Breakdown

Zener breakdown:

A strong reverse-bias applied to a p-n junction leads to high reverse current. When the increasing reverse bias attains a certain value, the junction breaks down and reverse current rises sharply. This specific value of the reverse bias voltage is called breakdown voltage (vz). The breakdown voltage depends upon the width of depletion layer, which in turn depends upon the doping concentration.

Zener Breakdown: Zener breakdown occurs in heavily doped p-n junction diodes which
have strong electric field across a thin depletion region. This field breaks the covalent bonds and generates electrons which are pulled into the conduction band and become available for conduction. Minority carrier thus generated quantum mechanically tunnel through the thin depletion layer. As the number of such free electrons becomes large, the reverse current through the Zener diode is becomes large. This breakdown (Zener effect) often occurs at a voltage as small as 8 volts. A resistance is connected in series with the Zener diode to limit the current flowing through it and protect it from damage due to excessive heating. The temperature coefficient of the Zener mechanism is negative: as the temperature of the p–n junction rises, the breakdown voltage decreases.

Avalanche Breakdown: Avalanche breakdown occurs in lightly-doped p-n junctions where the depletion region is relatively thick. Under reverse bias, small number of accelerated minority carriers enter the depletion region where electric field is very high. These carriers get accelerated, collide with atoms and knock off electrons from their bonds. Additional e-h pairs thus created are swept into the depletion region, where they too are accelerated and the process repeats itself. This impact ionization of e-h pairs causes Avalanche breakdown. It is similar to an avalanche where a small disturbance causes a whole mountainside of snow to come crashing down. The temperature coefficient of the avalanche mechanism is positive: as the temperature rises, the reverse breakdown voltage also rises.

Reverse bias is necessarily needed for Zener as well as Avalanche breakdown to occur. A typical Zener diode has either one or both of these breakdown mechanisms operating together. At low doping levels and higher voltages the avalanche mechanism dominates while at heavy doping levels and lower voltages the Zener mechanism dominates.