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Semiconductor Optical Detectors

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Semiconductor Optical Detectors

• Inverse device with semiconductor lasers

– Source: convert electric current to optical power

– Detector: convert optical power to electrical current

• Use pin structures similar to lasers

• Electrical power is proportional to i2

– Electrical power is proportional to optical power squared

– Called square law device

• Important characteristics

– Modulation bandwidth (response speed)

– Optical conversion efficiency

– Noise

– Area

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p-n Diode

• p-n junction has a space charge region at the interface of the two material types

• This region is depleted of most carriers

• A photon generates an electron-hole pair in this region that moves rapidly at the drift velocity by the electric field

• An electron-hole pair generated outside the depletion region they move by diffusion at a much slower rate

• Junction is typically reversed biased to increase the width of the depletion region

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p-n Diode

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Semiconductor pin Detector

• Intrinsic layer is introduced

– Increase the space charge region

– Minimize the diffusion current

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I-V Characteristic of Reversed Biased pin

• Photocurrent increases with incident optical power

• Dark current, Id: current with no incident optical power

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Light Absorption

• Dominant interaction

– Photon absorbed

– Electron is excited to CB

– Hole left in the VB

• Depends on the energy band gap (similar to lasers)

• Absorption ( requires the photon energy to be smaller than the material band gap

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Quantum Efficiency

• Probability that photon generates an electron-hole pair

• Absorption requires

– Photon gets into the depletion region

– Be absorbed

• Reflection off of the surface

• Photon absorbed before it gets to the depletion region

• Photon gets absorbed in the depletion region

• Fraction of incident photons that are absorbed

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Detector Responsivity

• Each absorbed photon generates an electron hole pair

Iph = (Number of absorbed photons) * (charge of electron)

• Rate of incident photons depends on

– Incident optical power Pinc

– Energy of the photon Ephoton= hf

• Generated current

• Detector responsivity

– Current generated per unit optical power

in units of m

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Responsivity

• Depends on quantum efficiency , and photon energy

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Minimum Detectable Power

• Important detector Specifications

– Responsivity

– Noise Equivalent noise power in or noise equivalent power NEP

– Often grouped into minimum detectable power Pmin at a specific data rate

• Pmin scales with data rate

• Common InGaAs pin photodetector

– Pmin=-22 dBm @B=2.5 Gbps, BER=10-10

• Common InGaAs APD

– Pmin=-32 dBm @B=2.5 Gbps, BER=10-10

– Limited to around B=2.5 Gbps