CMOS Image Sensor Pixel Negative Bias Charge Transfer
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Solution Overview
Problem
CMOS image sensors face limitations in charge transfer efficiency, blooming control, and dark current management due to the constraints on impurity concentration and gate insulation layer thickness, which affect the performance of charge transfer transistors.
Innovation Solution
Incorporating a negative bias for the charge transfer gate during the charge integration cycle and adding an additional n-type impurity doped region under the p-type impurity doped region to reduce band bending and enhance blooming control without compromising charge transfer efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the gate insulation layer thickness is reduced to improve charge transfer efficiency, then charge transfer efficiency is improved, but dark current increases due to increased band bending at the interface
Solution Approach 1:
The patent introduces a p-type impurity doped region specifically at the interface between the gate insulation layer and the substrate. This localized doping creates a region with different electrical properties (higher hole concentration) exactly where band bending occurs, thereby reducing dark current generation at the critical interface without affecting the overall gate insulation layer thickness or charge transfer efficiency
Solution Approach 2:
The patent changes the impurity concentration parameter by introducing a p-type doped region with a specific doping concentration at the interface. This parameter change modifies the electrical characteristics of the interface region, reducing band bending and associated dark current while maintaining the thin gate insulation layer structure needed for high charge transfer efficiency
2Object-generated harmful factors
If impurity concentration is increased to reduce band bending and dark current, then dark current is reduced, but charge transfer efficiency deteriorates due to increased scattering and reduced carrier mobility
Solution Approach 1:
The high impurity concentration is applied only locally at the interface region between the gate insulation layer and substrate, not throughout the entire device. This localized approach reduces dark current at the interface where band bending occurs most severely, while leaving the bulk regions with lower impurity concentrations that maintain high carrier mobility and charge transfer efficiency
Solution Approach 2:
The device structure is segmented into regions with different impurity concentrations: a high-doped interface region for dark current suppression and lower-doped bulk regions for maintaining carrier mobility. This segmentation allows each region to be optimized for its specific function without compromising the other
3Productivity
If a charge transfer transistor is used to transfer charge from photodiode to floating diffusion, then charge transfer is achieved, but blooming control becomes difficult due to the transistor gate region structure
Solution Approach 1:
The p-type impurity doped region is strategically positioned at the interface between the gate insulation layer and substrate, creating a localized region with enhanced hole concentration. This local modification reduces band bending and dark current generation at the critical interface without interfering with the transistor gate region's blooming control functionality
Solution Approach 2:
The p-type doped interface region acts as an intermediary layer that mediates between the gate insulation layer and substrate, reducing band bending and dark current generation at the interface. This intermediary structure does not interfere with the charge transfer transistor's blooming control capability while improving charge transfer efficiency
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 significantly reduces dark current generation and maintains excellent charge transfer efficiency while effectively controlling blooming, allowing for improved pixel performance.
Implementation Method 1
a charge transfer gate for transferring signal charge from the photodiode to a floating diffusion node
Implementation Method 2
sense light by converting impinging photons into electrons that are integrated in sensor pixels
Data Source
AI summary
A pixel of an image sensor includes a gate insulation layer formed over a substrate doped with first-type impurities, a transfer gate formed over the gate insulation layer, a photodiode formed in the substrate at one side of the transfer gate, and a floating diffusion node formed in the substrate at the other side of the transfer gate, wherein the transfer gate has a negative bias during a charge integration cycle.