Pixel Circuit Charge Accumulation Dark Current Reduction
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Solution Overview
Problem
Image sensors face challenges with increased dark current and reduced conversion efficiency due to the presence of a floating diffusion layer, which complicates the detection of photons and limits miniaturization, especially as the number of reads per frame increases.
Innovation Solution
A pixel circuit design that includes a photoelectric conversion portion, a control transistor, and an electric charge accumulation portion positioned between the control transistor and the photoelectric conversion portion, allowing for improved charge accumulation and voltage control, which reduces dark current and enhances conversion efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a floating diffusion layer is used to accumulate electric charge, then the photoelectric conversion can be achieved, but the dark current increases proportionally with the number of reads per frame
Solution Approach 1:
The patent extracts and removes the floating diffusion layer from the pixel structure, replacing it with a direct charge-to-voltage conversion mechanism. This eliminates the source of dark current while maintaining the photoelectric conversion function through a different architectural approach.
Solution Approach 2:
The patent changes the fundamental operating parameters by eliminating the floating diffusion layer and its associated readout mechanism. Instead, it uses a novel charge accumulation and voltage conversion approach that operates with different electrical characteristics, thereby eliminating dark current generation.
2Measurement precision
If the control transistor size is reduced to increase conversion efficiency, then the detection accuracy improves, but the gate length becomes insufficient to accommodate the electric charge accumulation portion and overflow barriers
Solution Approach 1:
The patent resolves the spatial conflict by transitioning to a different structural dimension or configuration. Instead of accommodating all components in the traditional planar gate length direction, it repositions the electric charge accumulation portion and overflow barriers in a different spatial arrangement that allows for shorter gate lengths while maintaining functional requirements.
Solution Approach 2:
The patent segments the pixel structure into distinct functional regions with optimized dimensions. The control transistor is separated and optimized independently from the charge accumulation and overflow barrier regions, allowing each component to have optimal dimensions for its specific function without being constrained by the traditional gate length requirement.
3Manufacturing precision
If the depth from the surface to the electric charge accumulation portion is increased to reduce impurity diffusion, then the overlap of impurities decreases, but the structural limitations prevent achieving sufficient depth
Solution Approach 1:
The patent extracts and removes the floating diffusion layer that created the structural depth limitations. This elimination allows for a reconfigured structure where impurity diffusion can be controlled more effectively without being constrained by the previous structural design that limited achievable depth.
4Ease of operation
If the overflow barrier width is reduced to achieve both reset voltage and saturated electric charge amount, then the reset function is improved, but the potential varies largely when LDD width varies
Solution Approach 1:
The patent segments the barrier structure into distinct functional zones with optimized dimensions. The overflow barrier is designed as a separate entity from the LDD region, allowing independent optimization of barrier width for reset function while the LDD width can be controlled separately to maintain potential stability.
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 design effectively eliminates dark current and improves the conversion efficiency of electric charge to voltage, enabling better photon detection accuracy and miniaturization of the control transistor.
Implementation Method 1
a photoelectric conversion portion that converts light incident along an optical axis to electric charge
Data Source
AI summary
Dark current of FD is eliminated in an image sensor, and conversion efficiency of converting electric charge to voltage is improved. A pixel circuit includes a photoelectric conversion portion, a control transistor, and an electric charge accumulation portion. The photoelectric conversion portion converts light incident along an optical axis to electric charge. The control transistor controls output voltage according to input voltage. The electric charge accumulation portion accumulates electric charge in a region positioned between the control transistor and the photoelectric conversion portion on the optical axis, and supplies a voltage according to the amount of accumulated electric charge as the input voltage to the control transistor.