The leakage of the 1N3595 diode is generally less than one picoampere even with 1mV of forward bias, so the circuit won't interfere with measurements of 10pA or more.
For electrical properties of MOS capacitors, the C-V curves were obtained by LCR meter (HP4284A), and picoampere meter (HP4140B) was used to measure the J-V curves.