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There's one (well, probably more, but right now I'm hung up on this one) aspect of P-N junction diodes that doesn't quite make sense to me.

I understand that doping introduces free electrons in the N-doped region and holes (or empty orbitals?) in the P-doped region, and that because of the density gradient, the free electrons will cross the junction, creating anions in on one side of the depletion zone. Those free electrons will then "fall" into the holes in the P-doped region, creating cations in the other side of the depletion zone, and that because these anions and cations are fixed in the lattice, an electric field will build up that will, at equilibrium, exactly equal the diffusion force.

But here's what's throwing me - in almost every explanation I read, they reference the holes diffusing as well and try as I might, I just can't "see" that. I do see the holes disappearing as they are occupied in the P-doped region by diffused electrons, but I can't grasp the idea that the holes are also moving across the junction to the N-doped region, effectively neutralizing the free electrons. There is one site that I've found that does not refer to hole diffusion in their explanation of the creation of a P-N junction diode, which is the HyperPhysics site hosted by Georgia State University. But since every other explanation I've seen does, I have to ask - are the holes really diffusing as well, or is it really just that cations are forming when the free electrons diffuse, and so we think of that as a hole being created?

Thanks.

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  • $\begingroup$ If an electron-hole pair (EHP) is created in the depletion region, the field through the depletion region will certainly separate the pair, the electron will propagate one way while the hole will propagate the other. Do you see this or does it throw you too? $\endgroup$ – Alfred Centauri May 9 '17 at 1:13
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Let us picture just the p-doped region. We have the atoms that make up the semiconductor, and on some lattice sites acceptor atoms. In a classical picture, an electron from a neighboring atom is attracted by this acceptor atom and forms a "hole", because the atom that electron came from is now missing an electron. Now to your question. In this state, an electron from any of the neighboring atoms might jump (thermally activated) to this atom that is missing an electron, thus effectively moving the hole one atom over. As this process continues and the hole is filled again and again by electrons from neighboring atoms, we might also view this as a hole diffusing about.

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