Classification of Metals, Conductors, and Semi-conductors

• Metals − Possess very low resistivity (or high conductivity)

• Semi-conductors − Possess resistivity or conductivity intermediate to metals and insulators

• Insulators − Possess high resistivity (or low conductivity)

• Semi-conductors are of two types:

  • Elemental semi-conductor − Example: Si and Ge

  • Compound semi-conductor − Example: CdS, GaAs, CdSe, InP, etc.

• Energy band diagram of metals or conductors
  

  • Conduction band is partially filled and the valence band is partially empty or the conduction and valence balance overlap.

  • Due to overlap, electrons can easily move into the conduction band. This situation makes a large number of electrons available for electrical conduction.

  • When the valence band is partially empty, electrons from their lower levels can move to higher levels making conduction possible.

• Energy band diagram for insulators
  

  • Large band gap E_g exists. (E_g > 3eV)

  • Since there are no electrons in the conduction band, no electrical conduction is possible.

  • The electron cannot be excited from the valence band to the conduction band by thermal excitation.

• Energy band diagram for semi-conductors
  

  • Energy band gap Eg is small. (Eg < 3eV)

  • At room temperature, some electrons from valence band cross the energy gap and enter the conduction band.