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Basics of Diode - Characteristics, Working Principle, Types, and Applications

Updated: Apr 6

What is a diode?

'Di' means two, and 'ode' means electrode. A diode is a two-terminal electrical or electronic device or component that contains two electrodes called the anode and cathode. It is a semiconductor device that allows current flow only in one direction and blocks the current flow in the opposite direction. A diode has high resistance in one direction and low resistance in another.


Diode
Diode

Diodes are also called rectifiers as they are used to convert the alternating current (AC) to the pulsating direct current (DC). Diodes are widely used in modern-day electronic circuits to prevent high voltage.


What are the materials used to produce diodes?

A diode is made of semiconductor materials, such as

  • Silicon

  • Germanium

  • Selenium

Different forms of diodes are available on the market

  • Metal Case

  • Stud Mount

  • Plastic case with band

  • Plastic case with chamfer

  • Glass case

Diode ratings are made based on the following parameters

  • Voltage

  • Diode Type

  • Current capacity


Symbol of the diode

In the below diagrammatic representation, it is clear that the diode has two terminals. The positive terminal is known as the anode, and the negative terminal is known as the cathode. The anode is represented by the arrowhead symbol, and the cathode is represented by the vertical line. When the diode is in a forward-biased condition, the current flows from the cathode to the anode.


Diagrammatic representation of diode, where anode is the arrow head(+ve) and the vertical line is cathode(-ve)
Symbol of Diode

Construction of the diode

The semiconductor materials are of two types, namely,

  • Intrinsic Semiconductor

  • Extrinsic Semiconductor


Intrinsic Semiconductor

An intrinsic semiconductor is a pure or undoped semiconductor free from any impurities and with low conductivity. At a given room temperature, the number of holes and the number of electrons are the same. The conductivity of an intrinsic semiconductor is based only on the room temperature.

Example: pure silicon and pure germanium, which are naturally available elements.


Extrinsic Semiconductor

An extrinsic semiconductor is an impure or doped semiconductor that contains impurities and has a higher conductivity compared to that of the intrinsic semiconductor. It is developed by adding impurities to the pure or intrinsic semiconductor to increase the number of holes or electrons. The conductivity of an extrinsic semiconductor is based on both the room temperature and the impurities added.

Example: Pure silicon and pure germanium that are doped with chemical impurities such as Boron, Aluminium,Phosphorous,Arsenic,Antimony, Indium, and Gallium


What are the two layers of a semiconductor diode?

  • P-type layer

  • N-type layer


P-type layer

When a trivalent impurity or acceptor impurity (boron, indium, aluminum, or gallium) is added to an intrinsic semiconductor, the number of holes will be in excess, and it will have a positive charge; hence, this type of layer is termed a p-type layer.


N-type layer

When a pentavalent impurity or donor impurity (Arsenic, Antimony, or Phosphorous) is added to an intrinsic semiconductor, the number of electrons will be in excess and it will have a negative charge, and hence this type of layer is termed the n-type layer.


Working Principles of Diode

In the n-type layer, electrons are the majority charge carriers, and holes are the minority charge carriers. In the p-type layer,holes are the majority charge carriers, and electrons are the minority charge carriers. Due to the concentration difference, diffusion occurs in majority charge carriers and recombines with minority charge carriers, which in turn are collected near the junction. This region is known as the depletion region.


Reverse Bias Condition

When the ptype of the diode (anode) is connected to the negative terminal of the battery and the nytpe of the diode (cathode) is connected to the positive terminal of the battery, then this type of connection is called a reverse bias condition.


Forward Bias Condition

When the ptype of the diode (anode) is connected to the positive terminal of the battery and the nytpe of the diode (cathode) is connected to the negative terminal of the battery, then this type of connection is called a forward bias condition.


Characteristics of Diode

The characteristics of the diode can be understood under the following topics:

  • Forward-Biased Diode

  • Reverse-Biased Diode

  • Zero-biased diode or unbiased diode


Forward-Biased Diode

  • In forward biasing, the n-type of the semiconductor is connected to the negative terminal of the battery, and the p-type of the semiconductor is connected to the positive terminal of the battery. This type of junction is said to be a forward-biased junction.

  • In the forward bias condition, the built-in electric field direction near the junction and the applied electric field direction are opposite to each other.

  • Therefore, the magnitude of the resultant electric field is less than that of the built-in electric field.

  • This results in less resistivity, and hence the depletion region is thinner.

  • At 0.6V in silicon, the depletion region resistance is completely negligible.


In forward bias diode, the depletion region is thinner
Forward Bias Diode


Reverse-Biased Diode

  • In reverse biasing, the n-type of the semiconductor is connected to the positive terminal, and the p-type of the semiconductor is connected to the negative terminal of the battery. This type of junction is said to be a reverse-biased junction.

  • In the reverse bias condition, the built-in electric field direction and the applied electric field direction are the same.

  • Therefore, the magnitude of the resultant electric field is higher than the magnitude of the built-in electric field.

  • This results in high resistivity, and hence the depletion region is thicker.


In Reverse Bias Diode, the depletion region is thicker
Reverse Bias Diode

Zero-biased diode or unbiased diode

  • If a semiconductor is not connected to an external source, it is called a zero-biased diode or an unbiased diode.

  • During doping, when there is an imbalance of holes and electrons, the electric field is developed between the n-type and p-type materials across the depletion layer.

  • The barrier potential of a silicon diode at room temperature is 0.7 V.


Ideal Diodes

  • Ideal diodes are diodes where current flow is allowed only in one direction (forward) and is not allowed in the reverse direction.

  • In a reverse-biased condition, the ideal diodes act as an open circuit, and the voltage across the diode is negative in this condition.


VI Characteristics of Ideal Diode
VI Characteristics of Ideal Diode


Types of Semiconductor Diodes


Applications of Semiconductor Diodes

  • A rectifier diode is used for the rectification of alternating current (A.C.).

  • LEDs are used to provide light.

  • Zener diodes are used for current and voltage stabilization in electronic systems.

  • Photodiodes are used to detect light.

  • Switching diodes are used in the circuits for fast switching.

  • A tunnel diode is a special diode used in the negative resistance region.

  • Rectifiers

  • Oscillators

  • Switches

  • Signal Limiters

  • Voltage Regulators

  • Signal Modulators

  • Signal De-modulators

  • Signal mixers

  • Clipping circuits

  • Clamping circuits

  • Logical gates


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