Capactance value became negative in Charge solver

- June 2, 2023 at 12:14 pm
  
  krishnanunnira.20
  Subscriber
  
  While simulating a p-i-n modulator in forward biased condition using a DC+ small signal ac signal, the capacitance value obtained is negative after a certain DC voltage and it will remain negative till some 20GHz frequency. Since capacitance cannot be negative what can I do to rectify the issue.
  Capacitance value is calculated in the same way as that of a traveling wave modulator example in Lumerical.
  Also how can I obtain the total resistance of the device in this case.
- June 2, 2023 at 9:19 pm
  
  Guilin Sun
  Ansys Employee
  
  negative capactance: by definition it is the difference of charges over voltage, delta_Q/delta_v. It is possible to have positive and negative numerically. Proper choice of delta_c might be important.
  https://optics.ansys.com/hc/en-us/articles/360042326774-Semiconductor-insulator-semiconductor-capacitor-SISCAP-Mach-Zehnder-modulator
  
  you may refer this example for resistance https://optics.ansys.com/hc/en-us/articles/360042328774-Traveling-Wave-Mach-Zehnder-Modulator
  as now you have voltage and current.

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