DIODES

What is a Diode?
Diode is an electronic component with active electrodes ,which allows the flow of current only in one direction.The two electrodes of the diode are Anode(+) and cathode(-).  Diodes are made up of semiconductor materials like silicon, Germanium.

The fundamental property of the diode is to allow the flow of current in only one direction.

How PN junction diode is formed?

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PN junction diode is made from a combination of 2 extrinsic semiconductors such as P-type semiconductor and N-type semiconductor. P-type semiconductor is formed when intrinsic semiconductor is doped with trivalent elements such as Al, Ga and Indium. N-type semiconductor is formed when intrinsic semiconductor is doped with pentavalent impurities such as Phosphorous, Arsenic or Antimony.
Here the joint between P-type and N-type semiconductors is called as PN junction. At the PN junction electrons from the N-type semiconductor will be attracted to the holes of P-type semiconductor. As a result the electrons and holes at the PN junction will disappear forming a layer called "depletion layer". At the same time P-type semiconductor become more negative and N-type semiconductor will become more positive. This results in a potential difference across PN junction.
This potential difference is called as 'Junction voltage or Barrier Voltage'. This junction voltage will prevents the charge carriers from flowing across the depletion region.
Diode can be used in both Forward Bias and Reverse Bias. The polarity of the applied voltage which allows the flow of current is known as 'Forward Bias'. The polarity of the applied voltage which doesn't allows the flow of current to flow is called as 'Reverse Bias'.

Forward Bias:

In Forward Bias positive terminal of the Battery is connected to the P-type material and negative terminal of the Battery is connected to the N-type material of the PN junction Diode. In forward Bias the diode acts as short circuit and start conducting , so that current will pass through Diode .But the diode starts conducting After the 'Cut in voltage or Break over voltage'.Cut in voltage is defined as the minimum voltage below which current is negligible above which the diode starts conducting.Cut in voltage for si is 0.7v and Germanium is 0.3v . 

Reverse Bias:

In Reverse Bias positive terminal of the Battery is connected to the N-type material and negative terminal of the Battery is connected to the p-type material of the PN junction Diode.In reverse Bias the diode acts as open circuit and no current will flow through the diode. 

V-I CHARACTERISTICS OF PN JUNCTION DIODE

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Diodes are mainly used as Rectifiers, Signal limiters, Voltage Regulators ,Switches ,Signal Modulators , Signal Demodulators,Clippers,Clampers.

TYPES OF DIODES: 

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1.LIGHT EMITTING DIODE

2.PHOTO DIODE
  

And also see : TRANSISTORS