18 December 2017

Intrinsic Semiconductor and Extrinsic Semiconductor

Intrinsic semiconductor

  • The semi conductor material in its purest form is called as intrinsic semi conductor.
  • The electron – hole pairs are created even at room temperature in an intrinsic semiconductor. 
  • When an electric field is applied to an intrinsic semi conductor material is applied, the conduction of current takes place by holes and electrons. 
  • Due to thermal energy, some of covalent bonds are broken and free electrons and holes are produced. 
  • Therefore the conduction of current is done by electrons and holes.  
  • However the current through intrinsic semiconductor in the external circuit is done by only electrons
  • The holes are move towards negative terminal as they are positive charged. 
  • As the holes moves from semi conductor material to negative terminal, the electrons enter the semiconductor material from negative terminal of supply and combine with holes therefore cancelling each other.
effect-of-semiconductor-on-intrinsic-semiconductor.png

Extrinsic semiconductor

  • The conductivity of semiconductor is increased by adding some impurities in pure semiconductor. 
  • When some impurities are added to pure semiconductor, it is called as extrinsic semiconductor. The process of adding impurities in the semiconductor is called as doping. 
  • One impurity atom is added for every 108 atoms of semiconductor. 
  • The number of free electrons or holes in the semiconductor crystal increases by adding impurities. 
  • However the free electrons or holes depend upon types of impurities added to pure semiconductor material. 
  • When some pentavalent impurities are added to pure semiconductor, large number of free electrons produced in the semiconductor. 
  • Similarly when some trivalent impurities are added to pure semiconductor, large number of holes produced in the semiconductor. 
  • There are two types of extrinsic semiconductor material.

P type semiconductor 

  • When a small amount of trivalent impurities are added to semiconductor material, it is called as P type semiconductor material. 
  • When a large number of trivalent impurities are added to semiconductor material, large number of holes is created in the semiconductor. 
  • Some of examples of trivalent impurities are
        Aluminium ( Atomic number 13 )
        Boron ( Atomic number 5 )
        Gallium ( Atomic number 31 )
         Indium ( Atomic number 49 )
  • The semiconductor material with trivalent impurity is shown in the Figure B.     
      
trivalent-impurity-added-to-silicon.png

  • When a small amount of trivalent impurities are added to silicon crystal, large number of holes are created in the crystal. 
  • The gallium is trivalent it means that it has three valance electrons whereas the silicon has tetravalent it means that it has four valance electrons. 
  • When a gallium impurity is added to silicon crystal, only three co – valent bonds are formed. 
  • As there are three valance electrons in the gallium, one hole is created in the silicon crystal.  
  • This short of electron is called as hole
  • Therefore we can say that each gallium atom creates one hole.  
  • As the impurities are added, more holes are created in the silicon crystal. 
  • There is more number of holes rather than electrons in the P type semiconductor material. As the holes are positive charged it is called as P ( positive ) type semiconductor material.
Majority carriers – Holes
Minority carriers – Electrons

Current flows due to P type material

  • The conduction of current in the P type material is done by holes
  • It does not mean that electrons do not play any important role for flow of current in the P type material. 
  • The electrons are minority carriers whereas the holes are majority carriers in the P type material. 
  • As the charge of holes is positive, they move towards negative terminal of the supply which is known as hole current.                    

effect-of-external-field-on-p-type-semiconductor.png

N type material

  • When a small amount of pentavalent impurities are added to semiconductor material, it is called as N type semiconductor material. 
  • As large number of pentavalent impurities is added to semiconductor material, large number of electrons is created in the semiconductor.  
  • Some of examples of pentavalent impurities are
        Phosphorus  ( Atomic number 15 )
        Arsenic ( Atomic number 33 )
        Antimony ( Atomic number 51 )
        Bismuth ( Atomic number 83 )
  • The semiconductor material with pentavalent impurity is shown in the Figure D.
                       pentavalent-impurity-added-to-silicon.png
  • When a small amount of pentavalent impurities are added to silicon crystal, large number of electrons are created in the crystal. 
  • The phosphorus is pentavalent it means that it has five valance electrons whereas the silicon has tetravalent it means that it has four valance electrons. 
  • When a phosphorus impurity is added to silicon crystal, four covalent bonds are formed. As there are five valance electrons in the phosphorus, one electron is created in the silicon crystal.  
  • Therefore we can say that each phosphorus atom creates one electron.  
  • As the pentavalent impurities are added, more electrons are created in the silicon crystal. 
  • There is more number of electrons rather than holes in the N type semiconductor material. As the electrons are negative charged it is called as N ( Negative ) type semiconductor material.
Majority carriers – Electrons
Minority carriers – Holes

Current flows due to N type material

  • The conduction of current in the N type material is done by electrons.
  • It does not mean that holes do not play any important role for flow of current in the N type material. 
  • The holes are minority carriers whereas the electrons are majority carriers in the N type material. 
  • As the charge of electron is negative, they move towards positive terminal of the supply which is known as electron current.                  

effect-of-external-field-on-n-type-semiconductor.png