Transistor for picoampers current source?

[carpenter]

I have constant current source with sense resistor, precision amplifier and N JFET transistor.
With 2N4392 my current source work from 1000pA. I need to improve the lower limit at least an order of magnitude to 100pA.
What transistor should I use?

 

[crutschow]

You might try a MOSFET such as contained in a CD4009. The data sheet states it has a typical gate input current of 10pA.