Pixel Array Conversion Gain Control via Dynamic Switching
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Solution Overview
Problem
Current dual conversion gain (DCG) image sensors face space inefficiencies due to the addition of capacitors and controlling transistors, which reduce the fill factor of photodiodes, particularly in small pixel size image sensors, leading to compromised performance under varying lighting conditions.
Innovation Solution
Implementing a pixel array with dynamically switchable connections for readout and source follower supply traces, allowing for multiple levels of conversion gain without the need for additional silicon space, by using switch circuitry to selectively couple and decouple traces from the floating diffusion node.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a capacitor and controlling transistor are added to implement dual conversion gain, then conversion gain performance is improved, but device area increases and fill factor decreases
Solution Approach 1:
The pixel cell is designed to perform multiple functions using shared circuit elements. The source follower transistor and supply trace serve dual purposes: normal operation mode and conversion gain mode. By dynamically switching the connection of the supply trace to the floating diffusion node, the same hardware infrastructure supports both low-gain and high-gain operations without requiring separate dedicated components for each function.
Solution Approach 2:
Instead of adding physical capacitor and transistor components, the patent creates a virtual capacitance effect by dynamically connecting the supply trace to the floating diffusion node. This copying approach replicates the functionality of a dedicated capacitor circuit using the existing supply trace infrastructure, achieving conversion gain without the area overhead of physical capacitor components.
2Adaptability or versatility
If additional capacitor and transistor components are used for dual conversion gain, then dual conversion gain capability is achieved, but manufacturing complexity increases
Solution Approach 1:
The pixel cell circuitry is designed with universal components that serve multiple functions. The source follower transistor and supply trace are configured to operate in different modes (normal gain and conversion gain) based on switching control, eliminating the need for separate dedicated components for each gain mode and simplifying the manufacturing process.
Solution Approach 2:
The patent implements dynamic switching of the supply trace connection to the floating diffusion node based on operational mode requirements. This dynamic reconfiguration allows the same static hardware to adapt between different gain modes, reducing manufacturing complexity compared to fixed architectures that would require separate components for each mode.
3Area of moving object
If pixel cell size is reduced, then image sensor integration density is improved, but space for conversion gain components becomes insufficient
Solution Approach 1:
The patent creates a virtual capacitance using the supply trace connection rather than requiring a physical capacitor component. This copying approach replicates the conversion gain functionality using existing trace infrastructure, enabling small pixel cells to achieve dual conversion gain capability without dedicating additional area to capacitor and transistor components.
Solution Approach 2:
The patent merges the conversion gain functionality with the existing supply trace infrastructure. By combining the capacitance function with the power supply distribution trace, the design eliminates the need for separate capacitor components, thereby preserving pixel cell area for photodetector elements while maintaining conversion gain capability.
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 approach enables efficient variation in conversion gain levels with minimal area overhead, improving signal-to-noise ratio across different lighting conditions while maintaining high fill factor, thus enhancing the performance of small pixel size image sensors.
Implementation Method 1
a branch portion of the source follower supply trace to increase a capacitance between the source follower supply trace and a readout trace
Data Source
AI summary
Techniques and mechanisms for a pixel array to provide a level of conversion gain. In an embodiment, the pixel array includes conversion gain control circuitry to be selectively configured at different times for different operational modes, each mode for implementing a respective conversion gain. The conversion gain control circuitry selectively provides switched coupling of the pixel cell to—and/or switched decoupling of the pixel cell from—a supply voltage. In another embodiment, the conversion gain control circuitry selectively provides switched coupling of the pixel cell to—and/or switched decoupling of the pixel cell from—sample and hold circuitry.