How depleton region penetrates more in lightly doped region and less in highly doped region?

Answer 1

A depletion region is formed when there is a junction of p and n type semiconductor impurities. As one moves across the junction from one end to the other, one sees a change in the free carrier density from naturally high on one side to naturally low on the other. It is true of both n and p type carriers in every juntion, albeit in opposite directions. This gradient of charges can create an avalanche of current except that it is counteracted by a strong electric field at the junction; the greater the gradient of mobile charges, the larger the electric field. The region of this electric field is called the depletion region because the free electric carriers are less dense in the region than they naturally occur. The loss of these carriers leaves charged semiconductor atoms in the region in their wake. By Coulomb's law, the total number of charged atoms required is fixed by the amount of electric field and the two are proportional. Hence, in a lightly doped semiconductor, the depth to which the electric field penetrates (i.e the depth of the depletion region) is larger because the density of charged atoms is low.