I became curious as to how this is possible after seeing this thread: https://www.physicsforums.com/showthread.php?t=562370 How are these electrometers able to have bias currents of 1-3 femtoamps? What kind of active leakage cancelation circuits are capable of this? Can this be used on other components (diodes, capacitors, Vds channels, etc.)? I tried to see if it was possible, so I did a simulation experiment. The simulations are attached below. I reverse biased a diode into a capacitor to collect the charge, and I see that in my simulation that there is a reverse bias of 2.5nA. I try to cancel the leakage current with a KCL at the diode node, and I do this with a current mirror with a leakage reference current using another diode. I had to adjust the biasing just right to eliminate the leakage current. I admit that this circuit is impractical and I just made it to see if the idea would work. Does anyone have any insight as to how this is done in these electrometers? Am I in the right direction?