Final answer:
In a pn junction diode, reverse bias breakdown can occur due to two primary mechanisms: Zener breakdown, which happens at low reverse voltages with high doping, and Avalanche breakdown, which occurs at higher voltages through a multiply ionization process.
Step-by-step explanation:
For a pn junction diode, there are two primary mechanisms that lead to reverse bias breakdown: Zener breakdown and Avalanche breakdown. The reverse bias configuration occurs when the positive side of the battery is connected to the n-type material of the diode, which widens the depletion layer and increases the potential energy difference across the p-n junction. Under normal reverse bias conditions, current flow is minimal.
Zener breakdown occurs at low reverse voltages and high doping levels. It is characterized by a sharp increase in current due to the strong electric field that pulls electrons from their valence bands and sends them across the barrier without needing any kinetic energy increase. This strong field also strips electrons away from the atom nucleus, a process known as field ionization.
Avalanche breakdown happens at higher reverse voltages and is caused by the ionization of atoms by the existing charge carriers. As these carriers accelerate in the electric field, they collide with lattice atoms, creating additional electron-hole pairs that, in turn, are accelerated. This chain reaction results in the multiplication of carriers, leading to a sudden surge in current known as the avalanche current.