Trench-Gate Source Follower for Low-Noise Scaled Pixels
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Solution Overview
Problem
Conventional CMOS image sensor pixel designs face a trade-off between image performance and noise performance due to the shared footprint of photodiodes and source-follower transistors, making it difficult to maintain acceptable noise performance as pixel dimensions decrease.
Innovation Solution
The integration of a trench-gate source-follower transistor, which features a buried-trench current channel and a trench-gate structure that increases the active region width without expanding the physical layout, allowing for improved noise performance and image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the active area of the source-follower transistor is increased to improve noise performance, then the signal-to-noise ratio is improved, but the photodiode area must be decreased, reducing full-well capacitance and dynamic range
Solution Approach 1:
The source-follower transistor is transitioned from a planar configuration to a three-dimensional trench-gate structure etched into the substrate. This vertical dimension allows the transistor to achieve larger effective active area through the trench walls and buried channel, improving noise performance without increasing the lateral footprint that would reduce photodiode area.
Solution Approach 2:
The trench-gate structure embeds the transistor channel within the substrate trench, creating a nested configuration where the active channel region is surrounded by the gate structure. This nesting enables the transistor to achieve larger effective width within the same lateral footprint, resolving the area trade-off between transistor and photodiode.
2Productivity
If pixel dimensions are decreased to increase pixel density, then more pixels can be integrated, but it becomes increasingly difficult to maintain acceptable noise performance
Solution Approach 1:
By moving the transistor channel into the vertical dimension through trench etching, the effective active area is decoupled from the lateral pixel dimensions. This allows small pixels to maintain adequate noise performance through the vertical trench structure while preserving lateral space for high pixel density integration.
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 trench-gate source-follower transistor enhances transconductance and signal-to-noise ratio, supporting higher frame rates and reduced power consumption while maintaining image quality, even at smaller pixel sizes.
Implementation Method 1
A contiguous doped region is implanted around the inner walls of the trench to form a buried-trench current channel. Activating the trench-gate causes current to flow between the gate and source regions via the buried-trench current channel
Implementation Method 2
A gate oxide layer can be disposed between the buried portion of the trench-gate and the buried-trench current channel
Data Source
AI summary
A trench-gate source-follower (TGSF) transistor is described, such as for integration with image sensor pixels. The TGSF transistor is at least partially built into a trench etched into a substrate. A contiguous doped region is implanted around the inner walls of the trench to form a buried-trench current channel. A trench-gate is formed to have at least a buried portion that fills the volume of the trench. A gate oxide layer can be disposed between the buried portion of the trench-gate and the buried-trench current channel. Drain and source regions are formed on either end of the trench-gate. Activating the trench-gate causes current to flow between the drain and source regions via the buried-trench current channel around the buried portion of the trench-gate. The geometry of the buried-trench current channel can effectively increase the width of the active region of the source-follower transistor without increasing its physical layout width.