KR20150144208A - Magnetoresistive random access memory using negative resistance - Google Patents

Abstract

Disclosed is a magnetic resistive memory device capable of preventing a random status conversion of MTJ during reading. The magnetic resistive memory device includes a memory cell having an access transistor and MTJ, a negative resistance connected in a row with the memory cell in reference of a sensing nod, and a reference cell unit for generating a reference voltage. Hereon, an initial voltage of the sensing nod during reading is determined as the reference voltage of the reference cell unit.

Description

[0001] MAGNETORESISTIVE RANDOM ACCESS MEMORY USING NEGATIVE RESISTANCE [0002] BACKGROUND OF THE INVENTION [0003]

The present invention relates to a magnetoresistive memory device, for example a STT-MRAM.

Random Access Memory (RAM) may be volatile or non-volatile. Volatile RAM loses information stored in volatile RAM each time power is removed, while non-volatile RAM can retain memory contents in non-volatile RAM even when power is removed from memory.

However, while non-volatile RAM has the advantage of being able to retain information without powering up, normal non-volatile RAM has slower write / read times than volatile RAM.

Magnetoresistive random access memory (MRAM) is a non-volatile memory technology with write / read times comparable to volatile memory. Unlike conventional RAM technology, which stores data such as electrical charges or current flow, the MRAM uses magnetic currents.

FIG. 1 is a view showing the structure of a general MTJ, and FIG. 2 is a diagram showing a structure of a memory cell of a general MRAM. 3 is a diagram illustrating an operation process of a general MRAM.

Referring to FIG. 2, the memory cell of the MRAM includes a magnetic tunnel junction (MTJ) and a transistor MN.

One end of the MTJ is connected to the bit line BL, the gate of the transistor MN is connected to the word line WL, and the source of the transistor MN is connected to the source line SL.

The MTJ comprises a pinned layer 100, a tunnel barrier layer 102 and a free layer 104, as shown in Figures 1 and 2.

The pinned layer 100 and the free layer 104 are made of a ferromagnetic material and can each have a magnetization direction and are separated by a tunnel barrier layer 102.

The pinned layer 100 is set to a specific polarity and the polarity of the free layer 104 can freely change to match the polarity of the external field that can be applied.

The change in polarity of the free layer 104 changes the resistance of the MTJ element. For example, the MTJ element has a low resistance state when the polarities are aligned, that is, when the magnetization directions coincide with each other (A in FIG. 1), and when the polarities are not aligned, 1, B, half-balanced state).

As to the structure of the entire MRAM, the MRAM includes a plurality of arrays, and each array includes a plurality of memory cells. At this time, each memory cell has the structure shown in FIG.

A read current I _READ flows from the bit line BL toward the source line SL when the voltage applied to the bit line BL is higher than the voltage applied to the source line SL. On the other hand, when the voltage applied to the bit line BL is smaller than the voltage applied to the source line SL, the read current I _READ flows from the source line SL toward the bit line BL.

That is, a read current (I _READ ) is applied to the memory cell for a read operation, and as a result, the resistance ( _MTJ ) of the _MTJ is varied. The MRAM senses the resistance ( _MTJ ) of the variable MTJ to determine a read operation.

Referring to FIG. 3, the read current (I _READ ) is generally selected to maximize the voltage at the sensing node. As a result, the read current (I _READ ) approaches the switching current I _{C -} . That is, non-destructive read margin becomes small, and arbitrary state conversion of MTJ may occur during a read operation. Particularly, in the manufacturing process, the switching current I _{C -} may change. When the switching current I _{C -} moves further to the right in FIG. 3, the read margin becomes smaller, Can be increased.

Korean Patent Publication No. 2004-0035850 (Published on April 29, 2004)

The present invention provides a magnetoresistive memory device, particularly a STT-MRAM, capable of preventing arbitrary state transition of the MTJ during a read operation.

