Zener Diode

A diode is a fundamental electronic component that allows the flow of electricity in one direction. A diode has low resistance to the flow of current in one direction and a very high resistance in the other. The diodes most commonly used are zener diodes, semiconductor diodes, etc.

What is Diode?

  • The semiconductor diode is a combination of two crystalline semiconductor materials. The two materials are known as p-type and n-type. This classification is based on the concentration of charge carriers in the material. The charge carriers in semiconductors are electrons and holes.
  • Through a process called doping, we are able to introduce certain desired properties in the form of impurities. Through doping, we can control the concentration of holes and electrons in a material.
  • The p-type semiconductor has a larger hole concentration, here p-type refers to the positive charge of the hole. This is done by introducing acceptor atoms which accept electrons to give rise to holes. The most commonly used impurity to dope silicon to p-type configuration is boron.
  • In contrast, n-type semiconductors have a larger electron concentration. This is done by introducing donor atoms as impurities. Here the n-type refers to the negative charge of the electrons.

What is Zener Diode?

  • It is a unique type of diode which allows the current to flow in one direction like a regular diode but it also permits it to flow in the opposite direction. It allows the flow in the opposite direction when the voltage is above a certain value known as Zener Voltage or Avalanche Point or Breakdown Voltage.
  • It consists of a highly doped, reverse biased (refer to an article on p-n junction diode) p-n junction diode while operating in the breakdown region.

Zener Diode Circuit

We can define zener diode as a single diode connected in a reverse bias. It can be connected in reverse bias positive in a circuit shown below,

Zener Diode

The symbol for Zener diode is represented as below,

Zener Diode Symbol

Avalanche Breakdown:

  • A conventional reverse biased diode, when subjected to its breakdown voltage allows a significant amount of current. But when this reverse breakdown voltage is exceeded, the diode experiences an avalanche breakdown.
  • Avalanche breakdown is a form of electric current conduction that allows very large current to pass through good insulators. This may permanently damage a conventional diode.
  • A Zener diode exhibits the same properties except it is designed to have a reduced breakdown voltage. In contrast to the conventional diode, it exhibits a controlled breakdown.
  • When we increase the voltage through Zener in reverse bias mode, first current increases uniformly with it but after it reaches the breakdown state, the current increases massively for a very small or negligible change in voltage. The change is sharper in Zener than the normal diode.

Causes of Breakdown:

  • The breakdown is caused by two effects, the Avalanche effect and the Zener effect. The Zener effect is dominant in voltages up to 5.6 volts and the avalanche effect takes over above that.
  • They are both similar effects, the difference being that Zener effect is a quantum phenomenon and the avalanche effect is the movement of electrons in the valence band like in any electric current. Avalanche effect also allows a larger current through the diode than the Zener effect.
  • The graph below will clearly illustrate the difference between the avalanche effect and the Zener effect.

Zener Breakdown

Applications of  Zener Diode

Zener diodes are usually used as voltage regulators in circuits since as you can see from the graph there is very little variation in voltage when compared to the increase in current. Conversely, very small changes in voltage can cause very large changes in current. It can also be used in surge protectors that we attach to our fridges and TVs to protect them from voltage fluctuations.

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