  # How to simulate the series resistance (including contact resistance) in a PN junction

Hi

I was reading the tutorial of Traveling wave modulator, and would like to simulate a waveguide modulator similar to the one on tutorial. There was a section talking about how to simulate Rslab. Here I have one question about the unit. The simulation was a 2D Y-normal simulation. But the results were extracted from the current and voltage. I wonder why a 2D simulation can give the current. Also, I found that the simulated resistance (1.8 ohm) was smaller than the values (10-15 ohm-mm, unit different again) I read on the papers. Finally, there should be contact resistance between Al and silicon. Therefore, I would like to know how to simulate this series resistance well.

https://kb.lumerical.com/en/modulators_charge-transport-simulation.html

Thanks

Hi @cmchang, my apologies for the long wait. As explained in this other post (What is the unit for contact current in DEVICE?), the current in a 2D simulation in DEVICE has a unit of Amp because the solver assumes that the length of the structure in the third dimension is equal to the “norm length”, a property of the solver region. In the online example of the modulator, the norm length is set to 1 cm (default value) which means the measured resistance actually has the unit of 1.8 Ohm-cm or 18 Ohm-mm which is not too far from the values you have mentioned (10 - 15 Ohm-mm).

Regarding the modeling of series resistance at contacts, you can use the contact-mini-circuit available at the electrical contacts. For example you can enable the series resistance and set the value to be equal to your contact resistance. Please note that the resistance value is absolute (Ohm) and not length dependent (Ohm-cm). So when you define the value of the resistance you will have to define it assuming that the device length is the third dimension is equal to “norm length”.