4 Biased diode

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A voltage V has been applied to the diode so that the chemical potentials at the two ends of the diode are different. Check out the variation with Vapplied of:

Forward bias or positive Vapplied, by reducing the barrier, allows electrons to flow easily from left to right over the barrier to the region where the chemical potential is lower. If you stand on your head, the same remarks hold for the holes. There can be a large current flow in the forward direction.

For reverse bias or negative Vapplied, electrons would like to flow from right to left where the chemical potential is lower, but there are so few electrons (minority carriers) available in the p-region that the reverse bias current remains very small.