Abstract:
High density magnetic random access memory (MRAM) is disclosed. In the MRAM, by using the multi-layered magnetic materials with different resistance characteristics, the magnetic tunnel junction (MTJ) cells are connected in parallel or in series, which are connected to a transistor, so as to be a control element for reading data without complicated reading procedure and timing, resulting in high density package of MRAM.

Description:
This application is a nonprovisional utility application of provisional application No. 60/408,904, filed Sep. 9, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high density magnetic random access memory (MRAM), and more specifically, to an MRAM comprising magnetic tunnel junction (MTJ) cells with different resistance characteristics and a transistor connected in parallel or in series with the MTJ cells, so as to be a control element for reading data without complicated reading procedure and timing, resulting in high density package of magnetic random access memory. 
     2. Description of the Prior Art 
     The MRAM has advantages such as non-volatility, high integrity, high access speed and strong radiation resistance. When reading the memory data, a voltage source is provided in a selected MTJ cell, so as to determine the digit value of the data by reading the sensing current. However, when writing the memory data, the conventional method employs two current lines (namely, a bit line and a write word line) to select a MRAM by inducing a magnetic field, so as to change the magnetization orientation of the magnetic material and update the data state. 
     It is known that the MRAM between the bit line and the write word line is a stacked structure formed of multi-layered magnetic materials. Basically, the MRAM comprises a soft magnetic layer, a nonmagnetic conductor or a tunnel barrier, and a hard magnetic layer. By the magnetization orientations of the two magnetic layers, which are in parallel in the same direction or in opposite directions, the data state is determined to be “1” or “0”. In the prior arts, such as U.S. Pat. No. 6,418,046 entitled “MRAM structure and system” filed by Naji and Peter K. (Gilbert, Ariz.) and “A low power 1 Mbit MRAM based on 1T1MTJ bit cell integrated with Copper Interconnects”, VLSI 2002 by M. Durlam et al., the MRAMs are composed of a MTJ cell and a transistor. Two adjacent transistors share the same source and isolation region such that the memory cell size formed thereof is 20 F 2  (wherein F is the characteristic size of technology node). However, compared with the dynamic random access memory (DRAM), such as an example described in “A Highly Manufacturable 110 nm DRAM Technology with 8 F 2  Vertical Transistor Cell for 1 Gb and Beyond”, VLSI 2002 by H. Akatzu et al., the size of the MRAM is more than two times of 8 F 2  of the DRAM. Besides, compared with the ferroelectric random access memory (FRAM), such as an example described in “Novel Integration Technologies for Highly Manufacturable 32 Mb FRAM”, VLSI 2002 by H. H. Kim, et al., the bit size of the FRAM has decreased to 15 F 2 . Therefore, it is obvious that the MRAM has fallen behind the DRAM and FRAM. Recently, an improved structure for the MRAM has been disclosed in U.S. Pat. No. 6,421,271, in which MTJ cells are connected in parallel to a transistor such that the MRAM is downsized. However, complicated reading process such as back-writing method (commonly used in DRAM) is required for successfully accessing the data state, therefore, it significantly reduces the operation speed and is hard to replace the static random access memory (SRAM). 
     Please refer to FIG. 1, it shows a schematic diagram of a conventional MRAM comprising an MTJ cell and a transistor according to U.S. Pat. No. 5,734,605. As shown in FIG. 1, the first write word line W 1  and the second write word line W 2  are perpendicular to and crisscross with the first bit line S 1  and the second bit line S 2 . And a plurality of MTJ cells  11  and transistors  13  are disposed between the write word lines and the bit lines. Each MRAM cell is composed of an MTJ cell  11  and a transistor  13  (i.e. 1T1MTJ structure). Since the transistor includes a source, a drain, a gate, and an isolation, etc., the transistor occupies a large area when desinging the layout. The memory cell area of this kind of structure is about 20F 2 , not comparable with the DRAM. Therefore, it is not competitive. 
