BIAS

In a p-n junction, when the n-type material is negative with respect to the p-type material, electrons flow easily from n to p. This is forward bias; the diode conducts well. When the polarity is switched so that the n-type material is positive with respect to the p-type material, it is in a state of reverse bias, and the diode conducts poorly.
When a diode is reverse-biased, electrons in the n-type material are pulled toward the positive charge, away from the junction. In the p-type material, holes are pulled toward the negative charge, also away from the junction. The electrons (in the n-type material) and holes (in the p-type material) become depleted in the vicinity of the junction. This impedes conduction, and the resulting depletion region behaves as a dielectric or electrical insulator.