Now imagine we also add phosphorus impurities to the other end of this crystal giving it conducting regions at both ends.
The extra electrons cannot cross the gap in the middle of the crystal. If, for instance, the extra electrons from the left moved toward the center, they would leave behind the phosphorus ions, each with a +1 net charge. The positive charge of all these ions would then act on the wandering electrons? negative charges (via an electric field) to pull them back into that end.
We are thus left with an electrically insulating gap in the center.
