Avalanche photodiode

Last updated December 10, 2023

An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of photomultiplier tubes. The avalanche photodiode was invented by Japanese engineer Jun-ichi Nishizawa in 1952.^[1] However, study of avalanche breakdown, microplasma defects in silicon and germanium and the investigation of optical detection using p-n junctions predate this patent. Typical applications for APDs are laser rangefinders, long-range fiber-optic telecommunication, and quantum sensing for control algorithms. New applications include positron emission tomography and particle physics.

Principle of operation

By applying a high reverse bias voltage (typically 100–200 V in silicon), APDs show an internal current gain effect (around 100) due to impact ionization (avalanche effect). However, some silicon APDs employ alternative doping and beveling techniques compared to traditional APDs that allow greater voltage to be applied (> 1500 V) before breakdown is reached and hence a greater operating gain (> 1000). In general, the higher the reverse voltage, the higher the gain. Among the various expressions for the APD multiplication factor (M), an instructive expression is given by the formula

M={\frac {1}{1-\int _{0}^{L}\alpha (x)\,dx}},

where L is the space-charge boundary for electrons, and $\alpha$ is the multiplication coefficient for electrons (and holes). This coefficient has a strong dependence on the applied electric field strength, temperature, and doping profile. Since APD gain varies strongly with the applied reverse bias and temperature, it is necessary to control the reverse voltage to keep a stable gain. Avalanche photodiodes therefore are more sensitive compared to other semiconductor photodiodes.

If very high gain is needed (10⁵ to 10⁶), detectors related to APDs called SPADs (single-photon avalanche diodes) can be used and operated with a reverse voltage above a typical APD's breakdown voltage. In this case, the photodetector needs to have its signal current limited and quickly diminished. Active and passive current-quenching techniques have been used for this purpose. SPADs that operate in this high-gain regime are sometimes referred to being in Geiger mode. This mode is particularly useful for single-photon detection, provided that the dark count event rate and afterpulsing probability are sufficiently low.

Materials

In principle, any semiconductor material can be used as a multiplication region:

Silicon will detect in the visible and near infrared, with low multiplication noise (excess noise).
Germanium (Ge) will detect infrared out to a wavelength of 1.7 µm, but has high multiplication noise.
InGaAs will detect out to longer than 1.6 µm and has less multiplication noise than Ge. It is normally used as the absorption region of a heterostructure diode, most typically involving InP as a substrate and as a multiplication layer.^[2] This material system is compatible with an absorption window of roughly 0.9–1.7 µm. InGaAs exhibits a high absorption coefficient at the wavelengths appropriate to high-speed telecommunications using optical fibers, so only a few micrometres of InGaAs are required for nearly 100% light absorption.^[2] The excess noise factor is low enough to permit a gain-bandwidth product in excess of 100 GHz for a simple InP/InGaAs system,^[3] and up to 400 GHz for InGaAs on silicon.^[4] Therefore, high-speed operation is possible: commercial devices are available to speeds of at least 10 Gbit/s.^[5]
Gallium-nitride–based diodes have been used for operation with ultraviolet light.
HgCdTe-based diodes operate in the infrared, typically at wavelengths up to about 14 µm, but require cooling to reduce dark currents. Very low excess noise can be achieved in this material system.

Performance limits

APD applicability and usefulness depends on many parameters. Two of the larger factors are: quantum efficiency, which indicates how well incident optical photons are absorbed and then used to generate primary charge carriers; and total leakage current, which is the sum of the dark current, photocurrent and noise. Electronic dark-noise components are series and parallel noise. Series noise, which is the effect of shot noise, is basically proportional to the APD capacitance, while the parallel noise is associated with the fluctuations of the APD bulk and surface dark currents.

Gain noise, excess noise factor

Another noise source is the excess noise factor, ENF. It is a multiplicative correction applied to the noise that describes the increase in the statistical noise, specifically Poisson noise, due to the multiplication process. The ENF is defined for any device, such as photomultiplier tubes, silicon solid-state photomultipliers, and APDs, that multiplies a signal, and is sometimes referred to as "gain noise". At a gain M, it is denoted by ENF(M) and can often be expressed as

{\text{ENF}}=\kappa M+\left(2-{\frac {1}{M}}\right)(1-\kappa ),

where $\kappa$ is the ratio of the hole impact ionization rate to that of electrons. For an electron multiplication device it is given by the hole impact ionization rate divided by the electron impact ionization rate. It is desirable to have a large asymmetry between these rates to minimize ENF(M), since ENF(M) is one of the main factors that limit, among other things, the best possible energy resolution obtainable.

