Buried Channel Transfer Gate for Image Sensor Blooming Control
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Solution Overview
Problem
Image sensors experience blooming current, where excess electrons from bright light spill into neighboring photodiodes, causing undesirable image artifacts, and existing solutions like blanket buried channels compromise pixel performance.
Innovation Solution
Implementing a buried channel transfer gate with a discontinuous buried channel region that extends from the floating diffusion, but not the photodiode, to manage blooming current without increasing dark current and maintaining photodiode performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a blanket buried channel is implemented to control blooming current, then blooming current is reduced, but dark current increases and pixel performance deteriorates
Solution Approach 1:
The patent divides the buried channel into two separate segments: one extending from the photodiode and another extending from the floating diffusion, with a gap between them. This segmentation allows the first segment to capture blooming current from the photodiode while the second segment directs it to the floating diffusion, preventing dark current generation in the overlap region and maintaining pixel performance.
Solution Approach 2:
The patent applies different functional qualities to different regions of the buried channel. The first buried channel portion is optimized for capturing blooming current from the photodiode, while the second buried channel portion is optimized for directing current to the floating diffusion. This local differentiation allows effective blooming control without compromising dark current performance.
2Object-affected harmful factors
If the buried channel extends from the photodiode to control blooming, then blooming current is directed away from neighboring pixels, but dark current is increased
Solution Approach 1:
The patent segments the buried channel into two distinct portions with a gap between them. The first portion extends from the photodiode to capture blooming current, while the second portion extends from the floating diffusion to direct it appropriately. This segmentation prevents the formation of a continuous conductive path that would generate dark current, thereby maintaining pixel reliability.
3Productivity
If a continuous buried channel is used to manage charge transfer, then charge transfer efficiency is improved, but blooming current control is compromised
Solution Approach 1:
The patent uses segmentation to create two separate buried channel portions that work in sequence. The first portion efficiently transfers charge from the photodiode while capturing blooming current, and the second portion directs the charge to the floating diffusion. The gap between segments prevents blooming current leakage while maintaining efficient charge transfer through coordinated operation of both segments.
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 effectively directs blooming current to the floating diffusion while minimizing dark current and maintaining high pixel performance, reducing image artifacts and full well capacity issues.
Implementation Method 1
The image pixels contain a photodiode (or other type of photodetector) for generating charge in response to light (e.g., by photoelectric conversion)
Data Source
AI summary
An image sensor pixel may include a photodiode, a floating diffusion, and a transfer gate. A buried channel may be formed under the transfer gate. The buried channel may extend from the floating diffusion to overlap a portion of the transfer gate without extending completely beneath the transfer gate or reaching the photodiode. The buried channel may provide a path for antiblooming current from the photodiode to reach the floating diffusion, while allowing for the transfer gate off voltage to remain high enough to prevent transfer gate dark current from flowing into the photodiode.