Image Sensor Reset-Sampling Capacitor for Leakage Noise Reduction
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Solution Overview
Problem
Image sensors with global electronic shutters face issues due to leakage current in shutter transistors, leading to dark current and noise, which affect image quality, especially when storing charge for extended periods.
Innovation Solution
The use of P-type shutter transistors in N-wells and a common reset-sampling capacitor shared among multiple image storage capacitors reduces leakage current and noise by employing differential amplification for noise reduction, allowing for efficient charge transfer and storage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If shutter transistors are used to transfer charge to image storage capacitors, then global electronic shutter functionality is achieved, but leakage current causes dark current and noise
Solution Approach 1:
The patent extracts the harmful leakage current path by introducing a separate reset capacitor that is isolated from the image storage capacitor during the charge transfer phase. The reset capacitor captures and holds the reset level voltage, preventing it from leaking into the image storage capacitor through the shutter transistor, thereby removing the source of dark current noise while preserving global shutter operation.
Solution Approach 2:
The reset capacitor acts as an intermediary element between the reset transistor and the image storage capacitor. It mediates the reset operation by storing the reset level voltage and providing a stable reference for correlated double sampling, while preventing direct connection and potential leakage between the reset circuitry and the image storage capacitor during charge transfer.
2Duration of action of stationary object
If charge is stored on image storage capacitors for extended frame times, then complete image capture is achieved, but voltage drift towards light-detected levels occurs due to shutter transistor leakage
Solution Approach 1:
The patent extracts the unstable voltage component by using the reset capacitor to capture and hold the reset level voltage separately. This isolated reset voltage serves as a stable reference that does not drift over time, allowing for accurate correlated double sampling even after extended charge storage periods, thereby preventing voltage drift effects from corrupting the stored image charge.
3Measurement precision
If multiple capacitors are used per pixel for correlated double sampling, then noise reduction is achieved, but device complexity increases
Solution Approach 1:
The patent merges the reset sampling function into a shared reset capacitor that serves multiple pixels, rather than requiring separate reset capacitors for each pixel. This consolidation maintains the correlated double sampling noise reduction capability while significantly reducing the total number of capacitors needed in the pixel array, thereby lowering device complexity.
Solution Approach 2:
The reset capacitor is designed with universal functionality to serve multiple pixels through the reset transistor network. A single reset capacitor can be sequentially connected to multiple pixels during the reset phase, allowing it to perform the same noise-reduction reference function for all pixels without requiring pixel-specific dedicated capacitors, thus reducing overall circuit complexity.
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 configuration minimizes dark current-induced noise and enhances image quality by maintaining voltage levels near the power supply voltage, even during extended exposure times, thereby reducing distortion and improving the dynamic range of captured images.
Implementation Method 1
a photodiode 102 coupled through a first selection transistor 104 controlled by a photodiode selection line 106 to a first pixel node 108
Implementation Method 2
CDS operation requires two read-outs per pixel: a dark current or reference level read out and a light-induced signal read-out
Data Source
AI summary
An image sensor has an array of pixel blocks, and each pixel block having associated shutter transistors with each coupled to transfer an image signal comprising a charge dependent on light exposure of a selected pixel onto an image storage capacitor of a plurality of image storage capacitors associated with the pixel block, the image storage capacitors of the pixel block configured to be read through a differential amplifier into an analog to digital converter. The differential amplifier of each pixel block receives a second input from a single reset-sampling capacitor associated with the pixel block. The single reset-sampling capacitor is loaded when the pixels of the pixel block are reset.