     Please refer to FIG. 2, it shows a schematic diagram of a conventional MRAM with a plurality of MTJ cells and a transistor according to U.S. Pat. No. 6,421,271. As shown in FIG. 2, a plurality of MTJ cells  20  are connected in parallel with each other and also connected to a transistor, therefore, significantly increasing the density. The gate of the first transistor Tr 1  connects to the first read word line WL 1 , and the drain of the first transistor Tr 1  connects to a plurality of MTJ cells  20  that are connected in parallel with each other. The gate of the second transistor Tr 2  connects to the second read word line WL 2 , and the drain of the transistor Tr 2  also connects to other MTJ cells  20  that are connected in parallel with each other, and use the same bit line BL with those MTJ cells  20  connected to the first transistor Tr 1 . However, since readout signal of the bit line is the equivalent resistance after the MTJ cells  20  are connected in parallel, it needs an additional readout process to filter the complex signals. The additional readout process requires destructive reading process, resulting in poor endurance of the memory cells and reducing the readout speed. 
     Accordingly, the present invention provides an MRAM to improve the drawbacks of uncapable of reducing size and increasing readout speed. The invention uses two bits to share one transistor and uses two MTJ cells with different resistance characteristics, which are connected in parallel or in series, so as to increase the package density and the readout speed, and to be a unified memory in replace of the conventional Flash, SRAM, and DRAM. 
     SUMMARY OF THE INVENTION 
     Accordingly, the primary object of the present invention is to provide a high density MRAM with reduced size. In the MRAM, by using the multi-layered magnetic materials with different resistance characteristics, the magnetic tunnel junction cells are connected in parallel or in series, which are connected to a transistor, so as to be a control element for reading data. And there is a write word line for providing the magnetic field of the writing operation for a plurality of MTJ cells. Besides, there is a read word line for controlling the readout signal. Further, there is a plurality of bit lines for reading the data. Therefore, the present invention can increase the packing density and provide a unified memory in replace of the conventional Flash, SRAM, and DRAM. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, spirits and advantages of the preferred embodiments of the present invention will be readily understood by the accompanying drawings and detailed descriptions, wherein: 
     FIG. 1 is a schematic diagram showing a conventional MRAM comprising a MTJ cell and a transistor; 
     FIG. 2 is a schematic diagram showing a conventional MRAM comprising a plurality of MTJ cells and a transistor; 
     FIG. 3 is a schematic diagram showing a high density MRAM comprising a two-bit memory cell of a series MTJ structure in accordance with the present invention; 
     FIG. 4 is a schematic diagram showing a R-H loop in two MTJ cells of the high density MRAM in accordance with the present invention; 
     FIG. 5 is a schematic diagram showing a series MTJ structure of the high density MRAM in accordance with the present invention; 
     FIG. 6 is a schematic diagram showing a two-bit memory cell of a parallel MTJ structure in accordance with the present invention; 
     FIG. 7 is a schematic diagram showing a parallel MTJ structure of the high density MRAM in accordance with the present invention; 
     FIG. 8 is a 3-D schematic diagram showing a 4×4 array of a series MTJ structure of the high density MRAM in accordance with the present invention; 
     FIG. 9 is a schematic diagram showing the writing operation of a series MTJ structure of the high density MRAM in accordance with the present invention; 
     FIG. 10 is a schematic diagram showing the reading operation of a series MTJ structure of the high density MRAM in accordance with the present invention; 
     FIG. 11 is a 3-D schematic diagram showing a 4×4 array of a parallel MTJ structure of the high density MRAM in accordance with the present invention; 
     FIG. 12 is a schematic diagram showing the writing operation of a parallel MTJ structure of the high density MRAM in accordance with the present invention; 
     FIG. 13 is a schematic diagram showing the reading operation of a parallel MTJ structure of the high density MRAM in accordance with the present invention; 
     FIG. 14 is a schematic diagram showing the writing operation of the second parallel MTJ structure of the high density MRAM in accordance with the present invention; 
     FIG. 15 is a schematic diagram showing the reading operation of the second MTJ structure of the high density MRAM in accordance with the present invention; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention can be exemplified by the preferred embodiments as described hereinafter. 
     Please refer to FIG.  3 . FIG. 3 is a schematic diagram showing a high density MRAM comprising a two-bit memory cell of a series MTJ structure in accordance with the present invention. Wherein the first MTJ cell R 1  and the second MTJ cell R 2  have characteristic curves with different resistances and are connected with each other through a transistor  30 . The transistor  30  is used for controlling readout signal through a read word line WL 1  at the gate terminal. One terminal of the first MTJ cell R 1  is connected to the first bit line BL 1  and one terminal of the second MTJ cell R 2  is connected to the second bit line BL 2 . The first write word line DL 1  passes the vicinity of the first MTJ cell R 1  and the second MTJ cell R 2 , so as to provide a magnetic field required for writing operation. The two-bit memory cells are magnetic materials of the high density MRAM. By changing the magnetization orientation of the materials, the data state of the two-bit memory cells can be updated, that is, reading or writing data. The above-mentioned transistor  30  is the switch and controlling element of the two-bit memory cells for reading data. 
