Self-Biased Varactor Capacitor for Image Sensor Dynamic Range
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Solution Overview
Problem
Existing image sensors with fixed condenser capacity in the sense node limit their dynamic range and frame rates, as they require dual capture under varying illumination conditions, which complicates auto-exposure and reduces effective fill factor.
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 multi-clocking and increased capacitance under high illumination, enabling high frame rates and expanded dynamic range without external biasing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a fixed condenser capacity is used in the sense node, then the circuit structure is simple, but the dynamic range is reduced and auto-exposure cannot be performed
Solution Approach 1:
The patent applies a varactor capacitor instead of a fixed capacitor, making the capacitance value dynamically adjustable based on voltage. This allows the sense node to adapt its capacity to different light conditions, expanding dynamic range while maintaining relatively simple circuit structure through voltage-controlled capacitance modulation.
Solution Approach 2:
The patent changes the capacitance parameter of the sense node by using a varactor capacitor whose capacitance can be varied by applying different voltages. This enables the system to adjust the effective capacity to match different exposure requirements, solving the limitation of fixed condenser capacity.
2Adaptability or versatility
If external biasing is applied to the gate of the varactor to form capacitance, then the dynamic range is expanded, but self-biasing control becomes difficult and additional row gate driver is required
Solution Approach 1:
The patent connects the varactor capacitor directly to the floating diffusion node, allowing it to self-bias using the voltage signals already present during photoelectric conversion. This eliminates the need for external biasing circuits and additional row gate drivers, achieving both dynamic range expansion and simplified control structure.
Solution Approach 2:
The patent merges the varactor capacitor control with the existing photoelectric conversion circuitry by connecting it to the floating diffusion node. This integration allows the varactor to be controlled by the same voltage signals used for signal readout, eliminating separate control circuits and reducing overall device complexity.
3Adaptability or versatility
If metal lines for connecting the varactor gate are added, then the varactor can be controlled, but the effective fill factor is lowered
Solution Approach 1:
The patent extracts the varactor gate connection from the traditional external biasing architecture and integrates it directly into the floating diffusion node. This eliminates the need for separate metal lines to connect to a dedicated gate terminal, thereby preserving effective fill factor while maintaining varactor control capability.
4Adaptability or versatility
If dual capture structure is used for low and high illumination, then reading under varying illumination is possible, but frame rate decreases due to multiple reads required
Solution Approach 1:
The patent uses a dynamically adjustable varactor capacitor that can be controlled in real-time during the photoelectric conversion process. This allows the system to adapt to different illumination conditions within a single capture cycle, maintaining high frame rates while handling both low and high illumination scenarios without requiring multiple reads.
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 maintains sensitivity under low illumination and increases capacitance under high illumination, achieving ideal Lin-Log sensitivity and allowing multi-capture while maintaining high frame rates even with a small photodiode.
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 varactor using a MOS capacitor is employed so that the dynamic range of the image sensor is expanded by controlling the condenser capacitor of the varactor
Data Source
AI summary
Provided are a pixel, a pixel array, an image sensor including the pixel array, and a method of driving the pixel array. The pixel includes a photoelectric converter, a capacitor, and a switching element. The capacitor accumulates electric charges converted by the photoelectric converter. The switching element outputs a potential of the capacitor. The switching element includes a transfer switching element transferring the electric charges, converted in the photoelectric converter, to the capacitor. The capacitor serves as a storage through multi-clocking of the transfer switching element.