Dear Shilendra,

Quantum mechanically, by the process doping, charge defects, such as polaron, bipolaron and soliton are introduced, which could then be available as the charge carriers. According to Band theory,  doping change host materials band structures by either taking electrons from the valence band or adding electrons to the conduction band. There are several process like Chemical doping, Electro-chemical doping, photo doping, Charge-injection doping, non-redox doping and Secondary doping, by which the conductivity of host materials increases.  

e.g. The protonic doping of poly-(aniline) base (PANI-EB) with aqueous protonic acids to produce conducting poly-(semiquinone) radical cations via the reaction (non-redox doping). 

Reference: F.Garnier, R.Hajlaoui, A.Yasser, P.Shrivastava: Science, 265, 1684 (1994)

With best wishes.

Thank You Dear Rinkesh for replying and discussion.

I will go through this reference.