A p-n junction is created when an n-type semiconductor and a p-type semiconductor are connected. The electrons from the n-type flow to the p-type while the holes from the p-type flow in the opposite direction. This is due to a concentration gradient. The flow of these free carriers creates a depletion region between the n-type and p-type regions, which stops additional flow of free carriers due to drift. An applied voltage can reduce the the effects of the depletion region and allow carrier flow to continue. The carrier flow creates current.