CMOS Active Pixel Structure with Dual Photodiode Zones
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Solution Overview
Problem
Conventional CMOS active pixel sensors face challenges in achieving high dynamic range and sensitivity while maintaining a simple design and avoiding bulkiness, as they often compromise on either sensitivity or dynamic range due to limitations in photodiode capacitance and noise issues.
Innovation Solution
A CMOS active pixel structure is proposed, combining a photodiode operating in photovoltaic mode with another operating in integration mode, where the photodiode in photovoltaic mode re-emits charge carriers, which are collected and accumulated by the integration mode photodiode, enhancing sensitivity and dynamic range without increasing bulkiness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the integration capacitance is increased to improve dynamic range, then the saturation level is improved, but the sensitivity deteriorates due to lower voltage for the same charge
Solution Approach 1:
The photodiode is divided into two distinct regions: a photovoltaic conversion zone for charge generation and a separate charge accumulation zone for charge storage. This segmentation allows the conversion zone to maintain low capacitance for high sensitivity while the accumulation zone provides high capacitance for extended dynamic range, resolving the contradiction between sensitivity and dynamic range.
2Measurement precision
If the photodiode surface area is increased to improve sensitivity, then the photon collection efficiency is improved, but the device complexity and bulkiness increase
Solution Approach 1:
The patent transitions from a planar photodiode structure to a three-dimensional stacked structure with vertically separated photovoltaic conversion zone and charge accumulation zone. This dimensional change allows both zones to occupy the same lateral footprint, improving sensitivity without increasing device area or complexity.
3Device complexity
If a single photodiode is used to simplify design, then the device complexity is reduced, but the ability to simultaneously achieve high sensitivity and dynamic range deteriorates
Solution Approach 1:
The dual-zone photodiode structure serves multiple functions within a single device: the photovoltaic conversion zone handles charge generation with high efficiency, while the charge accumulation zone provides extended dynamic range and temporary charge storage. This multi-functionality allows the single photodiode to achieve both high sensitivity and wide dynamic range simultaneously.
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 allows for improved sensitivity and dynamic range, enabling efficient charge accumulation and reduced noise, while maintaining a compact design by separating the depletion zones of the two photodiodes and optimizing charge carrier collection.
Implementation Method 1
a first photodiode operating in photovoltaic mode comprising a photovoltaic conversion zone defined by a doped zone of a second type forming a PN junction with the substrate
Implementation Method 2
at least one second photodiode operating in integration mode and reverse biased comprising a charge accumulation zone defined by a doped zone of the second type forming a PN junction with the substrate
Data Source
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AI summary
The invention concerns a structure of a CMOS active pixel, comprising a semi-conductive substrate (1) of a first type, at least one first photodiode operating in photovoltaic mode comprising a photovoltaic conversion area (2) defined by a doped area of a second type forming a PN junction with the substrate, said first photodiode re-emitting photoelectric charge carriers collected by the PN junction during the exposure of said first photodiode to a light, at least one second photodiode operating in integration mode and reverse-biased, said second photodiode comprising a charge accumulation area (3) defined by a doped area of the second type forming a PN junction with the substrate, said charge accumulation area being exposed to the charge carriers from the photovoltaic conversion area (2) in order to accumulate such charge carriers.