In a reverse-biased pn-junction, the depletion region is wider and the potential barrier is higher over equilibrium values. The electric field in the depletion region is from the n- to the p-region. When a sufficiently large reverse voltage is applied to a pn-junction, the junction breaks down and conducts a very large current. Of the two important breakdown mechanisms, Zener breakdown takes place in heavily doped diodes.
Usually, the energy that an electron can gain from even a strong field is very small. However, the depletion region is very narrow in a heavily doped diode. Because of this, the electric field across the depletion region is intense enough to break the covalent bonds between neighbouring silicon atoms and pull electrons out of their orbits. This results in conduction electrons and holes. In the energy band diagram representation, this corresponds to the transition of an electron from the valence band to the conduction band and become available for conduction.
The current increases with increase in applied voltage, but without further increase in voltage across the diode. This process, in which an electron of energy less than the barrier height penetrates through the energy bandgap, is called tunneling (a quantum mechanical effect). The creation of electrons in the conduction band and holes in the valence band by tunneling effect in a reverse- biased pn-junction diode is called the Zener effect.
[Notes : (1) Tunneling occurs only if the electric field is very high. The typical field for silicon and gallium arsenide is > 106 V / cm. To achieve such a high field, the doping concentrations for both p- and w-regions must be quite high (>1018 cm-3). Zener breakdown or Zener effect is named in honour of Clarence M. Zener (1905-93), US physicist, who explained the breakdown mechanism. (3) Avalanche breakdown occurs in diodes with a doping concentration of ≅ 1017 cm-3 or less. The carriers gain enough kinetic energy to generate electron-hole pairs by the avalanche process when the value of reverse | V | becomes large. An electron in the conduction band can gain kinetic energy before it collides with a valence electron. The high-energy electron in the conduction band can transfer some of its kinetic energy to the valence electron to make an upward transition to the conduction band. An electron-hole pair is generated. All such electrons and holes accelerate in the high field of the depletion region and, in turn, generate other electronhole pairs in a like manner. This process is called the avalanche process.]