     FIG. 4 is a schematic diagram showing a R-H loop in two MTJ cells of the high density MRAM in accordance with the present invention. At zero magnetic field, taking the first MTJ cell R 1  for example, there are two states for the resistance value, that is, R 1 max and R 1 min, indicating that the MTJ cell has non-volatile memory effect. In the present invention, however, two MTJ cells with different resistance characteristics (i.e., R 1  and R 2 ) are connected either in series or in parallel. More particularly, there are two states (R 1 max and R 1 min) for the first MTJ cell R 1  and two states (R 2 max and R 2 min) for the second MTJ cell R 2 . 
     When the two MTJ cells are connected in series, the overall equivalent resistance would be (R 1 max+R 2 max), (R 1 min+R 2 max), (R 1 max+R 2 min) or (R 1 min+R 2 min). That is, there are four states for the overall equivalent resistance when the two MTJ cells are connected in series. Similarly, there are also four states for the overall equivalent resistance when the two MTJ cells are connected in parallel. Therefore, no additional readout process and clock cycle are required when reading out the data. 
     Following are a few calculations based on the case in which the two MTJ cells are connected in series. Assuming the specific resistance value (RA) of the MTJ cell is 10 KΩ-μm 2 , the area of the first MTJ cell R 1  is 0.2592 μm 2  (0.36×0.72 μm 2 ), the area of the second MTJ cell R 2  is 0.1568 μm 2  (0.56×0.28 μm 2 ), and the magneto resistance (MR) value of the MTJ cell is 50%, there are four states (154 KΩ, 134 KΩ, 122 KΩ and 102 KΩ) for the two serial connected MTJ cells. On the other hand, there are also four states (36 KΩ, 30 KΩ, 27 KΩ and 24 KΩ) for the two parallel connected MTJ cells. 
     Besides, in the conventional structure, if the resistance value of the first MTJ cell R 1  is the same with that of the second MTJ cell R 2 , the specific resistance value (RA) of the MTJ cell is 10 KΩ-μm 2 , the area is 0.1568 μm 2  (0.56×0.28 μm 2 ), there are three states (48 KΩ, 38 KΩ, and 32 KΩ) for the two parallel connected MTJ cells. Therefore, additional readout process and clock cycle are required to figure out the bit state. 
     In addition to the connection of the first MTJ cell R 1  and the second MTJ cell R 2  shown in FIG. 3, this invention further includes the combination of more than the first MTJ cell R 1  and the second MTJ cell R 2  connection. Another embodiment of this invention is to apply the first MTJ cell R 1 , the second MTJ cell R 2 , and the third MTJ cell R 3  connected in series or parallel to make the equivalent resistance value divide into eight separated equivalent values. The separated equivalent value employs an appropriate reference generator to distinguish the individual bit state. By using more MTJ cells with serial or parallel connection, a high density and high access speed MRAM can be provided according to this invention. 
