High Dynamic Range Pixel Structure with Overflow Charge Storage
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Solution Overview
Problem
Pinned photodiodes have a restricted dynamic range, which is particularly problematic when pixel size is reduced, limiting their ability to resolve a wide brightness range compared to conventional photodiodes.
Innovation Solution
A pixel structure comprising a photo-sensitive element and a charge storage element with higher charge storage density, connected via a first transfer gate, allowing for increased dynamic range and improved performance in various operating modes by collecting and storing charges on the photo-sensitive element side of the transfer gate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a pinned photodiode is used in a pixel structure, then low dark current and correlated double sampling capability are achieved, but dynamic range is restricted
Solution Approach 1:
The pixel structure is segmented into two distinct charge storage regions: a first charge storage region (pinned photodiode) optimized for low dark current and correlated double sampling, and a second charge storage region (overflow capacitor) optimized for high charge storage capacity. This segmentation allows each region to specialize in its strengths while working together to achieve both low dark current and extended dynamic range.
Solution Approach 2:
The overflow capacitor is positioned adjacent to and works in conjunction with the pinned photodiode, creating a nested charge storage system where charges can be stored in either region depending on the light intensity. The smaller pinned photodiode is effectively nested within the larger pixel structure that includes the overflow capacitor, allowing the system to handle both small and large signal levels.
2Area of moving object
If pixel size is reduced, then higher resolution is achieved, but dynamic range capability deteriorates
Solution Approach 1:
Instead of increasing pixel area to improve dynamic range, the invention utilizes a temporal dimension by implementing dual charge storage regions that can be selectively filled and read out. The overflow capacitor provides an additional charge storage dimension that complements the pinned photodiode, allowing extended dynamic range without increasing the physical pixel footprint.
3Adaptability or versatility
If conventional photodiode is used instead of pinned photodiode, then dynamic range is improved, but dark current and noise performance worsen
Solution Approach 1:
Different regions of the pixel structure are assigned different functional qualities: the pinned photodiode region is optimized for low dark current and low noise through its buried structure and pinning contact, while the overflow capacitor region is optimized for high charge storage capacity. This local quality differentiation allows the system to achieve both low noise performance and extended dynamic range by placing each component in its optimal operational role.
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 significantly enhances the pixel's ability to resolve a wider brightness range, providing improved performance in small pixel structures and various operating modes, such as rolling shutter and global pipelined shutter, with the charge storage element contributing to a signal value read from the pixel.
Implementation Method 1
A pixel structure comprises a photo-sensitive element for generating charges in response to light
Data Source
AI summary
A pixel structure comprises a photo-sensitive element PPD for generating charges in response to light and a charge conversion element FD. A first transfer gate TX is connected between the photo-sensitive element PPD and the charge conversion element. A charge storage element PG is connected to the photo-sensitive element PPD. The charge storage element PG has a higher charge storage density than the photo-sensitive element PPD. The charge storage element PG is located on the photo-sensitive element PPD side of the first transfer gate TX and is arranged to collect charges generated by the photo-sensitive element PPD during an integration period. The charge storage element can be a photo gate, photodiode or capacitor. Arrangements are provided with, and without, a potential barrier between the photo-sensitive element PPD and the charge storage element PG.