According to an aspect of the present invention, there is provided a magnetoresistive memory device including: a memory cell having an access transistor and an MTJ; A negative resistance connected in parallel to the memory cell with respect to the sense node; And a reference cell unit connected to the memory cell and generating a reference voltage. Here, the initial voltage of the sense node is set to a reference voltage of the reference cell unit during a read operation.

A magnetoresistance memory device according to another embodiment of the present invention includes: a memory cell having an MTJ; A sensing node coupled to the memory cell; And a resistance element coupled to the MTJ. Here, the resistance according to the MTJ and the resistance element varies depending on the state of the MTJ, and a predetermined voltage is set as an initial voltage of the sensing node in a reading operation.

The magnetoresistive memory device according to the present invention can prevent the MTJ from being switched to any state during a read operation by connecting a negative resistance to the memory cell in parallel.

In addition, the magnetoresistive memory device sets the initial voltage of the sense node to a reference voltage, thereby preventing erroneous operation of the read operation regardless of the model parameter.

1 is a diagram showing a structure of a general MTJ.
2 is a diagram showing a structure of a memory cell of a general MRAM.
3 is a diagram illustrating an operation process of a general MRAM.
4 is a diagram showing a principle of a magnetoresistive memory device according to an embodiment of the present invention.
5 is a diagram showing a configuration of a magnetoresistive memory device according to an embodiment of the present invention.
6 is a diagram showing a circuit of a magnetoresistive memory device according to an embodiment of the present invention.
7 is a diagram showing the operation of a memory cell in the magnetoresistive memory device of FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The present invention relates to a magnetoresistive memory device, for example, a spin torque transfer magnetic random access memory (STT-MRAM), and more particularly, Errors can be prevented.

In general STT-MRAM, due to the process error (process mismatch) between MTJ and read circuit, unwanted MTJ state transition may occur during read operation. The present invention proposes a magnetoresistive memory device capable of preventing arbitrary state transition of the MTJ even in such a process error.

According to one embodiment, the magnetoresistive memory device of the present invention can prevent any state transition of the MTJ using a negative resistance.

In particular, the magnetoresistive memory device may use a negative resistance and set an initial voltage of a sensing node to a reference voltage of a reference cell to enable a normal read operation regardless of a model parameter.

Hereinafter, various embodiments of the magnetoresistance memory device and the method of operation of the present invention will be described in detail with reference to the accompanying drawings.

4 is a diagram showing a principle of a magnetoresistive memory device according to an embodiment of the present invention. 4, R _P means the resistance of the MTJ when the magnetization directions of the pinned layer and the free layer are the same (hereinafter referred to as "balanced state"), R _AP indicates the case where the magnetization directions of the pinned layer and the free layer are different , "Half-equilibrium state").

Referring to FIG. 4, a negative resistance may be connected in parallel to the MTJ of the memory cell. The negative resistance plays a role of preventing a malfunction in a reading operation.

In a general magnetoresistive memory device having no negative resistance, the resistance ( _MTJ ) of the _MTJ is set as shown in the graph of Fig. When a read current I _READ is applied to a memory cell for a read operation, the resistance R _{MTJ of the MTJ} is varied, and the magnetoresistive memory device senses the resistance R _MTJ of the variable _MTJ to determine whether the read operation is completed .

In this case, the read current (I _READ ) is generally selected so that the sense node voltage is maximized, which causes the read current (I _READ ) to approach the switching current of the MTJ, I _{C -} . That is, non-destructive read margin becomes small, and arbitrary state conversion of MTJ may occur during a read operation. Particularly, in the manufacturing process, the switching current I _{C -} may change. When the switching current I _{C -} moves further to the right in FIG. 3, the read margin becomes smaller, Can be increased.

