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.
The mechanism of avalanche breakdown occurs because of the reverse saturation current. The P-type and N-type material together forms the PN-junction. The depletion region develops at the junction where the P and N-type material contact.
The P and N-type materials of the PN junction are not perfect, and they have some impurities in it, i.e., the p-type material has some electrons, and the N-type material has some hole in it. The width of the depletion region varies. Their width depends on the bias applied to the terminal of P and N region.
The reverse bias increases the electrical field across the depletion region. When the high electric field exists across the depletion, the velocity of minority charge carrier crossing the depletion region increases. These carriers collide with the atoms of the crystal. Because of the violent collision, the charge carrier takes out the electrons from the atom.
The collision increases the electron-hole pair. As the electron-hole induces in the high electric field, they are quickly separated and collide with the other atoms of the crystals. The process is continuous, and the electric field becomes so much higher than the reverse current starts flowing in the PN junction. The process is known as the Avalanche breakdown. After the breakdown, the junction cannot regain its original position because the diode is completely burnt off.
The PN junction is formed by the combination of the p-type and the n-type semiconductor material. The combination of the P-type and N-type regions creates the depletion region. The width of the depletion region depends on the doping of the P and N-type semiconductor material. If the material is heavily doped, the width of the depletion region becomes very thin.
The phenomenon of the Zener breakdown occurs in the very thin depletion region. The thin depletion region has more numbers of free electrons. The reverse bias applies across the PN junction develops the electric field intensity across the depletion region. The strength of the electric field intensity becomes very high.
The electric field intensity increases the kinetic energy of the free charge carriers. Thereby the carrier starts jumping from one region to another. These energetic charge carriers collide with the atoms of the p-type and n-type material and produce the electron-hole pairs.
The reverse current starts flowing in the junction because of which depletion region becomes entirely vanishes. This process is known as the Zener breakdown.
In Zener breakdown, the junction is not completely damaged. The depletion region regians it original position after the removal of the reverse voltage
Equivalent Circuit of an Ideal Zener Diode and Actual Zener Diode
Ideal Zener Diode
The breakdown region of the ideal Zener diode in the VI curve is considered to be vertical. The graph shows that the voltage remains constant even after the variation of the current. Thus, the resistance of the Zener diode is neglected. The Zener diode in the breakdown region ideally behaves like a battery.
The equivalent circuit diagram is shown below.
Actual Zener Diode
The breakdown region graph of the actual Zener diode is not accurately vertical. The graph shows that the Zener diode has some resistance. The figure below shows that the Zener diode has some resistance Rz connected in series with the battery of voltage Vz.