Current conveyor and method for readout of MTJ memories

[0002] This invention relates to MTJ memories and more particularly, to apparatus and a method for reading data stored in MTJ memories.

[0003] Thin film Magnetic Random Access Memories (MRAMs) can be fabricated in a variety of memory cell embodiments, including a Magnetic Tunneling Junction (MTJ) cell. The MTJ cell essentially consists of a pair of magnetic layers with an insulating layer sandwiched therebetween. One of the magnetic layers has a fixed magnetic vector and the other magnetic layer has a changeable magnetic vector that is either aligned with or opposed to the fixed magnetic vector. When the magnetic vectors are aligned the resistance of the MTJ cell, i.e. the resistance to current flow between the magnetic layers, is a minimum and when the magnetic vectors are opposed or misaligned the resistance of the MTJ cell is a maximum.

[0004] Data is stored in the MTJ cell by applying a magnetic field to the MTJ cell directed so as to move the changeable magnetic vector to a selected orientation. Generally, the aligned orientation can be designated a logic 1 or 0 and the misaligned orientation is the opposite, i.e., a logic 0 or 1. Stored data is read or sensed by passing a current through the MTJ cell from one magnetic layer to the other. The amount of current passing through the MTJ cell, or the voltage drop across the MTJ cell, will vary according to the orientation of the changeable magnetic vector. Additional information as to the fabrication and operation of MTJ memory cells can be found in U.S. Pat. No. 5,702,831, entitled “Multi-Layer Magnetic Tunneling Junction Memory Cells”, issued Mar. 31, 1998, and incorporated herein by reference.

[0005] In the prior art, reading data stored in MTJ cells is achieved by passing a current through a series circuit including a load resistor and the MTJ cell. The current passing through the MTJ cell is controlled by a transistor with a bias voltage on the gate, and an output voltage is obtained at a junction between the load resistor and the current controlling transistor. Also, a bitline and a data line for the MTJ cell (and other MTJ cells in the array) are clamped at a desired voltage by the transistor. There are several major problems with this type of data readout including the fact that the load resistor must be much larger than the resistance of the MTJ cell, which makes operation at low supply voltages very difficult. Also, the operation of the circuit is dependent upon the clamping voltage provided by the transistor and the bias voltage. However, the clamping voltage is a function of the resistance of the MTJ cell, the bias voltage, and the load resistance, any or all of which can vary with a specific readout process, variations in the supply voltage, changes in temperature, changes in the resistance of the MTJ cell, etc. Also, the large load resistance and the other components in this prior art circuit require large chip areas preventing the fabrication of high density memory arrays. Also, the input impedance is high due to the presence of the load resistor.

[0006] Accordingly it is highly desirable to provide apparatus and a method of reading or sensing MTJ memory cells which overcomes these problems.

[0007] Referring to the drawings:

[0008] FIG. 1 is a schematic diagram of a prior art MTJ readout circuit;

[0009] FIG. 2 is a simplified schematic diagram of an MTJ readout circuit in accordance with the present invention;

[0010] FIG. 3 is a simplified schematic diagram of another embodiment of an MTJ readout circuit in accordance with the present invention;

[0011] FIG. 4 is a simplified schematic diagram of another embodiment of an MTJ readout circuit in accordance with the present invention;

[0012] FIG. 5 is a simplified schematic diagram of a complete readout circuit incorporating the MTJ readout circuit of FIG. 4; and

[0013] FIG. 6 is a more detailed schematic diagram of the complete readout circuit of FIG. 5.

[0014] Turning now to FIG. 1, a schematic diagram of a prior art MTJ readout circuit, designated 10, is illustrated. Circuit 10 includes a plurality of magnetic tunneling junction memory cells 11, 12, etc. As is well known in the art, each cell 11, 12, etc. includes an MTJ device, represented as a resistance R_MTJ, and a control or activating transistor. Only cell 11 of the plurality of memory cells will be discussed in detail herein, since the operation of all of the cells is similar. One terminal of cell 11 is connected to a current return, such as a circuit common or ground. The other terminal of cell 11 is connected to a bit line 13 which is connected to one or more memory cells (not shown) in a well known manner. The drain of a column decode transistor 14 is connected to bitline 13 and the gate is connected to a column decoder which selects one column at a time to be readout. The source of column decode transistor 14, and all of the other column decode transistors are connected to a data line 15.

