Concentration of electrons in the conduction band
At equilibrium, there is no net current flowing across p-n junction. Concentration of electrons in the conduction band in conduction band on p-side is small. These electrons can accelerate down the potential hill across the junction to the n-side resulting in a current Io that is proportional to their number. Concentration of electrons in the conduction band on n-side is large in comparison, because of the donor contribution. Though, only a small number of these electrons can flow to the p-side across the junction as they face a potential barrier.
If an external voltage Vi is now applied to crystal such that p-side becomes positive with respect to the n-side the electron energy levels would change as demonstrated in Fig b. Barrier at the junction is now lowered by an amount eVi resulting in a greatly enhanced current flow in the forward direction which is from the n-side to p-side. This change is barrier doesn't affect flow of electrons in the reverse direction, from p-side to the n-side, as the flow here is still down potential hill. So applied voltage causes a large net current flow in the forward direction