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Avalanche Photodiode (APD): From Theory to Practice in Optical Sensing


In the world of optical sensing, Avalanche Photodiodes (APDs) have emerged as a crucial technology that enhances the sensitivity and efficiency of light detection. Building on the principles of traditional photodiodes, APDs utilize the avalanche effect to amplify the photocurrent, making them suitable for a wide range of applications in fields like telecommunications, medical imaging, LIDAR, and scientific research. This article will delve into the theory and practical aspects of APDs, exploring their working principles, advantages, and real-world applications in optical sensing.


The process of detecting light—whether with our eyes, cameras or other devices—is at the heart of a wide range of civilian and military applications, including light or laser detection and ranging (LIDAR or LADAR), photography, astronomy, quantum information processing, medical imaging, microscopy and communications.

Light is also being used in  optical wireless communication, a form of free space communications consisting of a LASER at source and detector at the destination.  Both military and civilian users have started planning Laser communication systems from terrestrial short-range systems, to high data rate Aircraft and Satellite communications, unmanned aerial vehicles (UAVs) to high altitude platforms (HAPs), near-space communications for relaying high data rates from moon, and deep space communications from mars.

Detection of photons—the fundamental particles of light—is ubiquitous, but performance limitations of existing photon detectors hinders the effectiveness of the aforementioned applications.  The detectors which detect these signals are the most critical elements that determine the performance of wide range of civilian and military systems.


For deeper understanding of Photodiode technology and applications please visit: Photodiodes: Principles, Applications, and Future Trends

Photon Detectors

Photon detectors count photons of light. A photon detector has some surface that absorbs photons and produces some effect (current, voltage) proportional to the number of photons absorbed.

A photovoltaic cell consists of a layer of semiconductor (like selenium, Hg-Cd-Te, Cu2O, etc.) sandwiched between two metallic electrodes, with the exposed electrode thin enough to be transparent. Photons of light are absorbed by the semiconductor, forming electrons and holes that create a current proportional to the number of photons absorbed.

A phototube uses the photoelectric effect to generate a current from absorbed light. Light is absorbed by a metallic surface with a low work function. Electrons are emitted and attracted to a positively biased anode. Electronics measure the current, which is proportional to the number of photons absorbed.

 Semiconductor Single-Photon Avalanche Photodiodes (SPAD)

A photodiode in its simplest form is a p-n junction whose materials enable the right radiation frequency or wavelength to release electrons and create a photocurrent within the junction. When reverse-biased with the cathode voltage raised positive compared to the anode, this photocurrent
can be quickly extracted to produce a current proportional to the light level on the photodiode.

Adding an undoped region (“intrinsic” semiconductor) between the p- and n-type regions allows doping levels to be increased, causing higher levels of charge carriers and therefore greater
operation speed. A PIN junction, invented by Jun-ichi Nishizawa et al. in 1950, is also ideal for a photodiode. Adding an internal current gain region within a PIN photodiode turns it into an APD.

An avalanche photodiode (APD)

An avalanche photodiode (APD) is a type of photodiode that utilizes the internal multiplication of electrons through impact ionization to achieve high levels of gain. APDs are capable of detecting low levels of light with high sensitivity, making them suitable for use in applications such as optical communications, lidar, and medical imaging.

An avalanche photodiode (APD) is a type of semiconductor device that operates in reverse-bias mode and is capable of multiplying the number of photocarriers generated by incident light. This multiplication process, known as avalanche breakdown, can result in a significant increase in the signal-to-noise ratio of the device, making APDs particularly useful in low-light-level applications such as optical communications, lidar, and spectroscopy. APDs are commonly used as high-sensitivity detectors in fiber optic communication systems, where they are used to detect weak optical signals and amplify them to a level where they can be detected by standard electronics.

The basic structure of an APD is similar to that of a regular photodiode, consisting of a p-n junction or a p-i-n structure. However, in an APD, the electric field across the junction is typically much higher, causing the electrons that are generated by the incident photons to gain kinetic energy and collide with other atoms in the material. This results in the release of additional electrons, which in turn can collide with more atoms and release even more electrons, causing a cascade of electrons that results in an amplification of the original signal.

Invented by Nishizawa in 1952, the APD uses a careful doping structure to allow high-voltage application, creating high fields within the junction region. These high fields accelerate the photoelectrons, causing them to release other electrons through impact ionization and create a typical internal current gain of 100. Thus, a single photon can create 100 photoelectrons—however, this is still not enough to enable a simple single-photon detector.

One of the major advantages of APDs over normal photodiodes is their internal gain mechanism, which allows them to achieve higher sensitivity than normal photodiodes. This gain mechanism is achieved through the use of a high reverse bias voltage, which creates an electric field strong enough to ionize some of the atoms in the depletion region of the diode. When a photon is absorbed in the depletion region, it can create an electron-hole pair, which is then accelerated by the electric field and creates more electron-hole pairs through impact ionization. This avalanche process can create a large number of electron-hole pairs from a single absorbed photon, resulting in a higher signal-to-noise ratio and lower detection limits.

