Beginner's question about transistors

Thread Starter

gabrielw6

Joined Sep 21, 2013
2
Hey people,
I'm learning about FETs, and after remembering the classical image of a Bipolar transistor and of the FET, (image on http://www.electronics-tutorials.ws/transistor/tran_5.html I really like this site) I came up with a general question: in the bipolar, if both collector and emitter are with the same impurities, in the images symmetrical, why, or how, we differentiate the collector from the emitter? The same question on FETs, related to the drain and source. What am I misunderstanding? What is wrong or hidden?

Thanks in advance.
 

tshuck

Joined Oct 18, 2012
3,534
You are first assuming symmetrical layouts (as you noted), which, when implemented, are not symmetrical.

Were they symmetrical, you then have to worry about doping levels...e.g. the emitter is more heavily doped than the collector...

BJTs:

From this page.

JFETs


MOSFETs


Were they of the same doping levels and symmetrical, the drain and source could be swapped without regard, as is possible with some JFETs.
 
Last edited:

crutschow

Joined Mar 14, 2008
34,201
..............................
Were they of the same doping levels and symmetrical, the drain and source could be swapped without regard, as is possible with some JFETs.
But note that many FETs have the substrate connected to the source so you can't, in general, interchange source and drain.
 

tshuck

Joined Oct 18, 2012
3,534
But note that many FETs have the substrate connected to the source so you can't, in general, interchange source and drain.
Yes, that is true.

I meant this for discrete JFETs, but in updating my post with images, I messed up the arrangement and that caveat.

Were everything else the same, most MOSFETs still cannot swap drain and source, per your reasoning.
 

studiot

Joined Nov 9, 2007
4,998
why, or how, we differentiate the collector from the emitter?
Further to what has alredy been said, the collector (never the emitter I think) is often connected to the metal case or plate in power devices for heat dissipation purposes (it is assumed that power devices will be used in a configuration where the dissipated power is developed in the collector).
 

alfacliff

Joined Dec 13, 2013
2,458
newer designed rf power transistors have the emitter connected to the case for ease of heat sinking without having to insulate the case.
back in the old days, some fet's were interchangeable between source and drain.I dont think they are anymore. has to do with the internal topography.
 

ErnieM

Joined Apr 24, 2011
8,377
As far as good ole bipolar transistor go, yes, Virginia, you can swap collector and emitter and still get "transistor" action, albeit the current gain may be unity (or less).

If this was impossible the entire TTL family of devices simply would not work.
 

vk6zgo

Joined Jul 21, 2012
677
Back in the day,Philips & Mullard made some special true "Bipolar" transistors which were completely reversible.
I have no idea why!:D
 
Top