Semiconductor’s resistivity lies between the conductors and insulators. The property of resistivity is that it decides whether a material is a semiconductor. Semiconductors are the core fundamental materials which are used in solid-state electronic devices such as diodes, transistors etc. The atomic structure of the material decides whether the material will turn out to be a semiconductor, metal or insulator. 

  • Semiconductors could also be elements such as Ge, Si or compounds such as GaAs or CdS.
  • Semiconductor’s resistivity is less than insulators and more than conductors.
  • Semiconductors have negative temperature coefficients. 
  • The resistance in semiconductors  decreases with the increase in temperature and vice versa.
  • The properties of a Semiconductor changes when a suitable metallic impurity is added to it.

Semiconductor devices are substantially used in the field of electronics. The transistor replaced the bulky vacuum tubes, from which the size and cost of the devices decreased and this revolution has kept on increasing its pace and led to new inventions in integrated electronics. 

Classification/Types of Semiconductor

The semiconductors are of two types divided on the basis of purity:

  1. Intrinsic Semiconductors:

An intrinsic semiconductor is also called an i-type semiconductor or undoped semiconductor .

It is the purest semiconductor without any significant dopant species present ne = nh =ni  where, ne and nh = densities of electrons and holes respectively.

  1.  Extrinsic Semiconductors:

When the pure semiconductor is doped with impurity it is known as extrinsic semiconductor.

Extrinsic semiconductors are of two types depending upon which impurity is added

(a) n-type semiconductors (b) p-type semiconductors

NOTE: There is no addition of positive or negative charges,  Both of the semiconductors are electrically neutral.

Types of Semiconductor

  1. N- Type Semiconductor

A small amount of pentavalent impurity is added to a pure semiconductor to result in n type extrinsic semiconductor. The impurity added has five valence electrons.

Example, if an Arsenic atom is added to the germanium atom, four of the valence electrons join the Ge atoms while one electron remains as a free electron which can be seen in the diagram-

n-type semiconductor

These free electrons constitute electron current. They provide electrons for conduction.

  • The conduction takes place through electrons, the electrons are majority carriers and the holes are minority carriers.
  • When pentavalent impurity is added, the free electrons travel towards the positive electrode. This is called negative or N-type conductivity.
  1. P-Type Extrinsic Semiconductor

When a  small amount of trivalent impurity is added to a pure semiconductor it results in P-type extrinsic semiconductor. The added impurity has three valence electrons.

Example, if a Boron atom is added to the germanium atom, three of the valence electrons get attached with the Ge atoms, to form three covalent bonds. As there is no electron in boron remaining to form a covalent bond, the space is considered a hole. This is as shown in the following figure.

P-Type Extrinsic Semiconductor

When boron impurity is added in a small amount, It provides a number of holes which helps in the conduction which hold current.

  • In P-type extrinsic semiconductors, the holes are majority carriers while the electrons are minority carriers as the conduction takes place through holes.
  • The impurity added here provides holes which are called as acceptors, as they accept electrons from the germanium atoms.
  • The number of mobile holes remains equal to the number of acceptors, The P-Type semiconductor stays electrically neutral.
  • When an electric field is applied to a P-type semiconductor, to which a trivalent impurity is added, the holes travel towards the negative electrode, but at a slower pace than electrons. This is called P-type conductivity.
  • In the P-type conductivity, valence electrons move from one covalent bond to another.

Diode

Diodes hold the purpose of AC voltage rectification which is restricting the voltage to follow one direction only by using a filter or a capacitor, a dc voltage can be achieved. 

The types of diodes are:

  • Zener Diode-  

Zener diode is used in places where voltage regulation is needed. It is a reverse biased heavily doped p-n junction diode which is operated in breakdown regions.

Zener Diode

  • P-N junction Diode- 

Semiconductor diode is a p-n junction with metallic contacts provided at the end of the application for an external voltage. It is used in  optoelectronic devices or  photons and the entity is the photon. Examples of P-N junction diodes are solar cells, light-emitting diodes etc.  Junction diode is forward biased when the positive terminal of the external battery is connected to the p-side and negative terminal to the n-side of the diode.