# Calculate the capacitance and resistance for pn junction

#1

Hi, I have some questions in simulating the traveling wave modulator. I used the project file “tw_modulator_Rslab.lsf” to estimate the resistance. But I don’t know why the formula form is “Rslab = 0.1/Icathode +0.1/Ianode” ? In addition, the project file “twmod_ac_capaciatnce.lsf” mentions the "C_static = 0.5*(Cn+Cp)1e151e-6 ". Why can the Cn for negative charge and the Cp for positive charge be averaged to C_static? Thanks.

#2
1. In the resistance calculation part (tw_modulator_Rslab.lsf), the applied voltage was 0.1 V. As a result the resistance is calculated as R = R1 + R2 = V/Ianode + V/Icathode = 0.1/Ianode + 0.1/Icathode. Here R1 is the resistance from anode to WG and R2 is the resistance from cathode to WG.

2. In the capacitance calculation part (twmod_ac_capacitance.lsf) C = dQ/dV. dQ can be calculated in two ways, counting the negative charge on one side (Cn) or counting the positive charge on the other side of the depletion region (Cp). These two values should ideally be identical (and equal to Cstatic) but if a coarse mesh is used then there may be some slight mismatch. Averaging the two values allow us to reduce any numerical error coming from the meshing.