[0015] Data line 15 is connected to the drain of a transistor 16, the gate of which is connected to a bias voltage V_bias. The source of transistor 16 is connected through a load resistor 17 to a current supply 18. The source of transistor 16 is also connected to a data output terminal 19. Here it should be noted that load resistor 17 is chosen to be much larger than the resistance R_MTJto provide voltage gain for the circuit. Current source 18 supplies bitline current sunk by the resistance R_MTJ, depending upon its R_minor R_maxstate. Transistor 16 clamps data line 15 and the bitlines 13 at a desired voltage depending upon the bias voltage V_bias. Transistor 16 provides for low input impedance for current mode operation, since the impedance looking into transistor 16 from data line 15 is small. Additional information as to the construction and operation of prior art MTJ readout circuits of this type can be found in U.S. Pat. No. 5,793,697.

[0016] Several problems arise in this type of MTJ readout circuit, which produce a substantial deleterious effect on the operation of the memory circuit. Specifically, since load resistor 17 and memory cell 11 are connected in series in the current path of source 18, as the resistance R_MTJof memory cell 11 changes between R_minand R_maxthe voltage at terminals 15 and 13 changes by a more than desirable value due to the high input impedance at 15. Because these lines have a relatively high natural or inherent capacitance, large swings in the voltage require relatively long times (RC time constant of the lines) and seriously slow down readout times.

[0017] In addition, the clamping voltage (provided by transistor 16 and the voltage V_biasapplied to the gate) will vary with the readout process, the power supply (e.g. current source 18 and voltage V_bias), operating temperature, and the resistance R_MTJof cell 11. Large variations of the resistance R_MTJ(i.e. a large ratio between R_minand R_max) can render the circuit virtually useless, since voltage changes may extend beyond the operating region of the circuit. Such large variations of the resistance R_MTJwill change the current in bitline 15 and lead to output voltage variations which are undesirable. Further, because load resistor 17 is generally included as part of the integrated circuit, a very large resistance will require large chip area. Finally, because load resistor 17 must be very large, operation of the memory at low supply voltages (e.g. 1.8 volts) is very difficult.

[0018] To overcome the above described exigencies, a current conveyor for readout of a magnetic tunneling junction memory cell is disclosed. Turning now to FIG. 2, a simplified schematic diagram is illustrated of an MTJ readout circuit 25 in accordance with the present invention. An array 26 of MTJ memory cells 27 arranged in rows and columns is illustrated. A data readout line 28, which in this embodiment is a bitline connecting a column of MTJ memory cells 27 together in a well known fashion, is connected through a column decode transistor 29 to a current conveyor designated 30. For purposes of this description, column decode transistors 29 are considered to be a part of each data readout line 28 with which they are associated, since they are simply switches included to select a specific line. Further, since the column decode transistors 29 are well known in the art and not a part of this invention, in several of the following descriptions they are omitted for simplicity.

[0019] Current conveyor 30 includes a transistor 32 (which may be, for example, a field effect transistor, an HFET, a thin film transistor, or the like) having one current terminal (e.g. the source or drain) connected to data readout line 28 and the other current terminal (e.g. the drain or source) connected to a current source 33. The control terminal or gate of transistor 32 is connected to the output terminal of an operational amplifier 35. A negative input terminal of operational amplifier 35 is connected to receive negative feedback from data readout line 28 and a positive input terminal is connected to have a bias voltage V_biassupplied thereto. It should be noted that the negative input terminal of operational amplifier 35 has a very high (substantially infinite) input impedance so that little or no current flows from data readout line 28.

[0020] In operation, operational amplifier 35 compares the voltage V_blon data readout line 28 to V_biasand by means of the negative feedback clamps V_blto V_bias(since operational amplifier 35 looks essentially like a short circuit between V_blto V_bias), essentially providing current mode operation. Current conveyor 30 has a very low input impedance, isolating the data readout line 28 from the high output impedance of current source 33. The low input impedance combined with the clamping of V_bllimits the voltage swing of data readout line 28 and achieves high speed readout for very high density MTJ arrays. Thus, current conveyor 30 provides and maintains a constant bias across the MTJ memory cell 27 regardless of operating temperatures, changes in the supply voltage, and process conditions. Also, current conveyor 30 provides a small swing in the voltage on data readout line 28 to allow for high speed operation. Here it should be understood that the term “operational amplifier” is a generic term for any circuit which will provide the described operation and is not limited to any specific circuit.

