Talk:Field effect transistor

Most of what I've done here doesn't need any explanation, but there's one correction I think warrants a note. I replaced every instance of "glass" with "oxide". This is because the silicon dioxide layer under a gate is not glass. Glass would not work. A glass is an amorphous solid - irregular arrangement of atoms. The common usage of "glass" happens to be an instance of this. SiO₂ in MOSFETs is crystalline, not glass. -- Tim Starling

Small correction: amorphous oxides can work perfectly well as gate oxides. see link (http://www2.eng.cam.ac.uk/~www-edm/lsilicon.html#SiO). At least as far as I know, these films are almost entirely amorphous. The best films are done with ECR, but you can also use PECVD and get decent results. On physical grounds, I don't see any reason why a gate dielectric has to be crystalline in "work". --User:Dgrant

You're the man Dave, I'll take your word for it. Before you came along, I didn't know amorphous oxides were used for anything other than window glass :) I notice that the resistivity of the oxide in your reference is many orders of magnitude less than for crystalline silicon. That would degrade performance somewhat, but at least the breakdown voltage is still high. -- Tim Starling

"Oxide" is more correct than "Glass". A glass is not only amorphous but it also contains lots of impurities of highly mobile ions as softener, making it very inadequate as gate oxide. Dgrat is right about amourphous oxides, although they will result in very instable transistors. In organic transistors, even polymers are used as gate dieletric. --Qdr 17:12, 17 Jul 2004 (UTC)

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what does "whereas those to the left abstract from the body contact." mean? It doesn't make any sense to me, or at least is doesn't convey the indended meaning, in my mind. dave

It doesn't mean anything. I changed it to something which makes sense, and is probably right. I seem to remember seeing some FET-like structures with the body insulated from the backside, but I don't think they do that for MOSFETs. -- Tim Starling 00:39 May 14, 2003 (UTC)

You have that in SOI (Silicon on Insulator) FETs, but these work slightly different.--Qdr 17:12, 17 Jul 2004 (UTC)

Contents

1 mosfet symbols 2 MOSFET section 3 External links 4 Analog circuits 5 basic circuits

mosfet symbols

The schematic for your MOSFET shows a solid line connecting the Source and Drain. Does this not indicate a depletion mode MOSFET?. An enhancement mode MOSFET is symbolically shown with a dashed line between the Source and Drain.

The arrows for the 'metallurgical' contacts point at the bottom of the N diffusions. The metal contact is on the top surface. Shouldn't your arrows point to the upper surface of the N regions?.

All textbooks use different notations. I can't remember right now what is right, and what is depletion, enhancement, etc... However, should use standard IEEE conventional symbols, whatever those are, and we should state that they are the IEEE standard symbols. If there are some common "misuses" of the symbols out there, then we should mention that. So first thing I think is to check IEEE and see if there are standard symbols, and secondly to check something like Art of Electronics and see what it uses. dave 22:06, 16 Oct 2003 (UTC)

IEEE: http://ewh.ieee.org/soc/es/Nov1998/14/education/

You are right about the 'metallurgical' junctions, they are on top of the diffusion region and are not the same! Modern FETs do not only use high doping, but also metal silicides at this place.--Qdr 17:12, 17 Jul 2004 (UTC)

MOSFET section

I'm planning on doing a lot of work on the MOSFET section. I hope to discuss how MOSFETs are evolving to smaller and smaller submicron dimensions, and the problems designers are encountering...obviously non-technically. I've created two subcategories I want to expand upon--why MOSFETs are so popular and the problems with scaling. Rmalloy 13:47, 14 Jul 2004 (UTC)

I think some work needs to be done in the introduction to MOSFET...most important part. for later. Rmalloy 18:38, 14 Jul 2004 (UTC)

Thanks for your efforts! I assume at some point it would make sense to put all MOSFET stuff in separate article. Agreement? Pjacobi 19:26, 14 Jul 2004 (UTC)

I dunno, I'm new here and don't know what the protocols are. As long as info is easily accessible it makes little difference to me. I'm not going to touch stuff like that. Rmalloy 20:06, 14 Jul 2004 (UTC)

It could make sense to have a rather generic introduction to FETS on this page and move all the details (different types, processing, materials) to other pages. --Qdr 17:12, 17 Jul 2004 (UTC)

• • • • Splitup plan, please comment under each point if necessary. If no active disagreement is seen, I'll do the splitup around 2004-07-19 21:00 UTC. Pjacobi 18:49, 18 Jul 2004 (UTC)
        • The MOSFET specific parts of Field Effect Transistor will be moved.
        • This applies to section MOSFET (currently 1.1) and DMOS (currently 1.5).
        • It will go to http://en.wikipedia.org/w/wiki.phtml?title=MOSFET&redirect=no which is currently a redirect.
        • The JFET, MESFET, and HEMT sections are not yet substantial enough to be moved to separate articles.

