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Semiconductor Optical Detectors
Stephen Schultz
Fiber Optics
Fall 2005
1
Semiconductor Optical Detectors
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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
Stephen Schultz
Fiber Optics
Fall 2005
2
p-n Diode
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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
Stephen Schultz
Fiber Optics
Fall 2005
3
p-n Diode
Stephen Schultz
Fiber Optics
Fall 2005
4
Semiconductor pin Detector
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Intrinsic layer is introduced
– Increase the space charge region
– Minimize the diffusion current
Stephen Schultz
Fiber Optics
Fall 2005
5
I-V Characteristic of Reversed Biased pin
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Photocurrent increases with incident optical power
Dark current, Id: current with no incident optical power
Stephen Schultz
Fiber Optics
Fall 2005
6
Light Absorption
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Dominant interaction
– Photon absorbed
– Electron is excited to CB
– Hole left in the VB
Depends on the energy band gap
(similar to lasers)
Absorption (a) requires the photon
energy to be smaller than the
material band gap
hc


 Eg
hc
1.24
 m)

Eg Eg eV )
Stephen Schultz
Fiber Optics
Fall 2005
7
Quantum Efficiency
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Probability that photon generates an electron-hole pair
Absorption requires
– Photon gets into the depletion region
– Be absorbed
Reflection off of the surface
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Photon absorbed before it gets to the depletion region
  1  R)
  e a l
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Photon gets absorbed in the depletion region
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Fraction of incident photons that are absorbed
  1  ea d )
  1  R ) ea l 1  ea d )
Stephen Schultz
Fiber Optics
Fall 2005
8
Detector Responsivity
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Each absorbed photon generates an electron hole pair
Iph = (Number of absorbed photons) * (charge of electron)
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Rate of incident photons depends on
– Incident optical power Pinc
– Energy of the photon Ephoton= hf
Generated current
q
I ph   Pinc
hf
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Detector responsivity
– Current generated per unit optical power
q

 

AW
hf
1.24
 in units of m
Stephen Schultz
Fiber Optics
Fall 2005
9
Responsivity
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Depends on quantum efficiency , and photon energy
q

 

AW
hf
1.24
Stephen Schultz
Fiber Optics
Fall 2005
10
Minimum Detectable Power
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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
Stephen Schultz
Fiber Optics
Fall 2005
11