When P and N type semiconductors are connected together, some of the electrons drift from the N type into the P type to recombine with holes. This leaves positive ions on the N type and negative ions on the P type. Therefore an electric field exists across the depletion zone that stops further drift from electrons. This is also the potential that an external voltage source must exceed in order for current begin flowing.
But once current starts flowing, why does the built in voltage still remain at the depletion zone? Once current begins flowing, don't the electrons that drifted over the PN junction at the beginning now flow forward? Why is there still an electric field across the junction?