A depletion region consists of motionless charges such as positive ions and negative ions. The mobile charge carriers such as free electrons are absent in depletion region. The p-side of the depletion region has negative ions and n-side of the depletion region has positive ions.

The atom which loses an electron is called positive ion and the atom which gains an extra electron is called negative ion. Positive ions are ready to accept an electron and negative ions are ready to accept a hole.

Doping means adding impurities to the semiconductor to improve its electrical conductivity. When we add large amount of impurities to the semiconductor, it will produces large number of free electrons in the n-type semiconductor and large number of holes in the p-type semiconductor.

The large number of free electrons in the n-type semiconductor repels from each other and try to move towards p-side. However, before entering into p-side, the free electrons meet positive ions at the depletion region. We know that positive ions are ready to accept extra electrons. When the positive ions accept the extra electrons, they become neutral atoms. In this manner large number of free electrons fills the holes in positive ions and makes them neutral.

In the similar way, holes moving from p-side to n-side meets the negative ions and makes them neutral atoms. In this manner, free electrons and holes reduce the ions. Reduction of positive ions means reduction of depletion region. Thus, the depletion region decreases.