I'm looking for a diode-like component with a "resistance" (when linearized about V=0) of at least 20 Mohms at 75 deg C. The behaviour for larger voltages doesn't really matter so long as the diode is able to conduct about 50 uA by 2.5V. The capacitance must be no more than a few pF.
A regular signal diode like a 1N4048 has a linearized resistance of about 5 Mohms at room temperature, and much lower resistance at 75 deg C, so this obviously won't work. I've found a few "ultra-low leakage diodes" that could work but are quite pricey and not in the form-factor I'd like:
I also found this thread suggesting that using the base-emitter junction of a BJT may be preferable:
The ideal solution would cost less than $1 in QTY=10,000 and take up no more space than an 0603 or 0805 component (so putting 10 diodes in series is probably not feasible).