Talk:Band gap

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Old converstion between Tim and Dgrant[edit]

I think that image could be better. I don't think having multiple gaps all over the place really helps explain it. --User:Dgrant

*shrug* -- Tim
So you think that your picture is the best possible picture and that we shouldn't strive to get a better one, should one exist? --User:Dgrant
No, "shrug" as in "whatever". As far as I'm concerned it's just a matter of opinion, so I wasn't prepared to argue it. If you want to change it to something better, I won't stop you.
Great. Well, if I have time to find something more suitable, I'll put it on. Actually, I find the whole copyright thing annoying so I'll probably drawn one instead. --User:Dgrant

I just realized that I think I might have deleted some significant chunks in my recent edit of the Band gap article. I just finished reading the Wikipedia most common faux pas, and apparently is it not good to delete lots of material. --User:Dgrant

Hell no. In fact, I'm pretty annoyed at the overall quality of your recent edits. Your two new articles (direct bandgap and indirect bandgap) were not introduced properly, and were too hard to understand for someone who didn't already know it. Plus they were conceptually wrong - silicon absorbs light quite well, despite its indirect bandgap. And you don't use GaAs for fibre lasers, you use erbium doped glass.
Hmm, I wasn't too thrilled with the quality of the article when I first read it, which is why I made some changes. In fact many parts of it were rather shoddy. For example:
"An arbitrary figure can be chosen - say, 3 eV, above which the material will be called an insulator." arbitrary figure can be chosen??
Point taken.
"Band gap decreases with increasing temperature, due to thermal expansion" due to thermal expansion??? WTF
It's true. If you want to explain it better, fine. I didn't put it back in because I forgot, not because I disagree with it. Readers have asked about temperature dependence before (see Talk:Bipolar junction transistor), which is why I mentioned it. -- Tim
Actually you are right, it is true. I wasn't thining hard enough about that. I can't think of how to better explain it right now. Perhaps just saying that the bandgap is related to the lattice spacing, and that changes with temperature... --User:Dgrant
You clearly agree that these were not entirely correct as you did not put them back into the text word-for-word. So perhaps instead of attacking my changes you could have perhaps said something nice. (read Wikipedia:Wikipetiquette).
I'm quite within my rights to get annoyed when you yourself break the rules by reducing the size of an article and adding incorrect information. And I couldn't simply revert your article because that would have been in breach of the very rules you violated. You added some extra information, I was obliged to make sure that extra information remained. -- Tim
In the direct-bandgap article, I wasn't referring to a "fibre laser", and my original wording was "fibre-optic laser", which clearly implies a laser using in fibre-optics (communications), not a fibre laser. To be more clear, however, it should have read "solid-state laser for use in fibre-optics". --User:Dgrant
Expect some edits from me over the next hour or so. I'll try to fix what you've done. BTW, does the "most common faux pas" article mention leaving off signatures in talk pages? -- Tim (original article author)
Hmmm, it's interesting that you signed "original author" stating that you were the original article author. I thought this kind of thing didn't matter on Wikipedia. -- User:Dgrant
I'm rather protective of pages I start. Call it a character flaw. -- Tim

I'm actually wondering now if that figure is even correct at all. Did you get it from a source, or draw it yourself? It might actually be good to show one of those classic figures where they show the difference between a metal, insulator, and semiconductor, insofar as the bandgap. --User:Dgrant

I got it from lecture notes, so it's not very reliable. You made a good point about mobility. But I hadn't heard the one about number of carriers at melting point before. The usual definition (used obliquely in insulator) is conductivity at zero K. -- Tim
The mobility point I made was not explained very well. I used the phrase: "significant amount of carriers" which is again kind of vague. I only brought up the melting point issue because we want to talk about temperatures at which the material is in solid state, but I think that just makes it more confusing... The more I think about it, the more hard I think it is to define what an insulator is... Actually, conductivity at O K is a difficult thing to look at as well. Semiconductors become insulators and superconductors become VERY conducting.... --User:Dgrant

I think this statement is a bit vague: "In many devices this kind of conductivity is undesirable, and larger bandgap materials give better performance". I think we should say, "In a <blank> device, this kind of conductivity is better..." where <blank> is an example of one of these devices. --User:Dgrant

