Talk:Bipolar junction transistor: Difference between revisions

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i want to know about the phase shift caused by the BJT in CE mode

i want to know about temperature effect in bipolar junction transistor

someone should add a section on the small signal model.

voltage or current?

"very sensitive to the current passing through the base."

I've heard it's actually the voltage that does it, in BJTs *and* FETs. current is just a byproduct in BJTs. - Omegatron 20:37, Jul 28, 2004 (UTC)

You can't have one without the other. Whether you see it as current-sensitive or voltage-sensitive just depends on which model you use. As a first approximation, a BJT can be modelled as a current-controlled current source. The proportionality constant between the base current and the collector current is β or h_FE. This is taught in many lower-level electronics courses, since it is often sufficient for circuits operating in the forward-active mode. The Ebers-Moll and Gummel-Poon models use diode-like equations relating voltage to current. When using the current-controlled model, it's useful pedagogically to assign causation to the current, and when using the Ebers-Moll model it may be more useful to assign causation to the voltage. -- Tim Starling 01:02, Jul 29, 2004 (UTC)

Math models obscure the question as they simplify the concepts. Look to the full-blown details in order to understand how transistors actually work. As with any diode junction, the B-E junction contains an insulating "depletion layer" whose thickness is controlled by applied voltage. Make this insulating layer thicker, and the diode turns off (reverse biased), or make the layer thin enough, and charges are able to get across; the diode turns on. E.g. the diodes's current is controlled by the voltage applied to the diode's terminals. Now in a BJT there essentially are two currents in the same B-E diode junction: the base current and the Collector current, and... both are controlled by the thickness of the insulating B-E depletion layer. And the thickness of the B-E depletion layer is controlled by the applied voltage Vbe. THEREFORE, both the collector current Ic and the base current Ib are controlled by the Base-Emitter voltage Vbe. Things only SEEM confusing because Ib happens to be proportional to Ic. In reality Ib cannot affect Ic, instead both are affected by a third variable: the B-E voltage. Weird, eh? At the most fundamental level, BJTs and FETs are both controlled by voltage. However, in a BJT the main current must cross an insulating layer of variable thickness, while in an FET the main current remains in the conductive regions while the insulating regions encroach from the side. If a BJT acts like the dark lens in a pair of variable-density sunglasses, then an FET is more like a variable-aperature optical shutter or iris: a variable-sized empty hole. --Wjbeaty 21:29, Mar 20, 2005 (UTC)

Can't agree with this. Whether a BJT is a current or voltage controlled device depends upon whether the base-emitter junction is current or voltage fed. If we use a current source, then the b-e voltage will float to whatever is appropriate to allow that current to flow. If we use a voltage source, then a current appropriate to the voltage will flow. So, in truth, the collector current is dependent on *both* the base-emitter voltage and current. However, given that beta (the current gain) is relatively constant, then it's easier to consider a BJT as a current controlled device: increase the base current by x, and the emitter current increases by beta times X. Phil Holmes 15:45, 19 August 2005 (UTC)

I agree in principle with what you are trying to say here but I am uncomfortable with how you have said it. Specifically, the manner in which a two-port is driven does not in any way affect it's fundamental nature. Further, claiming that the collector current is dependent on both the base-emitter voltage and current is misleading. I am not aware of an equation for collector current that, to first order, has independent terms for both base-emitter voltage and current. But, you have also said in effect (and correctly) that the base-emitter voltage and base current are not independent. Thus, the collector current equation can be written as a function of base-emitter voltage OR base-emitter current.

Now, consider an ideal transconductance amplifier. By definition, the input current is zero so driving this amplifier with a current source is, let us say, a problem. In the non-ideal case however, the input current may be non-zero and is related to the input voltage (linearly in the case of a resistive input or more generally, non-linearly as in the case of BJT). Thus, by solving for the input voltage in terms of the input current, one can look at the non-ideal transconductance amplifier as a non-ideal current amplifier. This has nothing to do with the way the amplifier is driven - it is simply an alternate perspective on the transfer equations.

