Breakdown Mechanisms in Zener Diode

BREAKDOWN MECHANISMS IN ZENER DIODE

There are two types of mechanisms due to which breakdown occurs. They are

i. Zener breakdown

ii. Avalanche breakdown

Avalanche Breakdown

When the reverse bias voltage is increased, the field across the junction also increases. Thermally generated carriers crossing the junction acquire a large amount of kinetic energy from this field.

The velocity of these charge carriers increases and these electrons disrupt covalent bond by colliding with immobile ions and create new electron-hole pairs.

These new charge carriers again acquire sufficient energy from the field and collide with other immobile ions and generate further electron-hole pairs. This process continues and results in generation of avalanche of charge carriers within a short time. This mechanism of carrier generation is called as Avalanche multiplication.

This process results in flow of large amount of current at the reverse bias. Zener diodes having Zener Voltage above 6V exhibit avalanche breakdown and the diode has positive temperature coefficient i.e., the breakdown voltage increases when the p-n junction temperature is increased. Thus Zener diode has positive temperature coefficient.

Zener Breakdown

When P and N regions are heavily doped, strong electric field is created which results in direct rupture of covalent bonds. Hence new electron-hole pairs are generated and the reverse current begins to increase.

This process occurs at a reverse bias below 6V. Due to heavy doping, the depletion width becomes very small and the electric field across the depletion region becomes very high of the order of 10⁷ V/m. Thus Zener breakdown occurs.

For lightly doped diodes, the breakdown voltage is high and it is mainly due to Avalanche multiplication. Thus in Zener diode, breakdown occurs both in lower and higher breakdown voltages ie both Avalanche and Zener breakdown occurs. Such diodes are called Zener diodes.

Tepemrature Coefficient of Zener Diode

The percentage change in the Zener voltage V, for every °C rise/fall in temperature is called Temperature Coefficient (TC) of a Zener diode.

If V_z  is less than 6 V, the temperature coefficient is negative. For Zener diode with V_z greater than 6 V, the temperature coefficient is positive.