Question

Indicate if a pn junction can be obtained: a) by adding indium to germanium; (b) by adding aluminum to the vessel. Justify your answers.

Answer 1

A) Acceptor impurity atoms like indium have fewer valence electrons than the atoms they replace in the intrinsic semiconductor lattice. They accept electrons from the semiconductor's valence band. This provides excess holes to the intrinsic semiconductor. Excess holes increase the hole carrier concentration of the semiconductor creating a p-type semiconductor.

B) To make a p-type semiconductor extra materials like boron or aluminium are added to the silicon. These materials have only three electrons in their outer shell. When the extra material replaces some of the silicon it leaves a 'hole' where the fourth electron would have been if the semiconductor was pure silicon.