     Please further refer to FIG. 5, which is a schematic diagram showing a series MTJ structure of the high density MRAM in accordance with the present invention. As shown in FIG. 5, by using a two-bit cell sharing a transistor, the area occupied by each memory cell is reduced. Wherein the first MTJ cell R 1  is connected to the first bit line BL 1  and the second MTJ cell R 2  is connected to the second bit line BL 2 . Further, the first MTJ cell R 1  and the second MTJ cell R 2  are connected to a transistor  51 . The gate of the transistor  51  is connected to the first read word line WL 1  for controlling the readout signal. Another write word line DL 1  passes the vicinity of the first MTJ cell R 1  and the second MTJ cell R 2  so as to provide a magnetic field required for writing operation. The first two-bit memory cell  55  is therefore constructed. Besides, the transistor  52  is connected to the first bit line BL 1  and the second bit line BL 2  by the MTJ cells. The gate of the transistor  52  is connected to the second read word line WL 2 . Another write word line DL 2  passes the vicinity of the MTJ cells so as to provide a magnetic field required for writing operation. The second two-bit memory cell  56  is therefore constructed. Further, the transistor  53  is connected to a third bit line BL 3  and the forth bit line BL 4  by the MTJ cells. The gate of the transistor  53  is connected to the first read word line WL 1 . The write word line DL 1  passes the vicinity of the MTJ cells so as to provide a magnetic field required for writing operation. The third two-bit memory cell  57  is therefore constructed. Furthermore, the transistor  54  is connected to the third bit line BL 3  and the forth bit line BL 4  by the MTJ cells. The gate of the transistor  54  is connected to the second read word line WL 2 . The write word line DL 2  passes the vicinity of the MTJ cells so as to provide a magnetic field required for writing operation. The forth two-bit memory cell  58  is therefore constructed. The first two-bit memory cell  55 , the second two-bit memory cell  56 , the third two-bit memory cell  57 , and the forth two-bit memory cell  58  comprise one embodiment of the series MTJ structure of the present invention. 
     FIG. 5 shows a 4×2 array of the series MTJ structure according to FIG.  3 . In practical cases, the number of the memory cells is not limited to those shown in FIG.  5 . 
     Please refer to FIG. 6, which is a schematic diagram showing a two-bit memory cell of a parallel MTJ structure in accordance with the present invention. As shown in FIG. 6, the first MTJ cell R 1  and the second MTJ cell R 2  have characteristic curves with different resistance characteristics and are connected in parallel to a transistor  60 . The transistor  60  is used for controlling the readout signal through the first read word line WL 1  at the gate terminal. One terminal of the transistor  60  is connected to the first MTJ cell R 1  and the second MTJ cell R 2 , which are connected in parallel. While another terminal of the transistor  60  is grounded G 1 . One terminal of the first MTJ cell R 1  is connected to the first bit line BL 1  and one terminal of the second MTJ cell R 2  is also connected to the same bit line BL 1 . The write word line DL 2  passes the vicinity of the first MTJ cell R 1  so as to provide a magnetic field required for writing operation. Another write word line DL 1  passes the vicinity of the second MTJ cell R 1  so as to provide a magnetic field required for writing operation. 
     Please further refer to FIG. 7, which is a schematic diagram showing a parallel MTJ structure of the high density MRAM in accordance with the present invention. As shown in FIG. 7, one terminal of the first MTJ cell R 1  and one terminal of the second MTJ cell R 2  are commonly connected to a transistor  51 . The other terminal of the first MTJ cell R 1  and the other terminal of the second MTJ cell R 2  are commonly connected to the first bit line BL 1 . There are first write word line DL 1  and second write word line DL 2  pass the vicinity of the second MTJ cell R 2  and the first MTJ cell R 1  respectively, so as to provide a magnetic field required for writing operation. And one terminal of the first transistor  51  is connected to the first read word line WL 1 , thereby the first two-bit memory cell  75  is constructed. Besides, another terminal of the first transistor  51  is connected to the transistor  52 . The transistor  52  is connected to the first bit line BL 1  by the parallel connected MTJ cells. The third write word line DL 3  and the forth write word line pass the vicinity of the MTJ cells, so as to provide a magnetic field required for writing operation. The gate of the second transistor  52  is connected to the second read word line WL 2 , thereby the second two-bit memory cell  76  is constructed. Further, one terminal of the transistor  53  is connected to the second bit line BL 2  by the parallel connected MTJ cells. The gate of the transistor  53  is connected to the first read word line WL 1 . The first write word line DL 1  and the second write word line DL 2  pass the vicinity of the MTJ cells, so as to provide a magnetic field required for writing operation, a third two-bit memory cell  77  is therefore constructed. Furthermore, one terminal of the transistor  53  is connected to the forth transistor  54 . One terminal of the forth transistor  54  is connected to the second bit line BL 2  by the parallel connected MTJ cells. The gate of the transistor  54  is connected to the second read word line WL 2 . The third write word line DL 3  and the forth write word line DL 4  pass the vicinity of the MTJ cells, so as to provide a magnetic field required for writing operation, the forth two-bit memory cell  78  is therefore constructed. The above-mentioned read word lines are used for controlling the readout signal from the MTJ cells. The write word lines are used for providing one part of the magnetic field required for writing operation. The bit lines are readout lines and used for providing another part of the magnetic field required for writing operation. The first two-bit memory cell  75 , the second two-bit memory cell  76 , the third two-bit memory cell  77 , and the forth two-bit memory cell  78  comprise one embodiment of the parallel MTJ structure of the present invention. 
