APDS

APDs, or avalanche photodiodes, are highly sensitive semiconductor photodiodes that use avalanche multiplication to achieve internal signal amplification. They are designed to convert light into an electrical current with gain, enabling detection of very weak optical signals across a range of wavelengths.

Operation relies on reverse biasing the diode above its breakdown voltage, which accelerates carriers enough to cause impact ionization and generate additional charge carriers. This avalanche process provides multiplication gain. There are two main modes: linear-mode APDs operate with gain in a controlled, proportional manner suitable for analog detection, while Geiger-mode APDs (also called SPADs or single-photon avalanche diodes) are biased above breakdown to produce discrete pulses in response to individual photons and require quenching to stop the avalanche.

Materials and spectral coverage vary. Silicon APDs are common for visible to near-infrared light (roughly 0.4 to 1.0 micrometers), whereas InGaAs APDs cover near-infrared wavelengths up to about 1.7 micrometers, and InP-based devices are used for telecom wavelengths around 1.55 micrometers. APD performance is influenced by gain, noise (including the excess noise factor), dark current, afterpulsing, and timing jitter. Cooling is often employed to reduce dark current and afterpulsing, particularly in single-photon or high-sensitivity applications.

Applications of APDs include fiber-optic telecommunications receivers, LIDAR and time-of-flight measurements, fluorescence spectroscopy, astronomy, and certain medical imaging modalities. APD technology continues to evolve with improvements in noise, speed, integration with readout electronics, and the development of APD arrays for imaging.