Semiconductors- Types, Examples, Properties, Applications - Chemosmart

                  Semiconductors

             We know that, the conductivity of a normal metal decreases with an increase in temperature and addition of impurities. There are some materials or elements which may be insulator under normal conditions but which become conductors when the temperature is increased or when certain impurities are added. These materials are called as semiconductors.

 

Definition of semiconductor:

               A material whose electrical conductivity is less than that of a metal but greater than that of an insulator is called as semiconductor. 

The group four elements, mainly, silicon and germanium in their pure state are important semiconductors. These are the material whose electrical properties lie in between those of insulators and good conductors. 

Types of Semiconductors:

     There are mainly two types of semiconductors:

1) Intrinsic semiconductors

2) Extrinsic semiconductors. 

1) Intrinsic semiconductors:

               These are the materials which are insulators at absolute zero but become conductors of electricity at high temperature. 

Example: Graphite. 

              In terms of energy bands, they possess filled bands and there is very small energy gap between the filled and empty band. When temperature is increased the electrons of filled band get thermal energy and some of them are promoted into next higher empty band. This results in the conductivity of the material. 

                The most of this is that the conductivity of an intrinsic semiconductor increases with increase in temperature. This is due to the increase in the number of excited electrons with increase in temperature. 

2)  Extrinsic semiconductors:

         Some elements, which are normally insulators, can be made semiconductors when small amount of impurity is added to them. Such elements are called extrinsic or impurity semiconductors. Thus the conductivity of the same element can be improved by adding small amount of impurity, i.e. a small proportion of atoms of other elements which have valence electrons less or more than the parent element itself. The extrinsic semiconductors are divided into two types depending upon the nature of the impurity added. 

1) n- type extrinsic semiconductors

2) p- type extrinsic semiconductors. 

A) n- type extrinsic semiconductors:

       In n-type semiconductors the impurity provides an excess of electrons. Here an impurity atom to be added has more valence electrons than the parent insulator or a semiconductor atom. 

Examples: Phosphorus, Arsenic or antimony. Each P or As or Sb atom four covalent bonds with the surrounding four silicon or germanium atoms, by using its four valence electrons and fifth electron is left unused. 

            Thus there is an extra electron per atom at the lattice points occupied by P or As or Sb atoms compared to the lattice points occupied by silicon or germanium atoms. 

           According to Band theory, filled energy band of impunity lies in between filled and next empty energy band of parent metal. The extra electrons of impurity can easily be excited to the empty band by the application of heat or electric field. Crystals of pure elements of Si or Ge show increased conductivity due to the presence of extra electrons obtained from the donor impurity atoms and hence the name n-type semiconductors. It is also said that Si or Ge are doped with P or As or Sb which passes n-type semiconductivity. 

Effect of temperature on n-type semiconductor-

               The conductivity on n-type semiconductor increases with the increase in temperature. This is because the electrons of the impurity atoms possess higher energy than the energy of the filled band of the pure element. The tendency of passing these electrons into the higher band increases with increase in temperature. 

                     We know that the number of extra electrons depends upon the number of impurity atoms. Thus if only a small amount of impurity is added to the pure element then the number of extra electrons will be less. This results into the promotion of less number of electrons to the higher energy empty band and hence the increase in conductivity is limited. If there is a large increase in temperature, then the conductivity decreases. This is due to increase in atomic vibrations and electron scattering. 

B) p- type extrinsic semiconductors:

             In a p-type semiconductor the impurity provides an excess of holes. Here 'p' stands for the positive holes in pure element. These are obtained when an impurity atom to be added has less number of valence electrons than the parent atoms. 

Examples: Gallium, Indium etc. 

        In this case three valence electrons of each Ga or In atom form three covalent bonds with three silicon or germanium atoms and the fourth Si or Ge atom is linked with Ga or In atom by an incomplete bond containing only one electron. 

                Thus in this bond there is an electron deficiency in the lattice. This creates a positive hole in the valence band of silicon or germanium. The number of positive holes in Si or Ge are equal to the number of Ga or In atoms. Positive holes are the places where electrons are missing. 

             The electron from the neighbouring atom moves into the positive holes which appear to migrate. The conductivity is due to the movement of positive holes and hence the name p-type semiconductor. 

                According to the band theory: The impurity of Gallium contains an empty energy band just above the filled band of pure silicon or germanium element. The electrons from the filled band of Si or Ge are excited into the empty level of Ga. 

                Silicon or germanium show increased conductivity due to positive holes created Ga atoms and hence the name p-type semiconductors.

Effect of temperature on p-type semiconductor-

               Similar to n-type, the conductivity of p-type semiconductor increases with increase in temperature. The amount of impurity is also important in deciding the conductivity of the element. 

Applications of Semiconductors:

             Semiconductors possess a vast number of applications. The main basis of the modern technology and our high standard of living is the use of semiconductors. Following are some important applications of semiconductors. 

1) In the preparation of thermistors and photocells: Thermistors are used to control temperature in heating devices. Photocells are used for long distance signalling and automation. 

2) They are used in making Thermocouples which are used to produce electricity. 

3) In rectification of an A.C current: semiconductors are used to convert an alternating current into direct current by p-n junction diodes. 

4) In transistors: Transistors are used in the entire field of electronics, p-n-p or       n-p-n junctions can serve as a device of amplification and hence semiconductors are used in transistors. 

5) Semiconductors are used to control current intensity and voltage. 

6) They are used to convert heat energy to electrical energy. 

7) In some chemical reactions semiconductors are used as catalysts. 

8) In television sets etc. 

Important points regarding semiconductors:

1) Generally elements having four valence electrons are moderate conductors of electricity. 

2) Their electrical conductivity is in between that of an insulator and a good conductor. 

3) The valence band is completely filled. 

4) The energy gap between the valence and conductance band by heating or by applying electrical field. 

5) Semiconductors are insulators at very low temperature but become conductors of electricity with increasing temperature i.e. eelectrical conductivity increases with increasing temperature due to availability of more and more number of electrons in the conduction band. 

6) The electrical conductivity at room temperature is from weak to moderate. 

7) The conductivity increases with the addition of impurity like arsenic or indium.