How the junction works

When the N-type material is negative with respect to the P-type, electrons flow easily from N to P. The N-type semiconductor, which already has an excess of electrons, gets even more; the P-type semiconductor, with a shortage of electrons, is made even more deficient. The N-type material constantly feeds electrons to the P-type in an attempt to create an electron balance, and the battery or power supply keeps robbing electrons from the P-type material. This is shown in Fig. 1A and is known as forward bias.
When the polarity is switched so the N-type material is positive with respect to the P type, things get interesting. This is called reverse bias. Electrons in the N-type material

Fig. 1: At A, forward bias of a P-N junction; at B, reverse bias. Electrons are shown as small dots, and holes are shown as open circles.

are pulled towards 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) are the majority charge carriers. They become depleted in the vicinity of the P-N junction (Fig. 1B). A shortage of majority carriers means that the semiconductor material cannot conduct well. Thus, the depletion region acts like an insulator.