Light signal detector – APD
An avalanche photodiode uses the avalanche process to provide additional performance and sensitivity. Once the photodiode absorbs photons, the collision of electrons within the semiconductor breaks down the solid structure of atoms freeing up more electrons. This leads to a sudden rapid increase in the electronic current. Furthermore, a high reverse bias condition translates into a greater level of sensitivity for APDs. Charge carriers are pulled away from one another by the high electric field once a photon enters the depletion region and creates a hole electron pair. The process results in high velocity within the departing charge carriers which eventually collide with the atomic lattice creating further hole electron pairs. The process then repeats itself. Meanwhile, a photomultiplier tube, PMT, converts photons into electrons using a photocathode through the photoelectric effect. A photocathode is a negatively charged electrode coated with a photosensitive compound (Polyakov, 2013). When struck by a photon, the absorbed energy causes electron emission.
An avalanche photodiode has a higher level of sensitivity when compared to a photomultiplier tube due to avalanche gain. This allows it to substitute PMTs in low light detection applications. APDs also have significantly higher quantum efficiency and a large measurement range from 300mm to the NIR range. More so, the deep depletion range of the APD eliminates performance degradation witnessed in PMTs. However, avalanche photodiodes produce high levels of noise than the PMT, which delivers low noise and low jitter detection. The PMT also supersedes the APD in terms of linear gain. The avalanche process creates an output that is not linear. In some instances, the APD requires a much higher operating voltage than the PMT. This makes it less convenient to use, especially when coupled up with lower reliability.
References
Polyakov, S. (2013). Photomultiplier Tubes – an overview | ScienceDirect Topics. Retrieved 31 January 2020, from https://www.sciencedirect.com/topics/physics-and-astronomy/photomultiplier-tubes
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