I am trying to implement following measurement sequence om MOS capacitors using E4980A with option 001 ( SN: MY46100815; FW: A.02.13; option 001,002,301 - according system info ):
1) measure C-V; then 2) apply voltage pulse to the capacitor for a given time and 3) measure again C-V.
During the voltage bias pulse I would like to measure dc current through the capacitor, Idc, to be able to roughly evaluate the charge passed through the structure. In order to do that that I compose a list with voltage bias points depending on the voltage bias pulse duration and a chosen sweep delay value. The list is executed in VDID measurement mod, LIST:MOD:SEQ. Test AC signal during the pulse is set by :VOLTage:LEVel to 0 as recommended. The data is read with FETC? to obtain voltage and current values. Then with :LIST:SEQuence:TSTamp:DATA the time stamp data is obtained for the each measurement point. The time stamps agree with the desired pulse duration, so no problems on time side. Voltage readings are also consonant with the set voltage bias value. What bothers me is the Idc values. Idc I get for applying voltage biases from 3 to 15 V are always in the range of several nA to 10-30nA without even an average increase with the applied bias. These current values are also not consistent with the standard DC I-V measurements done with SMU (Keythley 236 in this case) using the same voltage range and measurement setup. Trying to find how accurate is the VDID measurement mode I attached 1GOhm resistor and s instead of something ~1nA at 1V I obtain ~4nA. Increasing Vbias does not change current. Moreover the current at open circuit at bias off is also ~4nA.
So my question is what is the lowest current value that can be measured in VDID mode. I wasn’t able to find that in the manual.
Another question is whether this particular E4980A with 001 could work in DC current sweep regime or DC current interface is needed.