Pixel Circuit Segmentation for CMOS Radiation Detection
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Solution Overview
Problem
Current CMOS imagers face challenges in increasing pixel size while maintaining sensitivity and energy resolution for radiation counting, as reducing parasitic capacitance to suppress noise limits the size of the detection node, making it difficult to achieve high sensitivity and high aperture ratio.
Innovation Solution
A pixel circuit design that includes a photoelectric conversion unit, an electric charge accumulation unit in a polygonal area adjacent to the conversion unit, and transfer units to efficiently transfer charges, allowing for larger pixel sizes without compromising sensitivity or aperture ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the detection node size is reduced to minimize parasitic capacitance, then sensitivity and noise performance improve, but pixel area and aperture ratio deteriorate
Solution Approach 1:
The pixel circuit is divided into distinct functional regions: a photoelectric conversion region for generating charge carriers, a charge accumulation region for storing signal charges, and a minimal detection node for reading out signals. This segmentation allows the pixel area to be expanded while keeping the detection node small to maintain low parasitic capacitance and high sensitivity.
Solution Approach 2:
The patent utilizes vertical stacking of functional layers to accommodate larger pixel area while maintaining a compact detection node. By extending the photoelectric conversion region and charge accumulation region in the vertical dimension, the design achieves larger effective pixel area without increasing the lateral size of the detection node, thus preserving low parasitic capacitance.
2Object-affected harmful factors
If the detection node size is reduced to minimize parasitic capacitance, then noise performance improves, but aperture ratio deteriorates
Solution Approach 1:
The pixel structure is segmented into a large photoelectric conversion area that captures incident photons efficiently (high aperture ratio) and a separate minimal detection node that reads out the accumulated charge with minimal parasitic capacitance (low noise). This spatial separation resolves the contradiction between aperture ratio and noise performance.
Solution Approach 2:
A charge accumulation region is introduced as an intermediary between the photoelectric conversion region and the detection node. This intermediary allows the detection node to remain small for low noise while the photoelectric conversion region can be large for high aperture ratio, as the accumulation region buffers the charge transfer process.
3Measurement precision
If pixel size is increased to improve energy resolution and counting rates, then sensitivity is maintained, but parasitic capacitance increases
Solution Approach 1:
The pixel circuit is segmented into functional regions where only the detection node contributes significantly to parasitic capacitance. By keeping the detection node minimal while allowing the overall pixel size (photoelectric conversion region + accumulation region) to increase, the design achieves improved energy resolution and counting rates without proportionally increasing parasitic capacitance.
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 enables larger pixel sizes, improving energy resolution and counting rates in radiation detection while maintaining sensitivity and aperture ratio, effectively addressing the limitations of existing CMOS imagers.
Implementation Method 1
Photons incident on a silicon substrate of the pixel circuit generate pairs of electrons and holes, and then the photodiode accumulates the electrons in a node on the side of cathode of the photodiode
Data Source
AI summary
In a photoelectric changing unit, a photoelectric conversion unit converts light into electric charge, and an electric charge accumulation unit accumulates the electric charge in a polygonal area whose plurality of sides are adjacent to the photoelectric conversion unit on a light receiving surface. A voltage generation unit accumulates the electric charge and generates a voltage according to an amount of the accumulated electric charge. A first transfer unit transfers the electric charge from the photoelectric conversion unit to the electric charge accumulation unit when an instruction on a transfer to the electric charge accumulation unit is issued. A second transfer unit transfers the electric charge from the electric charge accumulation unit to the voltage generation unit when an instruction on a transfer to the voltage generation unit is issued.