Introduction
In 1952, Jun-ichi Nishizawa designed a new type of diode that could handle weak light by using the photoelectric effect. This component, known today as the APD, helped move electrical systems forward, especially in early fiber-optic communication. His work with semiconductor diodes created a strong base for many modern applications.
The APD uses a special layer where photons trigger electrons, causing collisions among carriers in an avalanche. This builds up charge quickly, increasing current to improve detector speed. Since 2020, adding graphene has helped maintain performance and slow down degradation in high-speed communication systems.
What is an Avalanche Photodiode?
The APD is a sensitive type of photodiode that uses the photoelectric effect to convert optical signals into electric current. Unlike conventional photodiodes, such as PIN models that operate in linear mode, its gain is much higher. It can detect faint, weak light by boosting the output using an internal amplification process.
It works under reverse bias, where a breakdown in the depletion region causes an avalanche of charge. The entire field inside the device helps extend the reach of the electric force. This reach-through design allows even small inputs to trigger large responses through the detector. You can also read about step recovery diode.
Avalanche Photodiode symbol
Construction of Avalanche Photodiodes
The construction of an APD follows a configuration that’s similar to a PIN photodiode, but with key differences. It contains four regions: p+, i, p, and n+, with p+ and n+ being heavily doped. The p and i layers are lightly doped, and the intrinsic layer plays a major role in sensitivity.
Here, the p+ region acts as the anode, while the n+ side works as the cathode, forming the terminals. Due to high resistivity, the reverse bias is mostly applied across the p and n+ sides. As reverse bias is increased, the width of the depletion layer also becomes thinner compared to standard PIN devices. You can also read about varactor diode.
Working Principle
When incident light penetrates the p+ region, it gets absorbed in the resistive p area, creating electron-hole pairs. These pairs are generated near the depletion layer under maximum reverse voltage, causing carriers to drift quickly. A strong electric field already exists near the pn+ junction.
With high velocity, these charge carriers collide with atoms and produce new pairs through avalanche breakdown. This chain reaction builds a huge number of carriers, increasing the photocurrent as a result. The saturation velocity supports this process throughout the field efficiently.
Types of Avalanche Photodiodes
Silicon APD
The Silicon Avalanche-Photodiode is sensitive to wavelengths in the visible and near-infrared range of 400-1100 nm. Its architecture is i-p-n+, with weakly doped p and i regions and strongly doped p+ and n+ ends. The intrinsic layer is the material where photon absorption and electron-hole pair generation begins.
This layer is key for the Avalanche multiplication process, supported by a strong electric field. Contacts are connected to ensure efficient charge collection in the photodiode. Si is used in this silicon APD because it is stable, fast, and necessary for modern light-detection applications.
InGaAs APD
The InGaAs-Avalanche-Photodiode works well in the near-infrared and short-wave infrared range of 900-1700 nm. The primary difference between it and a silicon photodiode is the addition of a multiplication layer. This layer is doped with indium phosphide (InP) to improve performance and gain.
The intrinsic part of this device is made from indium gallium arsenide (InGaAs) material, which is optimized for specific wavelength sensitivity. The p region is lightly doped and supports strong amplification of the electric field. As a result, it provides improved signal response with less noise and higher strength.
e APD
The mid-infrared wavelength range of 800 nm to 1800 nm is the sensitivity range of the Germanium-Avalanche-Photodiode (Ge APD). Although the intrinsic layer is made of germanium, its structure is comparable to that of silicon APDs.
This photodiode has a smaller bandgap compared to silicon, which is the main difference in design. At longer wavelengths, the Ge-built layer facilitates efficient avalanche multiplication.
Avalanche Photodiode Characteristics
Avalanche photodiodes are high-speed diodes that are designed for detecting low light levels with an internal gain method. They work under reverse voltage, which increases sensitivity compared to PIN photodiode types. These are mainly used in the precise measurement of optical signals.
They are ideal for long-distance communication and applications needing high accuracy over a wide range of wavelengths. Some photodiodes are optimized for short or near-infrared regions, making them useful in specialized families. They’re often used in long-distance optical links where a fast and sensitive response is required.
Difference Between PIN Photodiode and Avalanche Photodiode
| Feature | PIN Photodiode | Avalanche Photodiode (APD) |
|---|---|---|
| Sensitivity | Low Sensitivity | Higher Sensitivity |
| Internal Gain | No internal gain | Has high internal gain via avalanche process |
| Design Structure | Simple design and structure | Complex design and structure |
| Noise Level | Lower Noise compared to APD | High Noise due to Avalanche process |
| Operation Voltage | Low Operation voltage | High Operation voltage |
Advantages and Disadvantages of Avalanche Photodiodes
Advantages of avalanche photodiodes include:
It has high sensitivity and can detect low-intensity light with great accuracy.
The gain level is large, and even a single-photon can generate a huge number of charge carrier pairs.
Offers a quick response time, making it ideal for fast systems.
Delivers high performance in light-detection applications.
Despite the high voltage needed, it may be used in systems where output gain is essential.
Disadvantages of avalanche photodiodes include:
Need a high reverse-bias voltage in order to function properly.
The output is not linear, which may affect measurement accuracy.
Generates a high range of noise during functioning.
Lower reliability, so it is less convenient and not used regularly.
The diode utilizes complex systems, making it harder to maintain.
Applications of Avalanche Photodiodes
Because of their great sensitivity and quick reaction, APDs are often utilized as receivers and detectors in optical communications.
Avalanche photodiode technology is used in devices such as PET scanners, barcode readers, and laser scanners.
Laser microscopy and laser range-finding tools benefit from precise imaging and a quick voltage response.
Tools like optical-time domain reflectometers (OTDR) and speed guns use reverse bias to detect reflected radiation.
In research, analyzer bridge circuits and antenna testing setups make use of carrier pair formation from light signals.
Scanner systems, both handheld and industrial, work efficiently using this diode due to its strong photodiode gain.
The junction breakdown in APDs allows for better signal capture from laser pulses in fiber networks.
Designers favor it for its wide bandwidth, especially where compact optical detection is required.
Applications also extend to domain testing and controlled charge measurements in specialized environments.
Conclusion
The Avalanche Photodiode (APD) is a powerful and highly sensitive light detector that plays a vital role in modern optoelectronic systems. With its ability to amplify weak light signals through the avalanche multiplication process, it outperforms conventional photodiodes, especially in high-speed and low-light applications.
Whether built from silicon, InGaAs, or germanium, each type of APD is tailored for specific wavelength ranges and performance needs. Despite some complexity and higher voltage requirements, APDs continue to be essential in fields like fiber-optic communication, medical imaging, laser detection, and precision measurements, making them a cornerstone of advanced photonic technologies.