What is Diode?

A diode is a device which has two active electrodes and two terminals and allows the current to pass only in a single direction. These devices are used to protect circuits by limiting the voltage and also turn AC into DC. Semiconductors like silicon and germanium are used to make most of the diodes. Even though they transmit current in a single direction, the way with which they transmit differs. There are different kinds of diodes and each type has its own applications.

Diode Symbol

Diode Symbol

Types of Diodes

  1. Light Emitting Diode
  2. Laser diode
  3. Avalanche diode
  4. Zener diode
  5. Schottky diode
  6. Photodiode

Types of Diode with Symbols

Light Emitting Diode (LED)

When electric current between the electrodes passes through this diode, light is produced. In other words, light is generated when a sufficient amount of forwarding current passes through it. In many diodes, this light generated is not visible as they are frequency levels that do not allow visibility. LEDs are available in different colors. There are tri-color LEDs which can emit three colors at a time. Light color depends on the energy gap of the semiconductor used.

Laser Diode

It is a different type of diode as it produces coherent light. It is highly used in CD drives, DVDs and laser devices. These are costly when compared to LEDs and are cheaper when compared to other laser generators. Limited life is the only drawback of these diodes.

Avalanche Diode

This diode belongs to a reverse bias type and operates using the avalanche effect. When voltage drop is constant and is independent of current, the breakdown of avalanche takes place. They exhibit high levels of sensitivity and hence used for photodetection.

Zener Diode

It is the most useful type of diode as it can provide a stable reference voltage. These are operated in reverse bias and break down on the arrival of a certain voltage. If current passing through the resistor is limited, a stable voltage is generated. Zener diodes are widely used in power supplies to provide a reference voltage.

Schottky Diode

It has a lower forward voltage than other silicon PN junction diodes. The drop will be seen where there is low current and at that stage, voltage ranges between 0.15 and 0.4 volts. These are constructed in a different way in order to obtain that performance. Schottky diodes are highly used in rectifier applications.


A photo-diode can identify even a small amount of current flow resulted from the light. These are very helpful in the detection of the light. This is a reverse bias diode and used in solar cells and photometer. They are even used to generate electricity.

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Diode Characteristics

Following are the characteristics of diode:

  • Forward-biased diode
  • Reverse-biased diode
  • Zero biased diode

Diode Characteristics

Forward-biased Diode

There is a small drop of voltage across the diode when the diode is forward-biased and current is conducting. For silicon diode, the forward voltage is 690mV and for germanium, 300mV is the forward voltage. The potential energy across the p-type material is positive and across the n-type material, the potential energy is negative.

Reverse-biased Diode

A diode is said to be reverse-biased when the battery’s voltage is dropped completely. For silicon diode, the reverse voltage is -20μA and for germanium, -50μA is the reverse voltage.  The potential energy across the p-type material is negative and across the n-type material, the potential energy is positive.

Zero-biased Diode

When the diode is zero-biased, the voltage potential across the diode is zero.

Diode Applications

Following are the applications and uses of diode:

  • Diodes as a rectifier
  • Diodes in the clipping circuit
  • Diodes in clamping circuits
  • Diodes in logical gates
  • Diodes in reverse current protection


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