avalanche photodiodes (original) (raw)

Acronym: APD

Definition: photodiodes with internal signal amplification through an avalanche process

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Related: single-photon avalanche diodessilicon photomultipliersphotodiodeselectronics for photodetectionphotomultipliersphototransistorsresponsivitysingle-photon detectorsphoton countingquantum efficiencyquantum noise

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Contents

What is an Avalanche Photodiode?

An avalanche photodiode is a semiconductor-based photodetector (photodiode) which is operated with a relatively high reverse voltage (typically tens or even hundreds of volts), sometimes just below breakdown. In this regime, carriers (electrons and holes) excited by absorbed photons are strongly accelerated in the strong internal electric field so that they can generate secondary carriers. The avalanche process, which may take place over a distance of only a few micrometers, for example, effectively amplifies the photocurrent by a significant factor, although not as much as in a photomultiplier. Therefore, avalanche photodiodes can be used for very sensitive detectors, which need less electronic signal amplification and are thus less susceptible to electronic noise. Compared with ordinary photodiodes, they are suitable for extending the detectable light levels by roughly an order of magnitude.

Typical applications of avalanche photodiodes include receivers in optical fiber communications, range finding, imaging, high-speed laser scanners, laser microscopy and optical time-domain reflectometers (OTDR). Special Geiger-mode variants are used as single-photon detectors.

Responsivity

avalanche photodiodes

Figure 1: Avalanche photodiodes. Source: Excelitas Technologies

The current amplification process strongly increases the responsivity of an APD. Note, however, that the amplification factor and thus the responsivity depends strongly on the reverse voltage, and may also substantially vary from device to device. Therefore, it is common to specify a certain voltage range within which all devices reach a certain responsivity. For precise measurements of low light powers, avalanche diodes are hardly suitable, since their responsivity is not nearly as well-defined as that of a p–i–n diode, for example.

Quantum Efficiency

Despite the high responsivity, the quantum efficiency of an APD is not necessarily high — certainly below 100% and possibly lower than for other photodiodes. This means that some of the incident photons do not contribute to the photocurrent, even though other photons do very much so, triggering an electron avalanche.

Materials and Wavelength Ranges

Silicon

Silicon-based avalanche photodiodes are sensitive in the wavelength region from ≈ 450 to 1000 nm (sometimes up to 1100 nm), with the maximum responsivity occurring around 600–800 nm, i.e., at somewhat shorter wavelengths than for silicon p–i–n diodes. Depending on the device and the reverse voltage applied, the multiplication factor (also called gain) of silicon APDs can vary between 50 and 1000.

Materials for Longer Wavelengths

For longer wavelengths up to roughly 1.7 μm, APDs based on germanium or indium gallium arsenide (InGaAs) are used. These have lower current multiplication factors of 10 to 40.

InGaAs APDs are significantly more expensive than those based on germanium, but exhibit superior noise performance and a higher detection bandwidth. Their high absorption coefficient allows the use of a rather thin absorbing layer.

Another possibility is to use germanium/silicon (GeSi) devices, where radiation is absorbed in germanium, and the carriers are transferred into a silicon region for charge multiplication [11, 18].

A special variant for infrared applications are III–V nanowire detectors, which are built from vertical (or occasionally lateral) III–V semiconductor nanowires — typically using a narrow-bandgap absorber such as InGaAs or GaAsSb, and a wider-bandgap multiplication region (e.g., GaAs or InP). Specific advantages result from such device architectures: The tiny avalanche volume reduces effects of detrimental charge traps (causing afterpulsing) and increases speed and timing accuracy through an ultralow capacitance. III–V nanowires can also be selectively grown or integrated on silicon with good material quality.

Less common semiconductor materials for APDs are gallium nitride (GaN) for ultraviolet light and HgCdTe for the mid-infrared up to wavelength of ≈14 μm (used under cryogenic conditions).

Detection Bandwidth

The detection bandwidth achievable with avalanche diodes can be quite high, although there is an inherent trade-off between bandwidth and amplification factor. On the other hand, the enhanced responsivity can allow the operation with a smaller shunt resistor than can be used with an ordinary photodiode, and that effect may compensate for a possible speed disadvantage of an avalanche diode.

Detection Noise

The large responsivity of an APD can help to reduce detection noise, since it much reduces the impact of electronic noise in the subsequently used photodiode preamplifier. Therefore, the noise performance of photodetectors with APDs can be better than that of devices with ordinary p–i–n photodiodes when electronic noise is a limiting factor.

On the other hand, the avalanche process itself is subject to substantial amplification noise (multiplication noise), which can offset the mentioned advantage. The resulting excess noise is quantified with the excess noise factor ($F$); that is the factor by which the electronic noise power is increased compared with that of an ideal photodetector. The amount of excess noise depends on many factors: the magnitude of the reverse voltage, material properties (in particular, the ionization coefficient ratio ($\kappa$)), and the device design. The excess noise factor increases with increasing amplification factor, as obtained for increasing reverse voltage. Therefore, the reverse voltage is often chosen such that the multiplication noise approximately equals the noise of the electronic amplifier because that setting minimizes the overall noise.

