Because of the insulating gap in the silicon bar, if we connect a battery to both ends of the crystal, no electrical current will flow.
This is equivalent to an electrical switch in the "open" (unconnected) position.
So the question is how to "close" the switch and make current flow.
The answer is to provide something that will pull the extra electrons into the insulating gap with a force that is stronger than the attraction they have to the positive phosphorus ions at the ends.
