CMOS Image Sensor Pixel Unit With Separate Storage Capacitors
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Solution Overview
Problem
Conventional CMOS image sensors suffer from low Signal-to-Noise Ratio (SNR) due to signal attenuation during the reading process, particularly in 8T-type global exposure CMOS image sensors where the first and second sampling capacitances share the exposure signal, leading to a decay of the useful signal.
Innovation Solution
The proposed pixel unit for CMOS image sensors includes a photoelectric conversion unit, an isolation transistor, a storage unit with separate first and second storage capacitors, and a reading unit, where the storage unit is controlled to charge or discharge these capacitors independently, allowing for separate storage and reading of reset and exposure signals without attenuation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the first and second sampling capacitances share the exposure signal in 8T-type global exposure CMOS image sensors, then the device complexity is reduced, but the signal-to-noise ratio deteriorates due to signal attenuation and decay of the useful signal
Solution Approach 1:
The patent divides the shared sampling capacitance into separate first and second sampling capacitances, with each capacitance dedicated to storing reset signals and exposure signals respectively. This segmentation prevents signal attenuation and maintains high signal-to-noise ratio while reading out images through separate paths.
2Measurement precision
If separate first and second storage capacitors are used to store reset and exposure signals independently, then the signal-to-noise ratio is improved by preventing signal attenuation, but the device complexity increases
Solution Approach 1:
The patent designs the pixel unit with multi-functional transistors that can operate in different modes. For example, the third transistor can function as either an amplifier transistor or a transmission transistor depending on the operating phase, reducing the need for separate dedicated transistors and mitigating the increase in device complexity.
3Device complexity
If the storage node FD is used to store signals temporarily during photoelectric conversion, then the signal processing is simplified, but the reset noise increases affecting image quality
Solution Approach 1:
The patent extracts the reset signal storage function from the shared sampling capacitance and places it in a dedicated first sampling capacitance. This separation allows the storage node FD to focus on photoelectric conversion while the first sampling capacitance handles reset signal storage, reducing reset noise interference in the image signal path.
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 solution improves the SNR of the image signals by preventing signal attenuation during the reading process, ensuring that both the reset and exposure signals are read out without decay, thereby enhancing the quality of the image generated by the CMOS image sensor.
Implementation Method 1
The photodiode PPD is adapted for performing photoelectric conversion and converting optical signals to electrical signals when the pixel unit is exposed
Data Source
AI summary
A CMOS image sensor, a pixel unit and a control method thereof are provided. The pixel unit includes: a photoelectric conversion unit, an isolation transistor, a storage unit and a reading unit, wherein a first terminal of the isolation transistor is connected to the photoelectric conversion unit, a second terminal of the isolation transistor is connected to the storage unit and the reading unit; and wherein the storage unit comprises a first switch unit, a second switch unit, a first storage capacitor, a second storage capacitor and a reset unit, the first switch unit is adapted for controlling the first storage capacitor to be charged or discharged, the second switch unit is connected to the second storage capacitor and is adapted for controlling the second storage capacitor to be charged or discharged. The pixel unit improves signal-to-noise ratio of image signals generated by the pixel unit.