Sense Module Discharge Circuit for NAND Flash Memory Reliability
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Solution Overview
Problem
In NAND type flash memory devices, the miniaturization of memory cells and introduction of stacked structures lead to reduced on-current and increased off-leakage, making it challenging to establish a sufficient on/off ratio, which affects operation reliability due to disparities in process, temperature, and transistor characteristics.
Innovation Solution
The semiconductor memory device employs a sense module configuration where the sense node is initially charged and then discharged using a sense transistor with a threshold voltage, allowing the discharge voltage to be set at a level that includes the threshold voltage disparity, thereby reducing the influence of local disparities and improving reading operation reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If memory cells are miniaturized and arranged in stacked structures, then storage capacity increases, but on-current decreases and off-leakage increases
Solution Approach 1:
The sense node is preliminarily charged to a voltage level that includes the threshold voltage disparity before the sensing operation. This preliminary charging action ensures that when the sense transistor turns on, the voltage at the sense node is already at an appropriate level to compensate for threshold voltage variations, thereby maintaining reliable sensing despite miniaturization effects
Solution Approach 2:
The invention changes the voltage parameter of the sense node by charging it to a level that incorporates the threshold voltage disparity. This parameter change allows the sensing circuit to operate reliably even as transistor characteristics vary due to miniaturization and stacking, effectively compensating for the degraded on/off ratio
2Reliability
If sense node is discharged using sense transistor with threshold voltage, then threshold voltage disparities are canceled out, but discharging time increases
Solution Approach 1:
The sense node is preliminarily charged to the appropriate voltage level before discharge begins. This preliminary action ensures that the discharge process starts from the correct voltage point, allowing the sense transistor to cancel threshold voltage disparities efficiently without requiring excessive discharge time
Solution Approach 2:
The sensing operation uses feedback through the sense transistor, where the transistor's threshold voltage characteristics are utilized to cancel out disparities. The sense transistor automatically adjusts its conduction based on the voltage at the sense node, providing natural feedback that compensates for threshold variations without requiring additional time
Data Source
AI summary
A semiconductor memory device includes a plurality of memory cells, a plurality of bit lines, each of which is electrically connected to a string of the memory cells, and a sense module provided for each of the bit lines. Each sense module includes a sense transistor that is configured to turn on and off to indicate whether or not data is stored in a memory cell that is targeted by a reading operation, the sense transistor having a threshold voltage level and a gate that is connected to a sense node, the sense node being connected to a discharge line through a series of transistors including the sense transistor so that prior to a sensing operation the sense node can be discharged to a level that is set in accordance with a threshold voltage thereof.