Transistor question -theory

[lerameur]

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

 

[Jon Kirwan]

On Fri, 19 Feb 2010 12:11:19 -0800 (PST), lerameur
<lerameur@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.

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

 

[Tim Wescott]

On Fri, 19 Feb 2010 12:11:19 -0800, lerameur 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??

The doping levels are different between the emitter and collector, and in
many transistor processes the emitter is smaller than the collector,
making it harder for the carriers to 'find' the emitter when it's
operated in reverse.

Most bipolar transistors _can_ be operated in 'reverse', but with a
severely degraded current gain -- I think a current gain less than unity
is not uncommon.

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.

Most FETs operate this way, at least to a first order. However, many
power FETs have diodes that are 'built in' as a side effect of the
manufacturing process, so you can't just freely turn any old FET around.

"Electronic switches" are commonly implemented as complementary small
signal FETs in parallel, with appropriate gate control to turn them both
on at the same time.

Now, if I'm lucky Jim Thompson hasn't answered by the time I press
"send", so I can get credit for an incomplete explanation before someone
who actually knows his stuff comes on line with the real deal.

--
www.wescottdesign.com

 

[Tim Wescott]

On Fri, 19 Feb 2010 12:33:14 -0800, Jon Kirwan wrote:

On Fri, 19 Feb 2010 12:11:19 -0800 (PST), lerameur <lerameur@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.

--
www.wescottdesign.com

 

[George Herold]

On Feb 19, 3:33 pm, Jon Kirwan <j...@infinitefactors.org> wrote:

On Fri, 19 Feb 2010 12:11:19 -0800 (PST), lerameur

leram...@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.

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- Hide quoted text -

- Show quoted text -

Caveat (I probably know less than Jon about transistors)

But I’ve always wanted to try that experiment. I think the biggest
thing to look out for (besides the much reduced beta) will be the
reverse bias Vbe voltage. Is this a zener breakdown?

So keep the supply voltage low and it might work..... Sounds like a
project for a late Friday afternoon....

George H.

 

[krw]

On Fri, 19 Feb 2010 15:16:06 -0600, Tim Wescott <tim@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@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)

 

[Shaun]

"lerameur" <lerameur@yahoo.com> wrote in message
news:a02c684f-f6c2-4671-866f-82a5a1c06f67@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

 

[whit3rd]

On Feb 19, 12:11 pm, lerameur <leram...@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.

 

[Jasen Betts]

On 2010-02-19, lerameur <lerameur@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@netfront.net ---

 

[George Herold]

On Feb 19, 3:11 pm, lerameur <leram...@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.