Moving electrons and moving holes? How does a hole move?

[KingNothing]

Hi all. I understand that a hole is the absence of an electron. I have come across textbooks with instances where they talk about movement of minority carriers, and movement of majority carriers. Since the hole has to be one of them, they are referring to moving holes.

But, how can a hole move? I can understand movement of an electron just fine.

 

[Jaynte]

Imagine you have a hole with electrons around it, then one electron moves into that hole which in turn creates a new hole where that electron was, hence the hole moves :)

 

[cabraham]

The process involves quantum mechanics, qm, which nobody fully understands. Personally, I believe that a hole is not on equal footing w/ an electron. It is called a "virtual particle" as opposed to an electron which is a "real particle".

But a hole, I believe, is still more than just the absence of an electron. If a hole was a mere void, then energy would not be released when electrons recombine w/ a hole. A vacancy in an atomic shell that is called a hole, due to absence of an electron, becomes filled due to an electron recombining.

Energy in the form of a photon is then released. An LED works this way. If a hole was just a void & nothing more, well, why are photons emitted when an electron occupies the vacancy formerly a hole? Of course, a hole, again, is not a true particle in the fullest sense as is an electron.

But I cannot accept the idea that a hole is nothing more than a void, due to an electron migrating away from the atom. There is energy in the electron shells, with or without an electron present. The fact that photons are emitted whenever electrons recombine w/ holes tell me that this is not an intuitive phenomena. There's more than meets the eye going on here.

Did I help?

Claude

 

[KingNothing]

But a hole, I believe, is still more than just the absence of an electron. If a hole was a mere void, then energy would not be released when electrons recombine w/ a hole. A vacancy in an atomic shell that is called a hole, due to absence of an electron, becomes filled due to an electron recombining.

Did I help?

Yup, I actually understand that. Sorry for not stating it more clearly. I do understand that a hole is the absence of an electron from where it would "normally" be in a valence shell.

Where do the photons go in devices that are not LEDs? Certainly the interaction is happening in PN junctions everywhere. Is this lost as heat?

Imagine you have a hole with electrons around it, then one electron moves into that hole which in turn creates a new hole where that electron was, hence the hole moves :)

I can't totally buy this. There is no guarantee that because an electron filled a hole, there must have been a hole created somewhere else. Maybe it was an extra electron to begin with?

By this definition, holes don't really move at all. They just disappear and reappear in different places.

 

[carlgrace]

Yup, I actually understand that. Sorry for not stating it more clearly. I do understand that a hole is the absence of an electron from where it would "normally" be in a valence shell.

Where do the photons go in devices that are not LEDs? Certainly the interaction is happening in PN junctions everywhere. Is this lost as heat?

Photons are only created in what are called "direct bandgap" semiconductors. These are semiconductors where the bottom of the conduction band (as in least energy) corresponds to the same momentum as the top of the valence band. GaAs and GaN are examples of direct bandgap semiconductors. In the case of silicon (and other devices that are not LEDs) the energy is dissipated through lattice vibrations. These vibrations can be treated mathematically as if they were particles and in that case they are called "phonons".

I can't totally buy this. There is no guarantee that because an electron filled a hole, there must have been a hole created somewhere else. Maybe it was an extra electron to begin with?

By this definition, holes don't really move at all. They just disappear and reappear in different places.

In some sense you are right. There *is* no guarantee. Sometimes the holes are filled by free electron and sometimes free electrons fall into what are called "traps" which are holes or allowable energy states. This can be troublesome in certain devices. Generally, though, the effect of this is minor and holes act just like a positive electron (except with lower mobility to account for the jumping in and out of valence shells). The effect is minor because the "extra electrons" due to dopants is TINY compared to the number of lattice sites in a semiconductor. So, what you said is totally possible, but it just doesn't happen all that often.

 

[carlgrace]

The process involves quantum mechanics, qm, which nobody fully understands. Personally, I believe that a hole is not on equal footing w/ an electron. It is called a "virtual particle" as opposed to an electron which is a "real particle".

But a hole, I believe, is still more than just the absence of an electron. If a hole was a mere void, then energy would not be released when electrons recombine w/ a hole. A vacancy in an atomic shell that is called a hole, due to absence of an electron, becomes filled due to an electron recombining.

Energy in the form of a photon is then released. An LED works this way. If a hole was just a void & nothing more, well, why are photons emitted when an electron occupies the vacancy formerly a hole? Of course, a hole, again, is not a true particle in the fullest sense as is an electron.

But I cannot accept the idea that a hole is nothing more than a void, due to an electron migrating away from the atom. There is energy in the electron shells, with or without an electron present. The fact that photons are emitted whenever electrons recombine w/ holes tell me that this is not an intuitive phenomena. There's more than meets the eye going on here.

Did I help?

Claude

The term "virtual particle" has a technical definition in physics and a hole does not qualify. A hole is absolutely a particle in the same way as an electron as far as . If you look at traffic from an airplane you see holes propagating down the freeway as people speed up and then are forced to slow down. Try looking some time.