If the base-emitter of a BJT is driven directly by a current source, it is reasonable to adopt the current amp perspective. Since the voltage developed at the input is relatively small and nearly constant over a wide range of input current, the BJT is an (approximately) ideal linear current amplifier from this perspective. If a voltage source is used instead, it is reasonable to adopt the transconductance amp perspective. However, since the source current varies greatly and non-linearly over a small range of input voltage, the BJT is a highly non-linear non-ideal transconductance amplifier from this perspective. Alfred Centauri 20:57, 19 August 2005 (UTC)

I think the only difference between what you've said and what I said is that I said *both* and you, in effect, replaced that with *either*. On reflection, I'd go along with that. Phil Holmes 08:30, 22 August 2005 (UTC)

mnemonic

i can't ever remember from the schematic symbol which side is emitter/collector and whether it is npn/pnp. any silly mnemonics for remembering or should i make one up? - Omegatron 16:21, Oct 18, 2004 (UTC)

It's not much of a mnemonic, but the terminal with the arrow is the emitter, and the arrow points in the direction of current flow. So if the arrow points from emitter to base, it implies the emitter is injecting positive charge into the base. Emitter positive, base negative, therefore pnp. -- Tim Starling 03:45, Oct 19, 2004 (UTC)

yeah, not much of one. something like e is for arrow and the N in the middle of PNP stands for "pointing iN". it has to be something you can just remember without thinking about it. but that is a really stupid attempt. ummmm... i dunno. i'll just stare at them until i remember i guess. :-) - Omegatron 13:41, Oct 19, 2004 (UTC)

Never Points iN and Points iN Proudly. that is a good one. and a bow/archer "emits" arrows? i guess that works. - Omegatron 14:50, Oct 19, 2004 (UTC)

The one I used was Not Pointing iN for NPN, and you could use Pointing iN Permanently for PNP if you need to remember both. This assumes you can remember that the emitter is the one with the arrow on. This last could'nt be more obvious really- its a bit like the symbol for a male! (who is normally the 'emitter' in sexual relations! --Light current 00:55, 5 February 2006 (UTC)

doping

In the illustration, there seems to be 4 types of semiconductor - p+, p-, n+, n-. What does this signify? The diagram looks to be more informative than the usual NPN or PNP layer discription. Where can I find more info? Thanks.

p⁺ means strongly doped with acceptor impurities, p^- means weakly doped with acceptor impurities, n⁺ and n^- mean strongly and weakly doped with donor impurities, respectively. The region of very strong doping leading up to the metal is called an ohmic contact. Strong doping reduces the diodic effect of touching metal and semiconductor, see Schottky barrier. As for more information, a google search didn't turn up much for me. You might want to try a textbook, e.g. ISBN 0471333727. -- Tim Starling 00:03, Nov 23, 2004 (UTC)

I've beefed up the sections on doping in the semiconductor article - this explains the n+, p+ nomenclature. --Phil Holmes 21:26, 29 August 2005 (UTC)

models

this should have some small- and large- signal models and such. - Omegatron 02:24, Mar 4, 2005 (UTC)

I'll draw up some diagrams for ones that we decide to use. Actually, this could be a good test for Wikipedia:Modular electronics diagrams. I'm going to start working on it. (Since I need to re-learn this for a project anyway). - Omegatron 16:44, Mar 6, 2005 (UTC)

Here are two active-mode models:

			C
			αiE
B
	iE ↓		Is/α
			E

B						C
	iB ↓				βiB
		File:ES wire NE.png
				E

What do you think? - Omegatron 18:09, Mar 6, 2005 (UTC)

Uses and possible technical details

Well I was thinking that a uses section for BJTs could be added to the article.

Uses - Current sources - TTL, transistor transistor logic

Technical Detail

-Low input impedance when compared to FETs and MOSFETs. Input impedance depends on amplifier configuration. - Output impendance is low. Also is dependent on amplifier configuration.

Amplifier configurations

Links to the following configurations maybe.

