Talk:Bipolar junction transistor

Put your text for the new page here. 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

Contents

1 voltage or current? 2 mnemonic 3 doping 4 models 5 Uses and possible technical details 6 Amplifier configurations

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)

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)

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)

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:

		Missing image ES_term_S.png Image:ES_term_S.png	C
		Missing image ES_cIs_NS.png Image:ES_cIs_NS.png	αiE
B	Missing image ES_term_E.png Image:ES_term_E.png	Missing image ES_wire_NSW.png Image:ES_wire_NSW.png
	iE ↓	Missing image ES_diode_NS.png Image:ES_diode_NS.png	Is/α
		Missing image ES_term_N.png Image:ES_term_N.png	E

B	Missing image ES_term_E.png Image:ES_term_E.png	Missing image ES_wire_SW.png Image:ES_wire_SW.png		Missing image ES_wire_ES.png Image:ES_wire_ES.png	Missing image ES_term_W.png Image:ES_term_W.png	C
	iB ↓	Missing image ES_diode_NS.png Image:ES_diode_NS.png		Missing image ES_cIs_NS.png Image:ES_cIs_NS.png	βiB
		Missing image ES_wire_NE.png Image:ES_wire_NE.png	Missing image ES_wire_ESW.png Image:ES_wire_ESW.png	Missing image ES_wire_NW.png Image:ES_wire_NW.png
			Missing image ES_term_N.png Image:ES_term_N.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