Amorphous Silicon Photosensor Diode Junction Photocurrent Amplification
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Solution Overview
Problem
Existing photosensors using amorphous silicon films struggle to amplify photocurrent effectively due to high dark currents, which obscure the current increase caused by light irradiation, especially with thin films where the resistance change is buried in bias currents.
Innovation Solution
A photosensor design incorporating a thick amorphous silicon film and an n-type amorphous silicon film doped with phosphorus, forming a diode junction that amplifies photocurrent by increasing the resistance and allowing for a significant difference between light and dark current measurements, achieved by optimizing the thickness of the amorphous silicon film and using a capacitive element to enhance voltage changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a thin amorphous silicon film is used for photosensor, then the device size is reduced, but the resistance change due to light irradiation becomes small and is buried in bias currents, reducing measurement precision
Solution Approach 1:
The patent introduces a diode structure formed by an n-type amorphous silicon film and a p-type amorphous silicon film as an intermediary mechanism. This diode amplifies the photocurrent generated in the thin amorphous silicon film through photoconduction, converting the small resistance change into a large current signal that can be easily detected above the bias current level.
Solution Approach 2:
The patent changes the electrical parameters of the amorphous silicon film by controlling its thickness and doping characteristics. By optimizing the film thickness to be thin enough for small size but thick enough to generate sufficient photocurrent, and by introducing doped regions to form the diode junction, the system achieves both miniaturization and high detection precision.
2Measurement precision
If a thick amorphous silicon film is used to increase photocurrent, then the photocurrent amplification is improved, but the dark current increases, reducing the signal-to-noise ratio
Solution Approach 1:
The patent divides the amorphous silicon film into multiple functional regions with different thicknesses and doping levels. The structure includes a thin intrinsic or lightly-doped region for photocurrent generation, and separately positioned doped regions (n-type and p-type) that form the diode junction. This segmentation allows each region to be optimized for its specific function while working together to amplify photocurrent without proportionally increasing dark current.
Solution Approach 2:
The patent applies different doping concentrations and film thicknesses to different local regions of the photosensor. The amorphous silicon film has varying properties: thinner and lightly-doped in the photocurrent generation region, and specifically doped (n-type and p-type) in regions forming the diode junction. This local quality variation enables high photocurrent amplification while controlling dark current through the diode's rectifying properties.
3Measurement precision
If a diode is formed using n-type amorphous silicon film doped with phosphorus, then photocurrent amplification is enhanced, but the device complexity increases
Solution Approach 1:
The patent designs the amorphous silicon film to serve multiple functions simultaneously. The same film structure provides both the photocurrent generation function (through photoconduction in the amorphous silicon) and the current amplification function (through the diode effect formed by doped regions). The n-type and p-type doped regions serve dual purposes: creating the diode junction for amplification and providing electrical contacts. This multi-functionality reduces the need for separate components and simplifies the overall device structure 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 solution achieves a 10,000-fold photocurrent amplification, allowing for effective light detection by isolating the current increase due to photoconduction from the bias current, thereby improving the sensitivity of the photosensor.
Implementation Method 1
an amorphous silicon film which is a light-dependent variable resistance element
Implementation Method 2
a diode is formed by the thick amorphous silicon (a-Si) film and the n-type amorphous silicon film (n+a-Si) doped with phosphorus. Also, with the use of the diode in a forward bias, a high-sensitive photosensor that obtains a large photocurrent amplification effect can be realized
Data Source
AI summary
A photosensor includes a lower electrode formed of a metal film, an amorphous silicon film disposed on the lower electrode, an n-type amorphous silicon film disposed on the amorphous silicon film, an upper electrode that is disposed on the n-type amorphous silicon film, and receives a first reference voltage, a capacitive element connected between the lower electrode and a second reference voltage, a switch circuit that inputs a first supply voltage to the lower electrode in an on-state, and puts the lower electrode into a floating state in an off-state, and a detector circuit that detects a voltage change in the lower electrode after irradiating the amorphous silicon film for a given period with light when the switch circuit is on.