Why there is a sharp cut off of the charged region outside the depletion region, like on this image?

For example why don't electrons on the conduction band in the n-type side rush towards the positively charged area making the whole piece positively charged somewhat, not just at the area near the depletion region?

The source of the confusion is that I know if you charge up a regular conductors the internal currents will uniformly distribute the charge along the whole piece, while insulators are only locally charged up, since they cannot carry current. Semiconductors here seem to act like insulators, but diodes do carry current when used. How?

  • $\begingroup$ Because the depletion region has done its job and equilibrated the Fermi energy between regions of different doping. If the Fermi levels are equilibrated then there is no driving force for net carrier redistribution. $\endgroup$ – Jon Custer Jul 25 '15 at 22:27
  • $\begingroup$ @JonCuster the question is not about why does the charge transfer between the pieces stop. The question is. when the pieces are in equilibrium why don't the remaining charge carriers distribute themselves uniformly in each half. Why there aren't any electric field? The + ions are a little closer than the - ions so there should be a little net force from + charges. Or let's see another example: you ground the negative end of a 9V battery and connect the other into piece of semiconductor, will the whole piece charge up to 9V or it only does that right next to the battery only like an insulator? $\endgroup$ – Calmarius Jul 26 '15 at 6:41

Okay, I think I found it out. What confused me is that images of the depletion region are exaggerated. The depletion region actually very thin (in the order of microns) and the two planes of charges are very tightly placed.

This means there is a negligible electric field outside the depletion region for the same reason why there aren't electric field outside the capacitor plates: the two charges are so close together that their effects outside the region cancel.

  • $\begingroup$ I disagree - it has nothing to do with how thick or thin the depletion layer is. It has to do with how much potential difference is needed to bring the Fermi levels in to alignment. $\endgroup$ – Jon Custer Jul 26 '15 at 14:23
  • $\begingroup$ @JonCuster Can you explain it? $\endgroup$ – Calmarius Jul 27 '15 at 9:00

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