Band theory of Semiconductors

Semiconductors: There’s a short band gap present in between conduction band the valence band. Therefore, when the trigger or incitation process is done, then electron’s small fraction with sufficient energy jumps.

Nevertheless, the conductivity of these substances can be increased by accelerating the doping or the temperature.

Below is the table of conduction band, energy gap of insulator and semiconductor of different elements:

ELEMENT ∆E(kJ/mol) of the energy gap # of electrons/cm^3 in the conduction band insulator, or conductor?
C (diamond) 524 (big band gap) 10-27 insulator
Si 117 (smaller band gap, but not a full conductor) 109 semiconductor
Ge 66 (smaller band gap, but still not a full conductor) 1013 semiconductor

In semiconductor’s case, the Valence band (V.B) and the vacant conduction band (C or C.B) or the (unoccupied band) and consist of the energy gap (Eg) of nearly 1 eV as shown in the picture.

The energy band which divides the 2 bands V i.e. valence band and C i.e. conduction band is known as the Forbidden band (F).

Very common eg of semiconductors are – (Si) Silicon (14) and (Ge) Germanium (32) with energy gaps or energy of approximately 1.12 eV and 0.75 eV in respective manners.

Energy bands in semiconductors

Description of Semiconductors on the Basis of Band Theory:

  • At absolute zero temperature, in semiconductors, you’ll witness a little forbidden gap between valence band the conduction. All the electrons present in the V.B remains bound and there are no free electrons available in conduction band.
  • When you talk of semiconductors, the forbidden energy gap in between conduction band and valence band is not more than 3 eV. As soon as temperature rises, number of electrons acquires external energy to intersect the forbidden gap between valence band and conduction band. At this time, these electrons will transfer into conduction band.
  • Simultaneously, electrons create energy levels which are vacant in V.B, where different valence electrons can be in motion. Therefore, this process gives rise to conduction possibility because of electrons present in the C.B along with the vacant space present in V.B.

Hence, at the above-mentioned absolute zero temperature, semiconductors act like conductors.

Characteristics of Semiconductors:

  1. Temperature rises and hence the conductance too
  2. The energy gap is very small and the V.B and the C.B are very close in semiconductors
  3. Valence band’s electrons can be easily incited to C.B


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