     FIG. 7 shows a 2×4 array of the parallel MTJ structure according to FIG.  6 . In practical cases, more memory cells can be employed to form an MRAM. 
     Please refer to FIG.  8 . FIG. 8 is a 3-D schematic diagram showing a 4×4 array of a series MTJ structure of the high density MRAM in accordance with the present invention. By designing a plurality of different sizes of series MTJ cells, a plurality of different resistances between the MTJ cells are achieved. 
     As shown in FIG. 8, the first MTJ cell  81  and the second MTJ cell  82  with different sizes are connected in series with two terminals of the transistor  83 , while another terminal of the transistor  83  is connected to the first read word line WL 1 . One terminal of the MTJ cell  81  is connected to the first bit line BL 1 , one terminal of the MTJ cell  82  is connected to the second bit line BL 2 . The first write word line DL 1  passes the vicinity of the two MTJ cells for providing a part of magnetic field for writing operation. The above-mentioned first MTJ cell  81 , second MTJ cell  82 , transistor  83 , first bit line BL 1 , second bit line BL 2 , first read word line WL 1  and first write word line DL 1  comprise a two-bit memory cell  85 . Also, FIG. 8 shows the array of a plurality of the memory cells  85 . As shown in FIG. 8, a plurality of MTJ cells with different sizes, a plurality of transistors, a plurality of write word lines, a plurality of read word lines and a plurality of bit lines achieve the high density MRAM. 
     FIG. 9 shows the writing operation of a series MTJ structure. The MTJ cells are isolated when the transistor is turned off. The writing operation is performed by cross selection. 
     When writing data “1”, the current on the first write word line DL 1  flows from right to left so as to generate a magnetic field on the hard axis of the MTJ cell, while the current on the second bit line BL 2  flows upwards from bottom to top so as to generate a magnetic field along the first direction on the easy axis of the MTJ cells. Data is written into the MTJ cell  82  only when two current paths crisscross on the MTJ cell  82 . However, in practical cases, the current flow direction is not limited to the above-mentioned directions. 
     On the other hand, when writing data “0”, the current on the first write word line DL 1  flows from right to left so as to generate a magnetic field on the hard axis of the MTJ cells, while the current on the third bit line BL 3  flows downwards from top to bottom so as to generate a magnetic field along the second direction on the easy axis of the MTJ cells. The MTJ cell  93  on which two current paths crisscrossed is written with data “0”. Similar to the conventional 1T1MTJ structure, the writing operation of the MTJ cells as shown in FIG. 9 is performed by a bit line and a write word line crisscrossed on a MTJ cell so as to update the data. 
     Please also refer to FIG. 10, which shows the reading operation of a series MTJ structure. As shown in FIG. 10, the transistor  83  is turned on by the read word line WL 1 . A voltage current is provided from the first bit line BL 1 , and the second bit line BL 2  is grounded. The sense current flows through the left MTJ  81 , the transistor  83  and the right MTJ  82 , and the ground terminal. The reading process is performed by comparing the sense current with a reference current generated by a reference generator  100 . Therefore, the MRAM can read the two-bit data simultaneously without complicated reading clock or timing to distinguish individual bit data. A high density with high reading throughput MRAM can be achieved. 
     FIG. 11 is a 3D schematic diagram showing a 4×4 array of a parallel MTJ structure of the high density MRAM in accordance with the present invention. By designing two different sizes of parallel MTJ structure, two different resistances are achieved, that is, different R-H loops in two MTJ cells of FIG. 4 can be achieved. As shown in FIG. 11, the MTJ cell  81  and MTJ cell  82  are connected in parallel to one terminal of a transistor  83 , while another terminal of the transistor  83  is grounded (G 1 ). Ground G 1  is also the ground of each MTJ cell. The gate of the transistor  83  is used for controlling the readout signal through the first read word line WL 1 . One terminal of the MTJ cell  81  and one terminal of the MTJ cell  82  are connected to the first bit line BL 1 . Beside, the first write word line DL 1  passes the vicinity of MTJ cell  82 , so as to provide a magnetic field required for writing operation. Further, the second write word line DL 2  passes the vicinity of MTJ cell  81 , so as to provide a magnetic field required for writing operation. Therefore, the MTJ cell  81 , MTJ cell  82 , transistor  83 , each bit line, write line, ground connected thereof comprise a pair of memory cell  110  and the array of a plurality of memory cells comprises the embodiment of parallel MTJ structure of the high density MRAM according to the present invention. 
