1.06 Avalanche Breakdown & Zener Breakdown.

It is important to point out that when we are describing the conduction properties of materials, we are considering fairly normal operating conditions and not situations involving extreme voltages. Air for instance is generally 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 to a large applied voltage. These are zener breakdown and avalanche breakdown.

Zener 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. i.e. Electrons are transferred from the valence to the conduction band. In this situation the current is still controlled by the limited number of free electrons produced and so it is possible to cause Zener breakdown without damaging the semiconductor.

Avalanche breakdown.

Avalanche breakdown occurs when the applied voltage is so large, that the electrons that are pulled from their covalent bonds, are accelerated to great velocities. These electrons then collide with the silicon atoms and knock off more electrons. The electrons liberated are then also accelerated and subsequently collide with other atoms. Each collision produces more electrons, which leads to more collisions. The net result is that the current in the semiconductor rapidly increases and the material can quickly be destroyed by the heat that this produces.

avalanche breakdown