States in the conduction and valence bands are extended states while the states in impurity levels are localized states. Band gap remains the same in case of doped semiconductors also. Only Fermi level position shifts towards conduction or valence bands in extrinsic range. When donors or accepter levels get completely ionized, Fermi level position again comes back to the center of the energy gap of the semiconductor.Band gap is defined for the extended states and not for localized states. Impurity concentration in n or p type material is of ppm level and hence they can not form extended states. When impurity concentration becomes very high, a new alloy or compound is formed which has a new band gap. Technically, doping word is used only for impurity concentrations till band gap does not change on incorporation of impurities.

your question is not clear..whether you are asking why to create donor and acceptor levels or how to create.

if it is why....in case of  semiconductors, to enhance the conductivity we dope with an impurity of third or fifth order. so that it will change to p type or n type. Adding impurity will introduce new energy eigen states in the bandgap. so that bandgap decrease thereby allowing carrier flow for less amount of energy. 

if it is how...physically we add impurities to semiconductors by using various techniques. donor levels will be nearer to conduction band and acceptor levels will be nearer to valance band.

I think you are struggling with the fact, that the doping adds additional bands for conduction within the band gap, and this contradicts the principle of it being a band gap, where no energy levels are possible ?.

Now, the band gap being a forbidden zone is only the case for the pure semiconductor, meaning when no impurities are added. Adding impurities, like doping, adds extra conduction and valence bands. For n-doping, a (small) band full of electrons is added just below the top of the band gap, for p-doping a (sall) band full of holes is added just above the band gap.

 Dear Henri ,

Thanks,that is right.

Regards,

Abbas

States in the conduction and valence bands are extended states while the states in impurity levels are localized states. Band gap remains the same in case of doped semiconductors also. Only Fermi level position shifts towards conduction or valence bands in extrinsic range. When donors or accepter levels get completely ionized, Fermi level position again comes back to the center of the energy gap of the semiconductor.Band gap is defined for the extended states and not for localized states. Impurity concentration in n or p type material is of ppm level and hence they can not form extended states. When impurity concentration becomes very high, a new alloy or compound is formed which has a new band gap. Technically, doping word is used only for impurity concentrations till band gap does not change on incorporation of impurities.