When a heavily doped junction is reverse biased, the energy bands become crossed at relatively low voltages (i.e., the n-side conduction band is lowered in energy than the p-side valence band). The crossing of the bands aligns the large number of empty states in the n-side conduction band opposite the many filled states of the p-side valence band. As the barrier separating these two bands is narrow, tunneling of electrons can occur. Tunneling of electrons from the p-side valence band to the n-side conduction band constitutes a reverse current from n to p; the is the Zener breakdown .
The basic requirement for tunneling current are a large number of electrons separated from a large number of empty states by a narrow barrier of finite height. Since the tunneling probability depends upon the width of the barrier (d in fig.(b)), it is important that the metallurgical junction be sharp and the doping high , so that the transition region W extends only a very short distance from each side of the junction.
For lightly doped junctions, electron tunnelling is negligible. If the electric field E in the transition region is large, an electron entering from the p side may be accelerated to high enough kinetic energy to cause an ionizing collision with the lattice. A single such interaction results in carrier multiplication; the original electron and the generated electron are both swept to the n side of the junction, and the generated hole is swept to the p side. The degree of multiplication can become very high if carriers generated within the transition region also have ionizing collisions with the lattice and create an EHP (Electron Hole Pair); each of these carriers has a chance of creating a new EHP, and each of those can also care an EHP, and so forth. This is an avalanche process, since each incoming carrier can initiate the creation of a large number of new carriers.