On the other hand, in the magnetoresistance memory device of the present invention, the negative resistance may be connected in parallel to the memory cell as shown in FIG. According to one embodiment, the resistance (R _AP ) of the MTJ in the semi-equilibrium state and the resistance (R _P ) of the MTJ in the equilibrium state having the negative value when the negative resistance is connected in parallel are parallel to the negative resistance A negative resistance can be used in the magnetoresistive memory device so that the resistance according to the connection to the positive terminal has a positive value.

The negative resistance connected in parallel to the sense line capacitor C _{SL results} in an unstable system and the positive resistance connected in parallel to the capacitor C _SL is a stable system, Lt; / RTI >

Small initial voltage (V _init) of the MTJ is unstable systems increases exponentially with the voltage (V _MTJ) of the MTJ to the V _DD direction and may enhance the voltage of the detection node, in a stable system voltage (V _MTJ of the MTJ May be reduced in the direction of the ground voltage to lower the voltage of the sense node. As a result, arbitrary state transition of the MTJ may not occur. In another aspect, the magnetoresistive memory device may perform a non-destructive read operation.

According to one embodiment, the magnetoresistive memory device may have a simple circuit structure using the negative resistance, but not sensitive to the model parameters. For example, the present invention proposes a magnetoresistive memory device capable of preventing erroneous operation of a read operation even when a tuning magneto-resistance (TMR) as a model parameter is set to be smaller than expected.

According to another embodiment, the magnetoresistive memory device can prevent an erroneous operation during a read operation by setting the initial voltage of the sense node to have the voltage of the reference cell portion. If the magnetoresistive memory device is configured such that the initial voltage of the sense node is not set, the initial voltage of the sense node starts at a voltage close to the power supply voltage (V _DD ). In this case, the negative resistance is connected in parallel to the MTJ A malfunction may occur at the time of reading operation.

Therefore, the magnetoresistive memory device of the present invention can prevent the erroneous operation during the read operation by setting the initial voltage of the sense node to the voltage of the reference cell portion. A detailed description thereof will be described later.

5 is a diagram showing a configuration of a magnetoresistive memory device according to an embodiment of the present invention. Of course, the magnetoresistive memory device includes many memory cells and reference cell portions, but only one memory cell and reference cell portion are shown in FIG. 5 for convenience of explanation.

Referring to FIG. 5, the magnetoresistance memory device of the present embodiment may include a memory cell 500, a reference cell portion 502, a negative resistance portion 504, and a read operation determination portion 506.

The memory cell 500 includes a circuit in which the access transistor and the MTJ are connected in series. The voltage of the sense node S is changed according to the resistance of the memory cell 500. [ According to one embodiment, the initial voltage of the sense node S may be set to the reference voltage of the reference cell portion 502.

The reference cell unit 502 outputs a reference voltage for judging the completion of the read operation and is connected to the memory cell 500.

The negative resistance portion 504 may include a negative resistance connected in parallel with the MTJ of the memory cell 500. Here, the resistance of the MTJ in the semi-equilibrium state and the resistance due to the negative resistance may have a negative value, and the resistance of the MTJ in the equilibrium state and the resistance according to the negative resistance may have a positive value. As a result, the voltage of the sense node S rises in the half-balanced state and decreases in the balanced state.

5, the negative resistance unit 504 is a separate element from the memory cell 500. However, the negative resistance unit 504 may be a part of the memory cell 500. FIG.

The read operation determination unit 506 compares the voltage of the sense node S corresponding to the voltage of the memory cell 500 with the reference voltage of the reference cell unit 502 and determines whether or not the read operation is completed .

In summary, the magnetoresistive memory device of this embodiment uses the negative resistance portion 504 connected to the memory cell 500 to prevent the arbitrary state transition of the MTJ, and the initial voltage of the sense node S to the reference cell portion 502 Of the reference voltage.

FIG. 6 is a diagram showing a circuit of a magnetoresistive memory device according to an embodiment of the present invention, and FIG. 7 is a diagram showing the operation of a memory cell in the magnetoresistive memory device of FIG.

Referring to FIG. 6, a memory cell 500 includes a transistor MT connected in series and an MTJ.

