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 V_i 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 eV_i 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