Image Sensor Pixel Circuit With Back-Gate Noise Correction
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Solution Overview
Problem
Electronic circuits with image sensors, particularly those using '3T'-type or capacitive transimpedance amplifier (CTIA) pixel architectures, face significant noise issues due to the resetting of photodiodes, which is greater than other noise sources by a factor of ten, limiting the accuracy of illumination level measurement.
Innovation Solution
The electronic circuit design incorporates an amplifying transistor with a back gate voltage control mechanism, where the threshold voltage varies based on the back gate voltage, and a control circuit adjusts this voltage to minimize noise. This design includes a sampling capacitor and transistor to store correction values, and a differential amplifier to align the output voltage with a reference voltage, reducing thermal noise impact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If photodiode resetting is performed to enable voltage measurement after pixel reset, then illumination level measurement becomes possible, but thermal noise increases by a factor of ten compared to other noise sources
Solution Approach 1:
The patent applies preliminary action by performing correlated double sampling: first sampling the voltage across the photodiode before reset, then sampling again after reset, and finally subtracting the two samples to eliminate the reset-induced thermal noise while preserving the illumination measurement
Solution Approach 2:
The patent uses feedback by continuously monitoring the output voltage and using it to control the back gate voltage of the amplifying transistor, creating a feedback loop that stabilizes the threshold voltage and reduces noise impact on measurements
2Adaptability or versatility
If new organic photosensitive materials are used to replace conventional semiconductors, then material versatility is improved, but the ability to use pinned diode architecture is lost
Solution Approach 1:
The patent achieves universality by designing a pixel architecture that works with both conventional semiconductor materials and new organic photosensitive materials. The amplifying transistor with back gate control and correlated double sampling approach provides a universal solution that does not depend on pinned diode architecture, enabling the same pixel design to function with different material types
3Device complexity
If '3T'-type or CTIA pixel architectures are used to eliminate transfer gates, then device complexity is reduced, but thermal noise from photodiode resetting increases significantly
Solution Approach 1:
The patent applies preliminary action by performing the first voltage sample before the photodiode reset occurs, capturing the state before the harmful reset noise is introduced. This preliminary sampling, combined with the post-reset sampling and subsequent subtraction, eliminates the reset-induced thermal noise while maintaining the simplified pixel architecture
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 effectively reduces the impact of thermal noise, improving the accuracy of illumination level measurement by correcting random thermal noise and maintaining rapid operation, applicable to both '3T' and CTIA pixel architectures.
Implementation Method 1
image acquisition cells, called pixels, comprising a photodiode used in reverse
Implementation Method 2
an amplifying transistor having: its gate connected to the first node, a conduction node coupled to an output of the cell, and a node for controlling a back gate voltage, the amplifying transistor being configured so that its threshold voltage varies according to the back gate voltage
Implementation Method 3
This resetting induces random noise having a standard deviation equal to √{square root over (kT/C)}, where k is Boltzmann's constant, T the temperature
Data Source
AI summary
An electronic circuit includes image acquisition cells, wherein each cell has a photodetector coupled to a first node of the cell, and an amplifying transistor having a gate connected to the first node, a conduction node coupled to an output of the cell, and a node for controlling a back gate voltage. The amplifying transistor is configured so that its threshold voltage varies according to the back gate voltage. A control circuit adjusts a voltage applied to the control node of the back gate voltage of the amplifying transistor of one of the cells according to a comparison of the voltage present at the cell output and a reference voltage.