detecting circuit for detecting data levels according to magnetization orientation of MTJ are used instead of sense amplifiers. Accordingly, the structures of MRAMs shown in FIGS. 24 through 27 will not be explained.  
         [0138]    data detecting circuits shown in FIGS. 24 through 27 have the same structures of data detecting circuits shown in FIGS. 16 and 20.  
         [0139]    [0139]FIG. 28 is a timing diagram during the read operation of an MRAM cell array for detecting four data A, B, C and D.  
         [0140]    This read operation is divided into an initial section T 0 , a memory cell selecting section t 1 , a sense amplifier enable section t 2  and a read terminating section t 3 .  
         [0141]    In the initial section t 0 , bit lines and word lines maintain a low level voltage not to read data, and a sense amplifier is disabled.  
         [0142]    In the memory cell selecting section t 1 , a large word line voltage is applied to a selected word line WL so that the read operation can be performed in Section  5 -C of FIG. 8, thereby resulting in the difference of magnetization orientation. A small word line voltage is applied to a non-selected word line WL so that the read operation can be performed in Section  5 -B of FIG. 8. Here, since a large word line voltage is applied to a selected word line, currents I 1  and I 2  are simultaneously generated. As a result, data stored in MRAM cell can be read. The data is stored in the MRAM cell, according to magnetization orientation of MTJ. Since a small word line, voltage is applied to a non-selected word line, only current I 2  is generated, and thereby, an MRAM cell is turned-on regardless of the magnetization orientation of MTJ. As a result, a current corresponding to data stored in an MRAM cell connected to a selected word line WL is applied to a bit line. Then, a sense amplifier enable section t 2  is entered.  
         [0143]    In the sense amplifier enable section t 2 , if a current needed tor read flows in a bit line, a sense amplifier activating signal SEN for activating a sense amplifier of a data detecting circuit is applied to a sense amplifier at the starting point of t 2 . Then, output signals of sense amplifiers SAa, SAb and SAc are generated by this sense amplifier activating signal SEN, thereby resulting in generating 2 bit data X and Y.  
         [0144]    In the read terminating section t 3 , the next cycle is prepared.  
         [0145]    [0145]FIG. 29 is a timing diagram during the write operation of an MRAM cell array for detecting four data A, B, C and D.  
         [0146]    The write operation is divided into an initial section t 0 , a write section t 1  and a write terminating section t 2 .  
         [0147]    In the initial section t 0 , a ground voltage is applied to selected and non-selected word lines WL. When a memory cell selecting section t 1  is entered, a voltage is applied to a selected word line WL.  
         [0148]    In the write section t 1 , a large word line voltage is applied to a selected word line WL so that bit line current and word line current needed to write flow in a bit line and a word line. A bit line current becomes larger so that a word line current needed to write cannot flow in the non-selected word line WL. In other words, different voltages A, B, C and D needed to write are respectively applied to a bit line. Thereby, data can be written in the MRAM cell according to magnetization orientation based on current directions between a bit line BL and a cell plate CP.  
         [0149]    Thereafter, in the write terminating section t 2 , a ground voltage is applied to a word line.  
         [0150]    As described above, the magnetization orientation is determined by directions of a word line current and a bit line current. The direction of the bit line current is maintained at one direction, and the magnetization orientation may be determined by changing the direction of the word line current. When the direction of the word line current corresponding to a logic value of “0” is determined, a current flows in only a bit line BL of an MRAM cell for writing a logic value of “0”. On the contrary, when the direction of the word line current corresponding to a logic value of “1” is determined, a current flows in only a bit line BL of an MRAM cell for writing the logic value of “1”. Accordingly, the magnetization orientation is differently regulated according to directions of the word line current and the bit line current, and thus a plurality of data may be written to each MRAM cell, As described earlier, the present invention discloses an MRAM cell for storing data according to the magnetization orientation of an MTJ, thereby improving a process.  
         [0151]    In addition, since data is read/written according to the magnetization orientation of an MTJ in an MRAM cell, the size of cell can be reduced, and the sensing margin can be improved.  
         [0152]    While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and described in detail herein. However, it should be understood that the invention is not limited to the particular forms disclosed. Rather, the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined in the appended claims.