BIAS:

In a p-n junction, whenever the n-type material is negative with respect to the p-type material, the electrons flow simply from n to p. This is forward bias; the diode conducts well. Whenever the polarity is switched therefore the n-type material is positive with respect to the p-type material, it is in a condition of reverse bias, and the diode conducts very badly.

Whenever a diode is reverse-biased, then electrons in the n-type material are pulled to the positive charge, away from the junction. In p-type material, holes are pulled to the negative charge, and also away from the junction. The electrons in the n-type material & holes in the p-type material become depleted in the locality of the junction. This impedes conduction, and the resultant depletion area behaves as a dielectric or electrical insulator.