A small quantity of electrons on the n-side of the junction are attracted by the "holes" in the p-side. They drift across the junction, and fill available holes. This causes a region across the junction to be depleted in carriers of current (that is, electrons and "holes"). This region is called the DEPLETION LAYER, because it has been depleted in majority carriers. The depletion zone is a few micrometers thick, and, since it has no majority carriers, acts as an insulating barrier.
It is important to realise that when electrons moves from the n-side of the junction to the p-side, the n-side becomes positively charged (since neutral atoms have lost electrons), while the p-side of the junction now becomes negatively charged (since neutral atoms have now acquired negatively charged electrons). A potential difference now exists across the junction. This potential difference is about 0.6 V for silicon, and about 0.2 V for germanium. This is called the BARRIER POTENTIAL.