Reference Resistor Value Adjustment for Memory Cell Read Accuracy
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Solution Overview
Problem
In large-capacity memory devices, the variation in resistance values of magnetic tunnel junction (MTJ) elements makes it challenging to accurately determine the resistance state, leading to reduced yield due to the reference resistor being set as a middle value between the mean resistance values of high- and low-resistance states, which does not account for standard deviations.
Innovation Solution
The resistance value of the reference resistor is set as Rref1, calculated using a specific formula that matches the normal distribution of low- and high-resistance values, ensuring it is smaller than the mean low-resistance value and greater than the mean high-resistance value, thereby maintaining a high yield even in large-capacity memory devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the reference resistor value is set as the middle value between the mean resistance values of high- and low-resistance states, then the resistance state can be determined for most MTJ elements, but the determination accuracy deteriorates in large-capacity memory devices due to resistance value variations
Solution Approach 1:
The patent changes the reference resistor value parameter from the conventional middle value to a specifically calculated value (Rref1) that accounts for the normal distribution characteristics of MTJ resistance values. This parameter change resolves the contradiction by positioning the reference value to achieve both high determination accuracy and high yield simultaneously, even in large-capacity memory devices with significant resistance variations.
2Quantity of substance
If the memory device capacity is increased to large-capacity, then more storage elements can be accommodated, but the resistance value variations increase making accurate determination difficult
Solution Approach 1:
The patent applies feedback by using the calculated reference resistor value (Rref1) that is derived from the statistical characteristics (mean and standard deviation) of the MTJ resistance distribution. This feedback mechanism allows the reference value to adapt to the actual resistance variations in large-capacity devices, enabling accurate determination despite increased quantity and corresponding variations.
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 allows for accurate read operations in large-capacity memory devices by ensuring the resistance value of the reference resistor is correctly positioned relative to the distribution of MTJ elements, enhancing the reliability of resistance state determination.
Implementation Method 1
Magnetic tunnel junction (MTJ) elements are widely used as the magnetoresistance devices. An MTJ element has two state, namely low-resistance state and high-resistance state. Depending on the state of the MTJ element, the resistance of the MTJ element becomes a low resistance or a high resistance state.
Data Source
AI summary
A memory device according to an embodiment includes: a plurality of memory cells including a storage element having a first and second terminals; a reference resistor having a third and fourth terminals; a first current source electrically connected to the first terminal of the storage element in the selected memory cell; a second current source electrically connected to the third terminal; and a determination circuit that determines the greater one among a resistance value of a storage element of selected one and a resistance value of the reference resistor, the resistance value of the reference resistor being smaller than a middle value between a mean value of first resistance values obtained from the storage elements in the high-resistance state and a mean value of second resistance values obtained from the storage elements in the low-resistance state, and greater than the mean value of the second resistance values.