The Avalanche Breakdown and Zener Breakdown are two different mechanisms by which a PN junction breaks. The Zener and Avalanche breakdown both occur in diode under reverse bias. The avalanche breakdown occurs because of the ionisation of electrons and hole pairs whereas the Zener diode occurs because of heavy doping. These are explained below in details.
For thicker junctions, the breakdown mechanism is by the process of Avalanche breakdown. In this mechanism, when the electric field existing in the depletion layer is sufficiently high, the velocity of the carriers (minority carriers) crossing the depletion layer increases. These carriers (electrons and holes) collide with the crystal atoms. Some collisions are so violent that the electrons are knocked off the crystal tom thus creating electron-hole pairs.
As the pair of electron hole is created in the midst of the high field, they quickly separate and attain high velocities to cause further pair generation through more collision. It is a cumulative process and it goes on. As the breakdown voltage is going to achieve the field becomes so large that the chain of collisions can give rise to an almost infinite current with a very slight additional increase in the voltage.
This process is known as Avalanche breakdown. Once the avalanche breakdown occurs the junction cannot come to its original position. Thus, the diode is said to be burnt off.
The Zener breakdown takes place in a very thin junction, i.e., the depletion layer is narrow when both the sides of the junction are very heavily doped. In the Zener breakdown mechanism, the electric field becomes as high as 107 V/m in the depletion layer with only a small applied reverse bias voltage.
In this process, it became possible for some of the electrons to jump across the barrier from the valence band in p material to some of the unfilled conduction band in n-material. This process is known as Zener Breakdown. In this process, the junction is not damaged. The junction regains its original position when the reverse voltage is removed. This process is used in the Zener diode.
However, if the number of electrons jumping across the barrier (i.e., the flow of current) increases beyond the rated capacity of the Zener diode, then Avalanche breakdown takes place which destroys the junction.
Thus, the Zener breakdown does not result in the destruction of the diode as long as the current through the diode is limited by the external circuit to a level within its power handling capabilities. Whereas, the Avalanche breakdown destroys the diode.
Equivalent Circuit of an Ideal Zener Diode and Actual Zener Diode
Ideal Zener Diode
For an ideal Zener diode in the VI graph, the breakdown region is considered to be vertical. This shows that the voltage is constant even if the current changes. Therefore, the Zener resistance is neglected. Thus, a Zener diode operating in the breakdown region ideally acts like a battery.
The equivalent circuit diagram is shown below.
Actual Zener Diode
For an actual Zener diode in the V-I graph, the breakdown region is not quite vertical. It shows that a Zener diode has a resistance. Hence, an actual Zener diode is considered as a resistance Rz connected in series with a battery of voltage Vz as shown in the figure below.