Fig 1(a)
Zener diode is fabricated by heavily doping both p-, and n- sides of the junction.Due to this, depletion region formed is very thin (<1/1000000 m) and the electric field of the junction is extremely high(~5000000v/m) even for a small reverse bias voltage of about 5V.The I-V chracteristics of a zener diode is shown in Fig 1(b).
It is seen that when the applied reverse bias voltage(V) reaches the breakdown voltage of the zener diode, there is a large change in the current.In other words, the zener voltage remains constant, eventhough current through the zener diode varies over a wide range.This property of the zener diode is used for regulating supply voltages so that they are constant.
Let us understand how reverse current suddenly increases at the breakdown voltage.We know that reverse current is due to the flow of electrons(minority carriers) from p->n and holes from n->p. As the reverse bias voltage is increased,the electric field at the junction becomes significant.When the reverse bias voltage V=break down voltage of zener diode, then theelectric field strength is high enough to pull valence electrons from the host atoms on the p-side which are accelerated to n-side.These electrons account for high current observed at the breakdown.The emission of electrons from the host atoms due to the high electric field is known as internal field emission or field ionisation.The electric field required for field ionisation is of the order of 10^6 V/m.