Conversion noise, Fano factor

The noise term for an APD may also contain a Fano factor, which is a multiplicative correction applied to the Poisson noise associated with the conversion of the energy deposited by a charged particle to the electron-hole pairs, which is the signal before multiplication. The correction factor describes the decrease in the noise, relative to Poisson statistics, due to the uniformity of conversion process and the absence of, or weak coupling to, bath states in the conversion process. In other words, an "ideal" semiconductor would convert the energy of the charged particle into an exact and reproducible number of electron hole pairs to conserve energy; in reality, however, the energy deposited by the charged particle is divided into the generation of electron hole pairs, the generation of sound, the generation of heat, and the generation of damage or displacement. The existence of these other channels introduces a stochastic process, where the amount of energy deposited into any single process varies from event to event, even if the amount of energy deposited is the same.

Further influences

The underlying physics associated with the excess noise factor (gain noise) and the Fano factor (conversion noise) is very different. However, the application of these factors as multiplicative corrections to the expected Poisson noise is similar. In addition to excess noise, there are limits to device performance associated with the capacitance, transit times and avalanche multiplication time.^[2] The capacitance increases with increasing device area and decreasing thickness. The transit times (both electrons and holes) increase with increasing thickness, implying a tradeoff between capacitance and transit time for performance. The avalanche multiplication time times the gain is given to first order by the gain-bandwidth product, which is a function of the device structure and most especially $\kappa \,$ .

Related Research Articles

A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a major technology used in digital imaging.

<span class="mw-page-title-main">Diode</span> Two-terminal electronic component

A diode is a two-terminal electronic component that conducts current primarily in one direction. It has low resistance in one direction and high resistance in the other.

A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts.

<span class="mw-page-title-main">Photodiode</span> Converts light into current

A photodiode is a semiconductor diode sensitive to photon radiation, such as visible light, infrared or ultraviolet radiation, X-rays and gamma rays. It produces an electrical current when it absorbs photons. This can be used for detection and measurement applications, or for the generation of electrical power in solar cells. Photodiodes are used in a wide range range of applications throughout the electromagnetic spectrum from visible light photocells to gamma ray spectrometers.

In electronics, an avalanche diode is a diode that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration and resulting hot spots, so that the diode is undamaged by the breakdown. The avalanche breakdown is due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers, which in turn create more ionization. Because the avalanche breakdown is uniform across the whole junction, the breakdown voltage is nearly constant with changing current when compared to a non-avalanche diode.

A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suzuki when they were working at Tokyo Tsushin Kogyo, now known as Sony. In 1973, Esaki received the Nobel Prize in Physics for discovering the electron tunneling effect used in these diodes. Robert Noyce independently devised the idea of a tunnel diode while working for William Shockley, but was discouraged from pursuing it. Tunnel diodes were first manufactured by Sony in 1957, followed by General Electric and other companies from about 1960, and are still made in low volume today.

A single-photon avalanche diode (SPAD) is a solid-state photodetector within the same family as photodiodes and avalanche photodiodes (APDs), while also being fundamentally linked with basic diode behaviours. As with photodiodes and APDs, a SPAD is based around a semi-conductor p-n junction that can be illuminated with ionizing radiation such as gamma, x-rays, beta and alpha particles along with a wide portion of the electromagnetic spectrum from ultraviolet (UV) through the visible wavelengths and into the infrared (IR).

An IMPATT diode is a form of high-power semiconductor diode used in high-frequency microwave electronics devices. They have negative resistance and are used as oscillators and amplifiers at microwave frequencies. They operate at frequencies of about 3 and 100 GHz, or higher. The main advantage is their high-power capability; single IMPATT diodes can produce continuous microwave outputs of up to 3 kilowatts, and pulsed outputs of much higher power. These diodes are used in a variety of applications from low-power radar systems to proximity alarms. A major drawback of IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process.

Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There are a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or by various performance metrics, such as spectral response. Semiconductor-based photodetectors typically use a p–n junction that converts photons into charge. The absorbed photons make electron–hole pairs in the depletion region. Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.

Indium gallium arsenide (InGaAs) is a ternary alloy of indium arsenide (InAs) and gallium arsenide (GaAs). Indium and gallium are group III elements of the periodic table while arsenic is a group V element. Alloys made of these chemical groups are referred to as "III-V" compounds. InGaAs has properties intermediate between those of GaAs and InAs. InGaAs is a room-temperature semiconductor with applications in electronics and photonics.

An electron avalanche is a process in which a number of free electrons in a transmission medium are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the medium, thereby ionizing them. This releases additional electrons which accelerate and collide with further atoms, releasing more electrons—a chain reaction. In a gas, this causes the affected region to become an electrically conductive plasma.

Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche process occurs when carriers in the transition region are accelerated by the electric field to energies sufficient to create mobile or free electron-hole pairs via collisions with bound electrons.

