An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode that takes advantage of the photoelectric effect to convert light into electricity. Studying them from a functional perspective, they can be regarded as the semiconductor analog of photomultipliers. Avalanche Photodiodes were invented by a Japanese engineer, one Jun-ichi Nishizawa, back in the mid-nineties, 1952 to be specific. However, the study of the avalanche breakdown and the investigation of optical detection using p-n junctions and microplasma defects in Silicon and Germanium predate this patent.
How APDs Work
For the avalanche photodiode to work, carriers (electrons and holes) excited by absorbed photons are enormously augmented in the strong internal electric field, which brings forth the generation of secondary carriers. The avalanche process then follows. It may take place over a short distance from the photons (a few micrometres). This process will effectively amplify the photocurrent by a significant factor, but not as much as in a photomultiplier. Through this science, avalanche photodiodes can be used for very sensitive detectors, which require less electronic signal amplification and are thus less susceptible to electronic noise.
Avalanche Photodiodes have particular properties, some of which are discussed below.
Amplifying the current could strongly increase the responsivity of an Avalanche Photodiode. However, it is worth noting that the amplification factor and the responsivity depend strongly on the reverse voltage and could be different across different devices. Therefore, it is common to specify a certain voltage range within which all devices reach a certain responsivity. Avalanche diodes are hardly suitable for precise measurements of low light powers since their responsivity is not nearly as well defined as a p–i–n diode.
Materials & Wavelength Ranges
Silicon-based avalanche photodiodes are most sensitive in a particular wavelength range. The range could start from something around 450 and ends at 1000 nm, but in special cases, they can exceed 1100 nm with the maximum responsivity occurring around 600–800 nm, which is at somewhat shorter wavelengths than for silicon p–i–n diodes. Depending on the quantity of voltage applied and the type of device, the multiplication factor of silicon avalanche photodiodes, also known as gain, can vary from fifty to a thousand. For longer wavelengths of up to roughly 1.7 μm, germanium or indium gallium arsenide based APDs are used. They have lower current multiplication factors of 10 to 40.
Despite its high responsivity, APDs don’t necessarily have high efficiency. It could even be possible that it is lower than for other photodiodes, which 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.
In terms of detection bandwidth of an APD, the ceiling can be relatively high, although there can be a trade-off between bandwidth and the amplification factor. Nevertheless, the enhanced responsivity provides room for the operation with a smaller shunt resistor than usable with an ordinary photodiode, which may pay off for a possible speed disadvantage of an avalanche diode.