Fine, go for it. Just remember: make it better, don't delete it. -- Tim

Audience ?, conductivity undesirable ?[edit]

Just a couple of nit-picky points I would like to make. The first one is that I'm confused as to who we are aiming the article at. You explain that "e" is the exponential "function" yet just state chemical formulae (GaAlAs etc.) without explanation. If you assume the target reader is newbie enough to not know what "e" means in the expression, they are unlikely to know what some of those chemicals are. Secondly, this phrase seems to make no sense to me whatsoever: "Conductivity is undesirable, and larger band gap materials give better performance." Conductivity is what makes, for example, the internet possible. I fail to see how conductivity can therefore be undesirable! That sentence needs either removing or a big revision to make clear exactly what is meant, particularly making reference to what situations high conductivities are undesirable. I understand that some people will find some of this very nit picky, but it's something I've picked up from my project supervisor!  :P Lateralis 19:49, 12 October 2006 (UTC)

Charge Carrier Question[edit]

"The only available carriers for conduction are the electrons which have enough thermal energy to be excited across the band gap,"

Is this correct? It is my understanding that the conductivity of intrinsic semiconducter material is due to thermally generated electron-hole pairs so that the charge carriers are an equal number of electrons (in the conduction band) and holes (in the valence band). Alfred Centauri 16:20, 26 May 2005 (UTC)

Yes, if you want to nit-pick. -- Tim Starling 09:48, Jun 23, 2005 (UTC)

how do we knows emiter and collecter of a transister?

See transistor -- Tim Starling 09:48, Jun 23, 2005 (UTC)

Band Gap Question[edit]

It would be really nice if someone would explain where the band gap arises from. It has something to do with the periodic potential (in the free electron model it disturbs the free-electron energy band -> bragg reflection creates the bonding and antibonding states at the edges of brillouin zones and the energy gap is the "area" between bonding- and antibonding state energies at the edge) but why aren't there any quantum states in the gap? Would be really helpful. Sorry for my english, hope you can understand :) --Risto 28.1--Tiresais 17:13, 18 July 2006 (UTC)0, 2005

to Risto; the bandgap is the electron-conductance of material as defined by its atomic structure, where:

e (-Eg/kT)

e is the exponential function Eg is the band gap energy k is Boltzmann's constant T is temperature

Electron transfer is dependant upon two points material band gap and temperature.

Yes, I do know that the atomic structure defines the size of the bandgap, I do know that the probability for an electron to be at a certain state is given by Fermi-Dirac distribution, but what I'd like to know is where the band gap arises from quantum mechanically. Or didn't I just understand what you tried to say :) --Risto
Only two electrons can occupy one energy band in quantum mechanics. The proximity of atoms in a solid mean that there must be many levels bunched together (otherwise how could the electrons exist?). The closeness of these levels give rise to the appearance of 'bands' of energies within which electrons can occupy. In insulators and semi-conductors, energy is reqiured to free the electron from the valence bands in the lattice, hence the band-gap is the energy to free the electrons from the lattice structure. Is that the answer you were looking for?--Tiresais 14:07, 29 June 2006 (UTC)

I didn't make the original question, but you explained it pretty clear I think! As to his last question: "but why aren't there any quantum states in the gap" - The answer is of course that it wouldn't be a gap of there were (occupiable) states in it! But I would like to rephrase the question as: What, from a QM point of view, determines the size of the bandgap? Is it purely atomic structure? I would think it has to do with the ease in which orbitals can be mixed, or is that incorrect? Is there some general rule to understand this? -- Mipmip 08:44, 18 July 2006 (UTC)
Presumably the size of the band gap depends on how strongly an electron is held in the lattice structure of the material. MY A level course didn't cover what effects this strength, however. sorry I can't help any further... Might be worth checking round related articles in lattice structure and bond strength. --Tiresais 17:13, 18 July 2006 (UTC)

Boron nitride[edit]

Please would someone good at editing tables add the band gap for cubic boron nitride to the table of band gaps. Thanks DFH 17:11, 29 August 2006 (UTC)

Done. Maaf 23:31, 21 February 2007 (UTC)

Disputed definition[edit]

The definition provided here is incomplete, as it only refers to the definition of band gap as it relates to semiconductor physics. Band gaps also refer to any particle or quasiparticle, not just electrons. For example, photons (see photonic crystal) and phonons both exhibit band gaps for certain materials. I suggest the definition be altered to

In solid state physics and related applied fields, the band gap, also called an energy gap or stop band is a region where a particle or quasiparticle is forbidden from propagating. For insulators and semiconductors, the band gap generally refers to the energy difference between the top of the valence band and the bottom of the conduction band.

