RRAM Cell Structure With Differential Threshold Voltages
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Solution Overview
Problem
Existing RRAM devices face issues with leakage current in unselected cells due to transistor leakage, leading to misreading of logic states as technology nodes shrink, which affects the reliability and accuracy of data storage.
Innovation Solution
Implementing a 2T1R configuration with transistors having different threshold voltages, where a first transistor with a lower threshold voltage is coupled in series with a resistor and a second transistor with a higher threshold voltage, effectively limiting leakage current by maintaining the active transistor of unselected cells in an off state.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single transistor is used in each RRAM cell, then the device structure remains simple, but leakage current in unselected cells increases leading to misreading of logic states
Solution Approach 1:
The single transistor is segmented into two separate transistors with different functions: a first transistor for accessing the RRAM cell and a second transistor for controlling leakage current. This segmentation allows each transistor to be optimized for its specific function, resolving the contradiction between structural simplicity and read accuracy.
Solution Approach 2:
Different threshold voltages are assigned to different transistors based on their specific functions. The first transistor has a lower threshold voltage for efficient cell access, while the second transistor has a higher threshold voltage to suppress leakage current in unselected cells, achieving local optimization of electrical characteristics.
2Reliability
If transistor threshold voltages are increased to reduce leakage current, then leakage current decreases, but read margins deteriorate due to reduced drive current
Solution Approach 1:
The transistor control function is segmented into two transistors, allowing the first transistor to provide sufficient drive current for reading (maintaining read margins) while the second transistor specifically suppresses leakage current through its higher threshold voltage and strategic placement in the current path.
Solution Approach 2:
Different threshold voltage parameters are assigned to different transistors: the first transistor uses a lower threshold voltage parameter to ensure adequate drive current and read margins, while the second transistor uses a higher threshold voltage parameter to suppress leakage, allowing both requirements to be satisfied simultaneously.
Data Source
AI summary
A memory device includes at least a non-volatile memory cell. The non-volatile memory cell includes a resistor with a variable resistance, a first transistor, and a second transistor that are coupled in series. The first transistor has a first threshold voltage and the second transistor has a second threshold voltage that is different from the first threshold voltage. The first and the second transistors have their channels doped in respective different concentrations.