Image Sensor Pixel Charge Domain Summing
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Solution Overview
Problem
Existing charge summing methods in electronic image sensors increase floating diffusion capacitance, leading to reduced voltage changes and poor signal linearity due to additional transistors, and either increased capacitance or incomplete charge transfer.
Innovation Solution
The solution involves isolating summing transistors from floating diffusions, using summing nodes that connect across rows and columns without increasing capacitance, and employing a dual reset transistor system to manage charge transfer efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If summing transistors are connected directly to floating diffusions to enable charge summing, then charge summing capability is improved, but floating diffusion capacitance increases causing reduced voltage change
Solution Approach 1:
The patent divides the charge collection function into separate components: photodiodes collect charge, summing transistors transfer charge between pixels, and a shared floating diffusion node converts charge to voltage. This segmentation allows summing transistors to operate without being directly connected to the floating diffusion, avoiding capacitance loading while maintaining charge summing capability.
Solution Approach 2:
The patent introduces an intermediary mechanism where summing transistors transfer charge to a shared floating diffusion node that is isolated from direct transistor connections during the summing operation. This intermediary approach allows charge summation without the capacitance penalty of direct transistor-to-floating diffusion connections.
2Measurement precision
If summing transistors are connected to photodiodes to avoid increasing floating diffusion capacitance, then voltage change is preserved, but charge transfer becomes incomplete causing poor signal linearity
Solution Approach 1:
The patent employs dynamic control of transistor switching sequences. Transfer transistors are activated in a controlled sequence that ensures complete charge transfer from photodiodes to the shared floating diffusion node. The timing and sequence of transistor activation are optimized to achieve complete charge transfer while maintaining low capacitance on the floating diffusion node.
Solution Approach 2:
The patent performs preliminary charge collection in photodiodes before activation of summing and transfer transistors. This preliminary action ensures that charge is fully accumulated in the photodiodes before transfer begins, facilitating complete and accurate charge transfer to the shared floating diffusion node while maintaining signal linearity.
3Reliability
If extra amplifiers are inserted to isolate summing transistors from floating diffusions, then charge transfer is improved, but signal noise increases
Solution Approach 1:
The patent extracts the noise-generating amplifier component from the signal path and replaces it with a direct charge-to-voltage conversion at the shared floating diffusion node. By taking out the extra amplifier that would be needed to isolate summing transistors, the patent eliminates the primary noise source while maintaining effective charge transfer through optimized transistor switching sequences.
Solution Approach 2:
The patent uses the inherent capacitance of the shared floating diffusion node as a substitute for the isolation function that an extra amplifier would provide. This copying approach replaces the active isolation mechanism (amplifier) with a passive capacitive node that achieves the same isolation effect without adding noise.
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 minimizes floating diffusion capacitance, enhances signal response, and ensures complete charge transfer, resulting in improved image quality and reduced noise.
Implementation Method 1
Each pixel has one photodiode. Charge collected by the photodiode is transferred to a floating diffusion
Data Source
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AI summary
An image sensor includes (a) a plurality of pixels, wherein each pixel comprises:(i) at least one photosensor; (ii) at least one transfer gate connecting the photosensor to a floating diffusion; (iii) an output transistor connected to the floating diffusion; (iv) a first reset transistor connected between the floating diffusion and a summing node; (v) a second reset transistor connected to the summing node; and (b) a first summing transistor connecting together the summing nodes of two or more pixels.