Why deplation width of pn junction is increases with the less of impurities atom ?

Question

with no external voltage is given,in pn junction if there is a few impurities atom then the deplation width is increases?

Answer 1

Because the PN junction without a depletion region is an unstable state.  Meaning that there will be a voltage between the positive ion side and the negative ion side.    

As a result valence electrons will flow towards the junction and combine with the P (positive ions i.e. holes) resulting in a build up of negative ions on the P side and because the negative ions were pulled from the N side there will be a build up of positive ions on the N side.  When an equilibrium is reached between the P and N sides you will be left with a depletion region that is larger.

By the way, I believe that it is the impurities that will give you more valence electrons to work with which I guess would result in potentially larger depletion region.  Also I hear that increasing the impurities may increase the electron energy levels allowing for a larger depletion region.