Forward and Reverse battery bias

In diagram below(a) the battery is arranged that is why the negative terminal supplies electrons to the N-type material. These types of electrons diffuse toward the junction. The positive terminal eliminates electrons from the P-type semiconductor, forming holes that diffuse toward the junction. If the battery voltage is sufficiently great to overcome the junction potential (0.6V in Si), the N-type electrons and P-holes merge annihilating each other. This frees up space in the lattice for more carriers to flow toward the junction. So, currents of P-type and N-type majority carriers flow in the direction of the junction. The recombination at the junction permits a battery current to flow via the PN junction diode. Such type of a junction is said to be forward biased.

Figure: (a) Forward battery bias repells carriers toward junction, where recombination results in battery current. (b) Reverse battery bias attracts carriers toward battery terminals, away from junction. Depletion region thickness increases. No sustained battery current flows.

If the battery polarity is inverted like in Figure above (b) majority carriers are attracted away from the junction in the direction of the battery terminals. The positive battery terminal attracts N-type majority carriers, electrons, away from the junction. The negative terminal attracts P-type majority carriers, holes, away from the junction. This raises the thickness of the non conducting depletion region. There is no recombination of majority carriers; so, no conduction. This arrangement of battery polarity is known as the reverse bias.