Re: Transistor question -theory

On Fri, 19 Feb 2010 12:11:19 -0800 (PST), lerameur
<(E-Mail Removed)> 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.

Jon

>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

On Fri, 19 Feb 2010 15:16:06 -0600, Tim Wescott <(E-Mail Removed)>
wrote:

>On Fri, 19 Feb 2010 12:33:14 -0800, Jon Kirwan wrote:
>
>> On Fri, 19 Feb 2010 12:11:19 -0800 (PST), lerameur <(E-Mail Removed)>
>> 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-------|
| |>
| |
.-. |
| | |
| | |
'-' |
| |
| |
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