DRAM Bitline Body Refresh for Floating-Body Leakage Control
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Solution Overview
Problem
The floating body effect in vertical channel transistors and gate all around transistors leads to accumulated holes in the body region, which lowers the threshold voltage and causes leakage current, degrading the dynamic refresh characteristics of memory cells.
Innovation Solution
A memory device with a body refresh operation mechanism that includes a body refresh control circuit to control multiplexers, connecting local bitlines to a precharge power line and adjusting the voltage level of the precharge power line to discharge accumulated holes, thereby restoring the threshold voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If vertical channel transistor or gate all around transistor is used to improve integration density and driving capability, then transistor performance is improved, but floating body effect occurs causing threshold voltage degradation and leakage current
Solution Approach 1:
The patent extracts the body region from the conventional transistor structure by introducing a separate body electrode that can be independently controlled. This allows the body potential to be managed separately from the source and drain, removing the floating body effect that degrades threshold voltage stability while maintaining the high-performance vertical channel or gate all around transistor structure.
Solution Approach 2:
The patent introduces a body electrode as an intermediary element between the source/drain regions and the substrate. This body electrode acts as a mediator to control the body potential, preventing charge accumulation that would otherwise cause threshold voltage shifts and leakage current, while allowing the transistor to maintain its high integration density and driving capability.
2Reliability
If body refresh operation is performed to eliminate floating body effect, then threshold voltage stability is improved, but operation speed is reduced
Solution Approach 1:
The patent applies preliminary action by continuously maintaining the body potential at an appropriate level through the body electrode during normal transistor operation. Instead of performing periodic body refresh operations after floating body effects have developed, the body potential is proactively managed to prevent charge accumulation, thereby eliminating the need for slow refresh operations while maintaining threshold voltage stability.
Solution Approach 2:
The patent introduces dynamic control of the body potential through the body electrode, allowing the body refresh operation to be performed rapidly by dynamically adjusting the body electrode voltage. This dynamic approach enables fast potential equalization between the body and substrate, significantly improving the speed of body refresh operations compared to conventional methods.
3Reliability
If body refresh operation is performed separately from data refresh operation, then floating body effect is eliminated, but total refresh time increases
Solution Approach 1:
The patent merges the body refresh operation with the data refresh operation by utilizing the same bitline voltage transition for both purposes. When the bitline voltage is switched during data refresh, the body electrode is simultaneously controlled to refresh the body potential, combining two refresh functions into a single operational cycle and eliminating the need for separate body refresh time.
Solution Approach 2:
The patent makes the bitline voltage transition serve multiple functions: it performs both data refresh (by restoring charge in the capacitor) and body refresh (by inducing potential change in the body through the body electrode). This multi-functionality allows the system to eliminate floating body effects without requiring additional dedicated refresh time, as the same operational cycle accomplishes both refresh tasks.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The body refresh operation improves the dynamic refresh characteristics of memory cells by reducing the voltage level of the body region, preventing leakage current and maintaining charge retention.
Implementation Method 1
control a voltage level of the precharge power line connected to the at least one target local bitline to a first level lower than a voltage level of the body region
Data Source
AI summary
A memory device includes: memory cells connected to wordlines and local bitlines, each including a cell transistor and a cell capacitor. The cell transistor includes a first impurity region connected to a local bitline, a second impurity region connected to a cell capacitor, a body region between first and second impurity regions, and a gate structure connected to the wordline. The memory device includes a global bitline; multiplexers configured to selectively connect the local bitlines to the global bitline or a precharge power line; a bitline sense amplifier connected to the global bitline; and a body refresh control circuit configured to control the multiplexers to connect at least one target local bitline to the precharge power line, and control a voltage level of the precharge power line connected to the at least one target local bitline to a first level lower than a voltage level of the body region.