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Diode is one of the building blocks of electronic circuits. There are different types of diodes, each with its own operating characteristics and applications. The various diode types are easily identified by name, circuit applications and schematic symbol. It should be noted that the term diode, used by itself, refers to the basic pn-junction diode. Different diodes has different names, such as zener diode, light emitting diode and so on. In this section we will learn more about diodes.

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Diode Definition

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A Diode is a two-terminal device that acts as a one way conductor that is it permits current flow in only one direction. The most basic type of diode is the p-n junction diode.

Diode Symbol

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The schematic symbol for the p-n junction diode is given below. The p-type material is called the anode and the n-type material is called the cathode. 

Diode Symbol

Important rules about diodes:
  1. If the voltage on the anode is positive with respect to the voltage on the cathode, we say that the diode is forward biased. At forward biased condition the diode permits current flow.
  2. If the voltage on the anode is negative with respect to the voltage on the cathode, we say that the diode is Reverse biased. At reverse biased condition the diode inhibits current flow. 

PN Junction Diode

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A single crystal of silicon can be made in which part of it is n-type material and part p- type. Small pieces of such a crystal with leads attached to the p- and n- type region are called p-n junction diodes. The pn junction diode conducts in one direction only. Connect a silicon diode as shown below

a) Connect negative end or cathode of the diode to the negative terminal of the cell.

The lamb lights, a current flow and this is called forward biasing. The diode conducts as electrons and holes are able to cross over the junction between p and n region and complete the circuit. 

Forward Bias

b) Reverse the connection to the diode as shown in (b), where its negative end is joined to the positive terminal of the cell, This is called reverse biasing. The diode now acts as an insulator, no current can flow through the diode.  

Reverse Bias

Diode Types

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There are several different types of diodes, which are used for specific applications. These types of diodes are also called special purpose diodes or simply special diodes.
  • Zener diode
  • Varator diode
  • Schottky diode
  • Tunnel diode
  • LED
  • Laser diodes
Types of Diode
Zener diode is a silicon p-n junction semiconductor device, which is generally operated in its reverse breakdown region. It was created in the late 1950s by its inventor Clarence Zener. It got many applications in different industries because of its reasonable accuracy, low cost and size.
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Tunnel Diode

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Tunnel diode was invented by the Japanese scientist L.Esaki in 1958. The tunnel diode works under the principle of quantum- mechanical tunneling. Silicon, germanium and gallium arsenide are the commonly used semiconductor material for the construction of tunnel diode. The tunnel diode exhibits a negative resistance interval. Negative Resistance values range from 1 to 200 ohms for various types of tunnel diodes.
It is a type of diode, which is a widely used semiconductor metal contact. Schottky diode specifications are mainly depends on the type and resistivity of the silicon wafer and the metal contact used. It is mainly used as a rectifier for high frequency signal. The fabrication of a schottky diode is simpler than that of a p-n junction diode. Its barrier height is also less than the built in potential of a p-n junction.
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Laser diodes are the type of diodes having small size, which is an efficient sources of laser light. They are well suited for wide range of applications, the most popular being optical fiber communications and data storage on disks (say CDs and DVDs).
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The light emitting diodes are very simple devices as compared to laser diodes. It is a device which will convert current to light. It is used in LAN and WAN applications with multimode fiber because multimode fiber can have large angle of acceptance of light. The main advantages of LED are reliability, small size, linearity, low cost ect.
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Varactor Diode

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A varactor diode is a silicon diode that works as a variable capacitor in response to a range of reverse voltage values. Varactor diode mostly find applications in automatic frequency control circuits. Varactors are available with nominal capacitance values ranging from 1 to 500 pF, and maximum rated operating voltage extending from 10 to 100 volts.
More topics in Diode
Zener Diode Schottky Diode
Laser Diode Light Emitting Diode
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