- Common collector - Common emitter - Common base

How gain works

Please see that I've changed the sentence that used to say "The proportion of electrons able to run the base "gauntlet" and make it to the collector is very sensitive to the current passing through the base." That's not correct - I've now clarified that this is fairly constant, but that gain occurs as a result of the emitter being more highly doped. Phil Holmes 08:47, 22 August 2005 (UTC)

Good catch. I was just reading that paragraph a day or to ago and the word 'proportion' didn't jump out at me at all. But don't forget that the light doping and the thinness of the base is also important to the gain. What is required is that the the diffusion length is long compared to the thickness of the base. This ensures that the odds of a charge carrier from the emitter recombining in the base region before reaching the base-collector junction are small. The smaller the odds, the greater the 'gain'.

One thing that I don't see mentioned in the article is that once the charge carrier reaches the base-collector junction, it is the electric field within the depletion zone that sweeps the charge carrier into the collector region. In a way (this is sure to raise some eyebrows!), it can be said that the role of the forward biased base-emitter junction is to increase the reverse bias 'leakage' current through the base-collector junction.

On another note, there is a statement in the opening paragraph that I'm concerned about but that I'm hesitant to change. This statement is: "BJTs can be thought of as current-controlled resistors". This is really wrong technically yet if it helps newcomers grasp the concept I'm not going to bother with it. Alfred Centauri 20:41, 22 August 2005 (UTC)

I've always been in favor of the "controlled resistors" analogy for newcomers, but it's perfectly acceptable to add "this is only a crude analogy" or whatever you want to make it accurate. User:Omegatron/sig 21:33, August 22, 2005 (UTC)

I'm not sure there is a better analogy at the newcomer level. I've thought about something like "the BJT is to electric current what power brakes are to pedal pressure" but I not sure if that even makes sense. Is there any way that an analogy can be made between a BJT CE amplifier circuit and a power brake system? Alfred Centauri 21:53, 22 August 2005 (UTC)

You can make a good analogy to a Globe_valve where the plunger is controlled by a diaphragm. EGR valves work this way. There is a big flow that is stopped by the plunger, and then you can control the plunger by applying a constant pressure or vacuum to a diaphragm (FET) or a small, constant current to a diaphragm with small holes in it (BJT). I've been meaning to draw a picture of this for hydraulic analogy. User:Omegatron/sig 23:57, August 22, 2005 (UTC)

I just visited the hydraulic analogy page. I've often thought about this analogy so it is good to see you working on this. I have a suggestion for resistor. A small diameter pipe sounds like higher gauge wire. But a resistor is like an obstacle course for moving electrons so maybe a water resistor is like a restrictor plate. I'm picturing a disk with holes in it that fits inside the pipe. I doubt if such a thing is actually found in a hydraulic system but maybe it would help create a picture of electrical resistance in a resistor Alfred Centauri 03:06, 23 August 2005 (UTC)

The analogy of a BJT to a current controlled resistor is very poor. Alfred's comment towards the top of this page reminded me of how much that grated when I read the article for the first time. To explain. The resistor we would be refering to would be between the collector and emitter terminals. With no base current, we put a voltage varying between 1 and 5 volts (say) between those terminals, and see what current flows. In this state, it would be negligible. We now apply some base current and do the same. The result we will see is that beta times the base current will flow at 5 volts, and there will be very little difference at 1 volt. (The Early effect means there is some difference). This is very different behaviour from a current controlled resistor. With one of those, I'd expect to see "infinite" resistance with no base current, and a current at 1 volt that is 1/5 the current at 5 volts with base current applied. My suggestion would be to delete the analogy. Phil Holmes 11:44, 23 August 2005 (UTC)

We already agreed it's a poor analogy. I still think it's helpful for someone who has no concept of what it is. It's better than saying "a controlled current source" or "an amplifier". Do you have a better suggestion? User:Omegatron/sig 14:06, August 23, 2005 (UTC)

