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.
n
T
p
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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.
T
n
p
T T
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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.
T
T
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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.
