Steven Swift wrote...

When I needed diode clamps that would work with +/-100mV with less
than 10pA of leakage, I always used selected Jfets with some back
bias. The DPAD10, etc. series of diodes that were designed for this
were too expensive. But that was when I was at Fluke and I could
specify exactly what I wanted and the vendors would come knocking.
Most of the parts were specially selected from National process 50,
which was used to make 2N4416 RF transistors.

Yes, the PAD-1 and pn4117 JFET curves on my plot show your point,
http://www.picovolt.com/win/elec/com...de-curves.html
with about 150 to 200mV forward voltage at 1pA. In that setup,
the current for under 100mV was well below what I could detect.

The diode equation works over all ranges of the diode. We also used
2n2484 over 9 decades of current (pA to mA) of DC and tested them to
be very log-linear. At AC, as already stated, you have to include the
capacitance in the model. A finger print on the transistor body would
ruin the response.

Not to mention the long reverse-recovery time of those parts. I put
up my graph to counter Dave's claim of a 600mV "barrier potential",
not to argue that these diodes make good low-level RF rectifiers. :)


--
Thanks,
- Win