<span class="mw-page-title-main">Impact ionization</span>

Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. For example, in semiconductors, an electron with enough kinetic energy can knock a bound electron out of its bound state and promote it to a state in the conduction band, creating an electron-hole pair. For carriers to have sufficient kinetic energy a sufficiently large electric field must be applied, in essence requiring a sufficiently large voltage but not necessarily a large current.

Capacitance–voltage profiling is a technique for characterizing semiconductor materials and devices. The applied voltage is varied, and the capacitance is measured and plotted as a function of voltage. The technique uses a metal–semiconductor junction or a p–n junction or a MOSFET to create a depletion region, a region which is empty of conducting electrons and holes, but may contain ionized donors and electrically active defects or traps. The depletion region with its ionized charges inside behaves like a capacitor. By varying the voltage applied to the junction it is possible to vary the depletion width. The dependence of the depletion width upon the applied voltage provides information on the semiconductor's internal characteristics, such as its doping profile and electrically active defect densities.^, Measurements may be done at DC, or using both DC and a small-signal AC signal, or using a large-signal transient voltage.

In solid-state electronics, silicon photomultipliers (SiPMs) are single-photon-sensitive devices based on single-photon avalanche diodes (SPADs) implemented on common silicon substrate. The dimension of each single SPAD can vary from 10 to 100 micrometres, with a density of up to 10,000 per square millimeter. Every SPAD in a SiPM operates in Geiger mode and is coupled with the others by a metal or polysilicon quenching resistor. Although the device works in digital/switching mode, most SiPMs are analog devices because the microcells are read in parallel, making it possible to generate signals with a dynamic range from a single photon to 1000 photons for a device with just a square-millimeter area. More advanced readout schemes are used for lidar applications. The supply voltage depends on the APD technology used and typically varies between 20 V and 100 V, thus being from 15 to 75 times lower than the voltage required for traditional photomultiplier tube (PMT) operation.

The heterostructure barrier varactor (HBV) is a semiconductor device which shows a variable capacitance with voltage bias, similar to a varactor diode. Unlike a diode, it has an anti-symmetric current-voltage relationship and a symmetric capacitance-voltage relationship, as shown in the graph to the right. The device was invented by Erik Kollberg together with Anders Rydberg in 1989 at Chalmers University of Technology.

Resonant-cavity-enhanced photodetectors, also known as RCE photodetectors, are sensors designed to detect light or other forms of electromagnetic radiation. They achieve this by employing an optical cavity, a configuration of mirrors or other optical elements that forms a cavity resonator for light waves, allowing more efficient targeting of specific wavelengths.

A noise generator is a circuit that produces electrical noise. Noise generators are used to test signals for measuring noise figure, frequency response, and other parameters. Noise generators are also used for generating random numbers.

This article provides a more detailed explanation of p–n diode behavior than is found in the articles p–n junction or diode.

James Robert Biard was an American electrical engineer and inventor who held 73 U.S. patents. Some of his more significant patents include the first infrared light-emitting diode (LED), the optical isolator, Schottky clamped logic circuits, silicon Metal Oxide Semiconductor Read Only Memory, a low bulk leakage current avalanche photodetector, and fiber-optic data links. In 1980, Biard became a member of the staff of Texas A&M University as an Adjunct Professor of Electrical Engineering. In 1991, he was elected as a member into the National Academy of Engineering for contributions to semiconductor light-emitting diodes and lasers, Schotky-clamped logic, and read-only memories.

References

↑ "Jun-ichi Nishizawa – Engineer, Sophia University Special Professor – JAPAN QUALITY REVIEW". Archived from the original on 2018-07-21. Retrieved 2017-05-15.
1 2 3 Tsang, W. T., ed. (1985). Semiconductors and Semimetals. Vol. 22, Part D "Photodetectors". Academic Press.
↑ Tarof, L. E. (1991). "Planar InP/GaAs Avalanche Photodetector with Gain-Bandwidth Product in Excess of 100 GHz". Electronics Letters. 27 (1): 34–36. Bibcode:1991ElL....27...34T. doi:10.1049/el:19910023.
↑ Wu, W.; Hawkins, A. R.; Bowers, J. E. (1997). Park, Yoon-Soo; Ramaswamy, Ramu V (eds.). "Design of InGaAs/Si avalanche photodetectors for 400-GHz gain-bandwidth product". Proceedings of SPIE. Optoelectronic Integrated Circuits. 3006: 36–47. Bibcode:1997SPIE.3006...38W. doi:10.1117/12.264251. S2CID 109777495.
↑ Campbell, J. C. (2007). "Recent advances in Telecommunications Avalanche Photodiodes". Journal of Lightwave Technology. 25 (1): 109–121. Bibcode:2007JLwT...25..109C. doi:10.1109/JLT.2006.888481. S2CID 1398387.