What is a Semiconductor

Electronics is the field of engineering and applied physics that deals with the design and application of electronic circuits and devices. In electronic circuits, the operation depends on the flow of electrons and holes (appeared due to deficiency of electrons) in the form of current, etc.

On the basis of its electrical nature, materials are categorized into three types

  1. Conductor: It is that type of material which has a number of free electrons to conduct the electricity. The metals are a good conductor of electricity. The free electrons are also known as conduction electrons.
  2. Insulator: It is that type of material which does not have the free electrons in its volume and hence, it does not conduct the electricity at all. Materials such as wood, plastic rubber etc are the insulators.
  3. Semiconductor: Semiconductors are the most basic component of an electronic circuit.

In this type of material, there are no free electrons at the normal temperature. So, it behaves as an insulator in that condition. But when temperature of a semiconductor is increased, then it has free electrons and can behave as a conductor. Materials such as Si, Ge, As, etc are the semiconductors.

Types of Semiconductor

Semiconductors are of the following 2 types

  1. Intrinsic Semiconductor: A semiconductor in its pure state, is called intrinsic semiconductor or i-type semiconductor
  2. Extrinsic Semiconductor: A semiconductor doped with a suitable impurity to increase its conductivity, is called extrinsic semiconductor. The conductivity of resultant crystal depends on the nature and quantity of the impurity added.
  • The process of deliberate addition of desirable impurity atoms to a pure semiconductor to modify its properties in a controlled manner is called doping. The impurity atoms added to a pure or intrinsic semiconductor, are called dopants.

Types of Extrinsic Semiconductor

On the basis of doped impurity, extrinsic semiconductors are of two types

  1. n-type Semiconductor: Extrinsic semiconductor doped with pentavalent impurity like As, Sb, Bi etc. in which negatively charged electrons work as the charge carrier, is called n-type semiconductor. Every pentavalent impurity atom donates one electron into the crystal, therefore it is called a donor atom.
  2. p-type Semiconductor Extrinsic semiconductor doped with trivalent impurity like Al, B, etc in which positively charged holes work as charge carriers, is called p-type semiconductor. Every trivalent impurity atom has a tendency to accept one electron, therefore it is called an acceptor atom.

In electronic devices, the current is due to flow of charge carriers. It may be electrons or holes.

p-n Junction (or Diode)

p-n Junctions are formed by joining n-type and p-type semiconductor materials. It has two terminals; one is on p-side while other is on n-side.

Terms related to pn Junction diode

  1. Depletion Layer: At pn junction, a region is created where there is no charge carriers. This region is called depletion layer. The width of this region is of the order of 10-6 m.
  2. Potential Barrier: The potential difference across the depletion layer, is called potential barrier. The potential barrier for Ge is 0.3 V and for Si is 0.7 V.
  3. Forward Biasing: In this biasing, the p-side of the diode or pn junction is connected to the positive terminal and n side of the P N junction is connected to negative terminal of a battery. In this way, forward current flows due to majority charge carriers. The width of depletion layer decreases.
  4. Reverse Biasing: In this biasing, the p-side of the diode is connected to negative terminal and n-side of the diode is connected to positive terminal of a battery. In this way, reverse current flows due to minority charge carriers. The width of depletion layer increases.
  • In diode, there are two types of current naming forward current (due to majority charge carriers) and reverse current (due to minority charge carriers).

Some types of Junction Diode

LED (Light Emitting Diode) What is LED?

It is a heavily doped pn junction diode which converts electrical energy into light energy. This diode emits spontaneous radiation forward biasing. The diode is covered with a transparent cover so that the emitted light may come out. LEDs are used in electronic gadgets as indicator light. It is also used as rectifier which converts an alternating current (AC) into direct current (DC).

Zener Diode: What is Zener Diode?

It is a highly doped p-n junction diode which is not damaged by high reverse current. It is always used in reverse bias in breakdown voltage region and is chiefly used as a voltage regulator.

Tunnel Diode: What is tunnel Diode?

A tunnel diode is a p-n junction diode which makes use of the quantum mechanical phenomenon of the potential barrier penetration. It is p-n junction which is made from a heavily doped semiconductor.

Photo Diode: What is pohoto Diode?

It is a special type of p-n junction diode fabricated with transparent window to allow to fall on the diode. It is used in reverse biasing. When the light is incident on photodiode, electron-hole pairs are generated due to electric field of the junction. Due to the electric field of the junction, electrons and holes are separated before they recombine. It results into increase in emf. Now, if external circuit is completed using some load, a photocurrent flows through the circuit as well through the load.

Solar cell: What is solar cell

A junction diode in which one of the p or n section is made very thin (so that the light energy falling on the diode is not absorbed reaching the junction), can be used to convert light energy into electric energy. Such junction diodes are called solar cells. Most important application of solar cells is that set of solar cells can be used to charge batteries in the daytime to use them during the night.

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