Talk:Semiconductor

"What makes semiconductors useful for electronic purposes is that they have the property of being able to carry an electric current by electron propagation or "hole" propagation".

Can't this be said with fewer words:

"Semiconductors are useful for electronic purposes because they can carry an electric current by electron propagation or "hole" propagation".

S.

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this question about Valence band versus conduction band, and which one is highest or lowest energy? if you add energy, doesn't this excite electrons in the valence band moving them into the conduction band? wouldn't this imply that the conduction band is higher energy? Waveguy

I'm sorry I didn't make this very clear. If you look at the diagram:

The conduction band has a higher energy than the valence band. But the valence band is the highest of the filled bands, and the conduction band is the lowest of the unfilled bands. -- Tim Starling 04:35, Jul 16, 2004 (UTC)

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Not all semiconductors are inorganic

Defining metals and semiconductors by the temperature dependance of their resistances is a gross oversimplification. The reason semiconductors' conductivity increases with temperature is because thermal energy can promote electroncs across the gap, generating the charge carriers that are actually responsible for eletrical conductivity. It is also true that if you shine light on a semiconductor its conductivity will increase because UV/visible light can also promote electrons (especially in organic semiconductors which tend to absorp in the visible region), but you wouldn't define a semiconductor that way. Anyway, the dependance of conductivity on temperature is a function of doping. Overdoping will decrease the conductivity of a semiconductor, for example the conductivity of poly(thiophene) goes through a maximum as a function of dopant right around 1:2 dopant:thiophene unit. Therefore, fully doped poly(thiophene) will become less conductive when heated because the added charge carriers will cause overdoping. It is also worth mentioning that other non-semiconducting systems can show an increase in conductivity as a function of increasing temperature due to ionic conductivity which often becomes accessible to highly charged, non-conjugated (i.e. lacking a band structure) polymers at elevated tempertaures. It is far more accurate to define semiconductors by their band structure because this is in fact the scientific definition of a semiconductor, while the thermal definition is something that physicists often incorrectly mistake for the definition.Fearofcarpet 21:54, 17 Mar 2005 (UTC)

Semiconductor not defined only by energy bands

I added a few words to the definition. (Are those sentences getting too long?) If we place an electron deep within certain insulators (such as a hunk of resin,) the electron will become trapped in place and will not move significantly when an electric field is applied. On the other hand, if we place the same electron inside a semiconductor, the electron will be freely mobile and will easily flow during an electric field. As insulators, semiconductors behave like vacuums do: they lack trapping centers, and the charges injected by doping, etc., are free to move around. I'm not certain, but I think the definition of "semiconductor" leans more heavily on "lack of trapping centers" than it does on "small band gap." For example, doesn't diamond behave as a very good insulator? Yet diamond also lacks trapping centers, and a diamond-based transistor isn't impossible: see diamond-based semiconductors (http://www.google.com/search?q=%22diamond-based+semiconductor%22) --Wjbeaty 04:42, Apr 13, 2005 (UTC)

A diamond is simply a "wide bandgap semiconductor". As such it is less conductive than silcion, but still more conductive than an insulator. Its all arbitrary anyway. I believe a semiconductor is rather loosely "defined" as being somewhere between a "conductor" and an "insulator"

By the way, can we have the band diagram on the article page? I shouldn't have to come to the discussion page to find it ;-)

Also, I am thinking of creating a page that deals specifically with the band structure of a semiconductor. What do you all think?--darkside2010 17:46, 15 Apr 2005 (UTC)

"Its all arbitrary anyway." Not at all. While the division between conductor and insulator is certainly arbitrary, "semiconductor" is only arbitrary if we screw up the definition of that word! Is copper a semiconductor? How about polyethelene? If we refuse to draw a line in the sand, then everything is a semiconductor, or nothing is.

Here's another possibility: "a semiconductor is a conductive material whose low electrical resistance is caused by mobile charge carriers contributed by very small amounts of impurities." In other words, if the conductivity is caused by doping, then it's a semiconductor regardless of its band gap width. And notice that this wording excludes materials which are full of trapping centers (if the material is insulating yet is full of *immobile* charge carriers, then it's not a semiconductor.) --Wjbeaty 22:12, Apr 17, 2005 (UTC)

Fair point. An encyclopedia should, after all, define things, and do it well. After reading the opening paragraph of semiconductor again, I think that we could come up with a better definition. So what is it that makes a semiconductor a semiconductor? It has a band gap, sure, but so do all solids - it comes from the quantisation of allowable electron energies around the nucleus. The thing that's different about a semiconductor is that the Fermi level lies somewhere in the bandgap, rather than in an allowable band as it does for a conductor. But then, an insulator also has a mid bandgap Fermi energy, the difference is that the bandgap is so wide that, as it says in the definition, there are not enough electrons in the conduction band at room temperature to allow appreciable current to flow. So, I guess that means that I was half right; the distinction between semiconductor and insulator is somewhat arbitrary. (It comes down to that phrase "appreciable current flow"). But you are right, there is a clear distinction between conductor and semiconductor.

