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This question applies to a small enough scale. The depletion region in a diode is charge polarized, generating an electric field between positive and negative charge carriers (protons and electrons), "holes" and electrons contributed by P and N type materials, often silicon doped with phosphate and boron. When two diodes are sandwiched against one another, as in an NPN or PNP transistor, the negatively charged ends of the depletion regions will phase one another. If shrunk down to a small enough scale, would there be a repulsion effect between these layers (where negative phases negative in NPN and positive phases positive in PNP), shrinking the width of the depletion regions?

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No. If using the depletion approximation, there is no electric field outside the depletion region.

Based on a more accurate model, there is a very small field, so you would probably have some effect like this, but not enough to care about.

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  • $\begingroup$ i understand it is irrelevant for diodes, just wondering in theory $\endgroup$
    – user21427
    Dec 16, 2021 at 12:31
  • $\begingroup$ Best analyzed from equilibrium point of view. The electric field outside the depletion region disappears only as the depletion region has formed, and behaves like a charged capacitor and an open switch. The circuit has settled. When two diodes are sandwiched, as in a transistor, the electric forces will settle in a slightly different equilibrium. This is not noticeable at the scale of transistors. But, in theory, at a small enough scale, it must be noticeable and there must be a repulsion effect, just based on basic physics. $\endgroup$
    – user21427
    Dec 17, 2021 at 6:02
  • $\begingroup$ @user21427 The depletion region doesn't form the way it sounds like you are describing. You don't make junctions by bringing together a block of p-type and a block of n-type semiconductor. The closest real method would probably be epitaxial growth in MBE, but its still not the same. You have to remember that you can't scale p-n junctions down to arbitrarily small dimensions since solids are made out of particles of finite size. There is a minimum layer size for the band diagram you drew. If you have single layers of atoms, even just a single p-n junction would be different. $\endgroup$
    – Matt
    Dec 17, 2021 at 13:12
  • $\begingroup$ @user21427 There need not be a repulsion effect outside the depletion region. The charge is compensated and exactly cancels out. Where does the field come from? $\endgroup$
    – Matt
    Dec 17, 2021 at 13:13
  • $\begingroup$ re: "You don't make junctions by bringing together a block of p-type and a block of n-type semiconductor", from all I have read, sure sounds like bringing together a p-type block and n-type block must form a junction and depletion region. that it would not seems to contradict basic physics. then that it is not practically how it is done, is another thing. I might have reasoned in a way that is "impractical" but it is because the question was very much just in theory. $\endgroup$
    – user21427
    Dec 18, 2021 at 13:35

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