Photoconduction

1
Photoconduction

• Peter Bowyer
• School of Physics and Astronomy
• University of Southampton
• Highfield
• SO17 3JH

2
Aims of the experiment

• To examine signals from two semiconductor
  pn-junction photodiodes
• For different incident wavelengths of light
• To study the energy band-gap of the
  semiconductors
• Investigate the usage of a lock-in detector

3
PN Junctions
4
Photoconduction

• Photons with Energy gt EG incident on depletion
  layer
• Generate electron-hole pairs
• electrons drift towards n-type holes towards
  p-type
• Current flow proportional to rate of production
  of electron-hole pairs, hence proportional to
  light intensity

5
Laser Diodes

• Semiconductor based laser a pn junction
• Mass produced much cheaper than traditional
  lasers
• Found in CD/DVD drives, barcode scanners,
  hand-held laser pointers etc.
• Useful due to their size

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Lock-in detectors

• Used to extract signal from background noise
• Requires a trigger synchronised with the signal
  you want to extract
• Provided by a chopper to modulate the signal

7
The Experiments
..were not this impressive (www.electricstuff.co.u
k)
8
Experiment 1

• Band edge definitions
• Width difference between max. and min. photon
  energy
• Position half peak height

• Graphs
• Very sharp peak for GaAs/P diode
• Odd shape for Si diode red filter left in too
  long

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Experiment 1 Results

• Results
• Composition of GaAs/P photodiode found to be
  Ga5As3P2
• Si bandgap found at 1.24eV cf official value of
  1.17eV (6 error)
• Width of GaAs/P edge found to be 0.075eV (20
  error)

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Experiment 2

• Reference provided by fixed oscillator at 450Hz
• Frequency bandwidth order expected
• where is the time constant

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Experiment 2 Results
Dependence on frequency

• Accurate to within 60
• Errors
• Display on CRT wasnt stationery
• Detector saturated?
• But still of correct order

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Conclusion

• Experiment 1 produced results accurate to within
  20
• Experiment 2 gave disappointing experimental
  results
• However, introduced the equipment so served its
  purpose

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References

• http//www.puretechinc.com/industries/laser_tech.h
  tm
• http//www.bentham.co.uk/pdf/Lock-in20amplifier2
  0tutorial.pdf
• Second Year laboratory handbook