Question

in zener and avalanche breakdown diode current flow is due to

Answer 1

As mentioned, the Avalanche breakdown happens, due to the, well, the avalanche effect. If the reverse bias voltage is very high, then the depletion region widens, and the electric field is quite strong.
The minority charge carriers get accelerated in this depletion region, and gain kinetic energy. Once the field is strong enough, they can knock off, other electrons from the valency band (creating a hole and another conduction electron). The beauty is, one energetic electron (or hole) produces two MORE charge carriers.
So if you have 1 energetic electron to begin with, it would get converted into 2 electrons and one hole (so 1 -> 3). So the increase is exponential, just like an Avalanche.
The problem is, due to this 'impact' (this process is called Impact Ionization by the way), heat is generated, and this heat could destroy the diode.

Zener breakdown, as mentioned, only happens, when the doping concentration is extremely high (unfortunately, I haven't found exactly how high it must be, so if anyone

Answer 2

Explanation:

The main difference between Zener breakdown and avalanche breakdown is their mechanism of occurrence. Zener breakdown occurs because of the high electric field whereas, the avalanche breakdown occurs because of the collision of free electrons with atoms. Both these breakdowns can occur simultaneously.