The gain of an APD is typically expressed as the multiplication factor, which is the ratio of the number of output electrons to the number of input photons. APDs can achieve gains of several hundred or even thousands, depending on the design and operating conditions. However, the avalanche process also introduces noise and increases the dark current of the device, which can limit its performance in certain applications.


Advantages of APDs:

  1. High Sensitivity: APDs offer higher sensitivity compared to traditional photodiodes, making them ideal for detecting weak light signals, especially in low-light conditions.
  2. Low Noise: The avalanche multiplication mechanism enables APDs to achieve a low noise performance, ensuring accurate and reliable signal detection.
  3. Wide Spectral Response: APDs are capable of detecting light across a broad range of wavelengths, from ultraviolet (UV) to infrared (IR), enabling multispectral applications.

For a deeper understanding of APD and SPAD technology and applications please visit: Advancements in Avalanche Photodiode (APD) and Single-Photon Avalanche Diode (SPAD) Technology: Theory, Applications, and Future Prospects


APDs are preferred over normal photodiodes in applications that require high sensitivity to low levels of light. They are used in a variety of applications including telecommunications, medical imaging, lidar, and high-energy physics experiments. In telecommunications, APDs are used as receivers in fiber optic networks to detect weak signals transmitted over long distances.

APDs are widely used in optical receivers for high-speed, optical fiber-based communication and lidar applications, which require extremely sensitive photodiodes that are capable of detecting very low levels of light intensity — in some cases, detection down to a few photons or single-photon level.

In medical imaging, they are used in positron emission tomography (PET) and single-photon emission computed tomography (SPECT) to detect low levels of radiation emitted by radioactive isotopes. In lidar, they are used to detect weak reflections from distant objects, such as in autonomous vehicle navigation. In high-energy physics experiments, they are used to detect signals from subatomic particles produced in particle accelerators.

When operated in the Geiger mode, APDs can be used in single-photon detection, such as quantum key distribution and quantum imaging. The signal-to-noise ratio (SNR) in an APD-preamplifier module can be enhanced by the internal avalanche gain of APDs if the dominant noise source is the preamplifier. A significant increase in SNR, relative to pin photodiodes, can be achieved if the randomness in the impact ionization process is small. APDs typically have a higher SNR than pin photodiodes because APDs apply reverse voltage, which causes them to experience internal gain.


Challenges and Future Directions:

However, APDs also have some disadvantages compared to normal photodiodes. They are more complex to manufacture and require more precise control of the bias voltage to prevent breakdown. They also have higher dark current and noise levels due to the avalanche process, which can limit their performance in some applications. Overall, the choice between APDs and normal photodiodes depends on the specific requirements of the application and the trade-offs between sensitivity, complexity, and cost.

To improve the performance of APDs, various techniques have been developed, such as the use of separate absorption and multiplication (SAM) structures, which separate the regions of the device where the photon absorption and the avalanche multiplication occur. Other techniques include the use of quenching circuits to limit the duration of the avalanche process and reduce the noise, as well as the use of cooling to reduce the dark current and improve the signal-to-noise ratio.

Overall, APDs offer a high-gain and high-sensitivity solution for detecting low levels of light, and have found widespread use in a variety of applications. Ongoing research and development in this field is focused on improving the performance and reliability of APDs, as well as exploring new applications and areas of use.

  1. Excess Noise Factor: One challenge with APDs is the excess noise factor associated with the avalanche multiplication process. Ongoing research aims to minimize this factor to further improve APD performance.
  2. Integration with Nanophotonics: The integration of APDs with nanophotonics and photonic integrated circuits could lead to compact and highly efficient optical sensing systems.

Novel Photodiode Cuts Excess Noise, Offers High Detection Efficiency

The University of California, Los Angeles (UCLA), reported in Feb 2023 of developing a new avalanche photodiode (APD)  that offers high sensitivity and low noise levels. The new APD has a novel structure that reduces excess noise generated during the avalanche process. This allows the APD to achieve higher sensitivity at lower bias voltages, resulting in reduced power consumption and improved device reliability. The researchers have demonstrated that the new APD can achieve a 6dB improvement in sensitivity over traditional APDs, as well as a significant reduction in noise.

To build the APD, the researchers combined a semiconductor alloy with a wider bandgap semiconductor. The semiconductor alloy is based on a GaAsSb absorption region that has excellent detection efficiency at IR wavelengths up to 1700 nm.

“One of the long-standing limitations of infrared APDs is a relatively high added noise from the multiplication process that limits the maximum multiplication factor,” said professor Chee Hing Tan. “This, in turn, prevented infrared APDs from reaching the performance limit predicted by established models.

“Our breakthrough result, with an excess noise factor of 2.48, is approaching the theoretical lower limit of 2. This provides the pathway to realize extremely low-noise APD that I believe can generate step changes in optical communication and long-range lidar.”


Avalanche Photodiodes (APDs) have emerged as a breakthrough technology in optical sensing, offering high sensitivity, low noise, and a wide spectral response. From telecommunications to medical imaging and quantum research, APDs are revolutionizing how we detect and harness light in various applications. As ongoing research continues to refine their design and performance, APDs are expected to play an even more significant role in advancing optical sensing technologies and enabling groundbreaking discoveries in the future.



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