© copyright M.J.Morris 2003
It is important to point out that when we are describing the conduction properties of materials we are considering fairly normal operating conditions and we are not talking about situations involving extreme voltages. Air for instance is an excellent insulator, however in thunderstorms voltages in the order of a hundred million volts can force a current through the air in the form of a lightning bolt. It would not take such an extreme voltage to break down a small piece of silicon and force it to conduct electricity. There are two stages that occur as a material begins to breakdown due a large applied voltage. These are zener breakdown and avalanche breakdown.
In Zener breakdown the electrostatic attraction between the negative electrons and a large positive voltage is so great that it pulls electrons out of their covalent bonds and away from their parent atoms. ie Electrons are transferred from the valence to the conduction band. In this situation the current can still be limited by the limited number of free electrons produced by the applied voltage so it is possible to cause Zener breakdown without damaging the semiconductor.
Avalanche breakdown occurs when the applied voltage is so large that electrons that are pulled from their covalent bonds are accelerated to great velocities. These electrons collide with the silicon atoms and knock off more electrons. These electrons are then also accelerated and subsequently collide with other atoms. Each collision produces more electrons which leads to more collisions etc. The current in the semiconductor rapidly increases and the material can quickly be destroyed.