An avalanche photodiode (APD) can be defined as a highly sensitive semiconductor photodiode that exploits the photoelectric effect to convert light into electricity. Looking at APDs from a functional standpoint, they can be regarded as the semiconductor analog of photomultipliers. It (APD) was invented by Japanese engineer Jun-ichi Nishizawa in 1952, but the study of avalanche breakdown, microplasma defects in Silicon and Germanium and the investigation of optical detection using p-n junctions predate this patent.
How Does an Avalanche Photodiode Work?
In the functioning of avalanche photodiodes, carriers (electrons and holes) excited by absorbed photons are strongly accelerated in the strong internal electric field, which enables them to generate secondary carriers. The avalanche process then follows. It may take place over a distance of only a few micrometres. This process will effectively amplify the photocurrent by a significant factor, but 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.
Avalanche Photodiodes have particular properties which are discussed below.
The responsibility of an APD is strongly increased by the current amplification process. However, it is important to note that the amplification factor and thus the responsibility depends strongly on the reverse voltage, and also tends to differ from device to device. It is, therefore, common to specify a certain voltage range within which all devices reach a certain responsibility. Avalanche diodes are hardly suitable for precise measurements of low light powers since their responsibility is not nearly as well defined as that of a p–i–n diode.
Materials & Wavelength Ranges
Silicon-based avalanche photodiodes tend to be sensitive in the wavelength region from something close to 450 to 1000 nm and can exceed to 1100 nm at times 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 you apply as well as the type of device, the multiplication factor of silicon avalanche photodiodes, which in some circles is known as gain, can vary from between fifty and 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 the fact that it has a high responsivity, an APD doesn’t necessarily have a high efficiency. It is even possibly 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.
The achievable detection bandwidth with avalanche diodes can be quite high, although there can be an inherent trade-off between bandwidth and the amplification factor. However, the enhanced responsiveness can allow the operation with a smaller shunt resistor than usable with an ordinary photodiode, which may compensate for a possible speed disadvantage of an avalanche diode.