During formation of $p-n$ junction electrons from $n$ side and holes from $p$ side move towards each other and form a potential barrier across the junction. And the space-charge regions on both the sides of $\mathrm{p}-\mathrm{n}$ junction which has immobile ions and entirely lacking of any charge carries will form a region called depletion region of a diode. The number of ionized acceptors on the p-side equals the number of ionized donors on the $\mathrm{n}$-side.