Various engineering approaches can be used for reduced excess noise:

Geiger Mode for Single-photon Detection

When operated in the so-called Geiger mode with carefully designed electronics, avalanche photodiodes can be used even as single-photon detectors with dark count rates well below 1 kHz and with a photon detection efficiency of several tens of percent, sometimes even well above 50%. Geiger mode means that the diode is operated slightly above the breakdown threshold voltage, where a single electron–hole pair (generated by absorption of a photon or by a thermal fluctuation) can trigger a strong avalanche. In the case of such an event, an electronic quenching circuit reduces the voltage at the diode below the threshold voltage for a short time, so that the avalanche is stopped, and the detector is ready for detection of further photons after some recovery time of e.g. 100 ns. That dead time constitutes a substantial limitation of this technology. It limits the count rate to the order of 10 MHz, whereas an avalanche diode in linear mode (i.e., operated with lower reverse voltage) may be operated with a bandwidth of many gigahertz. Such devices also have a limited quantum efficiency, i.e., not every incident photon can trigger an avalanche.

Single-photon APDs are called SPADs, which means single-photon avalanche diodes. When optimized for high quantum efficiencies, they can be used in quantum optics experiments (for example, for quantum cryptography) and in some of the applications mentioned above if an extremely high responsivity is required. SPADs with optimized amplifier electronics are also available in CMOS integrated form, even as large photodiode arrays, e.g. for use as image sensors for single-photon 3D imaging via time-resolved detection [10].

Note that noise in single-photon detectors is characterized differently from that in linear-mode avalanche photodiodes. In Geiger-mode operation, the key figures of merit are photon detection efficiency, dark count rate and timing jitter, rather than the excess noise factor.

See the article on single-photon avalanche diodes for more details.

Avalanche Diode Modules

Avalanche diodes are available as part of modules, which in addition to the photodiode also contain additional electronic components. In particular, there can be a transimpedance amplifier integrated into the package, which not only reduces the number of parts required on a circuit board, but also improves the noise performance and results in a better combination of bandwidth and responsivity. Some modules have been specifically optimized for use in optical fiber communications systems and are fiber-coupled. It is also possible to integrate the quenching electronics required for Geiger mode operation.

Silicon Photomultipliers

An important difference between an avalanche photodiode and a photomultiplier is that the latter has a much larger active area. However, it is possible to construct so-called silicon photomultipliers, containing one or more arrays of silicon-based avalanche diodes, where the combined active area can be fairly large.

Apart from the large active area, silicon photomultipliers are also suitable for measuring photon numbers [13], even if the single diodes are not: one may count the total number of diodes which are triggered by a weak incident optical pulse. That number approximates well the photon number (multiplied by the quantum efficiency) provided that the probability of more than one photon hitting a single diode is sufficiently small.

See the article on silicon photomultipliers for more details.

Phototransistors

Another type of semiconductor-based photodetector, which also uses some kind of photocurrent amplification, is the phototransistor. Here, however, the amplification is based on different principles, and the operating characteristics are also quite different.

Frequently Asked Questions

What is an avalanche photodiode (APD)?

An avalanche photodiode is a semiconductor photodetector operated with a high reverse voltage. This causes charge carriers generated by light to trigger an internal avalanche of secondary carriers, effectively amplifying the photocurrent.

How does an APD differ from a standard p–i–n photodiode?

Unlike a standard photodiode, an APD provides internal current gain. This makes it far more sensitive for detecting low light levels, but its responsivity is less stable and the avalanche process introduces additional noise.

What determines the wavelength range of an APD?

The wavelength range is determined by the semiconductor material. Silicon is typically used for visible and near-infrared light (approx. 450–1000 nm), while materials like InGaAs or germanium are used for longer infrared wavelengths up to 1.7 μm.

Why is an APD noisier than a simple photodiode?

An APD is inherently noisier due to the random statistical nature of the avalanche multiplication process itself. This is called multiplication noise or excess noise, and it adds to the overall signal noise. On the other hand, the amplification reduces the impact of electronic noise in the following amplifier.

What is Geiger mode in an avalanche photodiode?

Geiger mode is an operating regime where the APD is biased slightly above its breakdown voltage. In this mode, a single absorbed photon can trigger a strong, saturated avalanche current, allowing the device to function as a single-photon detector.

What are single-photon avalanche diodes (SPADs)?

SPADs are avalanche photodiodes specifically designed to be operated in Geiger mode for detecting single photons. They are used in highly sensitive applications like quantum optics, lidar, and time-resolved imaging.

What limits the speed of an APD?

The detection speed (bandwidth) of an APD involves an inherent trade-off with its amplification factor (gain). While the avalanche process has a finite response time, the higher responsivity can allow for circuit designs that partially compensate for this limitation.

Suppliers

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Analog Modules

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The Models 7511B and 7510 are high gain low noise APD-preamp optical receivers. The compact construction (modified TO-8 header) and PCB mounting capability make them ideal for miniature applications.

Hamamatsu Photonics

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Hamamatsu Photonics avalanche photodiodes are photodiodes with internal gain produced by the application of a reverse voltage. They have a higher signal-to-noise ratio (SNR) than PIN photodiodes, as well as fast time response, low dark current, and high sensitivity. Spectral response range is typically within 200 to 1150 nm.

Bibliography

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