Learning Forum

[HAOYU LIU]

In the ssac mode in the charge, there is an option of small signal ac optical generation, can I view its optical signal waveform in the simulation? Also, in the boundary condition of electrode, it can also be applied ac signal. When I want to test the bandwidth of photodiode, which option should I choose? 

[Guilin Sun]

CHARGE is a carrier transportation solver, which does not have optical waveform included. It has electric static fields by solving Possion equation. Optical signal waveform should be viewed in optical sovers, such as FDTD, INTERCONNECT etc.

When ssac is chosen, the equations are linear: https://optics.ansys.com/hc/en-us/articles/360034917693-CHARGE-solver-introduction

So it only needs a small variation for the perturbation. https://optics.ansys.com/hc/en-us/articles/360034918833-Boundary-Conditions-in-CHARGE-Simulation-Object

You may choose "apply ac small signal" and by default it will automatically use the perturbation amplitude of 1mV.

The "small signal as optical generation"  is for optical generation:

https://optics.ansys.com/hc/en-us/articles/360034918633-CHARGE-solver-Simulation-object

 

[HAOYU LIU]

When testing the bandwidth with ssac, should I choose apply ac small signal？Because I see it doesn't, use it in the vertical example. Also, when I test my structure with ssac for the bandwidth. It decreases when increasing the bias voltage. It's strange and the simulation value is far over the theoretical value. My design is a uni traveling carrier photodiode. Are the ssac mode able to test the transit time bandwidth?

[Guilin Sun]

 

I guess when the “small signal ac optical genetation” is checked it will automatically be in the “as small signal” status. You can compare the result by enabling and not enabling the “ac small signal”.

When the bias voltage changes, the working condition is changed, so I believe it is normal the bandwidth change. I would not expect to have the same bandwidth at high voltage as the small signal case.

You may need to check the device size. For 2D simulation it assumes the norm length of 1cm. The actual device may not be that large in 3D simulation.

I am not sure what the transient time bandwidth mean. The bandwidth is obtained through “transient” stage, even when it uses small signal: it does not simulate the steady state for such result.