Doping is a process of mixing sum impurity in a iinsulator and then that acts like conductor...The materials which are said to behave like this are called semi conductor .
it is very often to know that when we add some impurity ,the bonding inside the semi conductor just breaks and rejoins itself with new commer electrons of the impurity...major semi conductors are GaAs,Si,Ge ...we can notice out that if 1 electron is added to silicon(si),it reffers to a conductivity of 10 million electrons...so just think of this example,when we add extra impurity...what would happen?????????resulting a great vacancy for electrons to continueue their activities.
Coming to depletion region...it is just a smaal region where ions are formed + on N types and - to P Types .though it is very small but it cannot be neglected ....Hence we improve the ratio of impurity added.....the free electrons available are also in a huge number so a litttle space for the Depletion region remains there but it never ends....i-e depletion region can not be removed finally