Split-Gate Memory Transistor Reference Stabilization
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Solution Overview
Problem
Conventional semiconductor memory devices face variations in reference current due to electron injection or removal from the floating gate, affecting the reliability and accuracy of data sensing.
Innovation Solution
A semiconductor memory device with a split-gate type memory cell transistor and a MIS (Metal Insulator Semiconductor) reference transistor without a floating-gate structure, using a single gate electrode to prevent threshold voltage variations and stabilize the reference current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a transistor with the same structure as the memory cell transistor (including floating gate) is used as the reference transistor, then the reference transistor can be easily manufactured with the same process, but the threshold voltage of the reference transistor varies over time due to electron injection or removal from the floating gate, causing reference current instability
Solution Approach 1:
The invention extracts the floating gate structure from the reference transistor, retaining only the essential MIS transistor components (gate insulating film, gate electrode, source/drain regions). This removal of the floating gate eliminates the source of threshold voltage variation while preserving the transistor's basic functionality for generating reference current.
Solution Approach 2:
The invention applies different structural qualities to different transistors in the same device. The memory cell transistor retains the full split-gate structure with floating gate for data storage functionality, while the reference transistor uses a simplified MIS structure without floating gate for stable reference current generation. Each transistor is optimized locally for its specific function.
2Adaptability or versatility
If a split-gate memory cell transistor with floating gate is used, then data can be stored and sensed, but the floating gate structure causes threshold voltage variations that affect the stability of the reference current used for sensing
Solution Approach 1:
The invention segments the transistor population into two distinct groups with different structures: memory cell transistors with split-gate structure for data storage, and reference transistors with simple MIS structure for stable reference current. This segmentation allows each type to be optimized for its specific function without compromising the other.
3Manufacturing precision
If the reference transistor has the same structure as the memory cell transistor, then manufacturing precision can be maintained, but the reference current varies due to defects in insulating film and voltage peripherally applied affecting the floating gate
Solution Approach 1:
The floating gate is extracted from the reference transistor structure, eliminating the vulnerable component that suffers from electron injection/removal due to insulating film defects and peripheral voltage effects. This extraction preserves manufacturing simplicity while eliminating the source of reference current variation that degrades sensing accuracy.
Data Source
AI summary
A semiconductor memory device has: a substrate; a memory cell transistor of a split-gate type formed on the substrate; and a reference transistor formed on the substrate and used for generating a reference current that is used in sensing data stored in the memory cell transistor. The memory cell transistor has a floating gate and a control gate, while the reference transistor is a MIS (Metal Insulator Semiconductor) transistor having a single gate electrode.