CMOS Image Sensor Four-Transistor Pixel Compression Control
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Solution Overview
Problem
Image sensors with four transistors experience malfunctions and high reverse current issues, particularly in high light intensities, leading to reduced dynamic range and contrast, whereas those with three transistors have unwanted effects like increased dark current and reduced performance.
Innovation Solution
A method for image acquisition using CMOS-type sensors with four transistors, involving a reset step, integration with compression signal application during the last quarter or eighth of the integration time, and reading the reference state before charge transfer, to isolate the read node and transfer charges effectively, thereby reducing capacitive coupling effects and improving dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If pixels with four transistors are used in global shutter devices, then dynamic range and contrast are improved, but malfunctions occur at high light intensities causing saturation
Solution Approach 1:
The integration period is segmented into two phases: a first integration period where charges are accumulated without compression, and a second integration period where compression is applied to reduce the photocurrent discharge effect. This temporal segmentation allows the sensor to capture both low-light details and high-light saturation information without malfunction.
Solution Approach 2:
The compression signal is applied preliminarily during the second integration period before the final charge transfer to the read node. This preliminary compression action reduces the photocurrent discharge effect that would otherwise cause saturation and malfunction at high light intensities.
2Device complexity
If pixels with three transistors are used in rolling shutter devices, then manufacturing complexity is reduced, but dark current increases significantly with temperature
Solution Approach 1:
A transfer transistor is introduced as an intermediary component between the photodiode and the read node, enabling controlled charge transfer. This intermediary allows the pixel to function with four transistors in a rolling shutter configuration, achieving both manufacturing simplicity and reduced dark current by utilizing the transfer transistor's ability to isolate the photodiode during non-exposure periods.
3Measurement precision
If compression signal is applied during the entire integration time, then light intensity discrimination is improved, but charge transfer accuracy is reduced
Solution Approach 1:
The integration time is segmented into two distinct periods: a first integration period without compression for accurate charge accumulation, and a second integration period with compression for improved light intensity discrimination. This segmentation ensures that charge transfer accuracy is maintained while still achieving enhanced dynamic range.
Solution Approach 2:
Compression is applied partially during only the second integration period rather than throughout the entire integration time. This partial application of compression provides sufficient light intensity discrimination while avoiding excessive compression that would compromise charge transfer accuracy.
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 method enhances the dynamic range and contrast of images by reducing the impact of capacitive coupling and dark current, allowing for better discrimination of light intensities and improved performance in high light conditions.
Implementation Method 1
A pixel of an image sensor essentially comprises a reverse-biased photodiode having its junction capacitance discharged by a photocurrent as a function of a received light intensity
Implementation Method 2
a transfer transistor being interposed between the cathode of the photodiode and a read node
Implementation Method 3
the pixel has been provided to be controlled during part of the integration time period, generally the second half thereof, to enable compression of the signal
Data Source
AI summary
A method for acquiring images using at least one CMOS-type sensor with four transistors including an acquisition node and a read node, where the read node can receive a compression signal, including a step of reading a reference state of the sensor; a reset step; an integration step, during which the sensor is exposed and during part of which the compression signal is applied to the read node; and a step of reading the data acquired during the integration step; the read node being, during the integration step, isolated from the acquisition node, except immediately before the application of the compression signal, at which time the acquisition node is connected to the read node long enough to enable a transfer of the charges present at the acquisition node to the read node.