Memory Device Drain Select Line Voltage Control
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Solution Overview
Problem
Nonvolatile memory devices, particularly flash memory, face a disturb phenomenon during read operations where unselected memory strings are affected by Hot Carrier Injection (HCI), leading to data retention issues due to voltage fluctuations and channel potential imbalances.
Innovation Solution
A memory device with a voltage generating circuit and control logic that applies distinct drain select line voltages to unselected memory strings during a read operation, adjusting the voltage levels and durations to prevent HCI, ensuring that unselected memory strings are not inadvertently activated, thereby maintaining data integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single drain select line voltage is applied to all unselected memory strings during a read operation, then the circuit design is simple, but Hot Carrier Injection causes disturb phenomenon and data retention issues
Solution Approach 1:
The patent segments the unselected memory strings into two groups: those sharing the same source select line as the selected string and those that do not. Different drain select line voltages are applied to each group, with the first group receiving a higher voltage to prevent HCI while the second group receives a lower voltage to minimize disturb. This segmentation resolves the contradiction by enabling differentiated voltage control without requiring completely separate control circuits for each string.
Solution Approach 2:
The patent applies local quality by providing different voltage levels to different groups of unselected memory strings based on their specific relationship to the selected string. The first group (sharing source select line) receives a first drain select line voltage level, while the second group (not sharing source select line) receives a second drain select line voltage level. This localized differentiation addresses the specific HCI risk in the first group while minimizing unnecessary voltage stress on the second group.
2Reliability
If higher drain select line voltage is applied to prevent HCI in unselected memory strings, then data retention is improved, but power consumption and voltage stress increase
Solution Approach 1:
The patent applies higher drain select line voltage only to the first group of unselected memory strings that share the same source select line as the selected string, where HCI risk is highest. The second group of unselected memory strings receives a lower voltage level, reducing unnecessary power consumption and voltage stress. This localized approach maintains data retention where needed while minimizing energy loss.
Solution Approach 2:
The patent changes the voltage parameter dynamically by applying different drain select line voltage levels to different groups of unselected memory strings during the read operation. The first group receives a first voltage level sufficient to prevent HCI, while the second group receives a second, lower voltage level. This parameter differentiation resolves the contradiction by optimizing the voltage level for each specific group's needs.
3Reliability
If drain select line voltage is applied during the entire read operation period, then unselected memory strings are fully protected from HCI, but the disturb phenomenon increases due to prolonged voltage exposure
Solution Approach 1:
The patent segments the voltage application strategy by time and group, applying the first drain select line voltage to the first group of unselected memory strings and the second drain select line voltage to the second group. This segmentation allows selective protection where needed while minimizing disturb exposure, resolving the contradiction between protection and disturb prevention.
Solution Approach 2:
The patent applies different voltage levels to different groups of unselected memory strings based on their specific vulnerability to HCI. The first group (sharing source select line) receives higher voltage protection, while the second group receives lower voltage to minimize disturb. This localized approach balances protection needs with disturb minimization.
Data Source
AI summary
A memory device includes a memory cell array having a plurality of memory strings and includes a voltage generating circuit configured to generate and apply a plurality of drain select line voltages, a plurality of source select line voltages, and a read voltage to the memory cell array during a read operation. The memory device also includes control logic configured to control the voltage generating circuit to generate a first drain select line voltage applied to a first unselected memory string among unselected memory strings among the plurality of memory strings and a second drain select line voltage applied to second unselected memory strings among the unselected memory strings during the read operation, wherein the second drain select line voltage is different from the first drain select line voltage.