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I was studying pn junctions and came across the concepts of diffusion currents. From what I understand is, when the junctions come together, since there is an excess of donors on the n side and an excess of a accdeptor impurities on the p side the concentration gradients force a cross junction diffusion phenomena. Now what I would like to know is that what are the particles that move to create the diffusion currents. The notes from where I'm learning says holes and electrons. Now the actual movement of electrons make sense, because atomic ionization is a pretty ordinary event, but what does it mean when they say that a "hole" crosses the junction from the p side to the n side? Does the entire acceptor atom separate itself from the crystal lattice of the semiconductor and cross the junction? It doesn't really make sense to me? Or is it some exotic effect which I'm unaware of?

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In hole movement, the particles that are moving are still electrons. When an electron moves to the conduction band (i.e. at any temp above 0 K), there is an empty state that is created in the valence band that was originally occupied by the electron. This empty state is the hole. If another electron from the valence band moves to occupy this hole, it creates another hole one atom over. A chain of such electron movements could be thought of as a hole moving.

I think this is simplifying things quite a bit, but it gets the gist, and I don't really understand the more advanced version. :)

Edit : Actually, I'll try to include the advanced version, by just quoting from the excellent Daniel Mittleman from this awesome thread on the very related topic of 'Are holes real?', since most people probably can't access that thread.

... no such thing as a lone electron or, ... a lone hole - inside any solid. Any charged particle will interact with all of the other electrons, and nuclei, in the solid ...

... [With] These interactions .. taken into account ... One ends up describing what are called 'quasi-particles', which are excitations of the solid that, in some way, resemble a lone electron or a lone hole ...

It is not just semantics - [electrons and holes] are both equally real excitations of the many-body state of the solid.

So, while the simple picture generally enough for getting the drift of things, there's more beyond it, and there's a bit more to holes than just the absence of electrons.

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  • \$\begingroup\$ Now I just wanted to clarify something else, a hole is different from an ionized donor particle. A hole is a broken/unformed covalent bond, right? If that is so, then when electrons diffuse to transport holes, it needs to be that a formed covalent bond from the other side gets broken, then the free electron created in the process moves via diffusion to the other side and satiates a hole completing an unformed covalent bond, right? Hence recombination of holes with diffused donor electrons can never transport holes, right? I know I'm asking the question and answering it myself, just clarifying \$\endgroup\$ – ubuntu_noob May 4 '14 at 23:20
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    \$\begingroup\$ That sounds okay to me but I think (for a solid) it's much better to think of the electrons and holes as occupying quantum states than to think in terms of the covalent bonds being broken and formed (which is more appropriate for just a molecule). \$\endgroup\$ – sgauria May 5 '14 at 1:04
  • \$\begingroup\$ Would upvote unless “another electron… moves to occupy this hole, it creates another hole one atom over”. Yet another apology of the checkers board heresy. States in the band structure are not “one atom over”, dammit. \$\endgroup\$ – Incnis Mrsi Aug 24 '16 at 19:35
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One of my instructors explained it this way:

Imagine taking a shovel outside, and digging a small hole. Now, a few feet away, dig a new hole and put the dirt back in the old one. Do this again a few times, in new locations, always putting the new dirt into the previous hole.

You only ever have one empty hole, but it "moves" around!

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  • \$\begingroup\$ Another example from one of my instructors. Take a carton of eggs. Open it, and cook the egg on the left. Now, take the next egg (just to the right of the one we just cooked, call it "egg2"), and move it to the left one. Now take egg3 and move it to where egg2 was. Now take egg4 and move it to where egg3 was. What you have is the "hole" moving to the right, even though it's actually accomplished by moving the eggs to the left. \$\endgroup\$ – ajs410 May 2 '14 at 21:00
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Just wanted to clarify something (the other answers already addressed that what moves are electrons, what holes are, and that ions have a fixed position).

Diffusion is simply the inevitable statistical result of the random motion of free particles whenever there is a density gradient. There are no special forces, just random movement. The inevitable result is that particles end up moving from high density areas to lower density areas. This is normally called "diffusion current", and it is true for any free particle, and doesn't need to have any charge.

In a semiconductor these particles (electrons/holes) do have charge, so what ends up happening is that this displacement creates electrical fields, which generate their associated drift currents. Static equilibrium is usually achieved when these two current components are equal.

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  • \$\begingroup\$ Now I just wanted to clarify something else, a hole is different from an ionized donor particle. A hole is a broken/unformed covalent bond, right? If that is so, then when electrons diffuse to transport holes, it needs to be that a formed covalent bond from the other side gets broken, then the free electron created in the process moves via diffusion to the other side and satiates a hole completing an unformed covalent bond, right? Hence recombination of holes with diffused donor electrons can never transport holes, right? I know I'm asking the question and answering it myself, just clarifying \$\endgroup\$ – ubuntu_noob May 4 '14 at 23:19
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Atoms in the lattice shouldn’t walk. Electrons occupy states in the band structure. Each state has the momentum and the energy. The hole is an empty state in the valence band (whereas most of them are occupied with electrons). Momentum of a hole is opposite to electron’s momentum in the respective state, because the hole is the absence of electron. Acceptor atoms have the atomic number 1 less than “correct” and thus, an electrically neutral p-type semiconductor is short of electrons to fill the valence band; hence, it has holes. Really, it’s almost all you should know, and don’t take seriously all the usual tales about eggs carton, checkers board, etc. You haven’t to know how states are localized with atomic precision; the Fermi–Dirac statistics permits us to operate without such a knowledge.

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