Lateral MISIM Photoconductor for X-ray Imaging
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Solution Overview
Problem
Existing radiography imaging systems face challenges with high dark currents and performance limitations in vertical photodiodes and lateral metal-semiconductor-metal (MSM) photoconductors, leading to reduced fabrication yield and increased costs, as well as non-uniformity and high dark currents at higher electric fields.
Innovation Solution
A lateral Metal-Insulator-Semiconductor-Insulator-Metal (MISIM) photoconductive element is integrated with a switching element, featuring a semiconducting layer, an insulator layer between the semiconducting layer and sensor contacts, and electrodes on the insulating layer, which reduces dark current and enhances electron hole pair collection efficiency, allowing for faster operation and improved manufacturing simplicity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a vertical photodiode structure is used, then photoresponse and quantum efficiency are improved, but fabrication complexity and manufacturing cost increase due to thick amorphous silicon layers, specialized doped contact layers, and complex RIE sidewall etching processes
Solution Approach 1:
The patent inverts the conventional vertical photodiode structure by using a lateral photoconductor geometry with top and bottom electrodes. This inversion eliminates the need for complex RIE sidewall etching processes and specialized doped contact layers, while maintaining effective photoresponse through the lateral collection of photogenerated carriers across the semiconducting layer.
Solution Approach 2:
The patent extracts and eliminates the problematic thick amorphous silicon contact layers and specialized doped layers from the structure. By using a simplified lateral geometry with standard thin-film deposition, the patent removes the fabrication steps that cause low yield and high cost while preserving the essential photo detection function.
2Ease of manufacture
If a lateral metal-semiconductor-metal (MSM) photoconductor is used, then fabrication is simplified, but dark current increases and photoresponse uniformity decreases at higher electric fields
Solution Approach 1:
The patent introduces an insulating layer as an intermediary between the metal electrodes and the semiconducting layer. This insulating layer acts as a mediator that reduces dark current by blocking direct metal-semiconductor junction leakage while still allowing photogenerated carriers to be collected through the lateral electric field, thus improving reliability without sacrificing fabrication simplicity.
3Speed
If higher electric fields are applied to increase carrier collection speed, then operation speed improves, but dark current increases in conventional MSM structures
Solution Approach 1:
The insulating layer serves as a mediator that decouples the relationship between electric field strength and dark current. It allows high electric fields to be applied for fast carrier collection while blocking the direct metal-semiconductor junction leakage paths that would otherwise generate high dark current, enabling high-speed operation with low dark current.
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 MISIM photoconductive element achieves higher performance and lower costs compared to traditional photodiodes and MSM structures, with improved quantum efficiency and dynamic range, enabling better image quality and reduced manufacturing complexity.
Implementation Method 1
a semiconducting layer for absorbing photons
Implementation Method 2
an insulator layer placed between the semiconducting layer and lateral sensor contacts performs the function of reducing the dark current of photoconductor
Data Source
AI summary
This disclosure is directed at a photoconductive element for a digital X-ray imaging system which consists of a detector element comprising a semiconducting layer for absorbing photons, an insulator layer on at least one surface of said semiconducting layer and at least two electrodes on one surface of said insulator layer; and a switching element wherein at least one layer within said switching element is in the same plane as at least one said layer within said detector element.