Cell String Selection Transistors With Split Threshold Voltage Control
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Solution Overview
Problem
Existing non-volatile memory devices face challenges in achieving high performance and reliability due to variations in threshold voltage levels of string selection transistors, which affect the accuracy and efficiency of data storage and retrieval operations.
Innovation Solution
A non-volatile memory device with a cell string that includes two string selection transistors connected in series, where each transistor is programmed to distinct threshold voltage levels using specific programming voltages, and a threshold voltage management circuit manages these levels to optimize selection and non-selection of cell strings, thereby improving voltage margins and reducing driving voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single string selection transistor is used in the cell string, then the device complexity is low, but the voltage margin of the string selection transistor is insufficient and reliability is reduced
Solution Approach 1:
The cell string is divided into multiple segments with separate string selection transistors (first SST and second SST) controlling different portions of the cell string. This segmentation allows independent threshold voltage control for each SST, improving the voltage margin and selection accuracy without requiring a single complex transistor
2Reliability
If multiple string selection transistors are used to improve voltage margin, then reliability is improved, but the device complexity increases
Solution Approach 1:
Different string selection transistors are assigned different threshold voltage characteristics tailored to their specific positions and functions in the cell string. The first SST has a first threshold voltage and the second SST has a second threshold voltage, optimizing each transistor's performance for its local requirements rather than using a uniform design
3Reliability
If high programming voltages are applied to all string selection transistors, then reliable programming is achieved, but the driving voltage is high and energy consumption increases
Solution Approach 1:
The threshold voltage parameters of string selection transistors are dynamically adjusted based on their position and function. By changing the threshold voltage parameter rather than uniformly applying high programming voltages, the system achieves reliable programming while reducing overall energy consumption and driving voltage requirements
Data Source
AI summary
A method of operating a non-volatile memory device which includes a cell string includes performing a programming operation on a first string selection transistor of the cell string, performing a first read operation on the first string selection transistor based on a first reference voltage, and performing a programming operation on a second string selection transistor of the cell string connected to the first string selection transistor to have: a first target threshold voltage level in response to that a first bit value obtained by the first read operation indicates a first logical value, and a second target threshold voltage level lower than the first target threshold voltage level in response to that the first bit value indicates a second logical value opposite to the first logical value.