Q: Zener and avalanche breakdown.
Ans : Breakdown due to breaking of covalent bond of depletion layer termed as zener breakdown (After the discovery, C. zener) and such a diode is zener diode.
Zener diodes with different breakdown voltages (for regulations of different voltages) can obtained by changing the doping concentration of its p-and n-sides.
There is another variant of zener like breakdown if there is low doping concentration as compare than zener diode. Such diodes will have relatively wider junction widths. At very high reverses bias, already existing electrons and holes are accelerated in the junction field and may undergo many collisions (like nuclear chain reaction) with the atom in the crystal.
These new electron-hole pairs created atoms by impact ionization also get accelerated in the junction field and collide further with the crystal atoms giving an increasing number of new electrons and holes. These bias beyond a certain critical value. This phenomenon is known as Avalanche breakdown and the device is referred to as Avalanche diode.
Zener breakdown occur in heavily doped semiconductor where depletion layer is thin avalanche breakdown occurs in lightly doped semiconductor.