     FIG. 12 is a schematic diagram showing the writing operation of a parallel MTJ structure of the high density MRAM in accordance with the present invention. FIG. 12 shows the writing operation of a parallel MTJ structure with all transistors turned off. The writing operation is performed by cross selection. 
     When writing data “1”, the current on the second write word line DL 2  flows from right to left so as to generate a magnetic field on the hard axis of the MTJ cell, while the current on the first bit line BL 1  flows upwards from bottom to top, so as to generate a magnetic field along the first direction on the easy axis of the MTJ cell. Data is written into the MTJ cell  81  only when two current paths crisscross on the MTJ cell  81 . 
     However, when writing data “0”, the current on the second write word line DL 2  flows from right to left so as to generate a magnetic field on the hard axis of the MTJ cell, while the current on the second bit line BL 2  flows downwards from top to bottom, so as to generate a magnetic field along the second direction on the easy axis of the MTJ cell. The MTJ cell  124  on which two current paths crisscross is written with data “0”. Similar to the conventional 1T1MTJ structure, the writing operation of the MTJ cell is performed by a bit line and a write word line crisscrossed on a MTJ cell so as to update the data. However, in practical cases, the current path directions are not limited to the above-mentioned directions. 
     Please refer to FIG. 13, which shows the reading operation of a parallel MTJ structure. In FIG. 13, taking the two MTJ cells on the top-left corner for example, the transistor  83  is turned on by the first read word line WL 1 . A voltage source is provided from the first bit line BL 1 . The sense current flows through the two parallel connected MTJ cells ( 81  and  82 ), the turned-on transistor  83  and the ground (G 1 ). The reading process is performed by comparing the sense current with the reference current generated by a reference generator  100 . Therefore, the MRAM can read the two-bit data simultaneously without complicated reading clock or timing to distinguish individual bit data. 
     As the above-mentioned high density MRAM, the memory cells are a plurality of MTJ cells with different size and interlaced arrangement. The MTJ cells include those disposed on the write word lines and bit lines. Another different embodiment of the present invention will be described in the following. 
     FIG. 14 is a schematic diagram showing the writing operation of the second parallel MTJ structure of the high density MRAM in accordance with the present invention. In FIG. 12, the MTJ cells, in which the currents on each write word line flow through, are the same size. However, in FIG. 14, along the path direction of the first write word line DL 1 , the second write word line DL 2 , the third write word line DL 3  and the forth write word line DL 4 , the MTJ cells disposed thereof are not the same size. Besides, when writing data, the first read word line WL 1  and the second read word line WL 2  are turned off, that is, each transistor is turned off. 
     FIG. 15 is a schematic diagram showing the reading operation of a parallel MTJ structure of the high density MRAM in accordance with the present invention, so as to change the arrangement sequence of the MTJ cells in FIG.  13 . In FIG. 13, the MTJ cells, in which the currents on each write word line flows through, are the same size. However, in FIG. 15, along the path direction of the first write word line DL 1 , the second write word line DL 2 , the third write word line DL 3  and the forth write word line DL 4 , the MTJ cells disposed thereof are not the same size. Besides, when reading the two MTJ cells on the top-left corner, the first read word line WL 1  is turned on, that is, each transistor connected to WL 1  is turned on. 
     As the above-mentioned, the invention uses two MTJ cells with different area and different resistance characteristics, which are connected in parallel or in series. On the other hand, by using the same area with different manufacturing process, MTJ cells with different resistance characteristics can be connected in parallel or in series. By using the above-mentioned method, there will be four states or more of the equivalent resistance value. Since no additional complicated readout process and timing are required, the present invention increases the package density and does not decrease the readout speed, and will be a unified memory in replace of the conventional Flash, SRAM, and DRAM. 
     Accordingly, it should be evident to those skilled in the art that minor variations may be in the disclosed embodiments without departing from the spirit and scope of the invention.