Pinned-Photodiode Pixel Global Shutter Dark Current Reduction
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Solution Overview
Problem
Current image sensors with global electronic shuttering face limitations such as temporal noise due to uncorrelated reset operations and the buildup of dark current during signal charge storage, which affects image quality.
Innovation Solution
An image sensor design featuring a photodetector, a storage region, a sense node, and an amplifier, where the storage region is shielded from additional photo-generated charge, allowing for true correlated double sampling and reduced noise through a doped charge shield and a lightshield configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If signal charge is held on the sense node from the end of the shutter window until readout, then global electronic shuttering is achieved, but dark current builds up as background and bright points during this time
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: photodiode for charge collection, storage region for charge holding, and sense node for signal readout. This spatial segmentation allows the charge to be transferred from the photodiode to the storage region, preventing dark current accumulation on the sense node while maintaining global shuttering capability
Solution Approach 2:
A storage region is introduced as an intermediary between the photodiode and the sense node. This intermediate storage region holds the transferred charge during the readout period, preventing direct holding on the sense node and thereby eliminating dark current buildup while preserving the global electronic shutter function
2Device complexity
If reference readout is done after signal readout with uncorrelated reset operations, then readout sequence is simplified, but temporal noise is added to the image
Solution Approach 1:
The reset operation is performed preliminarily on both the photodiode and sense node before the signal integration period begins. This preliminary resetting ensures that both nodes start from a known state, enabling correlated double sampling where the reference and signal readings are taken from reset operations that are temporally correlated, thereby eliminating temporal noise while maintaining readout simplicity
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 provides a global electronic shutter with low dark current storage and true correlated double sampling, significantly reducing noise and improving image quality.
Implementation Method 1
a photodetector for collecting charge in response to incident light
Implementation Method 2
allowing for true correlated double sampling and reduced noise through a doped charge shield and a lightshield configuration
Data Source
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AI summary
An image sensor includes a two-dimensional array of pixels having a photodetector for collecting charge in response to incident light; a storage region adjacent the photodetector that receives the charge from the photodetector; a sense node adjacent the storage region that receives the charge from the storage region and converts the charge to a voltage signal; and an input to an amplifier for sensing the voltage signal from the sense node.