Why zener diode works in reverse bias

Thus the holes diffuse to the n-side and the electrons diffuse to the p-side. This results in the accumulation of charges around the junction, forming a depletion region. An electric polarity or electric dipole is formed across the junction, causing the flow of flux from n side top side. This results in varying negative electric field intensity, generating an electric potential across the junction.

This electric potential is actually the threshold voltage of the diode and is around 0. This acts as a potential barrier for the flow of majority charge carriers and the device does not conduct.

Now when a normal diode is biased such that a negative voltage is applied to the n side and positive voltage to the p side, the diode is said to be in forward biasing condition. This applied voltage tends to decrease the potential barrier after it goes beyond the threshold voltage.

At this point and afterward, the majority carriers cross the potential barrier and the device starts conducting with the flow of current through it. When the diode is biased in reverse condition to above, the applied voltage is such that it adds to the potential barrier and hinders the flow of majority carriers.

However, it does allow the flow of minority carriers holes in n-type and electrons in p-type. As this reverse bias voltage increases, the reverse current tends to increase gradually.

At a certain point, this voltage is such that it causes the breakdown of the depletion region, causing a massive increase in the flow of current. This is where the Zener diode working comes into play. As stated above the basic principle behind the working of a Zener diode lies in the cause of breakdown for a diode in reverse biased condition. Normally there are two types of breakdown- Zener and Avalanche. This type of breakdown occurs for a reverse bias voltage between 2 to 8V.

Even at this low voltage, the electric field intensity is strong enough to exert a force on the valence electrons of the atom such that they are separated from the nuclei. This results in the formation of mobile electron-hole pairs, increasing the flow of current across the device.

This type of break down occurs normally for a highly doped diode with low breakdown voltage and a larger electric field. As temperature increases, the valence electrons gain more energy to disrupt from the covalent bond and less amount of external voltage is required. Thus Zener breakdown voltage decreases with temperature.

This type of breakdown occurs at the reverse bias voltage above 8V and higher. It occurs for a lightly doped diodes with a large breakdown voltage. These electrons are again accelerated and collide with other atoms. Because of this continuous collision with the atoms, a large number of free electrons are generated.

As a result, electric current in the diode increases rapidly. This sudden increase in electric current may permanently destroys the normal diode. However, avalanche diodes may not be destroyed because they are carefully designed to operate in avalanche breakdown region.

Avalanche breakdown occurs in zener diodes with zener voltage V z greater than 6V. The zener breakdown occurs in heavily doped p-n junction diodes because of their narrow depletion region. When reverse biased voltage applied to the diode is increased, the narrow depletion region generates strong electric field.

When reverse biased voltage applied to the diode reaches close to zener voltage, the electric field in the depletion region is strong enough to pull electrons from their valence band. The valence electrons which gains sufficient energy from the strong electric field of depletion region will breaks bonding with the parent atom. The valance electrons which break bonding with parent atom will become free electrons.

This free electrons carry electric current from one place to another place. At zener breakdown region, a small increase in voltage will rapidly increases the electric current.

The symbol of zener diode is shown in below figure. Zener diode consists of two terminals: cathode and anode. In zener diode, electric current flows from both anode to cathode and cathode to anode. The symbol of zener diode is similar to the normal p-n junction diode, but with bend edges on the vertical bar.

The VI characteristics of a zener diode is shown in the below figure. When forward biased voltage is applied to the zener diode, it works like a normal diode. However, when reverse biased voltage is applied to the zener diode, it works in different manner. When reverse biased voltage is applied to a zener diode, it allows only a small amount of leakage current until the voltage is less than zener voltage.

When reverse biased voltage applied to the zener diode reaches zener voltage, it starts allowing large amount of electric current. At this point, a small increase in reverse voltage will rapidly increases the electric current. Because of this sudden rise in electric current, breakdown occurs called zener breakdown. The current increases to a maximum, which is determined by the series resistor, after which it stabilizes and remains constant over a wide range of applied voltage.

The breakdown is either due to the Zener breakdown effect that occurs below 5. Both mechanisms result in the same behavior and do not require different circuitry; however, each mechanism has a different temperature coefficient. The Zener effect has a negative temperature coefficient while the impact effect experiences a positive coefficient. The two temperature effects are almost equal at 5. Zener diodes vary in specifications such as nominal working voltage, power dissipation, maximum reverse current, and packaging.

Some commonly used specifications include:. Zener diodes are used for voltage regulation, as reference elements, surge suppressors, and in switching applications and clipper circuits. The load voltage equals breakdown voltage VZ of the diode.

The series resistor limits the current through the diode and drops the excess voltage when the diode is conducting.

If the input voltage increases to a value higher than the Zener breakdown voltage, current flows through the diode and create a voltage drop across the resistor; this triggers the SCR and creates a short circuit to the ground.

The short circuit opens up the fuse and disconnects the load from the supply. Zener diodes are used to modify or shape AC waveform clipping circuits.

The clipping circuit limits or clips off parts of one or both of the half cycles of an AC waveform to shape the waveform or provide protection. By using the Co-Browse feature, you are agreeing to allow a support representative from Digi-Key to view your browser remotely. When the Co-Browse window opens, give the session ID that is located in the toolbar to the representative.