Definition of Diode

A diode is a semiconductor device which have two terminals, typically allowing the flow of current in one direction only.
A diode is a specialized electronic component with two electrodes called the anode and the cathode. Most diodes are made with semiconductor materials such as silicon, germanium, or selenium.
The fundamental property of a diode is its tendency to conduct electric current in only one direction.
Diodes are also known as rectifiers because they change alternating current (AC) into pulsating direct current (DC).
The diodes are rated according to their type, voltage, and current capacity.
Diodes have polarity, determined by an anode (positive lead) and cathode (negative lead). Most diode allow current to flow only when positive voltage is applied to the anode.
Diodes are available in various configurations. From left: metal case, stud mount, plastic case with chamfer, glass case.

Symbol of Diode


History of the semiconductor diode

The first diodes to be used were discovered back in the early 1900s when the technology of wireless was in its infancy.
The Cats Whisker was one of the first type diode to be used. It considered on a very thin piece of wire (the Cats whisker itself) that could be into a piece of semiconductor type material (typically a mineral crystal) to make a point contact type of diode.
This was widely used up until the middle to late 1920s when thermionic or valve technology became sufficiently cheap to be widely used for radios sets.

Types of diode with their uses

There are many types of diode which are manufactured and widely used in todays electronics. The different type of diodes are used for different applications.

  • Backward diode

  • This type of diode is sometimes also called the back diode. It is a form of PN junction diode that is very similar to the tunnel diode in its operation.
    A backward diode can be used in high speed switching applications.
    It also can be used for rectifying weak signals.

  • BARITT diode

  • This form of diode gains its name from the words Barrier Injection Transit Time diode.
    It is used in microwave applications and bears many similarities to the more widely used IMPATT diode.

  • Gunn diode

  • Although not a diode in the form of a PN junction, this type of diode is a semiconductor device that has two terminals.
    It is generally used for generating microwave signals.

  • Cats whisker

  • This type of diode was the earliest type to gain widespread acceptance. It considered of a small wire placed on a piece of mineral crystal.
    This created a small point contact diode, which although unreliable was sufficiently good to enable radio transmissions to be heard when used in a crystal set.

  • IMPATT diode

  • The IMPATT diode stands for IMPact Avalanche ionization Transit Time microwave diode.
    It is used in a some applications where a simple generator is required for microwave signals.

  • Laser diode

  • This type of diode is different to the ordinary light emitting diode in that it produces laser (coherent) light.
    They are used in many applications including CD and DVD drives.
    Although much cheaper than other forms of laser generators, these diodes are more expensive than ordinary LEDs.

  • Light emitting diodes

  • Light emitting diodes or LED is one of the most popular types of diode. When forward biased with current flowing through the junction, light is produced.
    The diodes use component semiconductors and can produce a variety of colours. Although the original colour was red.
    LED is used as a bulb in the homes and industries.
    These are used in the mobile phones to display the message.
    They are also used in the traffic light signals, motorcycles, cars etc.

  • Photodiode

  • When light strikes a PN junction it can create electrons and holes, causing a current to flow. As a result it is possible to use semiconductors to detect light.
    These types of diode can be used to generate electricity. For some applications, PIN diodes work very well as photodetectors.

  • PIN diode

  • This type of diode has areas of P-type and N –type silicon, but between then there is an area of intrinsic semiconductor (i.e., no doping).
    This increasing the size of what is is termed the depletion region.
    This type of diode is used in a number of applications including radio frequency switches and photodiodes.

  • PN junction

  • The standard PN junction may be thought of as the normal or standard type of diode in use today.
    These diodes can come as small signal types for use in radio frequency, or other low current applications, or other types may be high current and high voltage ones that can be used for power applications.

  • Schottky diodes

  • This type of diode has a lower forward voltage drop than ordinary silicon PN junction diodes. At low currents the drop may be somewhere between 0.15 and 0.4 volts as opposed to 0.6 volts for a silicon diode.
    To achieve this performance they are constructed in a different way to normal diodes having a metal to semiconductor contact.
    They are widely used as clamping dioded, and in RF applications.

  • Tunnel diode

  • It is a diode which has negative resistance due to the quantum mechanical effect called tunneling.
    Although not widely used today, the tunnel diode was used for microwave applications where its performance exceeded that of other devices of the day.

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