CMOS Image Sensor Heterojunction Amplifier Noise Reduction
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in reducing noise in photodiodes and amplifier transistors, limiting their sensitivity and dynamic range, especially in achieving photon counting with readout noise below 15 e−rms.
Innovation Solution
The use of a CMOS image sensor with a heterojunction amplifier transistor featuring a Ge layer and an SiGeSn layer as the channel region, which reduces noise by forming a high-quality quantum well structure without lattice mismatch defects, enabling readout noise of less than 0.15 e−rms and increased dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If conventional Si-MOSFETs are used for amplifier transistors, then device complexity is low and manufacturing is easy, but noise level remains high and readout noise cannot be reduced below 15 e−rms
Solution Approach 1:
The patent employs a heterojunction structure combining Ge and SiGeSn materials to form the amplifier transistor channel. This composite material approach creates a quantum well structure that suppresses noise while maintaining manufacturability through established semiconductor fabrication processes.
Solution Approach 2:
The invention changes the material composition parameters by introducing Sn into the SiGe layer to form SiGeSn, and adjusts the Ge/SiGeSn layer thickness ratio to optimize the quantum well effect. These parameter changes reduce noise while controlling device complexity.
2Ease of manufacture
If photodiode and amplifier transistor are formed on the same silicon substrate, then manufacturing process is simplified, but noise from both components cannot be reduced simultaneously to achieve photon counting sensitivity
Solution Approach 1:
The patent applies local quality by forming the amplifier transistor with a special heterojunction structure in a specific region of the silicon substrate, while keeping the photodiode structure conventional. This localized optimization reduces amplifier noise without complicating the overall integrated manufacturing process.
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
This configuration significantly reduces noise in CMOS image sensors, enabling photon counting and a substantial increase in dynamic range by utilizing the heterojunction between the SiGeSn and Ge substrates as the channel, outperforming traditional Si-MOSFETs.
Implementation Method 1
a heterojunction amplifier transistor having a Ge layer and an SiGeSn layer joined together as a channel region, and amplifies the electric signal resulting from conversion by the photoelectric conversion element
Data Source
AI summary
According to one embodiment, a CMOS image sensor includes a photoelectric conversion element and an amplifier transistor. The photoelectric conversion element converts incident light into an electric signal. The amplifier transistor has a heterojunction in which a Ge layer and an SiGeSn layer are joined together, as a channel region and amplifies the electric signal resulting from conversion by the photoelectric conversion element.