Energy Band Diagaram ( Conductor, Semiconductor and Insulator )

Valance electrons or band
  • The electrons in the outer most orbit of an atom are called as valance electrons.
  • The valance electrons have highest energy. 
  • This band may be partially or completely filled. 
  • The partially filled band can accommodate more number of electrons.
Conduction band
  • It is band of electrons which are responsible for conduction of current in the conductor. 
  • All the electrons in the conduction band are free. 
  • The conduction of current through material is not possible if the conduction band is empty. 
  • The insulators have empty conduction band where it is partially filled for conductors.

Forbidden energy band
  • It is gap between conduction band and valance band on the energy level diagram.
  • The width of forbidden energy gap indicates bondage of valance electrons to its atom. 
  • Less the forbidden energy gap, less tightly valance electrons are bound to electrons and vice versa.

Classification of material on the bases of energy bands
  • The electrons of the lower or last orbit are closely bound with nucleus there are they does not play important role in the conduction process. 
  • The valance band and conduction band plays important role to decide electrical behavior of the material.
  • It is substance or material in which electrical current easily passes through it i.e. copper and aluminum
  • The valance band and conduction band overlap each other which create small potential difference. 
  • This potential difference causes flow of electrons which constitute electrical current
  • The energy band diagram is shown in the figure A.


Semi conductor

  • It is substance or material in which small electrical current easily passes through it i.e. silicon, germanium. 
  • The conductivity of semi conductor lies between insulator and conductor. 
  • The valance electron is completely filled whereas the conduction band is almost empty as shown in the figure B. 
  • However the energy gap ( forbidden gap ) between valance band and conduction band is very small ( 1 eV ). 
  • The electrical current allows to flow if small electric field applied across it. 
  • The semiconductor behaves as an insulator at room temperature or low temperature. 
  • However some electrons cross conduction band causes small electric current even at room temperature. 
  • As the temperature increases, more electrons pass through valance band to conduction band resulting conductivity increases. 
  • The semiconductor has negative temperature co – efficient of resistance because as the temperature increases electrical conductivity of semi conductor increases or resistance decreases. 
  • Finally we can say that the semi conductor material has
  Empty conduction band

  Full valance band

  Small forbidden energy gap between valance band and conduction band



  • It is a substance or material in which electric current does not pass through it i.e. wood, glass, plastic etc. 
  • The valance band is full whereas the conduction band is empty as shown in the energy band diagram. 
  • The forbidden energy gap between valance band and conduction band is large ( 15 eV ). 
  • The electrons of the valance band require very high electric field in order to cross it and move in the conduction band. 
  • The electrons of the valance band do not have enough energy to cross and move towards conduction band therefore the conductivity of the insulator is almost zero. 
  • The resistance of the insulator decreases with increase in the temperature therefore it has negative temperature co – efficient.


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