The conductivity arises form mobile charge carriers of the semiconductor. 

Types of matter

Mobile charge carriers



Ionized gas

Electrons & positive charged ions


Position ions and negative ions



Bohr's Atomic Model


The atomic model is proposed by Bohr in 1913. The Bohr has made following assumptions.

  • The atom has massive positive charged nucleus.
  • The electrons revolve round their nucleus in circular orbits. The centrifugal force between electron and nucleus is balanced by electrostatic pull.


Harmonic Resonance


  • Sometimes harmonic resonance occurs between power capacitor and transformer which may cause high magnification of harmonics. 
  • The resonance can happen at one particular frequency is called as resonance frequency ( fr )  in a system in which inductive reactance       ( XL ) and capacitive reactance ( XC ). 
  • The net impedance becomes low when the inductive reactance XL is equal to capacitive reactance XC


Atoms, Molecules & Ions



  • The smallest particle of any object is called as atoms. 
  • Every object is made of atoms. 
  • However, the entire world is made of matters. 
  • The atoms are very small and they measured in the nanometers.



  • According to Bohr , an atom is composed by number of electrons moves in circular or elliptical orbits around the center nucleus. 
  • The nucleus consists of protons and neutrons.
  • Mass of electron = 9.1 × 10 – 31 kg
  • Charge of electron = 1.6 × 10 – 19 coulomb


It is defined as the flow of charge from high density region to low density region without any external applied field. 

  • The flow of charge is due to non - uniform distribution of charge carriers in the semiconductor crystal. 
  • This will result in flow of current without any external applied field. 


Drift & Drift Velocity

  • The directed motion of the charge carriers ( electrons + holes ) in the semiconductor done mainly by ( 1 ) Charge drift ( flow ) under the influence of electric field ( 2 ) Charge drift from high charge density to low charge density