Global Shutter CMOS Sensor Pixel Circuit with Charge Overflow Management
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Solution Overview
Problem
Standard CMOS image sensors suffer from 'rolling shutter' artifacts due to sequential pixel scanning, while global shutter methods are often complex and costly, and CCD sensors are prone to saturation artifacts, limiting their use in compact and portable applications.
Innovation Solution
A pixel circuit with a pinned photodiode, transfer gates, and a capacitor node connected via barriers to manage charge overflow, allowing for global electronic shutter operation with high dynamic range and reduced blooming, using a merging switch for charge readout and storage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If global shutter is implemented in CMOS by transferring charge to registers, then rolling shutter artifact is eliminated, but saturation artifacts occur and transfer time increases
Solution Approach 1:
The pixel is divided into multiple independent photodiodes (first and second photodiodes) that can be reset independently, allowing simultaneous exposure of all pixels without requiring complex row-by-row transfer mechanisms. This segmentation enables global shutter functionality while simplifying the control circuitry.
Solution Approach 2:
The invention nests the reset mechanism within each pixel unit, where each photodiode has its own reset transistor integrated directly in the pixel. This eliminates the need for external register transfers and complex gate control, reducing device complexity while maintaining global shutter capability.
2Object-affected harmful factors
If overflow barriers and doped regions are used to reduce blooming, then excess charge is removed, but photocharges are lost and blooming is not fully prevented
Solution Approach 1:
The invention introduces a transfer gate as an intermediary mechanism between the photodiode and the overflow path. This transfer gate provides controlled charge redistribution, allowing excess charge to be managed without complete loss and preventing blooming through regulated charge flow rather than abrupt barriers.
Solution Approach 2:
The invention changes the operational parameters by using transfer gates that can be dynamically controlled to adjust charge flow. By modifying the gate voltage states, the system can regulate charge distribution adaptively, preventing blooming while retaining useful photocharges that would otherwise be lost to fixed barriers.
3Reliability
If charge storage capacity is increased to handle high dynamic range, then saturation artifacts are reduced, but pixel area increases
Solution Approach 1:
The invention merges multiple photodiodes (first and second photodiodes) within a single pixel unit, allowing them to share common readout circuitry and control mechanisms. This combining approach increases the effective charge storage capacity for high dynamic range imaging while maintaining a compact pixel area through shared resources.
Solution Approach 2:
The common readout circuitry and control structures serve multiple photodiodes simultaneously, providing multi-functional capability. This universality allows increased charge storage capacity without proportional area increase, as the same circuitry handles charge from multiple photodiodes.
4Productivity
If transfer and readout times are reduced for video applications, then frame rate is improved, but charge transfer accuracy may be compromised
Solution Approach 1:
The invention performs preliminary charge accumulation in multiple photodiodes during the exposure period, so that charge is already ready for rapid readout. This preliminary action eliminates the need for time-consuming charge transfer operations during the readout phase, enabling fast frame rates while maintaining accuracy through Correlated Double Sampling.
Solution Approach 2:
The invention replaces complex mechanical charge transfer mechanisms with electronic signal processing techniques, specifically Correlated Double Sampling. This substitution allows rapid readout of accumulated charge with high precision, achieving both fast frame rates and accurate charge measurement without physical charge movement during readout.
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
The solution enables high dynamic range imaging with reduced blooming and noise, allowing for efficient charge storage and readout without losing excess photocharges, suitable for video applications and fast image capture.
Implementation Method 1
Particular types of radiation sensors work under the photoelectric effect. Impinging photons are converted into electrons and are integrated (collected) in sensor pixels.
Data Source
AI summary
The present invention provides a pixel circuit comprising a pinned photodiode, at least one first transfer gate for electrically connecting the pinned photodiode to at least one storage node and at least one further transfer gate. The at least one further gate can connect the at least one storage node with at least one floating diffusion node. At least one merging switch is included for allowing connection between the at least one floating diffusion node with one or more capacitor nodes, which can accept charge that exceeds the maximum storage capacity of the storage node.