[sdiy] Does Source = Drain???

[David G. Dixon]

> > Every datasheet I've ever read on JFETs always says that the source
> > and
> > drain are interchangeable, right on the front page.  I don't think it
> > matters.  I'd say, orient them for maximum layout convenience.
> 
> Like David Ingebretsen, I've had similar questions recently, and have
> gone looking for answers... <snip>


First of all, good on ya, John, for not being as lazy as me and actually
doing some research!

I went back and checked the datasheets of all the JFETs I have ever used: 

J201 -- On page 1, just below the drawing of the device: "Note: Source and
drain are interchangeable."

2N5457 -- Ditto

2N5485 -- Ditto

2N4391 -- The datasheet I have on file doesn't mention it, although I found
a Philips datasheet from 1990 which refers to the device as "symmetrical"
and does say, at the bottom of page 1: "Note: Drain and source are
interchangeable."  Either they were lying, the others simply forgot to
mention it, or the design changed after 1990.  Linear's datasheet for the
PN4391 doesn't mention it, so perhaps there is something in there which
renders the JFET asymmetric.

In any case, you can see how one would draw the conclusion that JFET sources
and drains are interchangeable!