Transistor question -theory
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From: krw on 19 Feb 2010 19:17 On Fri, 19 Feb 2010 15:16:06 -0600, Tim Wescott <tim(a)seemywebsite.com> wrote: >On Fri, 19 Feb 2010 12:33:14 -0800, Jon Kirwan wrote: > >> On Fri, 19 Feb 2010 12:11:19 -0800 (PST), lerameur <lerameur(a)yahoo.com> >> wrote: >> >>>I have been analyzing the 'how to' of a transistor recently. Electron >>>flow, holes, dope material etc... I still do not understand why a >>>transistor can be reversed, anyway reverse based from all those web >>>pages. They all show a transistor either N-P-N or P-N-P, lets take NPN >>>or emitter-base-collector why cant we change it to collector-base- >>>emitter, isnt the emitter and collector both N, dopes with free >>>electrons?, shouldn't they act the same way?? >> >> Yes.... but the geometry of the emitter and the amount of dopant for the >> N material in it may differ from the collector's. Very old transistors >> were, I seem to recall, made from quite similar N materials for both the >> collector and emitter and were manually fused together. They were >> expensive, but they worked about like you suggest -- roughly equally >> either way -- if I got that much right. Perhaps someone with better >> knowledge about this can comment, though. >> >> But modern BJTs aren't built that way. They can diffuse different >> concentrations of dopants for the emitter vs the collector and the >> physical shape/size is also different. Which emphasizes one >> orientation/use over the other. You can still wire them up the other >> way, but the beta will probably be a lot lower. Some other features may >> be better, though. I suppose some designers take advantage of that. Not >> so often that I've seen it done much, though. > >IIRC from my college days, TTL logic uses the reverse gain of the input >stage to suck charge out of an intermediate stage's base, speeding up one >of the transitions. The forward gain of the multi-emitter transistor is intended to suck the current out of the base of the splitter transistor in the middle of the gate. The forward biased base-collector junction provides the current to turn the splitter on. | | | .-. .-. | | | | | | | | | | | '-' '-' | | | |/ | o-------| | | |> | | | | Splitter| | ----- |/ V A o------v v \--------| - | |>| o------------o O B o--------+ | | | |/ o-------| | |> | | .-. | | | | | | | '-' | | | | | (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
From: Shaun on 19 Feb 2010 21:56 "lerameur" <lerameur(a)yahoo.com> wrote in message news:a02c684f-f6c2-4671-866f-82a5a1c06f67(a)y17g2000yqd.googlegroups.com... > Hi, > > I have been analyzing the 'how to' of a transistor recently. Electron > flow, holes, dope material etc... I still do not understand why a > transistor can be reversed, anyway reverse based from all those web > pages. They all show a transistor either N-P-N or P-N-P, lets take NPN > or emitter-base-collector why cant we change it to collector-base- > emitter, isnt the emitter and collector both N, dopes with free > electrons?, shouldn't they act the same way?? > > this will lead me to another question, is there such devices that will > work they way I mentioned, meaning when you activate the base, the > current could flow in either directions ?? just like a mechanical > relay. > thank you > ken A Triac will allow current to flow in either direction, under most conditions. Some are better suited that others like alternistors, a variation of a triac. Shaun
From: whit3rd on 19 Feb 2010 22:16 On Feb 19, 12:11 pm, lerameur <leram...(a)yahoo.com> wrote: > etc... I still do not understand why a > transistor can be reversed, anyway reverse based from all those web > pages. They all show a transistor either N-P-N or P-N-P, lets take NPN > or emitter-base-collector why cant we change it to collector-base- > emitter, isnt the emitter and collector both N, dopes with free > electrons?, shouldn't they act the same way?? The doping is the same polarity, but not the same amount; the E doping is higher than B, with C doping usually inbetween those. So, reverse breakdown of BC junction is high (for PN2222, about 80V) and BE is low (about 7V), and 'emitter efficiency' of BE is high, of BC is low (this means lower reverse beta). The only real advantage of the reverse configuration is the saturation voltage of a switched-ON reverse transistor, which can be in the 10 mV range; before MOS switch devices became common, this was how a chopper transistor was operated.
From: Jasen Betts on 20 Feb 2010 02:40 On 2010-02-19, lerameur <lerameur(a)yahoo.com> wrote: > Hi, > > I have been analyzing the 'how to' of a transistor recently. Electron > flow, holes, dope material etc... I still do not understand why a > transistor can be reversed, anyway reverse based from all those web > pages. They all show a transistor either N-P-N or P-N-P, lets take NPN > or emitter-base-collector why cant we change it to collector-base- > emitter, isnt the emitter and collector both N, dopes with free > electrons?, shouldn't they act the same way?? if you're only using low voltages you can, it it will still work, just not as well. > this will lead me to another question, is there such devices that will > work they way I mentioned, meaning when you activate the base, the > current could flow in either directions ?? just like a mechanical > relay. several. --- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: George Herold on 22 Feb 2010 15:22
On Feb 19, 3:11 pm, lerameur <leram...(a)yahoo.com> wrote: > Hi, > > I have been analyzing the 'how to' of a transistor recently. Electron > flow, holes, dope material etc... I still do not understand why a > transistor can be reversed, anyway reverse based from all those web > pages. They all show a transistor either N-P-N or P-N-P, lets take NPN > or emitter-base-collector why cant we change it to collector-base- > emitter, isnt the emitter and collector both N, dopes with free > electrons?, shouldn't they act the same way?? > > this will lead me to another question, is there such devices that will > work they way I mentioned, meaning when you activate the base, the > current could flow in either directions ?? just like a mechanical > relay. > thank you > ken Just say a good thread about this on sci.electronics.design you might want to read it. George h. |