|Why can you force the current from the n-type semiconductor to the p-type after a p-n junction has been created but you can't force current from the p-type to the n-type?
Actually, you are asking about a current of electrons, which carry a negative charge. It's true that electrons can't be sent across the p-n junction from the p-type side to the n-type side. There are several things that prevent this reverse flow of electrons. First, there is an accumulation of negative charge on the p-type side of the p-n junction and this negative charge repels any electrons that approach the junction from the p-type end. Second, any electron you add to the p-type material will enter an empty valence level. As it approaches the p-n junction, it will find itself with no empty valence levels in which to travel the last distance to the junction. It will end up widening the depletion region—the region of effectively pure semiconductor around the p-n junction; a region that doesn't conduct electricity.