Pixel Sensor Floating Diffusion Node Potential Control for Dark Current Reduction
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Solution Overview
Problem
In very low light photography, dark current poses significant challenges due to the substantial background charge it generates, which is a fraction of the accumulated charge during exposure, necessitating a reduction in dark current to improve image quality.
Innovation Solution
A pixel sensor design that includes a transfer gate connecting a photodiode to a floating diffusion node, a reset circuit capable of applying different potentials, and a controller that adjusts these potentials to minimize dark current by reducing the voltage on the floating diffusion node during the exposure phase.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the CMOS imaging sensor operates at low temperatures and very low frame rates to reduce noise in very low light photography, then the image quality improves, but the dark current generates substantial background charge that degrades the signal
Solution Approach 1:
The patent applies preliminary action by resetting the floating diffusion node to a first potential level just before the accumulation phase begins, and then switching to a second lower potential during the accumulation phase. This preliminary reset action prepares the node to minimize dark current integration while maintaining readiness for photon detection, thereby reducing background charge before it can accumulate during the exposure period.
Solution Approach 2:
The patent changes the electrical potential parameter of the floating diffusion node dynamically: applying a first potential (higher level) when the transfer gate is conducting just prior to accumulation, then switching to a second potential (lower level) after the transfer gate is rendered non-conducting during the accumulation phase. This parameter change reduces the electric field that would otherwise drive dark current generation and integration, thereby reducing background charge while maintaining measurement precision.
2Measurement precision
If the transfer gate connects the photodiode to the floating diffusion node during the accumulation phase, then photon detection is enabled, but dark current integrates onto the floating diffusion node reducing signal quality
Solution Approach 1:
The reset circuit applies a first potential to the floating diffusion node just prior to the accumulation phase when the transfer gate is conducting, then switches to a second lower potential after the transfer gate is rendered non-conducting. This preliminary potential adjustment prepares the node to minimize dark current integration during the accumulation phase while maintaining the ability to detect photons effectively.
Solution Approach 2:
The patent dynamically changes the potential parameter of the floating diffusion node: applying a higher first potential when the transfer gate is conducting just before accumulation, then switching to a lower second potential during the accumulation phase after the transfer gate is non-conducting. This parameter change reduces the electric field strength that drives dark current integration, thereby reducing the quantity of dark current charge while preserving signal quality.
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 the background noise from dark current, enhancing image quality by minimizing the integrated dark current fraction during long exposures in low light conditions.
Implementation Method 1
a photodiode to a floating diffusion node... at least one of the pixel sensors including a transfer gate connecting a photodiode
Data Source
AI summary
A pixel sensor, an imaging array that includes such pixel sensors, and a method for operating an imaging array are disclosed. The pixel sensor includes a transfer gate that connects a photodiode to a floating diffusion node in response to a transfer signal, a reset circuit, and a controller. The reset circuit is adapted to apply either a first potential or a second potential to the floating diffusion node, the second potential being less than the first potential. The controller is configured to cause the reset circuit to apply the first potential to the floating diffusion node while the transfer gate is conducting just prior to a start of an accumulation phase, and then apply the second potential to the floating diffusion node after the transfer gate is rendered non-conducting, the second potential is less than the first potential.