Floating Diffusion Charge Transfer for Low Dark Current Image Sensors
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Solution Overview
Problem
Pixel crosstalk and dark current in image sensors lead to image degradation, as existing solutions often exacerbate these issues rather than fully mitigating them, limiting the performance of semiconductor image sensor devices.
Innovation Solution
The implementation of a floating diffusion operation in image sensors, where the timing of control signals for transistors is choreographed to reduce dark current, and additional capacitance is used to increase the full well capacity and reduce leakage, allowing for efficient sampling of image charge while minimizing dark current and motion blur.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If pixel isolation structures are added to reduce crosstalk, then pixel independence is improved, but device complexity and manufacturing difficulty increase
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: photodiode region for photon detection, floating diffusion region for charge accumulation, and transfer gate region for charge transfer. This spatial segmentation reduces crosstalk by isolating charge collection from charge transfer pathways, while maintaining manufacturing feasibility through standard CMOS process compatibility
Solution Approach 2:
A transfer gate is introduced as an intermediary component between the photodiode and floating diffusion. This intermediary enables controlled charge transfer only when activated, preventing spontaneous charge leakage and reducing dark current while maintaining simple pixel architecture
2Measurement precision
If pixel size is reduced to increase resolution, then image sensor resolution is improved, but dark current and crosstalk effects are exacerbated
Solution Approach 1:
The floating diffusion region is pre-configured with optimized doping profiles and geometric dimensions before charge transfer occurs. This preliminary structuring creates an electrostatic potential well that efficiently collects signal charge while repelling thermally generated dark current carriers, maintaining low dark current even in miniaturized pixels
Solution Approach 2:
The patent optimizes critical parameters including floating diffusion area (reduced to minimize capacitance and read noise), photodiode-to-floating diffusion distance (minimized for efficient charge transfer), and transfer gate threshold voltage (tuned for optimal charge transfer efficiency). These parameter changes enable high resolution with reduced dark current
3Speed
If charge transfer speed is increased to reduce motion blur, then motion blur is reduced, but dark current leakage increases
Solution Approach 1:
Charge transfer is performed through periodic activation of the transfer gate rather than continuous transfer. The transfer gate is pulsed at optimized timing to transfer accumulated charge rapidly to the floating diffusion, then remains inactive to prevent dark current leakage. This periodic action enables fast charge transfer while minimizing dark current exposure time
Solution Approach 2:
The transfer gate threshold voltage is dynamically adjusted during operation to optimize charge transfer efficiency. During the transfer phase, the gate is biased to enable rapid charge transfer; during the integration phase, the gate is biased to remain inactive and prevent dark current leakage. This dynamic control achieves fast transfer speed with minimal dark current
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 results in low dark current, wide dynamic range, and reduced motion blur, enhancing the overall performance of image sensors by effectively addressing pixel crosstalk and dark current challenges.
Implementation Method 1
a photodiode PD, a floating diffusion FD, and a transfer gate 212. The transfer gate 212 is coupled between the photodiode PD and the floating diffusion FD
Implementation Method 2
The floating diffusion FD may be a capacitor coupled between a node F and ground
Data Source
AI summary
An apparatus and method for a low dark current floating diffusion is discussed. An example method includes coupling a photodiode to a floating diffusion through a transfer gate where a gate terminal of the transfer gate is provided a first voltage, resetting the floating diffusion, repetitively sampling image charge on the photodiode a plurality of times, where the sampled image charge is coupled to the floating diffusion, and where the gate terminal of the transfer gate is provided a second voltage less than the first voltage during each sampling of the image charge, while repetitively sampling the image charge, coupling an additional capacitance to the floating diffusion, where a first capacitance voltage is applied to the additional capacitance during the sampling, and performing correlated double sampling of the sampled image charge.