Self-Biased Varactor Pixel Array for High Dynamic Range Imaging
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Solution Overview
Problem
Existing image sensors have a limited dynamic range and cannot perform auto-exposure due to fixed condenser capacity in the sense node, and require external biasing for varactor capacitance control, which reduces effective fill factor and frame rates.
Innovation Solution
A pixel array with a self-biased varactor capacitor that automatically adjusts capacitance based on the potential of the floating diffusion region, allowing increased sensitivity under low illumination and capacity under high illumination, enabling Lin-Log sensitivity and multi-capture functionality without external biasing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a varactor using a MOS capacitor is employed to control the condenser capacitor, then the dynamic range is expanded, but external biasing is required which makes self-biasing control difficult
Solution Approach 1:
The patent implements self-biasing control by connecting the control electrode of the varactor capacitor to the floating diffusion region. The voltage on the floating diffusion region automatically adjusts the capacitance of the varactor capacitor based on the accumulated electric charge, eliminating the need for external biasing circuits while maintaining expanded dynamic range capability
2Adaptability or versatility
If a row gate driver and metal lines are added to control the gate of the varactor, then capacitance control is achieved, but the effective fill factor is lowered
Solution Approach 1:
The patent merges the control electrode of the varactor capacitor with the floating diffusion region, combining two separate components into one integrated structure. This eliminates the need for additional row gate drivers and metal lines, thereby maintaining capacitance control capability while preserving the effective fill factor of the pixel array
Solution Approach 2:
The floating diffusion region serves dual functions: it acts as both the charge storage node for the photodiode and the control electrode for the varactor capacitor. This multi-functionality eliminates the need for separate control structures, maintaining pixel area efficiency while enabling dynamic capacitance control
3Adaptability or versatility
If dual-capture technology is used to read under low and high illumination, then reading capability is improved, but frame rate decreases due to multiple readings required
Solution Approach 1:
The patent implements dynamic capacitance adjustment through the self-biased varactor capacitor, which automatically changes its capacitance value based on the voltage level of the floating diffusion region. This dynamic adaptation allows the pixel to handle both low and high illumination conditions within a single capture cycle, maintaining high frame rates while expanding reading capability across different lighting conditions
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 self-biased varactor capacitor expands the dynamic range of the image sensor, maintains high sensitivity under low illumination, and achieves high frame rates by increasing capacitance only when needed, effectively addressing limitations of existing technologies.
Implementation Method 1
A pixel having such a structure causes electricity to flow by exciting electrons of the photodiode with incident light
Implementation Method 2
a variable capacitor connected to the capacitor, and having capacitance varied according to a potential of the capacitor
Data Source
AI summary
Provided are a pixel, a pixel array, and an image sensor including the pixel array. The pixel includes a photoelectric converter, a capacitor, a variable capacitor, and a switching element. The capacitor accumulates electric charges converted by the photoelectric converter. The variable capacitor is connected to the capacitor, and has capacitance varied according to a potential of the capacitor. The switching element outputs the potential of the capacitor.