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.

Conductor

• 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

Insulator

• 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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