Ge-on-Si Time-of-Flight Pixel Structure for Low Dark Current
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Solution Overview
Problem
Indirect time-of-flight sensors using Si as a detector material face challenges with low light absorption at 900 nm wavelengths, leading to higher dark current due to Ge's smaller bandgap and high density of states, which affects sensitivity and power handling.
Innovation Solution
A time-of-flight sensor design featuring a Si-based photocurrent collecting structure with epitaxially grown Ge-based photosensitive structure, including n-doped and p-doped regions configured to conduct electrons and holes respectively, with barriers in the conduction and valence bands to manage photocurrent, and optionally a charge sheet to reduce electric field strength and dark current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If Ge-based photosensitive structure is used to improve infrared sensitivity, then light absorption is improved, but dark current increases
Solution Approach 1:
The sensor is divided into two distinct functional regions: a Si-based photocurrent collecting structure and a Ge-based photosensitive structure. This segmentation allows each material to perform its optimal function - Si for low dark current and Ge for high infrared absorption - thereby resolving the contradiction between sensitivity and dark current
Solution Approach 2:
The patent employs a composite structure combining Si and Ge materials with different bandgap properties. The Si-based structure provides low dark current characteristics while the Ge-based layer enhances infrared absorption, creating a composite material system that achieves both improved sensitivity and controlled dark current
2Reliability
If p-doped region barrier for electrons is implemented, then charge carrier separation is improved, but device complexity increases
Solution Approach 1:
The patent applies local quality by implementing specific doping types in specific regions: p-doping in regions where electron barriers are needed and n-doping where hole barriers are needed. This localized doping strategy achieves effective charge carrier separation while maintaining manufacturing feasibility through standard semiconductor processing techniques
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 design enhances sensitivity to longer wavelengths, reduces dark current, and enables efficient energy and resource usage, contributing to green technology solutions by improving material consumption and ohmic losses.
Implementation Method 1
Indirect time-of-flight (iTOF) sensors may use infrared light, for example infrared light with a wavelength of about 900 nm
Implementation Method 2
the conduction band in the p-doped region includes a barrier for the electrons of the photocurrent and the valence band in the n-doped region includes a barrier for the holes of the photocurrent
Implementation Method 3
a Ge-based photosensitive structure epitaxially grown on the photocurrent collecting structure
Data Source
AI summary
A time-of-flight sensor includes at least one pixel, including: a Si-based photocurrent collecting structure and a Ge-based photosensitive structure epitaxially grown on the photocurrent collecting structure, wherein the photocurrent collecting structure includes an n-doped region and a p-doped region, wherein the n-doped region is configured to conduct electrons of a photocurrent to at least one n-contact and wherein the p-doped region is configured to conduct holes of the photocurrent to at least one p-contact and wherein the conduction band in the p-doped region includes a barrier for the electrons of the photocurrent and the valence band in the n-doped region includes a barrier for the holes of the photocurrent.