Back-Reflective APD Structure for Long-Wavelength Sensitivity
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Solution Overview
Problem
Conventional avalanche photodiodes (APDs) face limitations in sensitivity and speed due to wire-bonding and the trade-off between sensitivity and speed required for near-infrared spectral range, particularly for long wavelengths.
Innovation Solution
The photosensitive element features a semiconductor substrate with a radiation reflecting area on the opposite side of the radiation receiving area, doubling the radiation's path length within the substrate, and a thin silicon substrate to enhance absorption of long wavelengths, combined with flip-chip bonding for improved assembly and reduced package size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the substrate thickness is increased to improve absorption of long wavelengths, then sensitivity is improved, but the speed of the photosensitive element deteriorates
Solution Approach 1:
The patent introduces a radiation reflecting area on the back side of the substrate, creating an optical cavity that extends the radiation path length in the vertical dimension. This allows long wavelength radiation to be absorbed through multiple passes without increasing the physical substrate thickness, thus maintaining fast response speed while improving sensitivity for long wavelengths
Solution Approach 2:
The radiation reflecting area is integrated within the substrate structure itself, nesting the reflective function inside the existing photosensitive element. This compact integration allows the extended optical path to be achieved without adding external components that would increase overall device size or complexity
2Ease of manufacture
If wire-bonding is used to connect the APD, then the assembly is simple, but the sensitivity and fill factor are limited
Solution Approach 1:
The patent removes the wire-bonding structure from the photosensitive element design. By eliminating the wire-bonding step, the fill factor is improved as no space is occupied by wires, and sensitivity is enhanced by removing potential sources of noise and signal loss associated with wire bonds
Solution Approach 2:
The contact regions are directly integrated into the substrate surface, merging the electrical connection function with the photosensitive structure. This direct integration eliminates the need for separate wire-bonding assembly step while maintaining electrical connectivity
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 increases the absorption of long wavelengths, enhancing the sensitivity and speed of the photosensitive element while reducing substrate thickness, achieving a peak sensitivity of 73% (A/W) at 905 nm with a 30 µm substrate thickness and improving signal-to-noise ratio and fill factor.
Implementation Method 1
a radiation reflecting area is disposed on the side opposite to the radiation receiving area of the semiconductor substrate
Implementation Method 2
a multiplication region for multiplying charges generated from the incident radiation is formed at the first contact region
Data Source
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AI summary
The present invention relates to a photosensitive element comprising a semiconductor substrate with a radiation receiving area through which incident radiation can enter the photosensitive element. Further, the photosensitive element comprises a first contact region connected to a first contact and a second contact region connected to a second contact. A multiplication region for multiplying generated carriers under the incident radiation is formed at the first contact region when a voltage is applied between the first contact and the second contact. The first contact and the second contact are arranged on the side opposite to the radiation receiving area of the semiconductor substrate. Further, a radiation reflecting area is disposed on the side opposite to the radiation receiving area of the semiconductor substrate. Further, the present invention relates to an assembly method to provide a photosensitive element.