Pinned Photodiode Storage Capacity via Segmented Doping Profile
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Solution Overview
Problem
Existing photodiodes, such as those used in image sensors, have limited storage capacity, which restricts their ability to efficiently store and transfer charge during the accumulation and transfer phases.
Innovation Solution
A photodiode with a reinforced doping profile is created by adding a second zone with a higher dopant concentration and a shallower depth adjacent to the first zone, enhancing the pinching effect and allowing for increased storage capacity while maintaining the reverse bias potential at an appropriate value for complete charge transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If a single photodiode with a standard doping profile is used, then the device structure remains simple, but the storage capacity is limited
Solution Approach 1:
The photodiode is divided into two distinct photodiodes (first photodiode and second photodiode) with different doping profiles that are juxtaposed. The first photodiode has a standard doping profile while the second photodiode has a reinforced doping profile with higher dopant concentration and shallower depth. This segmentation allows each photodiode to contribute differently to the overall storage capacity while maintaining manageable individual structures.
Solution Approach 2:
The second photodiode introduces a localized region with reinforced doping characteristics (higher concentration C2 and reduced depth P2) adjacent to the first photodiode's accumulation zone. This local quality enhancement creates a pinned region that specifically increases storage capacity in a targeted area without requiring complete restructuring of the entire photodiode device.
2Quantity of substance
If the dopant concentration is increased to enhance storage capacity, then the storage capacity increases, but the reverse bias potential may become inappropriate for complete charge transfer
Solution Approach 1:
The reinforced doping is applied locally in the second photodiode region rather than uniformly across the entire structure. This localized enhancement increases storage capacity in specific pinned regions while leaving other areas with appropriate doping levels for charge transfer, thus maintaining the balance between storage and transfer functions.
Solution Approach 2:
By separating the photodiode into two distinct regions with different doping characteristics, the invention allows the first photodiode to handle charge transfer with its standard doping profile while the second photodiode provides enhanced storage capacity with its reinforced doping profile, preventing interference between these conflicting requirements.
3Quantity of substance
If a reinforced doping profile with higher concentration and shallower depth is added, then the pinching effect is enhanced and storage capacity increases, but the device structure becomes more complex
Solution Approach 1:
The complex reinforced doping profile is segmented into a separate second photodiode structure that is juxtaposed with the first photodiode. This segmentation allows the complex doping characteristics (higher concentration C2, shallower depth P2) to be implemented as a distinct module, making the overall device design and manufacturing more manageable despite the enhanced functionality.
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 modified photodiode achieves a 30% to 50% increase in storage capacity and ensures the diode remains fully pinned at the end of charge transfer, maintaining the integrity of charge transfer to the floating node, thus improving the overall performance of image sensors.
Implementation Method 1
When photons strike an upper surface 118 of the diode, electron-hole pairs are generated in the diode, more specifically in the regions 160, 110, 112, 120, and the electrons here are stored in the N region 110.
Implementation Method 2
The depth of the regions 110, 160 and the N, P dopant concentrations in these regions are chosen so that, when the charge transfer is complete, the gate of the transfer transistor being on (the potential Vp is applied to the N type region 110), the space charge zones of the two diodes meet. Thus, during the accumulation phase, the gate of the transfer transistor being off, no majority carrier can be extracted from the region 110.
Data Source
AI summary
In a photodiode formed by a region of a first type inside a region of a second type, of a semiconductor substrate, the region of the first type includes a first zone including a dopant of the first type having a first concentration and a first depth. The region of the first type also has a second zone adjacent to the first zone in the dopant of the first type has a second concentration higher than the first concentration and a second depth smaller than the first depth. A method for making such a diode is also disclosed.