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Phys 460 Lecture 19

( pdf version - 6 slides/page )
Wednesday, November 1, 2006
Lecturer: Richard Martin 		Homework 8

Reading: Kittel, Ch. 17, p 503-12; Added material in notes

Semiconductor Devices
Outline

  1. From previous lectures:
    • Part I: Crystal Structures, Diffraction, Reciprocal Lattice, Crystal binding
      Phonons, Dispersion curves,Thermal properties
    • Free electron gas
    • Energy bands for electrons in crystals:
      • Bloch theorem, Bands Ekn, gaps, metals vs. insulators
    • Semiconductors
      • Effective mass, negative electrons and positive holes
      • Law of mass action: np = "constant"
      • Doping by foreign atoms Binding of electrons or hole to impurity
      • Thermoelectric effects, Hall effect, Cyclotron resonance
  2. What is a semiconductor device?
    • Inhomogeneous material or doping
    • Control of currents or light
  3. Variations in band energies and Fermi energy
    • Band energies vary by - eV(r) due to electrostatic voltage V(r)
    • Fermi energy mu is the same everywhere in equilibrium
    • Concentrations: n = N0 exp( - (Ec - eV(r) - mu)/kB T), etc.
    • We can also define "Electrochemical potential" muelec = mu +eV
    • Then n = N0 exp( - (Ec - muelec)/kB T), etc.
    • Either is correct. We will use the constant mu
  4. p-n junction
    • Depletion region
    • Charges provide fields that shift bands
    • Rectification: (Diode behavior)
      Forward bias - exponential increase in current with V
      Reverse bias - saturation at small value
  5. Light emitting diodes - LEDs
    • p-n junction with forward bias
    • Electron hole recombination gives off light in many systems, e.g., GaAs
  6. Solar Cell
    • Reverse effect
    • Light creates electron-hole pairs that separate and give useful current
  7. Bipolar Transistor n-p-n p-n-p
    • Forward and Reverse biased junctions in series
    • Carriers emitted into into narrow base with forward biased junction
    • Actually are "swept" into reverse bias junction
    • Current amplification - small current controls large one
Email clarification questions and corrections to rmartin@uiuc.edu
Email questions on solving problems to xin2@.uiuc.edu