Explain term 'depletion layer' across a p-n junction

Rectifying action of p-n diode can be explained on the foundation of electronic structure of the semiconductor. When a pure semiconductor is doped to become n-type, Fermi level shifts up from the middle of the energy gap towards donor level. This is so because position corresponding to 505 probability of occupation moves up because of the relatively high concentration of donor electrons in conduction band. If crystal is p-type, Fermi level shifts down towards the acceptor level. When same crystal is doped to become ntype on one side and p-type on the other side, Fermi level has to be constant throughout the crystal in thermal equilibrium. This results in electron energy levels at the bottom of the conduction band in n-part to be lower than those in the p-part, by an amount equal to contact potential eV_o as demonstrated in Fig a. Contact potential at the junction gives rise to energy barrier or depletion layer.