The negative resistance portion 504 may include transistors M1, M2 and M3 constituting a negative resistance.

Transistor M1 may be an N-MOS transistor. Here, the gate of the transistor M1 may be connected to the drain of the sense node S and the transistor MT of the memory cell 500, and the source may be connected to the ground.

The transistors M2 and M3 may form a mirror structure as a P-MOS transistor. The drain of the transistor M2 may be connected to the drain of the transistor M1 and the drain of the transistor M3 may be connected to the drain of the transistor MT of the memory cell 500. [

The reference cell portion 502 may include a transistor RT and a resistor R _f as a reference cell, a negative resistance 600, an amplifier A 602, and a capacitor C.

The transistor RT may be an N-MOS transistor. At this time, the word line WL may be connected to the gate of the transistor RT and the gate of the transistor MT of the memory cell 500. As a result, the transistors MT and RT can be activated or deactivated simultaneously according to the word line signal applied through the word line WL.

The resistor _Rf may be connected to the source of the transistor RT and may be set to a resistance value when, for example, the resistance of the MTJ in the anti-parallel state and the resistance of the MTJ in the balanced state are connected in parallel.

The negative resistance 600 may include transistors MR1, MR2, and MR3.

The transistor MR1 may be an N-MOS transistor. Here, the gate of the transistor MR1 may be connected to the input terminal of the amplifier 602, for example, the (+) terminal, and the source may be connected to the ground.

The transistors MR2 and MR3 may form a mirror structure as a P-MOS transistor. Here, the drain of the transistor MR2 may be connected to the drain of the transistor MR1, and the drain of the transistor MR3 may be connected to the drain of the transistor RT of the reference cell.

According to one embodiment, the resistance of the negative resistance 600 may be half the resistance of the negative resistance portion 504.

The amplifier 602 is connected to the gate of the transistor MR1 and the transistor RT, and outputs a reference voltage capable of judging completion of the read operation. In addition, the amplifier 602 may serve to set the initial voltage of the sense node S together with the reference cell and the negative resistance 600. That is, the output voltage of the amplifier 602 is set to the initial voltage of the sensing node S.

According to one embodiment, the amplifier 602 has a feedback structure and the output terminal of the amplifier 602 can be connected to the capacitor C, the sense node S, and the read operation determination unit 506. Here, the read operation determination unit 506 may comprise a comparator.

Further, the connection between the output terminal of the amplifier 602 and the sensing node S can be switched by the switch SW.

Hereinafter, the operation of the magnetoresistive memory device of FIG. 6 will be described with reference to FIG. FIG. 7A shows the operation in the balanced state, and FIG. 7B shows the operation in the unbalanced state.

7A, when the switch SW is off and the word line WL is not activated, the voltage V _S of the sense node S can have a large value have.

Then, when a read current is applied to the memory cell 500 such that the word line WL is activated after the switch SW is turned on so that the MTJ state is in an equilibrium state, the memory cell 500 and the reference cell are operated So that the output of the amplifier 602 is provided to the sensing node S. That is, the sense node S is initialized to the reference voltage of the reference cell portion 502. [

Subsequently, the switch SW is turned off with the word line WL activated. At this time, since the read current is supplied to the memory cell 500 so that the MTJ state becomes an unbalanced state, The voltage V _S of the capacitor C is reduced. At this time, the comparator 506 compares the reference voltage with the voltage V _S of the sense node S.

Next, the comparator 506 provides a signal indicating completion of the read operation to the operation driver (not shown) when the difference between the reference voltage and the voltage V _S of the sense node S becomes equal to or greater than the reference value. The operation driver may disable the word line WL according to the provided signal to stop the read operation.

Referring to FIG. 7B, when the switch SW is off and the word line WL is not activated, the voltage V _S of the sense node _S may have a large value.