Sounds good, but I think also the other types of FET devices should be moved somewhere as they are way too specialized for a generic introduction. Maybe an article about "special" or "exotic" FETs? --Qdr 19:59, 18 Jul 2004 (UTC)

Lumping together JFETs and MESFETSs into Field effect transistor (exotic) (or [[Field effect transistor (bizarre)]?) will break my heart ;-). I'd vote keeping them (temporarily) in the main article, or as a second choice, make all separate articles, even when HEMT will be a short one. Pjacobi 21:34, 18 Jul 2004 (UTC)

I vote for separate articles, maybe that is also an incentive to extend the individual articles a little. And btw, there should also be a link to TFTs in the main FET article. --Qdr 22:17, 18 Jul 2004 (UTC)

I second this. With some diagrams all of these transistor types would be good separate articles. I think Field effect transistor should be a list of links. Maybe some generic discussion. Rmalloy 00:14, 19 Jul 2004 (UTC)

Rmalloy: There are some inaccuracies in your additions: The reason for using polysilicon as a gate material is the reduction of interface states and the self aligned S/D diffusion. Replacing it with metals (for example TaN, TiN) is subject of current research. Current gate oxide thicknesses are way below the 20nm you stated, I changed it to 2nm. But somebody should look up an accurate number. The problem with thin oxides is not breakdown, but leakage by quantum mechanical tunneling of electrons through the oxide. To remedy this, the industrie works on high-k dielectrics.--Qdr 17:41, 17 Jul 2004 (UTC)

QDR: First-off, I must admit that I haven't worked in this area for 2 years. But I think I am right about the polysilicon gate. The self-aligned S/D diffusion process would work equally well with a metal gate. I went back and looked at some stuff I wrote on www.everything2.com when I was a grad student in this area. Look at the article MOSFET that describes and shows diagrams of the fabrication process. The self-alignment process work the same if the gate were aluminum. I'm 99% sure that the reason you can't use aluminum is that there is a high-temperature annealing step after the gate is deposited, and this would melt aluminum. Now the reason the annealing must be done after the gate is deposited relates to the self-alignment process (S/D to be annealed created after gate), so we could be both right in a sense.

I looked up the melting points of the metals you mention, and they have very high melting points. When I was a grad student, I don't recall much effort into using these metals as gates. I don't know why, so I won't argue. But I do remember heavy emphasis on the silicides, like I wrote in this article. I know what surface states are, but I don't see how they relate.

2nm is the accurate number. I meant 20 angstroms. 2nm was considered an absolute cutoff.

Now my memory is that 2nm is still a long way for an electron to tunnel. I seem to recall that, like I said, the oxide broke down, creating states in the oxide that acted like rocks across a river that you can jump across, facilitating tunneling. So I won't argue with you here...we might both be right. And it's not worth arguing. But I'm 99% sure that 2nm was the cutoff, considered absolute by the chief technology officer of TSMC. And I agree about the high-K dielectrics, and mentioned it in the article. Rmalloy 18:54, 17 Jul 2004 (UTC)

I did some quick websearching. An Intel site claims successful operation with 1.2nm of gate oxide, so I guess I'll have to bite my tongue on 2nm. And I'm reminded of another key issue for gate materials--work function. The current setup, where the source, drain, and gate are all doped heavily at the same time gives the gates the proper work function for the transistor type--NMOS or PMOS. Successful metal gate processes would require two kinds of metals--one work function for each transistor-type. It's dawning on me that the choice of gate materials involves several issues, including all the ones we've mentioned and probably several more.

Feel free to clean up, correct, or add to anything I wrote. These issues are complex and there are many issues to discuss...maybe it would be best to avoid difficult issues like this altogether in an encyclopedia. Phew I'm glad I left this field! Rmalloy 16:35, 18 Jul 2004 (UTC)

The oxide thickness is a moving target, IMO around 2nm is a good guess. Intels 1.2 nm oxide was probably already nitrited oxide. There are many candidates for new metal gates, however the ones I quoted have been announced by the IMEC recently. And yes, there are various leakage mechanisms (poole frenkel, schottky emission into insulator valence band, direct tunneling, fowler-Nordheim) and they are enhanced by soft breakdown, however they are usually not regarded as breakdown itself.

I am pretty sure that the main incentive to use polysilicon gates was the self aligning SD process, back in the 70ies. I do not know the exact problem with Al gates and the self aligning process, but for example the spacer oxide would be pretty difficult to apply to an Al-Gate. Anyways, all of this is way too detailed for a Wikipedia article, the best way is to formulate it as generic as possible. --Qdr 19:59, 18 Jul 2004 (UTC)

Good point about the spacer. I have the melting thing stuck in my head...I must have picked it up somewhere. I agree this is all too detailed for Wikipedia, but I guess I thought some explanation for the polysilicon gate would be advisable. At first glance, polysilicon is a very strange choice for gate material. You obviously know what you're talking about, so don't hesitate to delete or change anything I wrote. Rmalloy 00:14, 19 Jul 2004 (UTC)

External links

IMHO the external link [1] (http://groups.yahoo.com/group/OrganicDevice/) is somewhat obscure - and I'm a member of that Yahoogroup! It's neither focused on FETs nor well known. I plan to remove the link when I have some better to provide. Pjacobi 19:26, 14 Jul 2004 (UTC)

Yeah, that's a dubious link. Interesting group though.

