A source of error?

[Martin Czech]

There are special field effect devices that have a drift zone
at the drain, in order to withstand higher Uds.
There are other technological measures with this kind of unsymmetry.

All other field effect devices are symetrical, ie. voltage polarities
determine what is source and what is drain.

Some MOSTFETS use the source terminal for the bulk, so this
is another kind of unsymmetrical behaviour.

I think all jfets are symmetrical, so there is no source or drain pin, really.

JFETS use a junction for gate isolation, the channel should never
be forward biased to the gate (except if you use a jfet as low
leakage diode). Are you sure that this is not your problem
with low voltages?

m.c.