Zener Diode: 

The diodes designed to work in the breakdown region are called Zener diodes. If the reverse voltage exceeds the breakdown voltage, the Zener diode will normally not be destroyed as long as the current does not exceed the maximum value and the device closes not overload.

When a thermally generated carrier (part of the reverse saturation current) falls down the junction and acquires energy of the applied potential, the carrier collides with crystal ions and imparts sufficient energy to disrupt a covalent bond. In addition to the original carrier, a new electron-hole pair is generated. This pair may pick up sufficient energy from the applied field to collide with another crystal ion and create still another electron-hole pair. This action continues and thereby disrupts the covalent bonds. The process is referred to as impact ionization, avalanche multiplication, or avalanche breakdown.

There is a second mechanism that disrupts the covalent bonds. The use of a sufficiently strong electric field at the junction can cause a direct rupture of the bond. If the electric field exerts a strong force on a bound electron, the electron can be torn from the covalent bond thus causing the number of electron-hole pair combinations to multiply. This mechanism is called high field emission or Zener breakdown. The value of the reverse voltage at which this occurs is controlled by the amount of doping of the diode. A heavily doped diode has a low Zener breakdown voltage, while a lightly doped diode has a high Zener breakdown voltage.

 

At voltages above approximately 8V, the predominant mechanism is the avalanche breakdown. Since the Zener effect (avalanche) occurs at a predictable point, the diode can be used as a voltage reference. The reverse voltage at which the avalanche occurs is called the breakdown or Zener voltage.

A typical Zener diode characteristic. The circuit symbol for the Zener diode is different from that of a regular diode and is illustrated in the figure. The maximum reverse current, I_Z(max), which the Zener diode can withstand is dependent on the design and construction of the diode. A design guideline that the minimum Zener current, where the characteristic curve remains at V_Z (near the knee of the curve), is 0.1/ I_Z(max).

Fig. 1 - Zener diode characteristic

The power handling capacity of these diodes is better. The power dissipation of a Zener diode equals the product of its voltage and current.

P_Z= V_Z I_Z

The amount of power that the Zener diode can withstand ( V_Z.I_Z(max) ) is a limiting factor in power supply design.

Fig. 1 - Zener diode characteristic

The power handling capacity of these diodes is better. The power dissipation of a Zener diode equals the product of its voltage and current.

P_Z= V_Z I_Z

The amount of power that the Zener diode can withstand ( V_Z.I_Z(max) ) is a limiting factor in power supply design.