Semiconductor Memory Device with Floating Body Transistor
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Solution Overview
Problem
Current semiconductor memory devices with a single memory transistor face limitations in differentiating threshold voltage between data states, require complex processes and high power supply voltages, and suffer from inefficiencies in writing and reading operations due to the need for specialized structures and processes.
Innovation Solution
A semiconductor memory device is designed using a storage transistor and an access transistor connected in series, with shared gate, bit, and source lines, allowing for the differentiation of threshold voltage between data states '1' and '0' through control of the access transistor's potential, enabling stable operation without requiring special processes or high power supply voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single memory transistor is used to constitute a memory cell, then device complexity is reduced, but the ability to differentiate threshold voltage between data states deteriorates
Solution Approach 1:
The single memory transistor is segmented into two transistors: a storage transistor for holding data and an access transistor for controlling read/write operations. This segmentation allows the storage transistor to maintain a floating body for charge accumulation while the access transistor provides controlled potential application, thereby achieving both simplified structure and effective threshold voltage differentiation between data states.
Solution Approach 2:
The access transistor serves multiple functions: it controls the potential applied to the storage transistor during read/write operations, acts as a switch for isolating the storage node during hold operations, and enables the floating body mechanism to function properly. This multi-functionality allows the two-transistor structure to achieve threshold voltage differentiation without requiring additional specialized components.
2Measurement precision
If a polysilicon pillar of special structure is formed, then threshold voltage control is improved, but ease of manufacture deteriorates
Solution Approach 1:
Instead of forming a polysilicon pillar with special structure, the invention changes the parameter of the storage transistor by creating a floating body region through selective oxidation and charge accumulation. The threshold voltage is controlled by varying the charge state of the floating body (accumulated holes or electrons) rather than by structural modification, thereby maintaining ease of manufacture using standard CMOS processes while achieving effective threshold voltage control.
3Reliability
If high power supply voltages are applied for initialization, then reliability of data writing is improved, but use of energy deteriorates
Solution Approach 1:
The invention performs preliminary action by pre-establishing the floating body structure during manufacturing and pre-configuring the access transistor to control potential application. This eliminates the need for high-voltage initialization procedures, as the floating body can be charged or discharged through normal operating voltages via the access transistor, thereby maintaining data writing reliability while reducing power consumption.
Solution Approach 2:
The floating body structure inherently maintains its charge state through the isolation provided by the oxide layer and the controlled potential application via the access transistor. The system self-regulates the charge accumulation and discharge processes during normal operation without requiring external high-voltage initialization, thereby achieving reliable data writing with lower power consumption.
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 solution allows for stable operation with a clear differentiation of threshold voltage between data states, simplifies the manufacturing process, reduces power consumption, and decreases chip area, while avoiding the need for specialized structures or high power supply voltages.
Implementation Method 1
by applying a high voltage between a source and a drain, impact ionization is caused in the vicinity of the drain, and holes generated thereby are accumulated in a body
Data Source
AI summary
The present invention aims at providing a semiconductor memory device that can be manufactured by a MOS process and can realize a stable operation. A storage transistor has impurity diffusion regions, a channel formation region, a charge accumulation node, a gate oxide film, and a gate electrode. The gate electrode is connected to a gate line and the impurity diffusion region is connected to a source line. The storage transistor creates a state where holes are accumulated in the charge accumulation node and a state where the holes are not accumulated in the charge accumulation node to thereby store data “1” and data “0”, respectively. An access transistor has impurity diffusion regions, a channel formation region, a gate oxide film, and a gate electrode. The impurity diffusion region is connected to a bit line.