[0021] Here it should be noted that current source 33 provides a current equal to the current required by an MTJ memory cell 27 to yield a voltage drop of V_biasacross the cell. When the resistance R_MTJof MTJ memory cell 27 is minimum, current conveyor 30 will maintain V_biasacross R_MTJby increasing the voltage at the gate terminal of transistor 32, and when R_MTJgoes to a maximum, current conveyor 30 will decrease the voltage at the gate terminal of transistor 32. Thus, the voltage at the gate terminal of transistor 32 is an indication of the state of MTJ memory cell 27 (i.e. the state of R_MTJ) Therefore, it can be seen that current conveyor 30 performs three functions: it clamps the voltage across R_MTJ(the MTJ memory cell) to the desired bias voltage (V_bias); it makes a small data readout line voltage swing possible for high speed readout and low current consumption; and it is a current-to-voltage converter (i.e. the change in current is proportional to the voltage at the gate terminal of transistor 32).

[0022] Turning now to FIG. 3, a simplified schematic diagram is illustrated of another embodiment of an MTJ readout circuit, generally designated 40, in accordance with the present invention. Circuit 40 includes a current conveyor 41 coupled to at least one MTJ memory cell 42 by a data readout line 43, generally as described above. In this embodiment, data readout line 43 is a bitline and includes a column decode switch or transistor 44. Also, a current source 45 is coupled to data readout line 43 through current conveyor 41, as described above. A load resistor 47 is coupled to a junction 48 between current source 45 and current conveyor 41. Load resistor 47 is much larger than R_MTJof MTJ memory cell 42. An enable switch 49, illustrated herein as a transistor with the gate connected to receive an “enable” signal, couples load resistor 47 to a current return, such as ground. An output signal V_ois available at a terminal 50 coupled to junction 48.

[0023] Circuit 40 has at least two advantages over MTJ readout circuit 25 of FIG. 2. Since the resistance R_Lof load resistor 47 is much larger than the resistance R_MTJof MTJ memory cell 42, small changes in the current I_blthrough data readout line 43 due to changes in the resistance R_MTJ(i.e. R_minto R_maxor vice versa) are amplified by load resistor 47. This feature is better understood by noting that the current I from current source 45 splits at junction 48 with part of the current, I_bl, flowing through data readout line 43 and the remainder of the current I_Lflowing through load resistor 47.

[0024] V_o=(I−I_bl)R_L=(I−V_bias/R_MTJ)R_L=I_LR_L

[0025] for

[0026] R_MTJ=R_min—I_L=I_Lmin

[0027] R_MTJ=R_max—I_L=I_Lmax

[0028] ΔI_L=I_Lmax−I_Lmin—ΔV_o=ΔI_LR_L

[0029] Thus, the larger R_Lis the larger ΔV_owill be. A second advantage is that V_ois a linear function of V_bias, since V_biasis a linear function of I, as explained above.

[0030] Turning now to FIG. 4, a simplified schematic diagram is illustrated of another embodiment of an MTJ readout circuit, generally designated 60, in accordance with the present invention. Circuit 60 includes a current conveyor 61 coupled to at least one MTJ memory cell 62 by a data readout line 63, generally as described above. Current conveyor 61 includes a transistor 62 and an operational amplifier 66 connected as described in conjunction with current conveyor 30 of FIG. 2. In this embodiment, data readout line 63 is a bitline and includes a column decode switch or transistor 64. Also, a current source 65 is coupled to data readout line 63 through current conveyor 61, as described above.

[0031] A second current conveyor 67 is coupled to a junction 68 between current source 65 and current conveyor 61. Current conveyor 67 is essentially the same as current conveyor 61 and includes a transistor 70 having a first current terminal (in this embodiment the drain terminal) connected to junction 68 and a second current terminal (the source terminal) connected to a current return, such as ground. Current conveyor 67 further includes an operational amplifier 71 having a first input terminal connected to junction 68 for negative feedback, a second input terminal connected to receive a second bias voltage V_bias2and an output terminal connected to the gate of transistor 70. An output signal V_ois available at a terminal 72 coupled to the gate of transistor 70.

