Drain Select Transistor Voltage Control for NAND Flash Reliability
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Solution Overview
Problem
The wide threshold voltage distribution of select transistors in NAND flash memory devices leads to operation errors due to repeated program operations, where threshold voltages are increased beyond target levels, affecting the reliability of the semiconductor device.
Innovation Solution
A semiconductor device with an operation circuit that performs repeated program and program verifying operations, initially applying a verifying voltage lower than the normal level to drain select transistors and incrementing it to the normal level during repeated verifying operations, ensuring the threshold voltages reach the target level without exceeding it.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If repeated program operations are performed on drain select transistors to increase threshold voltages to target level, then the threshold voltages reach the desired level, but the threshold voltage distribution becomes wide causing operation errors
Solution Approach 1:
The patent applies dynamics by making the verifying voltage level changeable during repeated program operations. Initially, a first verifying voltage level is used, and when threshold voltage distribution becomes wide, the verifying voltage is switched to a second level (higher than the first). This dynamic adjustment of the verifying voltage prevents operation errors while maintaining narrow threshold voltage distribution.
Solution Approach 2:
The patent changes the verifying voltage parameter from a fixed level to a variable level. The operation circuit monitors the threshold voltage distribution width and adjusts the verifying voltage level accordingly. When distribution width exceeds a threshold, the verifying voltage is increased to prevent further widening, thus resolving the contradiction between reaching target threshold voltages and maintaining narrow distribution.
2Reliability
If verifying voltage is increased to prevent threshold voltage distribution widening, then operation errors are prevented, but the complexity of voltage control increases
Solution Approach 1:
The patent implements feedback by having the operation circuit monitor the threshold voltage distribution width of drain select transistors during program operations. Based on this feedback, the circuit automatically adjusts the verifying voltage level. When the distribution width becomes wide, the feedback mechanism triggers an increase in verifying voltage to prevent operation errors, thus managing complexity through automated feedback control.
Data Source
AI summary
A semiconductor device and method of operating the same are provided. The semiconductor device may include a memory block including drain select transistors connected to bit lines, source select transistors connected to a common source line, and memory cells connected between the drain select transistors and the source select transistors. The semiconductor device may include an operation circuit configured to repeatedly perform a program operation and a program verifying operation to increase threshold voltages of the drain select transistors to a target level. The operation circuit may be configured to apply a level lower than a verifying voltage initially applied to the drain select transistors for the program verifying operation. The level being lower than a normal level. The operation circuit may be configured to increase the verifying voltage to the normal level whenever the program verifying operation is repeated.