Also I don't think that saying that the mobile charge carriers coming from dopants defines a semiconductor. It does define either an n-type or a p-type semiconductor, depending on the dopant species. But what about intrinsic silicon? Is that not also a semiconductor?

I'm trying to define things dynamically; based on "response to a change" rather than statically; based on "what it is." For a material we can ask what would happen if we take an ultra-pure sample and add tiny amounts of various impurities. If the material is a semiconductor, then certain impurities will create mobile charges. If the material is an insulator, then the impurities will only donate trapped and immobile charges. If the material is a conductor, then large numbers of mobile charges are present even in the pure material. Also, to be "semiconducting" the doping density which produces usable conductivity must be very small. That way the cloud of dopant-produced carriers acts like an easily compressed gas, and the voltages needed to create a depletion zone are easily reached by simple power supplies (try calculating the gate voltage required to sweep all the charges from an FET channel made of copper. Yeesh!) --Wjbeaty 09:31, Apr 19, 2005 (UTC)

As for immobile charge carriers, every solid is made up of (mostly) "immobile" charge carriers. All those protons and electrons that make up the atoms which make up the solid are "charge carriers"; that is, they carry either a positive or negative charge. So I don't think that path helps us to define a semiconductor. The points of distinction from conductors and insulators are mid band-gap Fermi level, and width of the band-gap respectively.darkside2010 12:39, 18 Apr 2005 (UTC)

But in semiconductor physics, the term "charge carriers" always implies current; a "carrier" is a mobile charge, not just a carrier of charge. (Sometimes they call them "current carriers," yet there's no such substance as "current," so the term "charge carrier" would be a bit less misleading to newbies.) Don't lose sight of my original point: as a conductor, silicon is very much like vacuum: if we inject charges into it, those charges will be free to move. But if we inject charges into an insulator (e.g. most plastics,) the charges won't move. I've seen it explained like this: an insulator is full of "trapping centers" caused by lattice defects, and if we inject charges into the material, the charges will bond with the trapping centers and become highly localized.

PS, you'll probably notice that I'm pushing hard for classical-physics models as opposed to QM concepts. Is wikiP a textbook for grad students, or a reference for the general public? Maybe QM is "more accurate," but if our goal is to inform the public, use of QM concepts is similar to use of Latin. At the very least, we need to include concepts graspable by the general public, who are usually regarded as being at the level of intelligent 6th-graders.

Wj, I'm not even sure where to begin. Are you suggesting that we should dumb down the "sum of human knowledge" because it contains concepts which may not be readily understandable to sixth graders? There are some phenomena in this world which cannot be understood without the use of quantum mechanical concepts. The concepts themselves are not difficult to understand. For example, one does not need to be able to solve Schrödinger's equation to be able to understand that electrons in solids are allowed to occupy certain bands of energies and not others. I find your last paragraph particularly strange after reading the rant on distortion of information in physics texts on your homepage.

Sorry, I just don't agree that a semiconductor is like a vacuum. What is the permittivity of a vacuum (free space)? Of silicon? I can't find the values on wikipedia, but I'm hoping that one day I can. The point is that they are not the same. (And what is the analogue of a "hole" in free space?) Even the effective mass of electrons is not the same in a vacuum and a semiconductor.

Earlier you asked what happens if we put an electron deep inside an insulator (resin). How are we going to put the electron there? The point is, we cannot put an electron deep inside an insulator. Why? Because current does not flow in an insulator.

Did you miss my question about intrinsic semiconductors? A semiconductor is not a semiconductor because it has dopants which produce mobile charge carriers (although, this effect is often used, see p-type and n-type). Intrinsic silicon is a semiconductor, even if it is 100% pure. When we introduce dopants, we are actually introducing trapping centres. So your idea to define a semiconductor based on presence or lack of trapping centres is simply wrong (and self-contradictory). Sorry.darkside2010 11:22, 19 Apr 2005 (UTC)