Intrinsic material, Electrical Engineering

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Intrinsic Material

  1. A perfect semiconductor crystal with no impurities or lattice defects.
  2. No carriers at 0 K, since the valence band is completely full and the conduction band is completely empty.
  3. For T > 0 K, electrons are thermally excited from the valence band to the conduction band (EHP generation).
  4. EHP generation takes place due to breaking of covalent bonds => required energy = Eg.
  5. The excited electron becomes free and leaves behind an empty state (hole).
  6. Since these carriers are created in pairs, the electron concentration (n/cm3) is always equal to the hole concentration (p/cm3), and each of these is commonly referred to as the intrinsic carrier concentration (ni).
  7. Thus, for intrinsic material n = p = ni.
  8. These carriers are not localized in the lattice; instead they spread out over several lattice spacings, and are given by quantum mechanical probability distributions.
  9. Note: ni = f (T).
  10. To maintain a steady-state carrier concentration, the carriers must also recombine at the same rate at which they are generated.
  11. Recombination occurs when an electron from the conduction band makes a transition (direct or indirect) to an empty state in the valence band, thus annihilating the pair.
  12. At equilibrium, ri =gi, where gi and ri are the generation and recombination rates respectively, and both of these are temperature dependent.
  13. gi(T) increases with temperature, and a new carrier concentration ni is established, such that the higher recombination rate ri(T) just balances generation.
  14. At any temperature, the rate of recombination is proportional to the equilibrium concentration of electrons and holes, and can be given by ri= αrn0p0 = αrni2=gi(5) where αris a constant of proportionality (depends on the mechanism by which recombination takes place).

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