CMOS Image Sensor Transfer Transistor Timing for Dynamic Range
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Solution Overview
Problem
CMOS image sensors face challenges in maintaining dynamic range in high illuminance environments, where existing technologies struggle to process signals without distortion, leading to reduced image quality.
Innovation Solution
The proposed image sensor design includes a pixel array with a photodiode, transfer transistor, reset transistor, drive transistor, and select transistor, along with an analog-to-digital converter and timing controller that maintains the transfer transistor in an active state during signal sampling, utilizing capacitors and switches to differentially amplify signals and improve capacitance at the sensing node, thereby enhancing dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the transfer transistor is turned off after sampling the first sensing signal, then the sampling process is completed efficiently, but the dynamic range is reduced in high illuminance environments
Solution Approach 1:
The transfer transistor's state is dynamically adjusted based on the sampling phase. During the first sensing signal sampling, the transfer transistor is turned off to improve efficiency. During the second sensing signal sampling in high illuminance environments, the transfer transistor is maintained in an on state to increase sensing node capacitance and expand dynamic range. This dynamic state adjustment resolves the contradiction between sampling efficiency and dynamic range.
Solution Approach 2:
The capacitance of the sensing node is changed by controlling the transfer transistor's state. When the transfer transistor is on, the sensing node capacitance increases, which reduces sensitivity and expands dynamic range for high illuminance environments. This parameter change allows the system to adapt to different lighting conditions while maintaining sampling efficiency through proper timing control.
2Reliability
If the sensing node capacitance is increased to reduce sensitivity, then the dynamic range improves in high illuminance environments, but the signal processing capability is reduced
Solution Approach 1:
The system uses periodic sampling actions with two distinct phases: first sampling the sensing signal with the transfer transistor off (higher sensitivity), then sampling the reference signal with the transfer transistor on (lower sensitivity, higher dynamic range). This periodic alternation allows the system to maintain both signal processing capability and dynamic range by capturing signals under different capacitance conditions.
Solution Approach 2:
The transfer transistor serves as an intermediary element that controls the connection between the photodiode and the sensing node. By selectively turning the transfer transistor on or off, the system can adjust the sensing node capacitance to match the lighting conditions, thereby maintaining both dynamic range and signal processing capability through intermediate states rather than fixed configurations.
3Reliability
If the transfer transistor is maintained in an on state during sampling, then the sensing node capacitance increases and dynamic range improves, but the sampling speed decreases
Solution Approach 1:
The transfer transistor is maintained in an on state only during the specific phase when sampling the second sensing signal in high illuminance environments, rather than being continuously on. This partial application of the on state achieves the necessary capacitance increase for dynamic range improvement while limiting the impact on overall sampling speed by keeping it off during other sampling operations.
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 design effectively increases the dynamic range of CMOS image sensors in high illuminance environments, reducing sensitivity and improving image quality by maintaining the transfer transistor's active state during signal sampling, which increases the capacitance of the sensing node and enhances signal processing capabilities.
Implementation Method 1
a pixel array including a plurality of unit pixels each having a photodiode
Data Source
AI summary
An image sensor includes a pixel array including a plurality of unit pixels, each having a photodiode, a transfer transistor, a reset transistor, a drive transistor, and a select transistor, an analog to digital converter for sampling an analogous sensing signal from the pixel array and converting the analogous sensing signal into a digital sensing signal, and a timing controller for forwarding a transfer signal which turns on the transfer transistor until after sampling the sensing signal.