Energy Bands and Charge Carriers in Semiconductors

In this post we will see why some materials are good conductors and why some are bad conductors.

Electrons are restricted to sets of discrete energy levels within atoms and between those levels large gaps exist. It is nearly the same for electrons in a solid. In the solid, the electrons are restricted to certain energies within a band (not an exact value as in the atom). This effect is caused by the wave function, since the electron is influenced by the neighbouring atoms. Summarized, the influence of neighbouring atoms on the energy levels of a particular atom can be treated as a small perturbation, giving rise to shifting and splitting of energy states into energy bands.


There are three types of bondings.

  • ionic bonding (example NaCl each Na atom is surrounded by six Cl atoms and vice versa ⇒ Na gives up its outer electron and Cl takes it ⇒ Na+ ion exerts an electrostatic attractive force upon its six Cl- neighbours) All electrons are tightly bound to atoms and all outer orbits of the atoms are completely filled. ⇒ No loosely bound electrons ⇒ good isolator
  • Metallic bonding: electrons are in a sea/cloud around the metallic atoms, normally they have more than three electrons on the outer shell which contribute to the sea/cloud.
  • Covalent bonding: (occur in diamond lattice semiconductors) an electron does not belong to one atom but to two atoms/ the bond. Due to this, one could assume that semiconductors are not conductive, but the electron can be excited out of a covalent bond and become a free electron when the temperature is large enough or if it is optically excited.


  • I do not understand the graphic on page 87, but I also have to say that I do not know the Schrödinger equation.
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