CLASSIFICATION OF MATERIALS BASED ON ENERGY BAND THEORY

• Based on the ability of various materials to conduct current, the materials are classified as conductors, insulators and the semiconductors.

• A  metal  which  is  very  good  carrier  of  electricity  is  called  conductor.  The copper and aluminium are good examples of a conductor

• A  very poor conductor  of electricity  is termed as  insulator.  The glass,  wood, mica, diamond are the examples of an insulator.

• A  metal  having  conductivity  which  is  between conductor  and  an  insulator  is called  semiconductor.  The  silicon  and  germanium  are  the  examples  of  a semiconductor.  This  does  not  conduct  current  at  low  temperatures  but  as temperature increases these materials behave as good conductors..

ENERGY BAND DIAGRAMS

CONDUCTORS

• In  the  metals  like  copper,  aluminium  there  is  no  forbidden  gap  between valence band and conduction band. The two bands overlap.

• Hence  even  at room temperature, a  large number of  electrons  are  available for conduction.

• So without any additional energy, such metals contain a large number of free electrons and hence called good conductors.

INSULATORS

• In  case  of  such  insulating  material,  there  exists  a  large  forbidden  gap  in between the conduction band and the valence band.

• Practically it is impossible for an electron to jump from the valence band to

• the   conduction   band.   Hence   such   materials   cannot   conduct   and   called insulators.

• The  forbidden  gap  is  very  wide,  approximately  of  about  7  eV  is  present  in insulators. For a diamond, which is an insulator, the forbidden gap is about 6 eV.

                            

• Such  materials  may  conduct  only  at  very  high  temperatures  or  if  they  are subjected to high voltage. Such conduction is rare and is called breakdown of an insulator.

• The other insulating materials are glass, wood, mica, paper etc.

SEMICONDUCTORS

• The  forbidden  gap  in  such  materials  is  very  narrow  as  shown  in  Fig.  Such materials are called semiconductors.

• The forbidden gap is about 1 eV.

• In  such  materials,  the  energy  provided  by  the  heat  at  room temperature  is sufficient to lift the electrons from the valence band to the conduction band.

INTRINSIC SEMICONDUCTORS

• A sample of semiconductor in its purest form is called an intrinsic semiconductor.

• The impurity content in intrinsic semiconductor is very small, of the order of one part in 100 million parts of semiconductor.