## Photonics

#### Calculate the bandwidth in 3d charge

• HAOYU LIU
Subscriber

For my photodiode design, I need to calculate the bandwidth with 3d transient charge. For the bandwidth of photodiode, it consists of  RC bandwidth and carrier transit time bandwidth. What are the R and C in the lumerical charge calculation? Also, for the 3d simulation, after the meshing status finishing, it goes in to the initializing step and the time is too long. After several days, it goes error for the initializing. How to achieve 3d bandwidth simulation in the charge?

• Khashayar Ghaffari
Ansys Employee

Hi,

Thank you for posting your question. Both ssac and transient CHARGE modes can obtain bandwidth including the internal RC effects. External R and C can be added to your contacts (under boundary conditions). Based on your description, here are my recommendations:

1 - Determining the best mode to run your bandwidth analysis in. If you expect the response to be linear, we recommend ssac mode as it's easier to achieve convergence in this mode. Please refer to the appendix in the following article ("Step 2.B using transient simulation") for a more detailed discussion: https://optics.ansys.com/hc/en-us/articles/360042957893-Vertical-photodetector

2 - Resolving convergence error. Obtaining convergence is easier in 2D simulation.. and also in steady-state. If possible I recommend testing your simulation with 2D steady-state first. This makes the troubleshooting easier. If using 3D is necessary, test the steady-state first before moving on to ssac or transient modes.

If the simulation works in steady-state and not in transient (or ssac) it's best to tinker with the respective settings (e.g. min and max time steps in the traient tab).

Best regards,

Khash

• HAOYU LIU
Subscriber

I can see the lumped impendence setting in the electrode node, for the C, how to set it? Does it use the junction capacitance of the photodiode to calculate?

• HAOYU LIU
Subscriber

Also, if I want to test the DC and AC saturation performance of the photodiode, how to do it? In the steady-state in charge, does it contains the space charge effect?