Semiconductors: Technological advancement that changed the world.

Semiconductors materials such as silicon (Si), germanium (Ge) and gallium arsenide (GaAs), have electrical properties somewhere in the middle, between those of a “conductor” and an “insulator”. They are not good conductors nor good insulators (hence their name “semi”-conductors). They have very few “free electrons” because their atoms are closely grouped together in a crystalline pattern called a “crystal lattice”.

by controlling the amount of impurities added to this intrinsic semiconductor material it is possible to control its conductivity.

 Various impurities called donors or acceptors can be added to this intrinsic material to produce free electrons or holes respectively.



This process of adding donor or acceptor atoms to semiconductor atoms is called Doping. The as the doped silicon is no longer pure, these donor and acceptor atoms are collectively referred to as “impurities”, and by doping these silicon material with a sufficient number of impurities, we can turn it into a semi-conductor.

History of Semiconductors

The term “semiconducting” was used for the first time by Alessandro Volta in 1782.

Michael Faraday was the first person to observe a semiconductor effect in 1833. Faraday observed that the electrical resistance of silver sulfide decreased with temperature. In 1874,Karl Braun discovered and documented the first semiconductor diode effect. Braun observed that current flows freely in only one direction at the contact between a metal point and a galena crystal.

In 1901, the very first semiconductor device was patented called "cat whiskers". The device was invented by Jagadis Chandra Bose. Cat whiskers was a point-contact semiconductor rectifier used for detecting radio waves.

A transistor is a device composed of semiconductor material. John Bardeen, Walter Brattain& William Shockley all co-invented the transistor in 1947 at BELL Labs.

What is semi conductors?  How P and N junction works? How transistors works?

  Silicon has four valence electrons in its outermost shell which it shares with its neighbouring silicon atoms to form full orbital’s of eight electrons The structure of the bond between the two silicon atoms is such that each atom shares one electron with its neighbor making the bond very stable.

N-type Semiconductor Basics

In order for our silicon crystal to conduct electricity, we need to introduce an impurity atom such as Arsenic, Antimony or Phosphorus into the crystalline structure making it extrinsic (impurities are added). These atoms have five outer electrons in their outermost orbital to share with neighbouring atoms and are commonly called “Pentavalent” impurities.

 The resulting semiconductor basics material has an excess of current-carrying electrons, each with a negative charge, and is therefore referred to as an N-type material with the electrons called “Majority Carriers” while the resulting holes are called “Minority Carriers”.

P-Type Semiconductor Basics

If we go the other way, and introduce a “Trivalent” (3-electron) impurity into the crystalline structure, such as Aluminium, Boron or Indium, which have only three valence electrons available in their outermost orbital, the fourth closed bond cannot be formed. Therefore, a complete connection is not possible, giving the semiconductor material an abundance of positively charged carriers known as holes in the structure of the crystal where electrons are effectively missing.

As there is now a hole in the silicon crystal, a neighbouring electron is attracted to it and will try to move into the hole to fill it. However, the electron filling the hole leaves another hole behind it as it moves. This in turn attracts another electron which in turn creates another hole behind it, and so forth giving the appearance that the holes are moving as a positive charge through the crystal structure (conventional current flow).

This movement of holes results in a shortage of electrons in the silicon turning the entire doped crystal into a positive pole. As each impurity atom generates a hole, trivalent impurities are generally known as “Acceptors” as they are continually “accepting” extra or free electrons.

Semiconductor Basics Summary

N-type (e.g. doped with Antimony)

These are materials which have Pentavalent impurity atoms (Donors) added and conduct by “electron” movement and are therefore called, N-type Semiconductors.

In N-type semiconductors there are:

• 1. The Donors are positively charged.
• 2. There are a large number of free electrons.
• 3. A small number of holes in relation to the number of free electrons.
• 4. Doping gives:
  •   positively charged donors.
  •   negatively charged free electrons.
• 5. Supply of energy gives:
  •   negatively charged free electrons.
  •   positively charged holes.

P-type (e.g. doped with Boron)

These are materials which have Trivalent impurity atoms (Acceptors) added and conduct by “hole” movement and are therefore called, P-type Semiconductors.

In these types of materials are:

• 1. The Acceptors are negatively charged.
• 2. There are a large number of holes.
• 3. A small number of free electrons in relation to the number of holes.
• 4. Doping gives:
  •   negatively charged acceptors.
  •   positively charged holes.
• 5. Supply of energy gives:
  •   positively charged holes.
  •   negatively charged free electrons.

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