Pixel Sensor Cell Dual-Well Electron Hole Collection
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Solution Overview
Problem
Conventional CMOS image sensor pixel cells have high complexity and inefficiency in collecting both electrons and holes, leading to reduced signal-to-noise ratio and increased substrate current, with additional defects from complex fabrication processes.
Innovation Solution
A pixel sensor cell design featuring n-type and p-type collection well regions beneath the substrate surface, coupled with a differential amplifier circuit to collect and utilize both electrons and holes, reducing collection area and substrate current while increasing signal output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If conventional single-sided electron collection is used, then device structure is simple, but signal output is limited to half of generated carriers
Solution Approach 1:
The pixel sensor cell is segmented into two independent collection regions: an n-type collection well for electrons and a p-type collection well for holes. This segmentation allows both types of charge carriers generated by incident photons to be collected separately and converted into electrical signals, doubling the usable signal output compared to conventional single-sided collection.
Solution Approach 2:
The invention merges electron collection and hole collection into a single integrated pixel sensor cell structure. Both the n-type collection well and p-type collection well are formed within the same substrate region, allowing simultaneous collection of both charge carriers from the same photon absorption event, thereby increasing signal efficiency.
2Quantity of substance
If dual NFET and PFET circuits are added to collect both electrons and holes, then signal output increases, but device complexity and fabrication difficulty increase
Solution Approach 1:
The invention extracts and eliminates the complex dual-transistor circuit structure (NFET and PFET) from the pixel sensor cell. Instead of using transistors to transfer and amplify signals from separate electron and hole collection regions, the design uses direct collection wells that can be read out with simpler circuitry, reducing device complexity while maintaining dual-carrier collection capability.
Solution Approach 2:
The invention changes the operational parameters by using direct collection wells with appropriate doping concentrations and depths. The n-type collection well is doped at 1×10^16 to 1×10^18 atoms/cm³ with a depth of 1 to 3 micrometers, while the p-type collection well is doped at 1×10^16 to 1×10^18 atoms/cm³ with a depth of 0.5 to 2 micrometers. These parameter optimizations enable efficient carrier collection without requiring complex transistor-based transfer mechanisms.
3Quantity of substance
If additional fabrication steps are added to form NFET and PFET, then dual carrier collection is achieved, but manufacturing defects increase
Solution Approach 1:
The invention removes the additional fabrication steps required to form NFET and PFET transistors within the photodiode region. By eliminating these complex fabrication processes, the substrate surface is exposed to fewer processing steps, reducing the introduction of manufacturing defects while maintaining the ability to collect both electrons and holes through the simplified collection well structure.
4Quantity of substance
If collection area is increased to improve signal output, then more carriers are collected, but pixel area and device size increase
Solution Approach 1:
The invention applies local quality optimization by creating highly doped collection regions with specific doping concentrations (1×10^16 to 1×10^18 atoms/cm³) and controlled depths (0.5 to 3 micrometers). The n-type collection well and p-type collection well are strategically positioned and dimensioned to maximize carrier collection efficiency within a compact area, allowing high signal output without increasing overall pixel area.
Solution Approach 2:
The invention utilizes the vertical dimension by forming collection wells at different depths within the substrate. The n-type collection well extends 1 to 3 micrometers deep while the p-type collection well extends 0.5 to 2 micrometers deep, allowing both electron and hole collection volumes to be stacked vertically rather than requiring lateral expansion. This dimensional approach doubles the collection capacity without increasing the horizontal pixel area.
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 design approximately doubles the output current, improves signal-to-noise ratio, and reduces the collection area by about 50% compared to conventional cells, while minimizing substrate current and fabrication defects.
Implementation Method 1
electrons generated by electromagnetic radiation impinging on the pixel sensor cell
Implementation Method 2
n-type collection well region formed beneath a surface of a substrate for collecting electrons
Implementation Method 3
holes generated by the impinging electromagnetic radiation
Implementation Method 4
p-type collection well region formed beneath the surface of the substrate for collecting holes
Implementation Method 5
circuit structure having a first input coupled to the n-type collection well region and a second input coupled to the p-type collection well region, wherein an output signal of the pixel sensor cell is the magnitude of the difference of a signal of the first input and a signal of the second input
Data Source
AI summary
The present invention is a pixel sensor cell and method of making the same. The pixel sensor cell approximately doubles the available signal for a given quanta of light. The device of the present invention utilizes the holes produced by impinging photons in a pixel sensor cell circuit. A pixel sensor cell having reduced complexity includes an n-type collection well region formed beneath a surface of a substrate for collecting electrons generated by electromagnetic radiation impinging on the pixel sensor cell and a p-type collection well region formed beneath the surface of the substrate for collecting holes generated by the impinging photons. A circuit structure having a first input is coupled to the n-type collection well region and a second input is coupled to the p-type collection well region, wherein an output signal of the pixel sensor cell is the magnitude of the difference of a signal of the first input and a signal of the second input.