Semiconductor Electronics: Materials, Devices And Simple Circuits (Physics) Class-12

Classification of Metals, Conductors, and Semi-conductors

• Metals − Possess very low resistivity (or high conductivity)
  
• Semi-conductors − Possess resistivity or conductivity intermediate to metals and insulators
  
• Insulators − Possess high resistivity (or low conductivity)
  
• Semi-conductors are of two types:
  
  • Elemental semi-conductor − Example: Si and Ge
    
  • Compound semi-conductor − Example: CdS, GaAs, CdSe, InP, etc.
    

• Energy band diagram of metals or conductors
  
  
  • Conduction band is partially filled and the valence band is partially empty or the conduction and valence balance overlap.
    
  • Due to overlap, electrons can easily move into the conduction band. This situation makes a large number of electrons available for electrical conduction.
    
  • When the valence band is partially empty, electrons from their lower levels can move to higher levels making conduction possible.
    

• Energy band diagram for insulators
  
  
  • Large band gap E_g exists. (E_g > 3eV)
    
  • Since there are no electrons in the conduction band, no electrical conduction is possible.
    
  • The electron cannot be excited from the valence band to the conduction band by thermal excitation.
    

• Energy band diagram for semi-conductors
  
  
  • Energy band gap Eg is small. (Eg < 3eV)
    
  • At room temperature, some electrons from valence band cross the energy gap and enter the conduction band.