[0032] MTJ readout circuit 60 has all of the advantages of MTJ readout circuits 40 and 25 plus some additional advantages. Specifically, current conveyor 61 clamps data readout line 63 to V_bias1(the bias on the operational amplifier of current conveyor 61). In this manner, absolute changes of R_MTJwill not change the current I_blin data readout line 63 and the read operation will be insensitive to R_MTJchanges as long as the R_MTJvariation range is within the linear range of current conveyor 61. Transistor 62 of current conveyor 61 is a source follower, that is, the impedance looking into the source terminal of transistor 62 from data readout line 63 is low. This eliminates voltage swings of data readout line 63 to a large extent and makes current mode operation possible.

[0033] In addition to the above advantages, second current conveyor 67 provides the following functions/advantages. Transistor 62 isolates data readout line 63 (bitline and dataline in this embodiment) from high impedance of current source 65 and output impedance of transistor 70, which provides a highly sensitive and high impedance at junction 68. Current conveyor 67 operates as a secondary clamp circuit and is responsible for sensing the same MTJ memory cell 62 current changes (as current sensor 61) and providing an output V_oat output terminal 72. Current conveyor 67 clamps junction 68 to a predetermined voltage such that transistor 62 and current source 65, which in this embodiment is a MOSFET current, are maintained and kept in deep saturation under all process, supply, and temperature conditions, plus all variations of MTJ memory cell 62 within the linearity limits of operational amplifier 71.

[0034] Another important advantage provided by second current conveyor 67 is that, like resistor 47 in readout circuit 40 of FIG. 3, Current conveyor 67 boosts the output voltage based on changes in current caused by changes in R_MTJof MTJ memory cell 62. Since transistor 70 operates in deep saturation, the resistance between the drain terminal and the source terminal (r_ds) is very much greater than R_MTJ. Small changes in the current in data readout line 63 (I_bl) due to R_MTJresistance changes (R_minto R_maxand vice versa) are amplified by transistor 70.

[0035] V₆₈=(I−I_bl)r_ds=(I−V_bias1/R_MTJ)r_ds

[0036] I−V_bias1/R_MTJ=I_ds

[0037] V₆₈=I_dsr_ds

[0038] where: r_dsand I_dsequal the resistance and current, respectively, between the drain and source of transistor 70.

[0039] It should be noted that the voltage at junction 68 (V₆₈) is clamped to V_bias2, so that the gate to source voltage (V_gs) of transistor 70 must vary in order to keep V₆₈clamped. Since V₆₈change (due to changes in R_MTJ) is large, V_gschanges will be large as well.

[0040] ΔV₆₈=(I_ds(max)−I_ds(min))r_ds

[0041] =[(I−V_bias1/R_max)−(I−V_bias1/R_min)]r_ds

[0042] Thus, the larger r_dsis the larger ΔV₆₈will be.

[0043] Most importantly, current conveyor 67 can be connected to provide an offset free output. When two MTJ readout circuits similar to circuit 60 are connected to operate differentially, as illustrated in FIG. 5, any input-referred offset at the input of operational amplifier 71 does not affect the output V_o. Referring additionally to FIG. 5, a simplified schematic diagram is illustrated of a differential readout circuit 75 incorporating two MTJ readout circuits 76 and 77, similar to MTJ readout circuit 60 of FIG. 4, connected in a differential fashion. In differential readout circuit 75, circuit 76 is connected to sense the current of a reference MTJ cell 78 (which will generally include a plurality of cells) and circuit 77 is connected to sense the current of a memory MTJ cell 79 (which will generally include a similar plurality of cells).

