COMMENT


A guide to avalanche photodiode sensor terminology


By Christian Rookes, VP marketing, Phlux Technology I


nfrared (IR) sensing underpins much of the technology you interact with every day, from high-speed fi bre-optic communications to autonomous navigation and biomedical imaging. Infrared light lies just beyond the visible spectrum, spanning roughly 700 nanometres (nm) to 1 millimetre (mm). Because IR wavelengths can pass through materials and atmospheric conditions that scatter visible light, they are well suited to thermal imaging, spectroscopy, LiDAR, remote sensing and optical communication systems where precision and reliability are essential. To convert invisible IR light into electrical signals, photodetectors are required. Among the available technologies, avalanche photodiodes (APDs) stand out for their ability to detect extremely low light levels while maintaining fast response. Understanding how APDs operate—and how their key parameters infl uence performance— helps you select and optimise devices for demanding sensing and communication applications. This guide introduces APD terminology with an emphasis on 1550 nm “eye-safe” devices for near-infrared detection.


Understanding avalanche photodiodes


An avalanche photodiode is a semiconductor device that converts incoming photons into electrical current with internal amplifi cation. When a high reverse-bias voltage is applied, a strong electric fi eld forms within the device. Photons absorbed in the active region generate electron–hole pairs, which are accelerated by this fi eld. As these carriers collide with the crystal lattice, they release additional charge carriers, triggering a chain reaction known as avalanche multiplication. This internal gain signifi cantly boosts the output current produced by weak optical signals, enabling APDs to detect light levels beyond the reach of standard photodiodes. As a result, APDs are widely used in fi bre- optic communication systems, laser range fi nders, medical imaging equipment and


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environmental sensors where both sensitivity and speed are required.


Key APD parameters you should know


Several technical parameters describe how an APD behaves in operation and how it will perform in a real system. Excess Noise Factor (ENF): ENF quantifi es the additional noise introduced by the statistical nature of avalanche multiplication. Because individual charge carriers experience different gain, fl uctuations occur that reduce the signal-to- noise ratio (SNR). Lower ENF values correspond to cleaner signals and improved detection performance.


Noiseless InGaAs APDs: Advances in indium gallium arsenide (InGaAs) materials and device design have enabled 1550 nm APDs with exceptionally low excess noise. These Noiseless InGaAs devices can achieve gain values above 100 without signifi cant SNR degradation, making them well suited to long-distance fi bre links, LiDAR receivers and photon-limited sensing applications. Dark current:


Even in the absence of light, a small current fl ows through every APD. This dark current arises from thermally generated charge carriers and contributes to background noise. Selecting low-leakage devices, optimising bias conditions, or using cooling techniques can reduce dark current and improve low- light performance. Gain:


Gain describes the ratio of output signal current to the photocurrent generated directly by incoming light. In APDs, gain is produced internally through avalanche multiplication. While higher gain improves sensitivity, it can also increase noise, power


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consumption and device instability. Careful control of the bias voltage allows gain and noise to be balanced for stable operation. Noise Equivalent Power (NEP): NEP defi nes the minimum optical power required to produce a signal equal to the detector’s noise level. A lower NEP indicates higher sensitivity and is particularly important in spectroscopy, astronomy and analytical instrumentation.


Materials and construction for 1550 nm APDs


Indium Gallium Arsenide (InGaAs): InGaAs is the material of choice for near- infrared detection, offering strong absorption from approximately 900 to 1700 nm. Its high quantum effi ciency, fast response and relatively low noise make it ideal for fi bre- optic communications, LiDAR systems, and scientifi c instruments operating at 1550 nm. Bare Die:


Some APDs are supplied as bare die, without a protective package. This allows direct wire-bonding or fl ip-chip attachment to substrates or printed circuit boards. Bare-die devices are particularly useful in compact, high-frequency, or thermally optimised designs where size and parasitics are critical. Fibre Pigtails:


Many APDs are available with integrated


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