CMOS Pixel Sensor Parasitic Photodiode Dynamic Range
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Solution Overview
Problem
CMOS imaging sensors face limitations in dynamic range due to maximum well capacity and noise, with existing solutions either increasing cost or introducing artifacts from multiple exposures.
Innovation Solution
Incorporating a parasitic photodiode with a higher light conversion efficiency into the pixel sensor, connected to a floating diffusion node, which allows for improved light detection without significantly increasing pixel size, and adjusting the relative efficiencies of the main and parasitic photodiodes to enhance dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large area photodiode is used to measure low light levels, then the dynamic range is improved, but the pixel size and cost increase
Solution Approach 1:
The pixel is divided into two photodiodes with different areas: a large area photodiode for low light detection and a small area photodiode for high brightness detection. This segmentation allows each photodiode to be optimized for its specific function without requiring the entire pixel to be large, thus improving dynamic range while controlling pixel size.
Solution Approach 2:
Different regions of the pixel have different properties: the large area photodiode has high light collection efficiency for dim locations, while the small area photodiode has lower collection efficiency but prevents saturation at bright locations. This local quality differentiation allows the pixel to handle both low and high light levels effectively.
2Measurement precision
If two different photodiodes are used for each pixel, then the dynamic range is improved, but the silicon area and device complexity increase
Solution Approach 1:
The patent combines the functions of two photodiodes into a single pixel structure by sharing common elements such as the floating diffusion node, readout amplifier, and control circuitry. Only the photodiode area and gate structures differ, reducing the overall complexity increase compared to fully independent dual-photodiode designs.
Solution Approach 2:
The floating diffusion node and readout circuitry serve both photodiodes, making these components multi-functional. The same readout amplifier processes signals from both the large and small area photodiodes, reducing the need for separate signal processing paths and lowering overall device complexity.
3Measurement precision
If multiple exposures are used to provide increased dynamic range, then the measurement precision is improved, but artifacts are introduced when the scene changes rapidly
Solution Approach 1:
The patent enables continuous simultaneous measurement by both photodiodes during a single exposure, eliminating the temporal discontinuity inherent in multiple exposure techniques. Both the large and small area photodiodes collect charge concurrently, ensuring that the scene remains unchanged throughout the measurement process and preventing artifacts from rapid scene changes.
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 extends the dynamic range of the pixel sensor, reducing noise and artifacts, while maintaining a compact sensor size and improving light intensity measurement accuracy across varying exposure levels.
Implementation Method 1
Each pixel sensor includes a photodiode that measures the image intensity at a corresponding point in the image. The photodiode accumulates charge at a rate determined by the light intensity emitted by the image at the corresponding point in the image
Implementation Method 2
The floating diffusion node includes a parasitic photodiode characterized by a second light conversion efficiency, the second light conversion efficiency is greater than or equal to 1/30 times the first light conversion efficiency
Data Source
AI summary
A pixel sensor having a main photodiode and a parasitic photodiode and a method for reading out that pixel sensor are disclosed. The parasitic photodiode also serves the function of a floating diffusion node in the pixel. The pixel sensor is read by first determining the exposure as measured by the parasitic photodiode and then determining the exposure as read by the main photodiode. One of the two exposure measurements is chosen as the pixel output. The main photodiode has a light conversation efficiency chosen such that one of the two measurements will provide a measurement of the exposure over a dynamic range that is greater than that of either the main photodiode or the parasitic photodiode utilized separately.