Frankly stuck for a good analogy. Perhaps it would be best not to bother with analogy and simply say that a BJT is a device that controls current flow between the collector and emitter by using a control signal between base and emitter? Incidentally, and you're probably aware of this, I found a useful reference when searching for source information to validate my memory on some of this. [[1]] Phil Holmes 15:52, 23 August 2005 (UTC)

The physical reason for gain

I was wondering if much more could be said on how current and voltage gain are obtained in a transistor. I have no idea myself, and i couldn't find any good explanation of it. I too think of a transistor as a variable resistance, but that analogy obviously breaks down when applied to an amplifying transistor circuit. I would greatly appreciate it if anyone could explain how gain is physically obtained. Fresheneesz 21:12, 10 February 2006 (UTC)

Does this not explain it ?

In normal operation, the emitter-base junction is forward biased and the base-collector junction is reverse biased. In an npn-type transistor for example, electrons from the emitter wander (or "diffuse") into the base. These electrons in the base are in the minority and although there are plenty of holes with which to recombine, the base is always made very thin so that most of the electrons diffuse over to the collector before they recombine with holes. The collector-base junction is reverse biased to prevent the flow of holes, but electrons are swept into the collector by the electric field around the junction. The proportion of electrons able to cross the base and reach the collector is approximately constant in most conditions. The heavy doping (low resistivity) of the emitter region and light doping (high resistivity) of the base region mean that many more electrons are injected into the base, and therefore reach the collector, than there are holes injected into the emitter. The base current is the sum of the holes injected into the emitter and the electrons that recombine in the base - both small proportions of the total current. Hence, a small change of the base current can translate to a large change in electron flow between emitter and collector. The ratio of these currents Ic/Ib, called the current gain, and represented by β or Hfe, is typically 100 or more.

--Light current 21:44, 10 February 2006 (UTC)

mm.. not really. I mean, it does indeed explain the general operation of a transistor, but the way its written is a bit hard to understand. On top of that, gain takes a huge backseat in this description. After reading that paragraph perhaps for the 4th time or so, I still can't imagine how gain would occur. Obviously small changes in the base voltage produce large changes in the amount of current allowed throgh the transistor - thats the whole point of the device. However, gain requires that *more* current or *more* voltage come out than comes in, which seems 120% unintuitive to me and I think is *so* important that it should have its own section on this page. Also I'm pretty sure that the base *current* is more of a byproduct than a cause, and that the base *voltage* is really what produces the transistor changes. Fresheneesz 22:25, 10 February 2006 (UTC)

OK then, does this explain it any better:

Math models obscure the question as they simplify the concepts. Look to the full-blown details in order to understand how transistors actually work. As with any diode junction, the B-E junction contains an insulating "depletion layer" whose thickness is controlled by applied voltage. Make this insulating layer thicker, and the diode turns off (reverse biased), or make the layer thin enough, and charges are able to get across; the diode turns on. E.g. the diodes's current is controlled by the voltage applied to the diode's terminals. Now in a BJT there essentially are two currents in the same B-E diode junction: the base current and the Collector current, and... both are controlled by the thickness of the insulating B-E depletion layer. And the thickness of the B-E depletion layer is controlled by the applied voltage Vbe. THEREFORE, both the collector current Ic and the base current Ib are controlled by the Base-Emitter voltage Vbe. Things only SEEM confusing because Ib happens to be proportional to Ic. In reality Ib cannot affect Ic, instead both are affected by a third variable: the B-E voltage. Weird, eh? At the most fundamental level, BJTs and FETs are both controlled by voltage. However, in a BJT the main current must cross an insulating layer of variable thickness, while in an FET the main current remains in the conductive regions while the insulating regions encroach from the side. If a BJT acts like the dark lens in a pair of variable-density sunglasses, then an FET is more like a variable-aperature optical shutter or iris: a variable-sized empty hole. --Wjbeaty 21:29, Mar 20, 2005 (UTC)

--Light current 22:36, 10 February 2006 (UTC)