[0044] Circuit 76 includes MTJ reference cell 78 coupled by a data line 81 to a current conveyor 82. Current conveyor 82 is connected to a junction 83, which is also connected to a current source 84 and a second current conveyor 85. A reference output voltage (V_oR) is available at the gate of transistor 86 of current conveyor 85. Similarly, circuit 77 includes MTJ data memory cell 90 coupled by a data line 91 to a current conveyor 92. Current conveyor 92 is connected to a junction 93, which is also connected to a current source 94 and a second current conveyor 95. A data output voltage (V_oD) is available at the gate of transistor 96 of current conveyor 95. Both circuits 76 and 77 are connected and operate as described in conjunction with circuit 60 of FIG. 4. Reference output voltage V_oRand data output voltage V_oDare connected to separate inputs of a differential amplifier 100, which provides an output signal V_oD−V_oR.

[0045] As stated above, a major advantage of differential readout circuit 75 is that it provides an offset free output V_oD−V_oR. This can be better understood by referring to the following. In differential readout circuit 75, in this preferred embodiment both transistors 86 and 96 operate in saturation so that the voltage dropped across each transistor 86 and 96 is approximately equal (and is designated V_thereinafter) and the beta of each transistor 86 and 96 is approximately equal (and is designated β hereinafter). The current flowing in transistor 86 (I₈₆) and in transistor 96 (I₉₆) is as follows:

[0046] I₈₆=β/2(V_oR−V_t)²—I₉₆=β/2(V_oD−V_t)²

[0047] V_oR=(2I₈₆/β)^½+V_t—V_oD=(2I₉₆/β)^½+V_t

[0048] V_oD−V_oR=(2I₉₆/β)^½+V_t−(2I₈₆/β)^½−V_t

[0049] V_oD−V_oR=(2I₉₆/β)^½−(2I₈₆/β)^½

[0050] Thus, it can be seen that the output signal V_oD−V_oRfrom differential amplifier 100 is independent of the voltage across the drain-source of transistors 86 and 96, which means that the output signal V_oD−V_oRis not affected by offset voltages in the operational amplifiers in current conveyors 85 and 95 as long as transistors 86 and 96 are in saturation.

[0051] Turning now to FIG. 6, a more detailed schematic diagram is illustrated of a complete readout circuit 110, constructed in the manner described in conjunction with differential readout circuit 75 of FIG. 5. Circuit 110 includes an array of MTJ memory cells arranged in rows and columns in a well known manner. For convenience of this description only five columns 112 through 116 are illustrated and only a small portion of each these columns is shown. Either of columns 112 or 113 is coupled through a pair of column decode transistors 120 to an MTJ readout circuit 121 (similar in construction and operation to MTJ readout circuit 60 of FIG. 4). Column 114 is coupled as a reference column through a transistor 122 to an MTJ reference circuit 123 (also similar in construction and operation to MTJ readout circuit 60 of FIG. 4). Either of columns 115 or 116 is coupled through a pair of column decode transistors 124 to an MTJ readout circuit 125 (similar in construction and operation to MTJ readout circuit 60 of FIG. 4).

[0052] Columns 112 through 116 are differentially connected by supplying a pair of differential amplifiers 130 and 131. The output signal of MTJ readout circuit 121 is supplied to the positive input of differential amplifier 130 and the reference signal from MTJ reference circuit 123 is supplied to the negative input. Similarly, The output signal of MTJ readout circuit 125 is supplied to the positive input of differential amplifier 131 and the reference signal from MTJ reference circuit 123 is supplied to the negative input. In this preferred embodiment, the value of the bias voltage applied to the first operational amplifier in the MTJ reference circuit 123 is selected so that the voltage of the reference signal from MTJ reference circuit 123 is placed at approximately the midpoint between the voltage of the output signal of MTJ readout circuit 121 and the voltage of the output signal of MTJ readout circuit 125. For all of the reasons explained above, the output voltages will retain this relationship under all process, supply, temperature, and MTJ resistance conditions.

[0053] Thus, a new and improved current conveyor is disclosed and described which greatly improves the operation and reliability of MTJ readout circuits. Because of the new and improved current conveyor, circuit operation and output signals are independent of all process, supply, temperature, and MTJ resistance conditions. Because of the new and improved current conveyor, voltage swings on data lines or bitlines are virtually eliminated so that the speed of the readout process is greatly increased. The new and improved current conveyor operates as a current-voltage converter to improve the operation and the voltage is amplified, to improve readout characteristics.

[0054] While I have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. I desire it to be understood, therefore, that this invention is not limited to the particular forms shown and I intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.