Pixel Structure Anti-Blooming Path for CMOS Image Sensors
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Solution Overview
Problem
Conventional CMOS image sensors fail to effectively control blooming, which leads to loss of details in high dynamic range scenes and issues with hot pixels at high temperatures, especially in automotive and surveillance applications.
Innovation Solution
A pixel structure is designed with a crystalline layer of a first doping type, including a photodiode region connected to a doped region of a second doping type as an anti-blooming path, with specific doping configurations and isolation structures to manage charge overflow and prevent blooming.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CMOS image sensors are used, then the device can capture images, but blooming occurs when pixels are filled with photo carriers causing charge spread to neighboring pixels
Solution Approach 1:
The pixel structure is segmented into distinct doped regions (photodiode region, well region, and doped region connected to power supply) with different doping types. This segmentation creates separate charge collection zones and overflow paths, preventing charge spread to neighboring pixels while maintaining effective photo carrier collection.
Solution Approach 2:
A doped region of second doping type is introduced as an intermediary between the photodiode region and the power supply. This intermediary region serves as a dedicated overflow path that intercepts excess photo carriers before they can spread to adjacent pixels, effectively controlling blooming while maintaining normal pixel operation.
2Temperature
If conventional CMOS image sensors are used, then the device can operate at high temperatures, but hot pixels become filled with dark current causing blooming in adjacent pixels
Solution Approach 1:
The doped region acts as an intermediary overflow path that captures dark current generated in hot pixels at high temperatures. By providing a dedicated discharge path connected to the power supply, excess dark current is diverted before it can spread to adjacent pixels, preventing temperature-induced blooming.
Solution Approach 2:
The doping concentration and type are strategically changed in different regions to create potential barriers and overflow paths. The well region and doped region have different doping types and concentrations compared to the photodiode region, creating electrical parameters that guide charge carriers along desired paths while blocking unwanted charge spread.
3Loss of information
If no anti-blooming path is provided, then the pixel structure is simpler, but details in dimly lit regions are washed out by blooming charges from ultra-bright regions
Solution Approach 1:
The pixel structure is divided into functionally distinct doped regions including a photodiode region for normal operation, a well region for charge storage, and a doped region connected to power supply for overflow. This segmentation creates dedicated paths that preserve detail information in dim regions while handling bright region overflow.
Solution Approach 2:
The doped region serves as an intermediary overflow path that intercepts excess charges from ultra-bright pixels. By providing this intermediate structure connected to the power supply, blooming charges are diverted before they can wash out detail information in neighboring dimly lit regions.
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 pixel structure effectively prevents blooming by directing excess photo electrons to a power supply, maintaining image quality in high dynamic range scenes and reducing the impact of hot pixels at elevated temperatures.
Implementation Method 1
A photodiode region is formed in the crystalline layer
Implementation Method 2
A doped region of a second doping type is formed in the crystalline layer and formed between the reset gate and the gate of the source follower. The first doping type is different from the second doping type, and the photodiode region is connected to the doped region under the top surface of the crystalline layer as an anti-blooming path.
Data Source
AI summary
A pixel structure of an image sensor is provided and includes following units. A crystalline layer of a first doping type is formed on a substrate. A photodiode region is formed in the crystalline layer. A gate of a source follower transistor is formed on a top surface of the crystalline layer. A reset gate is formed on the top surface of the crystalline layer. A doped region of a second doping type is formed in the crystalline layer and formed between the reset gate and the gate of the source follower. The first doping type is different from the second doping type, and the photodiode region is connected to the doped region under the top surface of the crystalline layer as an anti-blooming path.