NAND Flash Memory Adaptive Read Voltage Control
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Solution Overview
Problem
Existing NAND flash memory systems face challenges in determining an optimal read voltage efficiently, leading to inefficiencies in data retrieval due to variations in threshold voltage distributions of memory cell transistors.
Innovation Solution
A memory system that includes a controller and memory device capable of determining an optimal shift value of a read voltage on-chip, based on the number of memory cells in a predetermined range, using a voltage regulator to adjust read voltages dynamically.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fixed read voltages are used for data retrieval, then the memory system is simple to operate, but read errors increase due to threshold voltage variations in memory cells
Solution Approach 1:
The patent implements dynamic voltage adjustment by determining optimal read voltages based on real-time threshold voltage distribution characteristics of memory cells. The system transitions from fixed read voltages to adaptive voltages that change according to measured device characteristics, thereby improving read reliability without requiring overly complex manual control mechanisms.
Solution Approach 2:
The memory system performs self-characterization by automatically measuring threshold voltage distributions and determining optimal read voltages on its own. The system uses its own operational data (read results at different voltages) to characterize its state and adjust accordingly, eliminating the need for external intervention while maintaining high read reliability.
2Reliability
If multiple read voltages are tested to determine optimal voltage, then read operation reliability improves, but the time required for voltage determination increases
Solution Approach 1:
The system performs preliminary characterization operations during initialization or periodic intervals to establish baseline threshold voltage distribution data. This preliminary work enables the system to make informed voltage selections during actual data read operations without requiring extensive real-time testing, thus reducing operational time while maintaining accuracy.
Solution Approach 2:
The patent implements feedback mechanisms where read operation results at different voltage levels are analyzed to determine optimal voltages for subsequent operations. The system uses feedback from previous read attempts to characterize threshold voltage distributions and adjust future read voltages, progressively improving accuracy without requiring exhaustive voltage testing each time.
Data Source
AI summary
A memory device according to one embodiment includes a plurality of memory cells, a word line, and a controller. Each of the memory cells is configured to store multiple-bit data according to which of a plurality of states having different threshold voltages each of the memory cells is included in. The word line is connected to the memory cells. The controller is configured to count a number of memory cells having threshold voltages on a higher state side among the states, and to execute a read operation for the memory cells as a target, by using a read voltage that is shifted based on a result of the counting.