It does explain a transistor better. But I must be missing something fundemental, because I see no mention of gain in that description, nor can I see how gain arrises from that description. All of these descriptions seem to follow the variable-resistor analogy, while the concept of gain doesn't. Sorry if i'm being a pain in the ass, but I really think this is important. Fresheneesz 23:22, 10 February 2006 (UTC)

Well you do understand that current gain is the ratio of Ic/Ib ? Also, if you can control a variable resitor with a voltage or current, you can get gain , yes? --Light current 23:28, 10 February 2006 (UTC)

That must be what I don't understand. I understand the mathematics of Ic/Ib, but I don't understand why that it is the case. What I think I don't understand is that you can get gain with a voltage-based variable resistor. I would have thought that that is impossible. If the variable resistor analogy is held, then by, say, increasing the voltage in the base, the resistance through the CE junction goes down. But no matter how much one varies a resistor, it seems to me that the limits are a gain of 1 and a gain of 0 (when the resistance is infinity and 0 respectively). It seems to me that the resistance would have to be negative for one to get gain out of it (ie a battery or something). Fresheneesz 00:28, 11 February 2006 (UTC)

OK. If you have a variable resistor connected between a voltage supply and a load, then suppose somehow, you could alter the value of that variable resistor in sympathy with the input signal. Do you see how that will change the voltage across the output resistor?--Light current 01:00, 11 February 2006 (UTC)

Perhaps it might help if a sentence were added at the end of the paragraph originally quoted by Light Current. Put simply, this feature of the high doping of the emitter and low doping of the base means that a small change in current flowing between the base and emitter causes a larger change in current flowing between the collector or emitter: gain. --Phil Holmes 15:37, 11 February 2006 (UTC)

I think Fresheneesz must be thinking that the gain is happening from C to E or something, when in fact it is happening between B and C. But he is correct in thinking that it is the base emitter voltage that controls the collector current and the base currrent is just a by product!.--Light current 16:52, 11 February 2006 (UTC)

OH! Gain from B to C. That makes perfect sense. I've never thought of it like that. But looking back at the problem that confused me, It seems like the emitter and base get the same magnitudes of voltage. Heres a picture:

[In my book this is labeled as "common-base" configuration]

I can see how one would get gain using two different voltage sources (one at the emitter end, and one at the base end), but I can't see how gain could happen when the emitter and base voltages have the same magnitude. Fresheneesz 21:47, 11 February 2006 (UTC)

Well E and B have nearly (but not quite) the same voltages on them. Remember that there is an exponential dependence of the collector (or emitter) current on Vbe.--Light current 22:53, 11 February 2006 (UTC)

Volts must exist? Two majority carriers?

Light current - Your change of 'a voltage must be applied' to 'a voltage must exist' is interesting but rather opaque. What is your reasoning for making this change?

Also, isn't it true that a PN junction necessarily 'employs' both types of majority carriers? It might be misleading to make this statement especially in the opening paragraph. It is usually said that the BJT is a minority carrier device for the reason that the charge carrier current making the greatest contribution to the total electric current is the minority carrier in the base of the BJT. Alfred Centauri 13:52, 25 August 2005 (UTC)

a) I changed it to 'volts must exist' to remove the impression that you can just put any old battery across the BE junct. as the diagram shows. Because of the exponential relationship of current to voltage, current can vary tremendously with a few mV change. I admit its not the best wording! But I cant alter the diag to include a base resistor.

b) I mentioned 2 types of charge carrier to try to explain the name Bipolar Junction Transistor (as opposed to FET which has only one type of charge carrier) Again this may need rewording if you think its not clear.

