Winfield Hill wrote...

John Miles wrote...

Winfield Hill wrote...
http://www.picovolt.com/win/elec/com...de-curves.html ...
that part of my measurements cries out for further bench exploration.
It represents only one part, and is unconfirmed. Also, what happens
if the voltage is reversed? Are we to believe the diode is a 10M
resistor, shunted by a diode? I'm not comfortable with that.

I'm confused. Is there some reason to expect the semiconductor material
to be a perfect insulator with no resistivity at all? Nothing's
perfect, and those diodes probably aren't made in the most exacting
processes.

I would be blown away if you *couldn't* measure some ohmic current flow
in a diode at any particular voltage level.

Agreed. It's the rather low 10M value that raises my eyebrows.
Hence my suggestion that the measurements be revisited. Picked up
by John Jardine, who obtained similar values, copied below:

Test on a 1N4148.
ForwardV DiodeR
+50mV 8megs.
+30mV 9megs.
+20mV 10megs.
+10mv 12megs.
+5mV 21megs.
ReverseV
-5mV 21megs.
-10mV 30megs.
-30mV 270megs.

John also suggests the measurements may need further refinement.

Oops! I can think of several circuits I've designed over the years
using diodes for discharge protection that might not work exactly as
I intended, given this observation. And I recall several circuits
where I intentionally back biased the diode a few hundred millivolts
to insure an open circuit.

And others where I used a transistor collector or JFET gate instead.

Pease Porridge in the Feb 3rd issue of Electronic Design mentions
this problem, and Bob suggests using a transistor. "Using 2n3904s
as diodes is very important because most ordinary diodes are much
too leaky around +/-60mV to work well. Ordinary gold-doped 1n914s
and 1n4148s are quite unsuitable..."


--
Thanks,
- Win