Shared Photodiode Reset in 5T-4P CMOS Image Sensor
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Solution Overview
Problem
Current CMOS image sensors face challenges in achieving global shutter scanning with exposure time control, particularly in reducing interconnections and maintaining high performance while reading out individual photodiodes in shared pixel configurations, which affects conversion gain and fill factor.
Innovation Solution
The implementation of a CMOS image sensor with a pixel array featuring shared pixel units comprising four photodiodes and global shutter gates, where an image capture timing controller adjusts the global shutter pulse width to control exposure time and bin charge concurrently between photodiodes and a shared floating diffusion node, enabling efficient readout and improved image capture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional 4T pixel design is used, then pixel circuit functionality is achieved, but interconnections increase affecting pixel performance
Solution Approach 1:
The patent merges four separate photodiodes into a single shared pixel unit with a common floating diffusion node and reset transistor. This consolidation reduces the number of interconnections required while maintaining the functional capabilities of individual photodiodes, directly addressing the contradiction between pixel performance and interconnection complexity.
Solution Approach 2:
The shared floating diffusion node and reset transistor serve multiple photodiodes simultaneously, making these components universal for the entire pixel unit rather than dedicated to individual photodiodes. This multi-functionality reduces the overall interconnection count while preserving pixel performance through shared signal processing paths.
2Adaptability or versatility
If more interconnections are added for global shutter scanning, then exposure time control is achieved, but wiring complexity increases degrading conversion gain and fill factor
Solution Approach 1:
The patent combines global shutter gating functionality across all four photodiodes through shared control mechanisms. By using common gate structures and timing control for the shared floating diffusion node, the design achieves exposure time control without requiring separate wiring for each photodiode, thus reducing wiring complexity while maintaining adaptability.
Solution Approach 2:
The reset transistor and floating diffusion node are prepared in advance to handle simultaneous charge from multiple photodiodes. The preliminary setup of shared reset circuitry enables global shutter operation without adding complex wiring during the imaging process, as the infrastructure is already in place to manage concurrent charge binning.
3Measurement precision
If four separate pixels are used, then individual photodiode readout is achieved, but pixel area and fill factor decrease
Solution Approach 1:
The patent merges four photodiodes into a single pixel unit that shares the floating diffusion node and readout circuitry. This consolidation increases the effective pixel area and fill factor by eliminating redundant circuit elements, while individual photodiode readout is maintained through selective transfer gate control that directs charge from specific photodiodes to the shared node.
Solution Approach 2:
While merging the readout circuitry, the patent maintains segmentation of the photodiode array with individual transfer gates for each photodiode. This allows selective readout of individual photodiodes when needed, preserving measurement precision while benefiting from the area efficiency of shared circuitry.
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 solution enhances the CMOS image sensor's ability to perform global shutter scanning with exposure time control, reducing interconnections and maintaining high performance by allowing individual readout of photodiodes, thereby improving conversion gain and fill factor, and enabling efficient digital double sampling.
Implementation Method 1
The photodiode generates electric charges using absorbed light
Data Source
AI summary
An image sensor that provides global shutter scanning with exposure time control during image capture. The image sensor includes a pixel array with shared pixel units that each include four photodiodes with a floating diffusion node shared therebetween and respective global shutter gates disposed between each photodiode and a supply voltage of the pixel array. Moreover, an image capture timing controller controls an exposure time of each photodiode by adjusting a width of a global shutter reset pulse applied to the plurality of global shutter gates after each readout cycle during image capture to change the respective exposure time of each shared pixel unit.