Ge-on-Si Avalanche Photodiode Gain Control With Lower Noise
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Solution Overview
Problem
Conventional avalanche photodiodes (APDs) face challenges with high excess noise and bandwidth variation due to dependency on reverse bias voltage, lacking independent control over gain and bandwidth, and suffering from high noise due to carrier multiplication in germanium.
Innovation Solution
A novel lateral and vertical heterojunction Ge-on-Si APD design with four or three contacts, respectively, allows for independent control of the electric field in silicon and germanium, enabling gain tuning to minimize excess noise and maintain bandwidth, by using additional gain control contacts to tailor the electric field profile for optimal multiplication in silicon.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If carrier multiplication is performed in germanium to increase gain, then multiplication gain is improved, but excess noise increases
Solution Approach 1:
The device is segmented into distinct germanium and silicon regions with separate functions: germanium for carrier absorption and generation, silicon for low-noise multiplication. This spatial segmentation allows each material to perform its optimal function without the noise penalties of germanium multiplication
Solution Approach 2:
The silicon layer acts as an intermediary between the germanium absorption region and the readout circuitry. It receives photogenerated carriers from germanium and performs the multiplication function in a low-noise environment, mediating the transition from light detection to signal amplification
2Power
If reverse bias voltage is increased to control gain, then multiplication gain is improved, but bandwidth varies
Solution Approach 1:
The device separates the functions of gain control and bandwidth control into different regions and mechanisms. Gain is controlled by the electric field in the silicon multiplication region, while bandwidth is maintained by the properties of the germanium absorption region and the heterojunction interface
Solution Approach 2:
The invention changes the controlling parameter for gain from reverse bias voltage to electric field strength in the silicon region, achieved through front surface illumination. This parameter change allows independent optimization of gain and bandwidth characteristics
3Ease of operation
If additional gain control contacts are added to enable independent electric field control, then gain control is improved, but device complexity increases
Solution Approach 1:
The front surface illumination contact serves multiple functions: it controls the electric field in the silicon multiplication region, enables gain tuning, and maintains the heterojunction structure. This multi-functionality reduces the need for separate gain control contacts
Solution Approach 2:
The device uses the illumination light itself to control the gain through the photovoltaic effect and electric field modulation in the silicon region. The system is self-controlled through the incident light characteristics rather than requiring external bias control circuits
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 approach reduces excess noise while increasing multiplication gain, achieving lower noise power compared to ideal photodetectors with minimal bandwidth variation, enhancing the sensitivity and noise performance of APDs.
Implementation Method 1
the multiplication happens mostly in silicon to achieve lower excess noise
Implementation Method 2
A novel lateral and vertical heterojunction Ge-on-Si APD design
Data Source
AI summary
An avalanche photodiode includes a silicon layer on a substrate; a germanium layer on the silicon layer; a cathode and an anode on any of the silicon layer and the germanium layer; and a plurality of contacts on the germanium layer, in addition to the cathode and the anode. The silicon layer can include a highly doped region at each end, an intrinsic doped region in a middle, and an intermediately doped region between the highly doped region at each end and the intrinsic doped region, and the cathode and the anode are each at a respective a highly doped region at each end. The germanium layer can include a plurality of highly doped regions with each including one of the plurality of contacts.