Photodetectors are devices that convert light into an electrical signal. There are many different types of photodetectors, each with its own advantages and disadvantages. The most common type of photodetector is the silicon photodiode. Silicon photodiodes are used in a wide variety of applications, including optical fiber communication, medical imaging, and security systems. Other types of photodetectors include avalanche photodiodes, photoconductive cells, and photoemitters.
Avalanche Photodiode (APD)
An avalanche photodiode (APD) is a type of semiconductor device that converts light into an electrical signal. The APD consists of two regions: a p-n junction and an avalanche region. When the p-n junction is exposed to light, electrons are excited from the valence band to the conduction band. These excited electrons can then be accelerated by an applied electric field and collide with other atoms in the Avalanche region, causing additional electrons to be excited into the conduction band. This process amplifies the current flowing through the device and results in a large output signal. APDs have very high gain; they can detect single photons with high efficiency. However, APDs also have very high noise levels due to the random nature of electron avalanches; this limits their use in applications where low noise levels are required such as medical imaging or astronomical observations.
Photoconductive Cell (PC)
A photoconductive cell (PC) is a type of semiconductor device that converts light into an electrical signal. The PC consists of two regions: a p-type region and an n-type region. When the p-n junction is exposed to light, photons are absorbed by electrons in the valence band and promote them into the conduction band. This increasesthe number of free carriersin the device and allows current to flow through it when connected to an external circuit . PCs have relatively low sensitivity compared to other types of photodetectors; however, they have very fast response times on the order of nanoseconds which makes them useful for applications such as laser ranging or time-of-flight measurements .
Photoemitter
A photoemitter is a type of semiconductor device that emits electrons when exposed to light . The photoemitter consists of two regions: a cathode made from an n-type material ,andan emissive layer made froma material with a lower work function thanthe cathode . Whenlight strikes theatomic emitter ,electronsare emittedfromthecathodeand travel throughthe external circuittoanodesomewhereelseinthedevice . Photoemittershaveverylowefficiencycomparedtophotovoltaicdevicesbuttheyhavefastsurface recombination velocitieswhich makesthemusefulforhighspeedapplicationssuchas optoelectronicswitches .