Electronic Devices Review I

1
Electronic Devices Review I

• Optical Absorption
• Photoluminescence, photoconductivity
• EltEg, transmitted, appears transparent
• EgtEg, absorbed, appears opaque
• Photoluminescence photon absorbed
• Fluorescence (direct recombo fast)
• Phosphorescence (defect level (trap) - slow)
• Cathodoluminescence high E e- absorbed
• Electroluminescence Introduction of current
• LED current causes injection of minority
  carriers into regions of crystal where they
  recombine w/ majority carrier.

2
Electronic Devices Review I

• Carrier Lifetime Photoconductivity
• Photoconductivity Increase in conductivity due
  to excess EHP arising from optical luminescence
• Indirect Recombo Trapping
• Recombo level
• Hole (space) capture from valence band
• Electron capture from conduction bandReady to
  capture hole (space) again!
• Temporary trapping trapped _at_ center, excited
  before recombo
• Whats the difference between a trap and a
  recombination center? Hint probability

3
Electronic Devices Review I

• Steady State Carrier Generation Quasi-Fermi
  Levels
• Semiconductor _at_ equilibrium experiences thermal
  generation of EHPs. Why?
• Steady light shone on sample Optical generation
• Fermi levels equilibrium doping
• Quasi-fermi levels steady state optical
  generation
• Fn Fp ? Indication of deviation from
  equilibrium
• Application Photoconductive Devices

4
Electronic Devices Review I

• Diffusion of Carriers
• Diffusion
• Total current density
• Einstein relation
• Physical meaning of Voltage E-field
  relationship
• Mathematical meaning of equilibrium

5
Electronic Devices Review I

• Diffusion Recombo The Continuity Equation
• Continuity equation (4-31) ? J ? Diffusion
  equation (4-33)
• Steady state carrier injection Diffusion Length
• Meaning of steady state. Hint What happens to
  current?Solution to Diffusion equation4-36

6
Electronic Devices Review I

• Haynes-Shockley Experiment
• Independent measurement of D and m (minority
  carrier)
• E-field established on n-type bar (e.g.)
• Pulse of holes created in bar
• Pulse
• Drifts
• Spreads by diffusion
• Excess hole conc. monitored at some point down
  bar
• x/t gives measure of mobility
• Spreading of pulse measures diffusion coeff.

7
Electronic Devices Review I

• Gradients in Quasi-Fermi Levels
• Drift, diffusion, or combo of two ?
• currents ?? gradients of 2 quasi-fermi levels