Please remove it, I think its just a commercial plug - the whole yahoo group is. --Qdr 17:12, 17 Jul 2004 (UTC)

Analog circuits

I've added all I feel comfortable adding about MOSFETs in analog circuits. I wish someone could discuss analog stuff, since everything is so digital digital digital. Rmalloy 23:38, 14 Jul 2004 (UTC)

Added an explanation of how FETs actually work. Pinchoff and all that. --Wjbeaty 08:17, Apr 3, 2005 (UTC)

Sorry, I see that you had quite a bit of work with your recent addition. However, I don't think it belong to this article and thus reverted it.

*You explained the operation of a JFET only. This is a rarely used special type of FET, the explanation does not apply to common MOSFETS. Maybe you could improve the JFET section?

*The operation of the different types of field-effect transistors is already explained in the articles for the respective devices. There is no need for detailed information about the operation in the main article. See also thread above. --Qdr 11:57, 3 Apr 2005 (UTC)

"You explained the operation of a JFET only" What? Please justify such a statement. As I understand it, you're completely incorrect. Which part of my explanation do you think doesn't apply to all types of FETs? (Ah, I see one issue: the present MOSFET article only describes enhancement-mode devices, not MOSFETs in general. Perhaps I should move this analog/pinchoff explanation to both JFET and MOSFET articles rather than having a general FET explanation here.)

Also, please indicate where on WP the "pinchoff" phenomenon is explained. I don't get any search hits at all. This stuff is common to all FETs: it's the essence of the "analog mode" explanation of FET operation. Could you explain why it doesn't belong in the main article? Should there be a separate article to explain generalized FET operation (including pinchoff mechanism?)

I note that Rmalloy says above, "I wish someone could discuss analog stuff." I attempted to do so. Rather than removing it, shouldn't you post your own version of a general analog-based FET explanation? Or if you object to specific details of my analog explanation, please edit them. --Wjbeaty 02:22, Apr 4, 2005 (UTC)

A quesiton: "The MOSFET's strengths as the workhorse transistor in most digital circuits does not translate into supremacy in analog circuits, in which the bipolar junction transistor (BJT) has traditionally been seen as the transistor of choice, due largely to its high gain." This does not seem correct, since FETs have near-infinite gain - essentially no current flows into the gate. Glengarry 21:09, 15 Jul 2004 (UTC)

Bad subject-verb agreement for one thing :(.

Ok, this is not an area I know a lot about, but let me try to explain as best I can. "Gain" invariably means "small signal voltage gain" (output signal voltage / input signal voltage). The fact that a MOSFET gate allows no DC current isn't relevant (though its true). The job of analog circuits is to handle small signals.

Suppose a transistor is being used for amplification in an analog circuit, it is "DC biased" to put it in the high gain regime. A small signal voltage is applied between gate and source (or between base and emitter), creating a small signal current from drain to source (or from collector to emitter). The ratio of this small signal current to the small signal voltage is called "transconductance." My sense is that BJTs have substantially higher transconductance than MOSFETs. For a tiny bit of support, see http://www-inst.eecs.berkeley.edu/~ee130/SP03/homework/hw13soln.pdf. This small signal current may drive a resistive load, giving an small signal output voltage of the current times the resistance of the load. Thus you end up with a higher "gain" (output voltage/input voltage).

I'm really shaky on everything analog, and that's why I made such a vague statement. I just felt like it would be inappropriate to only talk about digital stuff when analog circuits are very important. I think what I wrote is essentially correct, but if someone wants to delete it that's fine. I'd rather someone teach me though! Rmalloy 00:42, 16 Jul 2004 (UTC)

The gain issue is easily misinterpreted and is always good to start a flame war at news://sci.electronics.* or http://www.diyaudio.com. I'd write something into article but for the fear of a edit war! In essence there are four quantities which can be seen as geen dV(out)/dV(in), dI(out)/dV(in), dV(out)/dI(in), and dI(out)/dI(in). Of course MOSFET score big on dV(out)/dI(in) and dI(out)/dI(in) in NF, as no input current flows, but the practical significance is more that there is moe leeway in designing the preceeding stage.

What's more the problem with MOSFETs in discrete designs, is the variabiliy of there threshold voltage.

Pjacobi 07:27, 16 Jul 2004 (UTC)

It's not true that no *small signal* input current flows into a MOSFET. Current is constantly charging and discharging the MOSFET gate, so dI(in) != 0. In fact, capacitors, like the MOS capacitor, are short-circuits to high-frequency current. I think this topic is probably best left alone in the article unless someone is an expert on the subject, so we avoid misinformation.Rmalloy 13:04, 16 Jul 2004 (UTC)

Agreed that the transconductance of a BJT is favorable compared to a MOSFET. I'll made the change in the article. Thanks, Glengarry 14:56, 16 Jul 2004 (UTC)

basic circuits

each basic circuit needs an article. common source, common drain, common gate, source follower amplifiers, etc. i can, of course, draw schematics. but i don't know these well enough to do the articles. i can start them with what i know... same for BJTs. - Omegatron 16:00, Jan 19, 2005 (UTC)