under reverse bias (applying a negative voltage to the p-side with respect to the n-side), the potential drop (i.e., voltage) across the depletion region increases. essentially, majority carriers are pushed away from the junction, leaving behind more charged ions. thus the depletion region is widened and its field becomes stronger, which increases the drift component of current (through the junction interface) and decreases the diffusion component. in this case, the net current flows from the n-side to the p-side. the carrier density (mostly, minority carriers) is small and only a very small "reverse saturation current" flows.