See Google's scholarly articles for phononic or photonic crystals. --h2g2bob 20:53, 21 February 2007 (UTC)

Your phrasing is fine by me. This whole article is an ugly duckling; you'd think it would mention the concept of periodic potential, Bloch's theorem, the Kronig Penny model, and the Pauli exclusion principle from which the model of forbidden and allowed energy bands arises... It was probably written by people with mostly electronics and very little physics background. -- mattb @ 2007-02-22T00:38Z

Remove table[edit]

I've removed the big table of band gaps (diff). It was unsourced - so there is no way of knowing if it is accurate or even if it is a copyright violation. --h2g2bob 18:25, 25 February 2007 (UTC)

Article is difficult to understand[edit]

Got my answer here instead. —Preceding unsigned comment added by (talk) 01:19, 22 April 2008 (UTC)

I agree. The above link is more useful than this wiki page. As for this wiki page; sometimes no information is more useful than poor information. At least if there were no page here, maybe someone who knew the theory well would think to him/herself, "No one has done anything here, maybe I should..."Eliasds (talk) 03:12, 28 April 2008 (UTC)

It's quite a common problem with wikipedia articles on complex subjects. These articles are often written by students or those with a private interest in the subject and as such there's a desire to go over the top in demonstrating your own knowledge. The aim of writing an article, of course, isn't to prove how clever you are or how many long words you can use, it's to communicate complex ideas in a way that people with little background knowledge of the subject can understand. Sometimes that gets overlooked. —Preceding unsigned comment added by (talk) 17:50, 5 October 2010 (UTC)

Infrared and ultraviolet[edit]

Which is the band gap for infrared light ?. And for ultraviolet ? --Nopetro (talk) 13:35, 20 May 2009 (UTC)

There's no answer to that question. Light does not have a band gap. Only materials can have a band gap. --Steve (talk) 12:00, 19 May 2016 (UTC)

intro seems to be at odds with first paragraph[edit]

why does it say it's the energy difference between the bottom of the valence band and top of the conduction band in the intro, but it reverses top and bottom in the first paragraph of "in semiconductor physics"? —Preceding unsigned comment added by (talk) 00:14, 26 October 2010 (UTC)

It seems to have been fixed. --Steve (talk) 12:00, 19 May 2016 (UTC)

Mathematical concept[edit]

The population of states here is described with Maxwell Boltzmann, but we are talking about electrons/holes. Therefore, shouldn't it be Fermi-Dirac statistics? That also connects directly to why the Fermi-level's probability of being filled is 1/2. — Preceding unsigned comment added by (talk) 10:55, 19 May 2016 (UTC)

You're right. Indeed that whole subsection has no useful and correct information that helps people understand the concept of "band gap". I deleted it. (I'm not totally opposed to having some discussion of the Fermi-Dirac distribution in this article in the future, but it might as well be rewritten from scratch.) --Steve (talk) 12:00, 19 May 2016 (UTC)

Should we have a paragraph on LEDs[edit]

Should we have a paragraph on LEDs since there is one on Photovoltaic cells ? If so perhaps also the table of band gaps could note which are direct or indirect, and possibly any major applications of the material eg LEDs or PV cells ? - Rod57 (talk) 19:02, 29 October 2016 (UTC)

Hmm, I would change it to a section called "implications for light absorption and emission" that says something like "Semiconductors tend to absorb and emit photons with energy above the bandgap, and be transparent for photons with energy below". The implications of this include the fact that diamond is transparent while silicon is opaque, why bandgap matters for solar cells, and why bandgap matters for LEDs and lasers. Just my opinion, not very well thought through. I do like the idea of saying somewhere the obvious fact that bandgap affects the color of LEDs and lasers if we don't already. --Steve (talk) 21:34, 29 October 2016 (UTC)

yes — Preceding unsigned comment added by (talk) 08:38, 6 February 2017 (UTC)