And the release of a photon (or a phonon) when holes and electrons recombine isn't anything "more than meets the eye". It can be explained using the Bohr model of the atom. An electron falling into a bound state must release energy because its momentum is changing.

 

[Bob S]

In semiconductors, electrons and holes move with a velocity proportional to the electric field.

v = μE

where μ is the mobility (cm²/Volt-sec) and E is the electric field (volts/cm)

Typical electron mobility μ for Si at room temperature (300 K) is 1400 cm²/ (V·s), and the hole mobility is around 450 cm²/ (V·s).

See http://en.wikipedia.org/wiki/Electron_mobility

Bob S

 

[cabraham]

The term "virtual particle" has a technical definition in physics and a hole does not qualify. A hole is absolutely a particle in the same way as an electron as far as . If you look at traffic from an airplane you see holes propagating down the freeway as people speed up and then are forced to slow down. Try looking some time.

And the release of a photon (or a phonon) when holes and electrons recombine isn't anything "more than meets the eye". It can be explained using the Bohr model of the atom. An electron falling into a bound state must release energy because its momentum is changing.

Referring to bold quote, you're putting a hole on equal footing w/ an electron regarding being a full particle, then using a car analogy as support. I don't think they are the same. Your analogy makes the case for a hole being merely a void. A void does not exist outside an array of true particles. If the highway had but one car on it, there is no void. But there is the one car. A hole can exist only when there are multiple cars w/ a space in between 2 of them. This analogy equates a hole with a mere void.

Regarding the Bohr model, I only brought up the release of energy to illustrate that there is more than meets the eye regarding electrons & vacant shell states. You claim that there isn't more than meets the eye, then invoke Bohr model. But the Bohr model is indeed "more than meets the eye". The vacant state in the shell has a specific energy. An electron filling that state releases some energy. The car analogy does not behave like that. When a car moves into an empty space, energy is not radiated.

That is what I meant with "more than meets the eye". I don't think the Bohr model fits the cars on a freeway example you gave. Just my 2 cents.

Claude

 

[carlgrace]

Referring to bold quote, you're putting a hole on equal footing w/ an electron regarding being a full particle, then using a car analogy as support. I don't think they are the same. Your analogy makes the case for a hole being merely a void. A void does not exist outside an array of true particles. If the highway had but one car on it, there is no void. But there is the one car. A hole can exist only when there are multiple cars w/ a space in between 2 of them. This analogy equates a hole with a mere void.

Regarding the Bohr model, I only brought up the release of energy to illustrate that there is more than meets the eye regarding electrons & vacant shell states. You claim that there isn't more than meets the eye, then invoke Bohr model. But the Bohr model is indeed "more than meets the eye". The vacant state in the shell has a specific energy. An electron filling that state releases some energy. The car analogy does not behave like that. When a car moves into an empty space, energy is not radiated.

That is what I meant with "more than meets the eye". I don't think the Bohr model fits the cars on a freeway example you gave. Just my 2 cents.

Claude

I agree, the analogy does not hold up... that is why it is an analogy and not an explanation. Of course a hole cannot exist without an electron, but as a charge carrier in a semiconductor it can act as if it were independent. The reason is there is a practically infinite (as far as our purposes usually go) number of electrons, so we can have many holes doing their thing. Minority carrier devices such as BJTs *depend* on holes acting as a particle. Certain classes of image senors collect holes, not electrons. They exist as a particle as much as an electron does, since an electron is *merely* a probabilistically located packet of charge that exhibits wave and particle-like qualities.

I guess my point is that it is useful to think of them as particles. Thinking of electrons as particles is also a useful analogy, but not one that is fundamentally more correct, in my opinion.

As for the *more than meets the eye* thing, I wasn't trying to be rude. I have studied enough over the years that this electron/hole behavior has become somewhat intuitive to me.

 

[Bob S]

The "hole" in semiconductors is analogous to a bubble in water or carbonated drink. The bubble is a void, has no mass, moves only under the influence of the gravitational force (analogous to the electric field), but moves in the opposite direction to (is "repulsed" by) the normal gravitational force. So without water, the bubble does not move. The water is equivalent to a sea of electrons, and the bubble a void in the sea. Gravity is equivalent to the electric field that attracts water and repels voids.

Bob S

 

[dlgoff]

Just to add to Bob S post,

In an intrinsic semiconductor like silicon at temperatures above absolute zero, there will be some electrons which are excited across the band gap into the conduction band and which can produce current. When the electron in pure silicon crosses the gap, it leaves behind an electron vacancy or "hole" in the regular silicon lattice. Under the influence of an external voltage, both the electron and the hole can move across the material. In an n-type semiconductor, the dopant contributes extra electrons, dramatically increasing the conductivity. In a p-type semiconductor, the dopant produces extra vacancies or holes, which likewise increase the conductivity. It is however the behavior of the p-n junction which is the key to the enormous variety of solid-state electronic devices.

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/intrin.html#c4"