SOLUTION

• At 0 K an intrinsic semiconductor behaves as an insulator.
• The crystal structure of Si (or Ge) is a tetrahedron with an atom at each vertex.
• Each Si atom in the crystal contributes four valence electrons so that the atom is tetravalent.
• The inert ionic core of the Si atom carries a positive charge of +4e.
• The binding forces between neighboring atoms result from the fact that each Si atom shares its four valence electrons, which are tightly bound to the nucleus, with the four neighboring atoms forming four covalent bonds.
• The covalent bond is a fairly strong bond, requiring a fair amount of energy to release an electron from it.
• At a very low temperature ( 0K), the intrinsic Si approaches the ideal structure, and the crystal behaves as an insulator since no free charge carriers are available for conduction to occur.

• The conduction band is totally unoccupied and the valence band has completely filled.
• The absence of electrons in the conduction band does not allow current to flow under the influence of an electric field. Therefore, they are insulators at low temperatures.