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Attributes: high speed and internal gainGood for communications
A thin side layer is exposed through a window to achieveillumination.3 p type layers follow this and terminate at the electrode.These p-type layers have different doping levels in order tomodify the field distribution across the diode.
1st p-type region is a thin layer2nd p-type region is a thick, lightly dope layer. (almostintrinsic)3rd p-type region is heavily doped layer.
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The diode operates in the reverse bias mode inorder to increase the field in the depletionregions.
Applying an adequate R.B. will force thedepletion region in the p-layer to reach-throughto layer.The field ultimately extends from -sidedepletion layer to the - side depletion layer.Absorption of photons and thereforephotogeneration takes place in the long layer.It is a uniform field in the layer due to thesmall net space charge density.
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Factors determining speed:1. Time it takes for photogenerated electron to
cross the absorption region ( layer) to the
multiplication layer (p layer).2. Time it takes for the avalanche process to
build-up in the p-region and generate EHPs.3. Time it takes for the last hole released in the
avalanche process to vacate the region.
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These EHPs can be accelerated by high fields tohigh kinetic energies to cause further impactionization releasing even more EHPs leading toan avalanche of impact-ionization process.
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APDs have a advantage over photodiodewithout internal gain for the detection of thevery low lights often encountered in optical
fiber communication.APDs generally provide an increase insensitivity of b/w 5 to 15 dB over pinphotodiode while often giving a wider
dynamic range as a result of their gainvariation with response time and reversebias.
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Gerd Keiser, Optical Fiber Communications:Fourth edition, Tata McGraw-Hill,2009,Page No. 222-231.
Internet.
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