See here for instance [2] Light current 15:52, 25 August 2005 (UTC)

Perhaps stating that current is carried by both holes and electrons would be a good wording? Phil Holmes 16:11, 25 August 2005 (UTC)

I've changed some wording related to my first question but I guess I got auto-logged out before I finished my edit. Alfred Centauri 18:35, 25 August 2005 (UTC)

Anybody know what is meant by 'high' doping and 'low' doping? Is the writer trying to differentiate between the majority carrier types in the base(P region) and the emitter(N region). If so, it is not explained very well. Light current 23:04, 25 August 2005 (UTC)

If I were to use those terms, I would be refering to high or low doping densities. The base region is lightly doped which means that the mobile hole and electron densities are relatively similar. That is, the majority carriers aren't that great of a majority. On the other hand, the emitter is highly doped meaning that there are very few minority carriers compared to the majority carriers. BTW, I like your addition of the word 'appreciable' but I would suggest that you parenthetically add what appreciable is in this context. I would think that something in the neighborhood of 1mA would be about right. Regarding your change of 'emitter' to 'base' - if it makes you feel better then so be it. Both are correct as long as $\beta$ is finite. Here's a question for you to ponder though. Let $\beta$ go to infinity and ask what current does $V_{BE}$ control? Alfred Centauri 01:44, 26 August 2005 (UTC)

I was thinking of the case when the collector current is small - but in that case, Ib = ~ Ie, so I'm not sure now! I guess I was thinking of the B-E junction alone.Light current 20:21, 26 August 2005 (UTC)

I'm not sure what your notation is here. Are these small-signal currents? Actually, it looks like the notation used on the original schematic that I have now modified. That is, for DC (constant) voltages and currents, the letter and subscripts should both be capitalized. So given that, you are correct that $I_{B}$ is proportional to $I_{E}$ as long as $\beta$ is finite and the BJT is in the active region of operation. So, it is equally correct to say that $V_{BE}$ controls $I_{B}$ or $I_{E}$ or $I_{C}$. However, if you let $\beta$ tend to infinity (which is not so farfetched - the small signal model for a JFET 'looks' like the small signal model of a BJT with $\beta$ set to infinity), $I_{B}$ is zero. In this limit, it is clear that fundamentally, $V_{BE}$ controls $I_{E}$ I'm teaching a junior level class on small signal modeling of transistor circuits this semester. It's good to clear out the cobwebs and review this stuff. It's amazing how much more sense it makes when you have to explain to someone else. I remember reading somewhere that a prominent theoretical physicist would, whenever he was stumped on some difficult problem, explain the problem to his dog. His dog would just sit there and look at him without interrupting and, as he would try to explain the problem to his dog, he would invariably see the resolution to his difficulty. Alfred Centauri 20:52, 26 August 2005 (UTC)

Yes, I was intending DC currents, but I've always been sloppy about the capitalisation I use (I just cant remember them all (any?). The problem does not practically occur in JFETs in that most reasonable voltages applied between gate and source won't cause a large current to flow and blow up your transistor junction( unless you apply the wrong polarity).-- that is the point I was tring to make. PS I must get a dog!!(or is WP just as good?):-) Light current 21:22, 26 August 2005 (UTC)

Text from Transistor

I did not know why I'm putting so much info on transistor, slaps forehead. The transistor page can get smaller if I link to bjt and fet. The following paste will be highly edited on transistor now. It is here for future use.

The bipolar junction transistor (BJT) was the first type of transistor to be commercially mass-produced. Bipolar transistors are so named because the main conduction channel uses both electrons and holes to carry the main electric current. Two p-n junctions exist inside the BJT, colector-base junction and base-emitter junction. When the BJT is not powered, the junctions are in unbiased thermal equilibrium with a depletion region formed at each junction. The arrangement of greatest interest is when the B-E junction is forward biased and the C-B junction is reverse biased because this makes amplification possible. Applying a forward bias voltage to the B-E junction unbalances the thermal equilibrium of the junction. In an NPN type BJT, the p-type base begins to inject surplus holes (holes not required to maintain thermal equilibrium) into the emitter where they quickly recombine in the n-type material at the emitter contact. Similarly, the emitter injects surplus electrons into the base. If not for the close proximity of the reverse-biased C-B junction, the B-E junction would behave like a diode–injected emitter electrons would recombine in the base and transistor action would not occur. However, in the NPN BJT, the electrons which are injected from the emitter into the base diffuse across the very narrow base region before most of them have time to recombine within the base region. The depletion region of the nearby reverse biased C-B junction contains an electric field, which sweeps any electrons close to it into the n-type collector, where they recombine with holes at the collector contact. Through this action, only a small amount of carriers in the base (holes) are needed to cause a large amount of carriers in the emitter (electrons) to flow through to the collector.

