Bi-directional Selector Element with Multiple Thresholds for MRAM Read Disturbance
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Solution Overview
Problem
Conventional two-terminal selector elements cause read disturbance in magnetic random access memory (MRAM) during the read operation due to high current levels associated with threshold voltages, which can unintentionally switch the resistance state of memory elements, necessitating a solution that minimizes this disturbance while being cost-effective.
Innovation Solution
A bi-directional two-terminal selector element with two selector devices coupled in series, each having different threshold voltages, is used in conjunction with a magnetic tunnel junction (MTJ) memory element, allowing for controlled current flow and reduced read disturbance by utilizing switching layers with varying compositions or thicknesses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional two-terminal selector element is used, then the device complexity is reduced and manufacturing is simplified, but read disturbance occurs due to high current levels that can unintentionally switch the resistance state of memory elements
Solution Approach 1:
The selector element is divided into two separate selector devices coupled in series, each with different threshold voltages. This segmentation allows the first selector device to block read currents below its higher threshold voltage, preventing read disturbance, while the second selector device operates at a lower threshold voltage to enable programming operations. The segmentation resolves the contradiction by creating distinct operational voltage windows for read and write operations.
Solution Approach 2:
The invention changes the voltage parameter characteristics by introducing two different threshold voltages (Vth1 and Vth2) through different switching layer compositions or thicknesses. The first selector device has a higher threshold voltage to block read currents, while the second has a lower threshold voltage for programming. This parameter differentiation allows the system to maintain simple two-terminal structure while achieving reliable read operations by exploiting the voltage-dependent switching behavior.
2Ease of operation
If a single threshold voltage selector device is used, then the operation is simpler, but it cannot distinguish between read and write operations, causing read disturbance
Solution Approach 1:
The selector element is segmented into two selector devices with different threshold voltages. The first selector device (higher Vth) acts as a gate for read operations, blocking currents below its threshold, while the second selector device (lower Vth) enables programming. This segmentation provides natural operational differentiation without complex control logic, resolving the contradiction between operational simplicity and read disturbance prevention.
Solution Approach 2:
The first selector device with higher threshold voltage serves as an intermediary protective element that mediates between the read current source and the memory element. It allows benign read currents to pass (by blocking them) while preventing harmful currents that could cause unintended switching. This intermediary structure simplifies operation control by providing passive current management rather than active control logic.
3Reliability
If the threshold voltage is lowered to enable read operations, then read disturbance is reduced, but programming voltage margin is reduced
Solution Approach 1:
The dual-selector structure segments the voltage requirements: the first selector device with higher threshold voltage ensures read reliability by blocking low-level read currents, while the second selector device with lower threshold voltage provides sufficient programming margin. The series configuration ensures that programming current must overcome both thresholds, providing adequate energy margin while the read operation is protected by the higher first threshold.
Solution Approach 2:
The invention changes the voltage parameter distribution by assigning different threshold voltages to the two selector devices. The first device operates at a higher voltage parameter (Vth1) to protect against read disturbance, while the second device operates at a lower voltage parameter (Vth2) to enable programming. This parameter differentiation resolves the contradiction by creating distinct voltage domains for read and write operations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The bi-directional selector element effectively minimizes read disturbance in MRAM by allowing controlled current flow through the use of multiple threshold voltages, ensuring that the resistance state of the memory element is accurately sensed without unintended switching, thereby enhancing the reliability of memory operations.
Implementation Method 1
each having different threshold voltages, is used in conjunction with a magnetic tunnel junction (MTJ) memory element, allowing for controlled current flow and reduced read disturbance by utilizing switching layers with varying compositions or thicknesses
Implementation Method 2
When the magnetization directions of the magnetic free and reference layers are substantially parallel or oriented in a same direction, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistance of the MTJ
Implementation Method 3
The electrical resistance of the MTJ is high when the magnetization directions of the magnetic free and reference layers are substantially anti-parallel or oriented in opposite directions
Implementation Method 4
Upon the application of an appropriate current through the MTJ, the magnetization direction of the magnetic free layer can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction of the magnetic reference layer
Data Source
AI summary
The present invention is directed to a memory device including a memory cell coupled to two wiring lines at two ends thereof. The memory cell includes a memory element, which includes a magnetic free layer and a magnetic reference layer with a tunnel junction layer interposed therebetween, and a bi-directional two-terminal selector element having multiple threshold voltages coupled to the memory element in series. The magnetic free layer has a variable magnetization direction substantially perpendicular to a layer plane thereof and the magnetic reference layer has a fixed magnetization direction substantially perpendicular to a layer plane thereof. In an embodiment, the bi-directional two-terminal selector element includes two selector devices with each selector device including two electrodes with a switching layer interposed therebetween. In another embodiment, the bi-directional two-terminal selector element includes a selector device incorporating therein two switching layers.