Global Shutter Pixel Array with Buffered Sampling for Fixed Pattern Noise
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Solution Overview
Problem
CMOS image sensors face challenges in implementing global shutter operation due to issues like parasitic light sensitivity and leakage current, which result in fixed pattern noise and noise increase in image capture, especially in high-speed applications.
Innovation Solution
A pixel design comprising a photo-sensitive element, a sense node, a transfer gate, a reset switch, and buffer amplifiers that allow for differential sampling and storage of radiation signals on a capacitor, enabling reduced fixed pattern noise and anti-blooming protection without separate anti-blooming gates, facilitating both triggered and pipelined global shutter modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a floating diffusion is used for signal storage during exposure, then pipelined shutter operation is enabled, but parasitic light sensitivity and leakage current increase causing fixed pattern noise and image gradient
Solution Approach 1:
The pixel circuit is segmented into distinct functional regions: a first buffer amplifier for signal buffering, a sample capacitor for charge storage, and a second buffer amplifier for output. This segmentation isolates the storage function from light-sensitive regions, preventing parasitic light sensitivity and leakage current from degrading the stored signal while maintaining pipelined operation capability.
Solution Approach 2:
A sample capacitor is introduced as an intermediary element between the floating diffusion and the output circuitry. This capacitor stores the signal charge in a controlled manner, decoupling the storage process from the light-sensitive photodiode region and eliminating the fixed pattern noise and gradient issues associated with direct floating diffusion storage.
2Reliability
If a separate anti-blooming transistor is added to drain excess charges, then anti-blooming protection is achieved, but device complexity increases
Solution Approach 1:
The transfer gate is designed to serve multiple functions: it transfers signal charges from the photodiode to the floating diffusion during normal operation, and it also drains excess charges during anti-blooming conditions. This multi-functionality eliminates the need for a separate anti-blooming transistor, reducing device complexity while maintaining protection capability.
Solution Approach 2:
The anti-blooming function is merged with the existing transfer gate circuitry. By controlling the transfer gate's operation mode, the same structure performs both signal transfer and excess charge drainage, consolidating functions and minimizing the total transistor count in the pixel circuit.
3Duration of action of moving object
If the floating diffusion stores signal during longer exposure times, then integration is improved, but light-dependent gradient increases with brighter areas near last rows
Solution Approach 1:
The signal charge is extracted from the light-sensitive floating diffusion region and transferred to a sample capacitor for storage. This extraction removes the stored signal from the path of parasitic light, preventing additional light-dependent gradients from developing during extended storage periods while allowing longer integration times at the photodiode.
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 significantly reduces fixed pattern noise and temporal noise, allowing for higher gain and improved image quality in high-speed cameras by storing radiation signals behind the first buffer amplifier and resetting the sense node during exposure, enabling noise-free image acquisition.
Implementation Method 1
a photo-sensitive element for generating charges in response to incident radiation
Data Source
AI summary
A pixel comprises a photo-sensitive element for generating charges in response to incident radiation and a sense node. A transfer gate is positioned between the photo-sensitive element and the sense node for controlling transfer of charges to the sense node. A reset switch is connected to the sense node for resetting the sense node to a predetermined voltage. A first buffer amplifier has an input connected to the sense node. A sample stage is connected to the output of the first buffer amplifier and is operable to sample a value of the sense node. A second buffer amplifier has an input connected to the sample stage. Control circuitry operates the reset switch and causes the sample stage to sample the sense node while the photo-sensitive element is being exposed to radiation. An array of pixels is synchronously exposed to radiation. Sampled values for a first exposure period can be read while the photo-sensitive element is exposed for a second exposure period.