CMOS Pixel Circuit With Dual Floating Diffusion for Low-Noise HDR
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Solution Overview
Problem
Existing CMOS image sensing devices face challenges in achieving high conversion gain with low noise and wide dynamic range, particularly in low light conditions, due to the limitations of pixel integration and signal processing circuits.
Innovation Solution
The device employs a semiconductor substrate with shared pixel regions, insulating films, and a dual floating diffusion region system, where a first floating diffusion region with low capacitance is used for high conversion gain and a second region with higher capacitance for low gain modes, along with a FinFET-type source follower transistor to enhance signal amplification and reduce noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single floating diffusion region is used in conventional CMOS image sensors, then the circuit structure is simple, but the conversion gain is limited and noise performance deteriorates
Solution Approach 1:
The patent divides the single floating diffusion region into two separate floating diffusion regions (first and second floating diffusion regions). Each region can be independently controlled through dedicated transfer transistors, enabling independent optimization of their capacitance values. This segmentation allows the first floating diffusion region to operate with low capacitance for high conversion gain while the second region can have higher capacitance for noise reduction, thereby resolving the contradiction between achieving high conversion gain and maintaining simple structure.
Solution Approach 2:
The patent introduces control transistors that dynamically switch the connection states between the photodiode, transfer transistors, and the two floating diffusion regions. By dynamically controlling which floating diffusion region receives charge from the photodiode, the system can adaptively optimize the conversion gain and noise performance based on imaging conditions. This dynamic control mechanism enables the system to achieve high conversion gain when needed while maintaining operational flexibility.
2Manufacturing precision
If pixel integration is increased to improve resolution, then more pixels can be packed, but noise increases and conversion gain decreases
Solution Approach 1:
By segmenting the floating diffusion region into two independently controllable regions, the patent enables selective charging of the first floating diffusion region with low capacitance. This segmentation allows each pixel to optimize its conversion gain independently, thereby maintaining high signal-to-noise ratio even as pixel integration increases and pixel size decreases.
Solution Approach 2:
The patent changes the capacitance parameter of the floating diffusion region by providing two regions with different capacitance values. The first floating diffusion region is designed with low capacitance to achieve high conversion gain, while the second region has higher capacitance. This parameter change enables the system to maintain optimal noise performance across different pixel integration levels by selecting the appropriate floating diffusion region for charging.
3Adaptability or versatility
If a dual mode operation is implemented with two floating diffusion regions, then both high conversion gain and wide dynamic range are achieved, but the circuit complexity increases
Solution Approach 1:
The patent designs the pixel circuit with two floating diffusion regions that can serve multiple functions. The first floating diffusion region with low capacitance is optimized for high conversion gain mode, while the second region with higher capacitance can handle larger signal ranges. Both regions share common control mechanisms and can be selectively activated, providing multi-functionality that enables the circuit to adapt to different imaging conditions without requiring completely separate circuit paths.
Solution Approach 2:
The patent employs dynamic control through control transistors that switch between charging the first or second floating diffusion region based on the desired operating mode. This dynamic switching mechanism allows the circuit to flexibly transition between high conversion gain mode and wide dynamic range mode, achieving adaptability while managing complexity through unified control architecture rather than separate dedicated circuits for each mode.
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
This configuration enables high conversion gain with low noise and a wide dynamic range, allowing the device to perform well in varying illumination conditions by switching between modes, thus enhancing image quality and resolution.
Implementation Method 1
a photoelectronic conversion element and a transfer transistor are disposed in at least one of the plurality of pixel regions, wherein the photoelectronic conversion element is connected to the first active region by the transfer transistor
Data Source
AI summary
An image sensing device includes: a substrate; and a plurality of pixels isolated by an insulating film disposed within the semiconductor substrate. The plurality of pixels share a first active region, wherein at least one of the plurality of pixels comprises a photoelectronic conversion element and a transfer transistor. The photoelectronic conversion element is connected to the first active region by the transfer transistor. The first active region and a first floating diffusion region are connected by the transfer transistor. The first floating diffusion region is connected to a gate of a source follower transistor, and the first floating diffusion region is connected to a second floating diffusion region through a first transistor. The first transistor is turned off when an operating mode is a first mode. The first transistor is turned on when the operating mode is a second mode different from the first mode.