CIE A Level Physics (9702) 2019-2021

Revision Notes

27.3.5 Semiconductors

Band Theory: The Effect of Temperature on Conductors & Semiconductors

  • The conductivity of conductors and semiconductors is highly dependent on temperature
    • For semiconductors: resistance decreases and conductivity increases with temperature
    • For metals: resistance increases and conductivity reduces with temperature
  • At absolute zero, the kinetic energy of all particles is zero, but conductivity is different for conductors and semiconductors:
    • In semiconductors: all electrons will be held tightly by atoms and none will be available to move into the conduction band, hence semiconductors behave as insulators at low temperatures
    • In metals: there will still be electrons in the conduction band, so, as long as there is a potential difference they will still be able to move and conduct

 

  • For intrinsic semiconductors:
    • An increase in thermal energy will excite more electrons from the valence band into the conduction band
    • This increases the conductivity of the semiconductor, hence semiconductors behave as conductors at high temperatures
  • For conductors:
    • In conductors, the conduction band is already full with electrons
    • Due to the overlapping valence and conduction bands, they already conduct close to maximum conductivity
    • Since there are already enough electrons in the conduction band, an increase in temperature only leads to increased lattice vibrations
    • Therefore, the collisions of conduction electrons with positive ions increases, which leads to higher resistance in metals

Band Theory: Light Intensity & LDRs

  • A light-dependent resistor (LDR) is a type of intrinsic semiconductor
    • Its resistance decreases as temperature increases
  • This is because when photons fall on the LDR, electrons absorb their energy and move from the valence band to the conduction band
    • The electrons in the conduction band can now form a current
  • Increasing the light intensity gives rise to more photons per unit time and hence increases the number of electrons promoted to the conduction band
  • Therefore, the resistance of an LDR will decrease as the intensity of light increases

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