Then, when a read current is applied to the memory cell 500 so that the word line WL is activated after the switch SW is turned on so that the MTJ state is in an anti-parallel state, the memory cell 500 and the reference cell So that the output of the amplifier 602 is provided to the sensing node S. That is, the sense node S is initialized to the reference voltage of the reference cell portion 502. [

Subsequently, the switch SW is turned off with the word line WL activated. At this time, since the read current is supplied to the memory cell 500 so that the MTJ state becomes an unbalanced state, The voltage V _s of the capacitor C increases. At this time, the comparator 506 compares the reference voltage with the voltage V _S of the sense node S.

The comparator 506 then provides a signal to the operation driver indicating the completion of the read operation when the difference between the reference voltage and the voltage V _S of the sense node S is above the reference. The operation driver may disable the word line WL according to the provided signal to stop the read operation.

In summary, the reference voltage of the reference cell unit 502 can be set as the initial voltage of the sense node S. As a result, a malfunction of the read operation can be prevented irrespective of the model parameters, and the circuit structure of the magnetoresistive memory device can be simplified.

It will be apparent to those skilled in the art that various modifications, additions and substitutions are possible, without departing from the spirit and scope of the invention as defined by the appended claims. Should be regarded as belonging to the following claims.

500: memory cell 502: reference cell part
504: Negative Resistance Unit 506: Read Operation Judgment Unit
600: Negative Resistance 602: Amplifier

Claims (12)

A memory cell having an access transistor and an MTJ;
A negative resistance connected in parallel to the memory cell with respect to the sense node; And
A reference cell part connected to the memory cell and generating a reference voltage,
Wherein an initial voltage of the sense node is set to a reference voltage of the reference cell unit during a read operation. The MTJ of claim 1, wherein when the state of the MTJ is in an equilibrium state, the negative resistance and the resistance of the MTJ have a negative value, and when the state of the MTJ is in an equilibrium state, And the resistance according to the state has a positive value. The method according to claim 1,
And a read operation determining unit comparing the reference voltage with the voltage of the sense node to determine the read operation. The method of claim 1, wherein the connection between the sensing node and the reference cell unit is switched by a switch,
Wherein the reference voltage is set to an initial voltage of the sense node as the switch is turned on and the switch is turned off after the reference voltage is set to an initial voltage of the sense node. Device. 2. The magnetoresistive memory device of claim 1, wherein the negative resistance is realized by a first transistor of a mirror structure and a second transistor connected to one of the first transistors. The method of claim 1, wherein the reference cell portion includes a reference cell having a reference cell transistor and a resistor, a negative resistance corresponding to half of the negative resistance, an amplifier, and a capacitor,
Wherein the amplifier has a feedback structure and the output of the amplifier is provided to the sense node. A memory cell having an MTJ;
A sensing node coupled to the memory cell; And
A resistance element coupled to the MTJ,
Wherein the resistance according to the MTJ and the resistance element is different according to the state of the MTJ and a constant voltage is set as an initial voltage of the sensing node during a read operation. 8. The magnetoresistive memory device of claim 7, wherein the resistive element is connected in parallel to the memory cell with respect to the sense node as a negative resistance. 8. The method of claim 7,
Further comprising a reference cell unit coupled to the memory cell and generating a reference voltage,
Wherein the constant voltage is the reference voltage, and the reference cell unit also includes a negative resistance. 10. The apparatus of claim 9, wherein the connection between the sense node and the reference cell is switched by a switch,
Wherein the reference voltage is set to an initial voltage of the sense node as the switch is turned on and the switch is turned off after the reference voltage is set to an initial voltage of the sense node. Device. 8. The method of claim 7, wherein when the state of the MTJ is in an equilibrium state, the resistance of the resistance element and the resistance of the MTJ has a negative value, and when the state of the MTJ is in an equilibrium state, And the resistance according to the state has a positive value. 8. The magnetoresistive memory device of claim 7, wherein the resistance element is realized by first transistors of a mirror structure and a second transistor connected to one of the first transistors.

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