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The P-N Junction. If the crystal was neutral before diffusion took place, it must be neutral afterwards

If the crystal was neutral before diffusion took place, it must be neutral afterwards. Further, because both regions were also originally neutral they must contain equal and opposite charges after diffusion. These charges have an attractive electric force between them and are not able to diffuse away from the vicinity of the junction. The two charges are concentrated immediately adjacent to the junction, and produce a potential barrier across the junction.

The polarity of the potential barrier is such as to oppose the further diffusion of majority charge carriers across the junction, but to aid the movement of minority charge carriers; this gives rise to a minority charge carrier current in the opposite direction to the diffusion current.

The difference in potential from one side of the junction to the other is called the height of the potential barrier and is measured in volts. The height of the potential barrier attains such a value that the majority charge carrier (diffusion) currents and minority charge carrier current are equal and so the net current across the junction is zero. Any charge carriers entering the region on either side of the junction over which the barrier potential is effective are rapidly swept out of it, and hence this region is depleted of charge carriers.

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