Avalanche Photodiodes | previous | next | feedback |
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Definition: photodiodes with internal signal amplification through an avalanche process
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, as it also occurs in photomultipliers. The avalanche process effectively amplifies the photocurrent by a significant factor. In this way, avalanche photodiodes can be used for very sensitive detectors, which need less electronic signal amplification and are thus less sensitive to electronic noise. However, the avalanche process itself is subject to quantum noise and amplification noise, which can offset the mentioned advantage. Tentatively, their noise performance is better compared with ordinary p-i-n photodiodes in the high-speed regime, but not for low detection bandwidths.
Silicon-based avalanche photodiodes are sensitive in the wavelength region from ∼450 to 1000 nm, with the maximum responsivity occurring around 600-800 nm, i.e., at somewhat shorter wavelengths than for silicon p-i-n diodes. For longer wavelengths, APDs based on germanium or indium gallium arsenide (InGaAs) are used.
Geiger Mode for Photon Counting
When operated in the so-called Geiger mode with carefully designed electronics, avalanche photodiodes can be used even for photon counting with dark count rates well below 1 kHz and with a quantum efficiency of several tens of percent, sometimes even well above 50%. The Geiger mode means that the diode is operated slightly above the breakdown threshold voltage, where a single electron-hole pair can trigger a strong avalanche. In the case of such an event, the electronics reduce the voltage at the diode below the threshold voltage for a short time, so that it can recover.
Photon-counting avalanche detectors with optimized amplifier electronics are also available in CMOS integrated form.
See also: photodiodes, photomultipliers, responsivity, photon counting, quantum efficiency, quantum noise
Categories: metrology, photonic devices
