Different Types of Diodes | Their Circuit Symbols & Applications

This article is all about types of diode with their symbol; Zener diode is discussed in detail

Zener diode is a heavily doped semiconductor p-n junction diode which is designed to operate in reverse direction. The diode is specially designed for optimizing the breakdown region.

Symbolic Representation

Table of Contents

Working Principle of Zener Diode

When a PN junction diode operates in reverse bias, the depletion layer becomes wider. If the reverse-biased voltage across the diode is increased continuously, the depletion layer becomes more and wider. Same time, there will be a constant reverse saturation current due to the flow of minority carriers. After a certain reverse voltage applied across the junction, the minority charge carriers get a sufficient amount of kinetic energy due to the strong electric field applied. Free electrons with sufficient kinetic energy collide with immobile ions of the depletion layer and knock out more free electrons. These newly created secondary electrons also get sufficient energy due to the same electric field and create more new free electrons by collision. Due to this phenomenon, very soon, a large number of free electrons get created in the depletion layer, and the entire diode becomes conductive. This causes a breakdown across the junction termed as an avalanche breakdown.

When a PN junction diode is highly doped, the concentration of impurity atoms is large in the crystal. This higher concentration of impurity atoms causes a higher concentration of ions in the depletion layer, henceforth for the same applied reverse voltage, the width of the depletion layer becomes thinner as compared to the normally doped diode. Due to this thinner depletion layer, potential gradient or electric field strength across the depletion layer is very high. If the reverse voltage is continuously increased, after a certain voltage, the electrons released from the covalent bonds within the depletion region and make the depletion region more conductive. This breakdown is called a Zener breakdown.

Difference between avalanche and zener breakdown

The reverse voltage at which this breakdown occurs is called Zener voltage.

It is because of very strong built in electric field of depletion layer.

Zener breakdown occurs at a lower voltages level then avalanche breakdown in a particular diode.

Zener breakdown is much sharper than avalanche breakdown.

The Zener voltage of the diode gets adjusted during diode manufacturing with the help of proper doping.

When a Zener diode is connected to a voltage source and if the source voltage is greater than Zener voltage, the voltage across Zener remains constant irrespective of the source voltage. Although current through the diode can be of any value depending on the load connected. That is the reason why we use Zener diode mainly for controlling voltage in different circuits. Hence suitable for voltage regulation.

Avalanche breakdown occurs because of the thermal collision, due to the rise of the temperature at the junction.

· Avalanche breakdown is not a sharp breakdown.

· Avalanche breakdown is not appropriate for voltage regulation.

Zener Diode Circuit

Zener Diode is nothing but a simple heavily p-n junction diode connected in reverse bias. Heavily doped regions are represented as p+ and n+ in the figure below.

Characteristics of a Zener Diode

To discuss the diode circuits we look through the graphical representation of the operation of the Zener diode. Normally, it is called the V-I characteristics of a Zener diode.

V-I characteristics

When the diode is connected in forwarding bias condition, it acts as a normal diode but when the reverse bias is applied and voltage increases than Zener voltage, a sharp breakdown takes place. In the V-I characteristics above Vz is the Zener voltage. It is also the knee voltage because at this point the current increases rapidly.

Here listed different types of diode with their symbolic representation