Storage Gate Pixel Charge Management for Noise Reduction
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Solution Overview
Problem
Conventional image sensors face limitations in signal-to-noise ratio (SNR) due to high fixed pattern noise and photon shot noise, especially at varying light levels, which affects image quality and is exacerbated by the scaling down of pixel sizes.
Innovation Solution
The implementation of a storage gate pixel design where photo-generated charge is stored and transferred to a diffusion region only if it exceeds the storage capacity or a predetermined threshold, enhancing the signal-to-noise ratio by managing charge accumulation and transfer based on signal intensity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional image sensor circuits are used, then the basic imaging function is achieved, but the signal-to-noise ratio is low due to high fixed pattern noise and photon shot noise
Solution Approach 1:
The pixel is divided into two separate photodiodes: a first photodiode for charge accumulation and a second photodiode for reset operation. This segmentation allows the accumulation photodiode to be optimized for signal collection while the reset photodiode handles noise reduction, thereby improving the overall signal-to-noise ratio by isolating and addressing different functional requirements separately
Solution Approach 2:
A storage node is introduced as an intermediary between the first photodiode and the readout circuitry. The storage node temporarily holds the accumulated charge and enables correlated double sampling operations, which effectively reduce fixed pattern noise and photon shot noise by allowing comparison between signal and reset levels, thus improving signal-to-noise ratio
2Productivity
If pixel sizes are scaled down to increase resolution, then more pixels fit in the sensor area, but noise performance deteriorates and signal-to-noise ratio decreases
Solution Approach 1:
By segmenting the pixel into multiple specialized photodiodes and a storage node, the design allows each component to be optimized for its specific function. This enables smaller pixel sizes to maintain adequate signal-to-noise ratio because the segmented architecture efficiently separates signal accumulation from noise-generating operations, allowing higher pixel density without proportional noise increase
Solution Approach 2:
The storage node performs preliminary charge storage and enables reset operations before readout. This preliminary action allows the system to establish a baseline reset level and perform correlated double sampling, which reduces noise effects even in small pixels where signal levels are low, thereby maintaining signal-to-noise ratio at higher pixel densities
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 approach effectively reduces noise and improves the signal-to-noise ratio across a range of signal intensities, leading to higher image quality and better performance even at smaller pixel sizes.
Implementation Method 1
each one of the cells including either a photogate, photoconductor, or photodiode overlying a charge accumulation region within a substrate for accumulating photo-generated charge
Data Source
AI summary
A storage gate pixel operates such that the storage gate is not required to have the same capacity as a photodiode of the pixel. This provides greater fill factor for the pixel and a higher signal to noise ratio.