SPAD Photodiode Grating for Low Jitter and High Efficiency
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Solution Overview
Problem
SPAD-type photodiodes face challenges in achieving high sensitivity and low jitter, particularly at long wavelengths, due to limitations in temporal resolution and sensitivity, often requiring high bias voltages and thick semiconductor material to absorb light effectively.
Innovation Solution
A photodiode design featuring a periodic structure with regularly spaced pads filled with low-index material, where the P-doped and N-doped zones extend under the entire structure, enhancing light absorption and reducing thickness while maintaining sensitivity, by creating a sub-wavelength resonant grating that concentrates the electromagnetic field in the avalanche zone.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the thickness of the photodiode is increased to absorb more photons, then sensitivity is improved, but temporal resolution deteriorates due to increased transit time uncertainty
Solution Approach 1:
The photodiode structure is segmented into distinct functional zones: a first zone with higher doping concentration for rapid carrier generation and transport, and a second zone with lower doping concentration for reduced jitter. This segmentation allows each zone to optimize for its specific function, achieving both high sensitivity and good temporal resolution simultaneously.
Solution Approach 2:
Different regions of the photodiode are assigned different doping concentrations tailored to their specific functions. The first zone (closer to the entrance surface) has higher doping for fast response, while the second zone (closer to the avalanche zone) has lower doping for low jitter. This local optimization resolves the contradiction between thickness requirements for sensitivity and thickness requirements for temporal resolution.
2Measurement precision
If the doping level is reduced to accelerate carriers towards the avalanche zone, then temporal resolution is improved, but the electric field strength decreases requiring higher bias voltages
Solution Approach 1:
The doping profile is segmented into two distinct zones with different doping concentrations. The first zone maintains higher doping to provide sufficient electric field for carrier acceleration without requiring excessive bias voltage, while the second zone uses lower doping to minimize jitter and improve temporal resolution. This segmentation eliminates the need for uniformly high bias voltage across the entire structure.
Solution Approach 2:
The doping concentration parameter is changed along the depth of the photodiode, transitioning from higher values in the first zone to lower values in the second zone. This parameter variation allows the electric field distribution to be optimized for both carrier acceleration and low jitter, reducing the overall bias voltage requirement compared to uniform low-doping designs.
3Measurement precision
If the thickness of the semiconductor layer is reduced to improve temporal resolution, then jitter is reduced, but photon absorption efficiency decreases
Solution Approach 1:
The photodiode is segmented into a first zone optimized for rapid carrier response (thinner, higher doping) and a second zone optimized for low jitter (thinner, lower doping), with the total thickness optimized for photon absorption. The periodic structure further segments the light path to enhance absorption in the thinner overall structure, achieving both temporal resolution and absorption efficiency.
Solution Approach 2:
A periodic structure is introduced on the surface of the photodiode, adding a spatial dimension to light-matter interaction. This periodic structure creates multiple reflection paths and enhances the effective optical path length within the thinner semiconductor layer, thereby improving photon absorption efficiency without increasing the physical thickness that would degrade temporal resolution.
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 photon absorption near the avalanche zone, allowing for reduced semiconductor material thickness, lower bias voltage, and improved temporal resolution with maintained sensitivity, resulting in a photodiode with low temporal dispersion and high quantum efficiency.
Implementation Method 1
creating a sub-wavelength resonant grating that concentrates the electromagnetic field in the avalanche zone
Implementation Method 2
enhancing light absorption and reducing thickness while maintaining sensitivity
Implementation Method 3
A photon absorbed by the layer of semiconductor material generates an electron-hole pair
Implementation Method 4
These charges have enough energy to excite other electrical charges by impact (impact ionization). This chain reaction is called the avalanche effect
Implementation Method 5
the spaces between the pads being at least partially filled with a material called a low index material, having a refractive index lower than that of the layer of semiconductor material
Data Source
Figure 1~3
Figure 4
Figure 5
AI summary
The invention relates to a SPAD photodiode (100) having a semiconductor layer (110) comprising an N-doped region (111) and a P-doped region (112) separated by an avalanche region (113). The semiconductor layer (110) is sandwiched between a periodic structure (120) and a low-index layer (130) having a refractive index lower than that of both the semiconductor layer and the periodic structure. The periodic structure (120) is deposited directly onto the semiconductor layer. The photodiode thus exhibits low temporal dispersion and high quantum efficiency without requiring a high charge acceleration voltage.