SPAD Photodiode with Segmented Doping for Deep Charge Collection
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Solution Overview
Problem
Existing SPAD photodiodes face challenges in collecting charges photogenerated in depth within the semiconductor substrate, as the electric field becomes attenuated beyond a certain distance from the PN junction, leading to inefficient charge collection, especially for radiations with higher wavelengths.
Innovation Solution
The design incorporates a semiconductor substrate with a low doping level and a structured N-type doped region extending deeply into the substrate, forming a non-parallel avalanche area, which enhances the electric field depth and facilitates efficient charge collection through a combination of epitaxial layers and trench formation filled with polysilicon, ensuring charges reach the avalanche area without significant delay.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional PN junction structure is used, then the device is simple to manufacture, but charge collection efficiency deteriorates for deep photogenerated charges
Solution Approach 1:
The semiconductor substrate is divided into multiple doped regions (first doped region, second doped region, third doped region) with different conductivity types and doping levels. This segmentation creates multiple depletion zones that work together to extend the electric field deeper into the substrate, enabling efficient collection of photogenerated charges from greater depths while maintaining a manageable structural complexity through systematic doping patterns.
Solution Approach 2:
Different regions of the substrate are doped with different conductivity types and doping levels to create locally optimized electric field distributions. The first doped region has a first conductivity type and doping level, the second doped region has a second conductivity type and doping level, and the third doped region has a third conductivity type and doping level. This local quality variation ensures that the electric field is appropriately distributed at different depths and locations, improving charge collection efficiency without requiring a uniformly complex structure throughout.
2Manufacturing precision
If the electric field extends deeper into the substrate, then charge collection efficiency improves, but the risk of spurious triggering increases
Solution Approach 1:
The third doped region is specifically designed with a conductivity type opposite to the second doped region and a doping level lower than that of the second doped region. This creates a localized electric field configuration that extends the field depth for charge collection while simultaneously creating a field distribution that prevents spontaneous avalanche breakdown. The lower doping level in the third region reduces the risk of spurious triggering by minimizing thermal generation of carriers in the deep substrate regions.
Solution Approach 2:
The doping level in the third doped region is deliberately set lower than that of the second doped region, and the conductivity type is reversed. This parameter change creates an electric field profile that extends deeper into the substrate for improved charge collection efficiency while maintaining field strengths below the avalanche threshold in regions where spurious triggering could occur. The parameter optimization balances deep charge collection with reliability by preventing spontaneous breakdown.
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 enables efficient collection of photogenerated charges regardless of their depth, reducing the mean time to collection and minimizing spurious triggering, thereby improving the photodiode's sensitivity and reducing crosstalk.
Implementation Method 1
When a photogenerated electric charge is injected into the depletion area, if the displacement speed of this charge in the depletion area is sufficiently high, that is, if the electric field in the depletion area is sufficiently intense, the photodiode is capable of avalanching. A single photon is thus capable of generating a measurable electric signal.
Implementation Method 2
the photodiode is capable of avalanching. A single photon is thus capable of generating a measurable electric signal
Data Source
AI summary
A SPAD-type photodiode including: a semiconductor substrate of a first conductive type having a front side and a back side; and a first semiconductor region of the second conductivity type extending in the substrate from the front side thereof and towards the back side thereof, the lateral surfaces of the first region being in contact with the substrate and the junction between the lateral surfaces of the first region and the substrate defining an avalanche area of the photodiode.