Germanium Photodiode JFET Structure for Low Dark Current Sensing
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Solution Overview
Problem
Existing optical image sensors face challenges with high dark current and low optical fill factor due to poor interfaces and surface quality between semiconductor layers, which affect their performance and reliability.
Innovation Solution
The development of germanium-based photosensitive devices with a doped silicon layer between the germanium layer and the silicon substrate, along with a junction field effect transistor (JFET) and pinned photodiode passivation layer, reduces leakage current and improves optical fill factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing optical image sensor structures are used, then manufacturing is simpler, but dark current is high and optical fill factor is low
Solution Approach 1:
A doped silicon layer is introduced as an intermediary between the germanium photodiode layer and the silicon substrate. This intermediate layer acts as a buffer that passivates interface states and reduces leakage current pathways, thereby lowering dark current without requiring fundamental changes to the overall sensor architecture.
Solution Approach 2:
The sensor employs a composite structure combining multiple semiconductor materials (germanium, silicon, doped silicon) with different properties. The germanium layer provides high optical absorption efficiency, while the doped silicon layer provides electrical passivation and interface quality improvement, creating a synergistic effect that reduces dark current while maintaining optical performance.
2Reliability
If existing optical image sensor structures are used, then device complexity is lower, but optical fill factor is reduced
Solution Approach 1:
The doped silicon layer serves as an intermediary that enables better optical coupling between the germanium photodiode and the silicon substrate. By improving the interface quality and reducing light scattering at the interface, more light reaches the photosensitive area, thereby increasing the optical fill factor.
Solution Approach 2:
The invention changes the doping parameters and material properties of the intermediate layer to optimize optical transmission. By adjusting the doping concentration and material composition of the silicon layer, the optical properties are tuned to maximize light transmission to the photodiode while maintaining electrical passivation.
3Reliability
If interface quality between semiconductor layers is poor, then manufacturing is easier, but leakage current increases
Solution Approach 1:
The doped silicon layer is formed in advance during the manufacturing process, before final device assembly. This preliminary action of creating a passivation layer early in the process prevents interface defect formation and ensures high interface quality without requiring additional complex processing steps later.
Solution Approach 2:
The doped silicon layer acts as an intermediary that simplifies manufacturing by providing a buffer zone that tolerates variations in interface quality. This intermediate layer compensates for manufacturing variations and ensures consistent low leakage current performance across production batches.
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 leakage current, enhances optical fill factor, and improves conversion gain and noise performance, leading to better sensitivity and reliability in optical image sensors.
Implementation Method 1
a doped silicon layer disposed between the silicon substrate and the germanium layer
Implementation Method 2
a junction field effect transistor (JFET)
Implementation Method 3
pinned photodiode passivation layer
Data Source
AI summary
Germanium-based sensors are disclosed herein. An exemplary germanium-based sensor includes a germanium photodiode and a junction field effect transistor (JFET) formed from a germanium layer disposed on and/or in a silicon substrate. A doped silicon layer, which can be formed by in-situ doping epitaxially grown silicon, is disposed between the germanium layer and the silicon substrate. In embodiments where the germanium layer is on the silicon substrate, the doped silicon layer is disposed between the germanium layer and an oxide layer. The JFET has a doped polysilicon gate, and in some embodiments, a gate diffusion region is disposed in the germanium layer under the doped polysilicon gate. In some embodiments, a pinned photodiode passivation layer is disposed in the germanium layer. In some embodiments, a pair of doped regions in the germanium layer is configured as an e-lens of the germanium-based sensor.