Nonvolatile Memory Device With Series String Selection Elements
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Solution Overview
Problem
Current nonvolatile memory devices face challenges in achieving high integration density and reliability due to limitations in controlling memory cell connections and threshold voltages, which affect data storage and retrieval efficiency.
Innovation Solution
The design includes multiple cell strings sharing a bit line, word lines, and selection lines, with string selection elements having different threshold voltages, allowing for selective connection to the bit line and improved control over memory cell operations, utilizing a combination of enhancement-mode and depletion-mode transistors to enhance channel length and reduce short channel effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple cell strings share a bit line to increase integration density, then integration density is improved, but control over memory cell connections becomes more difficult
Solution Approach 1:
The string selection device is divided into multiple string selection elements (first string selection element and second string selection element) connected in series. Each element controls a specific cell string independently, enabling fine-grained control over multiple cell strings sharing a bit line. This segmentation allows the device to maintain high integration density while simplifying the control mechanism for each individual string.
2Reliability
If string selection elements have different threshold voltages to improve selective control, then reliability is improved, but device complexity increases
Solution Approach 1:
Different string selection elements are assigned different threshold voltages (first threshold voltage for the first string selection element, second threshold voltage for the second string selection element). This local differentiation in electrical characteristics enables selective control of specific cell strings by applying appropriate voltages to the string selection line, improving reliability without requiring complex external control circuits.
3Reliability
If channel length is increased to suppress short channel effects, then reliability is improved, but device area increases
Solution Approach 1:
The patent utilizes vertical stacking of multiple cell strings and three-dimensional integration techniques to achieve effective channel length extension without proportionally increasing the planar device area. By transitioning from two-dimensional lateral scaling to three-dimensional vertical integration, the device suppresses short channel effects while maintaining compact footprint.
Data Source
AI summary
A nonvolatile memory device is provided. The device may include a plurality of cell strings that are configured to share a bit line, word lines, and selection lines. Each of the cell strings may include a plurality of memory cells connected in series to each other and a string selection device controlling connections between the memory cells and the bit line, and the string selection device may include a first string selection element with a first threshold voltage and a second string selection element connected in series to the first string selection element and having a second threshold voltage different from the first threshold voltage. At least one of the first and second string selection elements may include a plurality of switching elements connected in series to each other.