If the reverse-bias voltage across a p-n junction diode is increased, at a particular voltage the reverse current suddenly increases to a large value. This phenomenon is called breakdown of the diode and the voltage at which it occurs is called the breakdown voltage. At this voltage, the rate of creation of hole-electron pairs is increased leading to the increased current.
There are two main processes by which breakdown may occur. The holes in the n-side and the conduction electrons in the p-side are accelerated due to the reverse-bias voltage. If these minority carriers acquire sufficient kinetic energy from the electric field and collide with a valence electron, the bond will be broken and the valence electron will be taken to the conduction band. Thus a hole-electron pair will be created. Breakdown occurring in this manner is called avalanche breakdown. Breakdown may also be produced by direct breaking of valence bonds due to high electric field. When breakdown occurs in this manner it is called zener breakdown.
A diode meant to operate in the breakdown region is called an avalanche diode or a zener diode depending on the mechanism of breakdown. Once the breakdown occurs, the potential difference across the diode does not increase even if the applied battery potential is increased. Such diodes are used to obtain constant voltage output. Figure (45.16) shows the i-V characteristic of a zener diode including the breakdown region and a typical circuit which gives constant voltage Vo across the load resistance RL. Even if there is a small change in the input voltage Vi, the current through R remains almost the same. The current through the diode changes but the voltage across it remains essentially the same. Note the symbol used for the zener diode.
