In a reverse-biased pn-junction, the depletion region is wider and the potential barrier is higher over equilibrium values. The majority charge carrier concentration in each region decreases from the equilibrium values and the diffusion current across the junction is zero. Only a very very small current flows due to the motion of minority charge carriers. Thus, the principal characteristic of a pn-junction diode is that it rectifies, i.e., it conducts significantly in one direction only.
When a sufficiently large reverse voltage is applied to a pn-junction, there is an abrupt strong increase in the reverse current and its rectifying properties are lost. This is known as junction breakdown. The absolute value V of the voltage at which the phenomenon occurs is called breakdown voltage. The breakdown process is not inherently destructive and is reversible.
Two important breakdown mechanisms are the Zener breakdown (due to tunneling effect) and avalanche breakdown (due to avalanche multipli-cation).
