When a p-type impurity is added in a pure semiconductor, a large number of holes are created. Thus, each impurity atom is deficient of one electron and they accept the electrons which are loosely bound to other neighboring atoms. So, each impurity atom acquires negative charges. The number of negatively charged impurity atoms and the number of holes are equal in p-type semiconductor. Thus, it is electrically neutral. But when holes diffuse from the p-side to the n-side in a p-n junction or in other words we say that when an electron diffuses from the n-side to the p-side in a p-n junction, it leaves a vacant place in the n-side region and thus the n-side gets positively charged. Also, the p-side gets negatively charged due to the diffusion of electrons from the n-side.