Avalanche Photodiode APD

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Avalanche Photodiode (APD)

• Attributes high speed and internal gain
• Good for communications
• A thin side layer is exposed through a
  window to achieve illumination.
• 3 p type layers follow this and terminate at the
  electrode.
• These p-type layers have different doping levels
  in order to modify the field distribution across
  the diode.
• 1st p-type region is a thin layer
• 2nd p-type region is a thick, lightly dope
  layer. (almost intrinsic)
• 3rd p-type region is heavily doped layer.

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APD

• The diode operates in the reverse bias mode in
  order to increase the field in the depletion
  regions.
• Applying an adequate R.B. will force the
  depletion region in the p-layer to reach-through
  to layer.
• The field ultimately extends from -side
  depletion layer to the - side depletion
  layer.
• Absorption of photons and therefore
  photogeneration takes place in the long
  layer.
• It is a uniform field in the layer due to the
  small net space charge density.

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APD

• The E-field is at a maximum at the - side
  and a minimum at the - side.
• Drifting electrons arriving at the p-layer
  experience elevated fields and acquire enough
  kinetic energy (greater than Eg) to impact-ionize
  some of the Si covalent bonds and release EHPs.
• These EHPs can be accelerated by high fields to
  high kinetic energies to cause further impact
  ionization releasing even more EHPs leading to an
  avalanche of impact-ionization process.

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APD

• In summary a single elctron entering the p-layer
  can generate a large number of EHPs which
  contribute to an observed photocurrent.
• APDs have an internal gain mechanism.
• This avalanche multiplication corresponds to a
  quantum efficiency greater than unity.
• Why is photogeneration restricted to the
  region ?
• Electrons as a carrier in Si have a higher impact
  ionization efficiency than holes (less excess
  noise in avalanche multiplied photocurrent).

5
APD

• Factors determining speed
• Time it takes for photogenerated electron to
  cross the absorption region ( layer) to the
  multiplication layer (p layer).
• Time it takes for the avalanche process to
  build-up in the p-region and generate EHPs.
• Time it takes for the last hole released in the
  avalanche process to vacate the region.

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Avalanche Multiplication Factor
7
APD

• Response time of APD is longer than PIN diode
  however it offers gain.
• APD does not require additional amplification
  which introduces delay to PIN diode.
• Drawback of APD The peripheral edge of the
  p junction reaches avalanche breakdown before
  the p window of illumination area.
• Uniform avalanche multiplication is required to
  promote avalanching of primary photocurrent
  versus dark current (generates random EHPs).
  Remedy guard ring.

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