BREAKDOWN MECHANISMS IN ZENER DIODE

Avalanche breakdown:

                       The avalanche breakdown takes place when both sides of the junction are lightly doped and consequently the depletion region is large. When the reverse bias voltage is increased the accelerated free electrons collide with the semiconductor atoms in the depletion region. Due to the collision the covalent bonds are broken and electron - hole pairs are generated. These new charge carriers so produced acquire energy from applied potential and in turn produce additional carriers. This forms a cumulative process called as avalanche multiplication. This avalanche multiplication causes the reverse current to increase rapidly. This leads to avalanche breakdown. 

Zener breakdown:

                        This Zener breakdown mechanism is fundamentally different from avalanche breakdown. Zener breakdown occurs when the electric field in the depletion region increases to point where it can break covalent bonds and generate electron hole pair. The electrons generated in this way will be swept by the electric field in to N sides and holds in to P sides. Thus these electrons and holes constitute reverse current across the junction that helps to supports the external current. In terms of an energy band structure in this breakdown process an electron makes a transition from the valance band to the conduction band without the interaction of any other particle. Once the Zener effect starts a large number of carriers can be generated with a negligible increase in the junction voltage. In fact the zener process is quantum tunneling. However later on the breakdown in junction which may result from avalanche multiplication or tunneling was in general called Zener breakdown.

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