Solid-state imaging device holding circuit dark current noise reduction
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Solution Overview
Problem
Conventional solid-state imaging devices suffer from noise due to dark current, particularly in global shutter operations where charge is held for extended periods, leading to image quality degradation.
Innovation Solution
The implementation of a holding circuit outside the pixel circuit to store electric signals, utilizing a larger capacitor to reduce noise and allowing signals to be held for longer periods without degrading image quality, along with a difference circuit to enhance signal-to-noise ratio and a buffer circuit to manage signal transfer speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If the capacitor C is implemented as a floating diffusion unit to enable global shutter operation, then the ability to hold electric charge for extended periods is improved, but dark current increases causing white spot noise and degrading image quality
Solution Approach 1:
The patent extracts the charge holding function from the pixel circuit's floating diffusion unit and relocates it to a separate holding circuit. This separation removes the harmful dark current generation from the pixel circuit while preserving the global shutter capability. The holding circuit uses a transfer transistor and capacitor to store charge without the dark current issues that plague floating diffusion units during extended holding periods.
Solution Approach 2:
The patent introduces a transfer transistor as an intermediary component between the pixel circuit and the holding circuit. This transfer transistor enables the movement of electric charge from the pixel circuit to the holding circuit, allowing the system to achieve global shutter operation without requiring the pixel circuit itself to hold charge for extended periods, thereby avoiding dark current accumulation.
2Adaptability or versatility
If the electric signal is held in the pixel circuit for extended periods to enable global shutter operation, then the shutter flexibility is improved, but noise due to dark current increases degrading image quality
Solution Approach 1:
The patent segments the imaging system into distinct functional blocks: pixel circuits for light detection, transfer transistors for signal movement, and holding circuits for charge storage. This segmentation allows each component to perform its specialized function optimally - the pixel circuit focuses on detection while the holding circuit handles extended storage without dark current issues, maintaining shutter flexibility throughout.
Solution Approach 2:
The patent extracts the extended charge holding function from the pixel circuit and places it in a separate holding circuit. This extraction enables global shutter operation with extended holding times while removing the source of dark current noise from the pixel circuit, thereby maintaining shutter flexibility without the harmful side effects.
3Measurement precision
If a larger capacitor is used in the holding circuit to reduce noise, then the signal-to-noise ratio is improved, but the device area increases
Solution Approach 1:
The patent relocates the holding circuit from the two-dimensional pixel array plane to a separate dimension or layer outside the pixel circuit. This dimensional change allows the holding circuit to use larger capacitors for better noise performance without consuming valuable pixel array area, as the holding circuit occupies space in a different spatial dimension or external to the main array.
Data Source
AI summary
A plurality of pixel circuits arranged in rows and columns, and each of which outputs an electric signal according to an amount of received light; a first column signal line provided for each of the columns, and for sequentially transferring the electric signals from said pixel circuits in a corresponding column; and a holding circuit provided for each of the pixel circuits in each column, and which holds the electric signal transferred through the column signal line in the corresponding column are provided. A holding circuit includes a first capacitor which holds a first electric signal of the corresponding pixel circuit in a reset state; and a second capacitor which holds a second electric signal after the corresponding pixel circuit receives light. A difference circuit calculates a difference between two electric signals held by the first capacitor and the second capacitor in a same holding circuit.