diagram

that diagram looks like its meant to represent a particular highly complex process but it doesn't specify which one and really serves more to confuse then inform. imo the first diagram in the article should show a minimal bjt and possiblly have diagrams of more complex processes further down. Plugwash 03:15, 7 January 2006 (UTC)

Dopant concentrations

At the moment, part of the article reads "By varying the voltage across the base-emitter terminals very slightly, the current allowed to flow between the emitter and the collector, which are both heavily doped and hence low resistivity regions, can be varied." AFAIK, this is incorrect. The emitter region of the BJT is the highest doped region, whilst the collector region has a relatively low dopant concentration --Rspanton 00:22, 28 January 2006 (UTC)

Correct. I've deleted the reference to doping at this point to correct this. (The author probably meant that the bulk collector is highly doped to minimise collector resistance, which is true, but it's confusing to state that here). --Phil Holmes 16:41, 28 January 2006 (UTC)

The transistor models

I think the article should include the transistor's signal models, including the reference to the model's components such as gm, r0, rπ, Cμ, Cπ, etc... It should represent the transistor model to a full range of frequencies, from DC to the infinite. Afonso Silva 21:49, 3 February 2006 (UTC)

You are talking about a particular transistor model (the hybrid-pi model). Would it be best to start a new page about that model, or add it to this one? I am unsure about the relevant wikipedia policies. Rspanton 00:01, 5 February 2006 (UTC)

I think a new page on hybrid pi woud be appropriate.--Light current 00:50, 5 February 2006 (UTC)

Yes, that was an example, I think the article should include a section with a brief description of the models, with a picture and the description of the main parameters on every type of signals. Those models include not only the hybrid pi model but also the Ebers-Moll model or the T Model (I don't know more), that section would later include links to the main articles and also explain the advantages and applications of each model. The article lacks that kind of information and much more. By the way, Rspanton, what policies are you talking about? Afonso Silva 23:41, 7 February 2006 (UTC)

Yes . One page on Transistor 'hybrid pi' model, one page on Transistor 'T' model. (I think Ebers Moll is the same as the T model). I dont think there are any more common models!--Light current 00:42, 8 February 2006 (UTC)

Maybe I'm wrong, but I think the Ebers Moll and the T Model are separate models, the Ebers Moll model is used to predict the operation of the BJT in all modes (active and saturation). The T model is mainly used to analyze the BJT behaviour in AC operation. Afonso Silva 21:22, 15 February 2006 (UTC)

High-frequency behaviour

The article should also include a reference to the transistor behaviour at high frequencies, and the existance of parasite capacitances that influence the device behaviour. Afonso Silva 21:50, 3 February 2006 (UTC)

Alpha

In my ECE book, it says that alpha is different during DC operation and AC operation. It defines α_DC to be I_C / I_E and α_AC to be ΔI_{C/ / ΔIE. It goes on to say that the ac alpha is formally called the "common-base, short-circuit, amplification factor". This seems to be inconsistant with the article's "αF. Fresheneesz 20:46, 9 February 2006 (UTC)}

Forgive me if you already know this, but if you're ECE, you should get comfortable with the small signal model concept. In analog design, you have steady-state (DC) and small-signal (AC) models. You need DC and AC versions of each parameter, because each one is used for different purposes. DC is needed to find the operating point, output range, etc. AC is needed to find the gain. In reality AC and DC operation are happening at the same time; it's just more convenient to analyze them separately. - mako 21:33, 9 February 2006 (UTC)

picture of the structure section

I've replaced the original picture by a much more simple one, that illustrates the contant of the section much better. If someone disagrees with that, just revert the change.

Thank you! It looks fine. I wanted to do that but I don't have drawing skillz Snafflekid 23:40, 15 February 2006 (UTC)