Diode: Breakdown mechanism

Breakdown mechanism

The reverse current or the reverse saturation current due to the minority charge carriers is small. If the reverse bias applied to a p-n junction is increased beyond a point, the junction breaks down and the reverse current rises sharply. The voltage at which this happens is called the breakdown voltage and it depends on the width of the depletion region, which in turn depends on the doping level.

A normal p-n junction diode gets damaged at this point. Specially designed diodes like Zener diode can be operated at this region and can be used for the purpose of voltage regulation in circuits. There are two mechanisms that are responsible for breakdown under increasing reverse voltage.

Zener breakdown

Heavily doped p-n junctions have narrow depletion layers of the order of <10^-6 m. When a reverse voltage across this junction is increased to the breakdown limit, a very strong electric field of strength 3 × 10⁷ V m^–1 is set up across the narrow layer. This  electric field is strong enough to break or rupture the covalent bonds in the lattice and thereby generating electron-hole pairs. This effect is called Zener effect.

Even a small further increase in reverse voltage produces a large number of charge carriers. Hence the junction has very low resistance in the breakdown region. This process of emission of electrons due to the rupture of bands in from the lattice due to strong electric field is known as internal field emission or field ionization. The electric field required for this is of the order of 10⁶ V m^–1.

Avalanche breakdown

Avalanche breakdown occurs in lightly doped junctions which have wide depletion layers. Here, in this case, the electric field is not strong enough to produce breakdown. Alternatively, the thermally generated minority charge carriers accelerated by the electric field gains sufficient kinetic energy, collide with the semiconductor atoms while passing through the depletion region. This leads to the breaking of covalent bonds and in turn generates electron-hole pairs.

The newly generated charge carriers are also accelerated by the electric field resulting in more collisions and further production of charge carriers. This cumulative process leads to an avalanche of charge carriers across the junction and consequently reduces the reverse resistance. The diode current increases sharply.

For a reverse voltage of, (i) less than 4V → Zener effect predominates (ii) greater than 6V → Avalanche effect predominates (iii) between 4 and 6V → both effects are present.