MRAM Write Method Using Word Line Overdrive Voltage
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Solution Overview
Problem
Conventional MRAM memory devices face challenges in generating a high enough heating current to reach the high threshold temperature required for writing operations due to the low core voltage needed for reliability and integration constraints, leading to insufficient data writing.
Innovation Solution
The method involves applying a word line overdrive voltage higher than the core operating voltage to ensure a sufficient heating current is passed through the magnetic tunnel junction, allowing it to reach the high threshold temperature without increasing transistor size, and using a high power transistor with a low threshold voltage and source line overdrive voltage to manage current density and leakage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a low core voltage is used to power the MRAM cell, then power consumption is reduced and transistor reliability is improved, but the heating current magnitude becomes insufficient to reach the threshold temperature for writing operations
Solution Approach 1:
A word line overdrive voltage is applied before the actual write operation to pre-heat the magnetic tunnel junction. This preliminary heating action raises the baseline temperature of the MTJ, enabling subsequent write currents to reach the required threshold temperature even when operating at low core voltages that preserve transistor reliability
Solution Approach 2:
The writing process uses periodic pulsed voltages: a first voltage pulse for preliminary heating, followed by a second voltage pulse for the actual write operation. This periodic application of different voltage levels enables temperature control while maintaining low average power consumption and protecting transistor reliability
2Area of stationary object
If the size of the magnetic tunnel junction is reduced for higher integration density, then manufacturing precision and area are improved, but the heating current required to reach threshold temperature increases due to smaller thermal mass
Solution Approach 1:
The preliminary heating phase using word line overdrive voltage is particularly effective for small-area MTJs because it applies heat over an extended period before the write pulse, allowing the small thermal mass to accumulate sufficient temperature. This pre-heating action compensates for the reduced thermal mass of miniaturized MTJs without requiring excessive write current
3Productivity
If a high heating current is applied to reach the threshold temperature, then the writing capability is improved, but the power consumption increases and may damage the select transistor
Solution Approach 1:
By performing preliminary heating through word line overdrive voltage before the write operation, the magnetic tunnel junction reaches a higher initial temperature. This reduces the temperature differential that must be achieved during the write pulse, thereby reducing the peak write current and associated power consumption while maintaining effective writing capability
Solution Approach 2:
The separated two-phase voltage application (preheating phase followed by write phase) allows energy to be distributed over time rather than concentrated in a single high-power pulse. This periodic approach reduces instantaneous power consumption and average energy loss while achieving the necessary heating effect for successful writing
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
This approach enables high-density memory devices to be written with low power consumption while maintaining transistor reliability by ensuring the magnetic tunnel junction is heated to the necessary temperature for resistance variation.
Implementation Method 1
applying a single heating current pulse I heat through the magnetic tunnel junction 2 in order to heat it above a predetermined high threshold temperature
Implementation Method 2
Each MRAM cell 1 comprises a magnetic tunnel junction 2, represented by a resistance, electrically coupled at one end to a select CMOS transistor 3
Data Source
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AI summary
The present disclosure concerns a method of writing in a memory device (10) comprising a plurality of magnetoresistive random access memory cells (1) arranged in rows and columns, each cell (1) including a magnetic tunnel junction (2) having a resistance that can be varied during a write operation when the magnetic tunnel junction (2) is heated at a high threshold temperature, and a select transistor (3) electrically coupled to the magnetic tunnel junction (2); a plurality of word lines (WL) connecting cells (1) along a row; and a plurality of bit lines (BL) connecting cells (1) along a column; the method comprising supplying a bit line voltage (VBL) to one of the bit lines (BL) and a word line voltage (VWL) to one of the word lines (WL) for passing a heating current (Iheat) through the magnetic tunnel junction (2) of a selected cell (1); wherein said word line voltage (VWL) is a word line overdrive voltage (VWLo) being higher than the core operating voltage of the cells (1) such that the heating current (Iheat) has a magnitude that is high enough for heating the magnetic tunnel junction (2) at the predetermined high threshold temperature. The memory device can have a high-density and can be written with low power consumption.