PMOS Pixel Structure Reducing Cross Talk via N-Well Segmentation
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Solution Overview
Problem
Current CMOS image sensors built on p-type substrates suffer from high levels of pixel-to-pixel cross talk due to lateral diffusion of minority carriers, while those built on n-type substrates have lower cross talk but require re-engineering of support circuitry and are prone to dark current defects.
Innovation Solution
A CMOS image sensor with a PMOS pixel structure using holes as signal-charge carriers, featuring an n-type well spanning the imaging area on a p-type substrate, which reduces cross talk and dark current by driving excess carriers into the substrate, while maintaining standard CMOS circuitry and substrate biasing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If CMOS image sensors are built on p-type substrates, then circuit integration level is high, but pixel-to-pixel cross talk is high
Solution Approach 1:
The substrate is segmented into distinct regions: a first conductivity type substrate (p-type) for peripheral circuitry and a second conductivity type well region (n-type) for the pixel array. This spatial segmentation allows the pixel region to operate with low cross-talk characteristics while the peripheral region maintains high circuit integration benefits from p-type substrate processing.
Solution Approach 2:
Different conductivity types are applied to different locations within the sensor structure. The pixel array region uses n-type well on p-type substrate configuration to minimize cross-talk, while the peripheral circuit region utilizes p-type substrate for optimal circuit integration. This local differentiation of material properties resolves the contradiction between integration and cross-talk performance.
2Object-generated harmful factors
If CMOS image sensors are built on n-type substrates, then pixel-to-pixel cross talk is low, but re-engineering of support circuitry is required
Solution Approach 1:
The sensor is divided into an n-type well pixel array region (for low cross-talk) and a p-type substrate peripheral region (for standard CMOS circuitry). This segmentation allows each region to be optimized independently, eliminating the need to re-engineer the entire support circuitry while still achieving low cross-talk performance in the imaging area.
Solution Approach 2:
The n-type well is locally introduced only in the pixel array region rather than throughout the entire substrate. This localized modification provides the low cross-talk benefit where needed (in the imaging area) while preserving the p-type substrate characteristics in peripheral regions where standard CMOS circuitry benefits from p-type processing.
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 solution effectively reduces cross talk and the bulk-diffusion component of dark current, retaining the advantages of mainstream CMOS processing on p-type substrates with minimal modifications to existing circuitry and substrate biasing.
Implementation Method 1
lateral diffusion of minority carriers within the p-type substrates
Implementation Method 2
biased at predetermined potential with respect to the substrate for driving excess carriers into the substrate
Data Source
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AI summary
An image sensor with an image area having a plurality of pixels each having a photodetector of a first conductivity type, the image sensor includes a substrate of the first conductivity type; a first layer of the second conductivity type between the substrate and the photodetectors, spanning the image area and biased at predetermined potential with respect to the substrate for driving excess carriers into the substrate to reduce cross talk; one or more adjacent active electronic components disposed in the first layer within each pixel; and electronic circuitry disposed in the substrate outside of the image area.