ZENER BREAKDOWN and zener vs avalanche

               Zener breakdown occurs in specially developed heavily doped diode of N.Type and P.Type impurities. Zener diode specially designed to operate in the reverse bias region specially in the breakdown region  and is used in circuits of voltage regulators. The symbol of zener diode is shown below

                A Zener diode works as normal PN Junction diode in the forward bias, so it is generally used in reverse bias so in general the cathode is shown with the positive sign and anode is shown in negative. As the electrons move from the anode to cathode the current is measured from cathode to anode.

A zener diode skeleton is as shown

The Typical characteristics of a zener diode in both the forward and the reverse bias in comparison with the normal PN Junction diode is shown above. In the forward biasing condition there exists a cutin voltage after which the current starts passing through the diode and the concept of the forward biasing is same for the Zener diode and normal PN junction diode. Now consider the case of the reverse bias as the doping is very high and the depletion width is very small, there exists a very high electric field in the depletion region. 

                                                 Electric field is inversely proportional to distance

Now as we slowly increase the reverse bias region, initially there will not be any current and as we still increase the reverse bias voltage, at a particular voltage the electric field now present along with the applied field is very high so that the electric field removes the electrons from their orbits and there by creating the free charge carriers which results in a short circuit between the cathode and the anode, this condition is said to be the Zener breakdown. The zener breakdown occurs much before the avalache breakdown and as compared to the avalnche breakdown, here there does not exist the process of electrons hitting other atoms and dislodging the charge carriers, the device will be safe. As there are no collisions there will not be any heating effect on the device.

The voltage at which the breakdown occurs is called the zener breakdown. Now if we remove the biasing the zener diode will return to its normal state.

The zener diode has negative temperature coefficient as the temperature raises the electrons will already be available at some raised energy level and a small voltage is required for the breakdown whereas in the avalanche breakdown has a positive temperature coefficient as the temperature raises the avalanche breakdown voltage increases, as the temperature increases the collisions will happen at low kinetic energy and difficult to dislodge charge carriers at that energy so it requires more voltage for avalanche breakdown.