The symbol of zener diode is shown above. When we studied about the diode characteristics, it was found that under reverse bias, there is a small amount of current due to the drifting of the minority charge carriers. But as the reverse potential is increased beyond a level, suddenly the current increase, which is called as 'breakdown' and the corresponding voltage is called as breakdown voltage. Here zener diode is a device which operates on the reverse bias breakdown voltage.
There are two ways in which the breakdown can occur. When a reverse bias is given it hinders diffusion but it aids drifting. So the minority carriers namely electrons in 'P' type and holes in n-type get accelerated and they attain very high kinetic energy. These highly energetic charge carriers collide with valence electrons breaking their bond creating hole - electron pair, This is called as 'avalanche breakdown'. The high electric field inside the semiconductor may also break the valence bonds this is called as 'zener breakdown'. A diode operating in this type of zener breakdown is called as a zener diode.
The zener diode can be used as a voltage regulator. Once the breakdown occurs, the potential difference across the diode does not increase even if the applied battery potential is increased. The (V-I) graph shows the characteristics of a zener diode. Even if the applied Voltage (Vi) fluctuates, the current through the load resistance 'RL' remains the same. The voltage across the diode practically remains the same.
