Split Trunk Pixel Layout for CMOS Image Sensors
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Solution Overview
Problem
Conventional CMOS pixel cell arrays have a low fill factor and limited charge storage capacity, leading to inefficiencies and phenomena like 'blooming' due to the significant surface area required by non-photosensitive components and the limited capacity of storage nodes.
Innovation Solution
A split trunk pixel cell array architecture is introduced, where common components such as transfer and reset transistors are shared across multiple pixels, allowing for a separate active device area for each trunk, and the source follower transistor gate acts as a capacitor electrode, increasing charge storage capacity and fill factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If conventional CMOS pixel cell layout is used with separate transistors for each pixel, then charge transfer and reset functions are achieved, but fill factor is limited to approximately 50% due to significant surface area required by non-photosensitive components
Solution Approach 1:
The patent merges multiple transistor functions into shared structures. Specifically, the transfer transistor and reset transistor are combined into a single device with a shared gate structure, allowing both functions to be performed by one physical component. This merging eliminates the need for separate transistor instances in each pixel cell, thereby increasing the photosensor area while maintaining the necessary charge transfer and reset capabilities.
Solution Approach 2:
The shared transfer/reset transistor is designed to perform multiple functions: it acts as both a transfer transistor for moving charge from the photosensor to the readout circuit and as a reset transistor for clearing the photosensor node. This multi-functionality reduces the number of components needed per pixel cell, directly increasing the fill factor while preserving all necessary operational functions.
2Reliability
If floating diffusion region is used as storage node, then charge transfer is enabled, but charge storage capacity is limited causing blooming phenomenon
Solution Approach 1:
The patent segments the storage function from the transfer function by introducing a separate storage node (capacitor) distinct from the floating diffusion region. The floating diffusion region handles charge transfer operations, while the dedicated storage node provides expanded charge storage capacity. This segmentation prevents charge overflow into adjacent pixels (blooming) by providing an isolated storage region with sufficient capacity.
Solution Approach 2:
The patent introduces a transfer transistor as an intermediary component between the photosensor and the readout circuitry. This transfer transistor acts as a controlled gateway that regulates charge flow, preventing unwanted charge migration that causes blooming. The intermediary provides controlled access to the storage node while isolating the photosensor from direct exposure to readout circuit variations.
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 enhances the fill factor and quantum efficiency of the pixel cell array by optimizing the layout to maximize photosensor area and reduce fixed-pattern noise, while also increasing charge storage capacity without sacrificing fill factor.
Implementation Method 1
when incident light 187 strikes the surface of a photodiode photosensor 120, electron/hole pairs are generated in the p-n junction of the photodiode photosensor 120
Data Source
AI summary
A pixel array architecture having multiple pixel cells arranged in a split trunk pixel layout and sharing common pixel cell components. The array architecture increases the fill factor, and in turn, the quantum efficiency of the pixel cells. The common pixel cell components may be shared by a number of pixels in the array, and may include several components that are associated with the storage and readout of a signal from the pixel cells.