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.

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.

Steve.
--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA