## Photonics

#### Cathode current in charge simulation

• sagar
Subscriber

I am doing transient current mesurentment in charge simulation by appling -5 V at anode and 0 V at cathode, where cathode and anode are basically thin gold layer placed on top of the material and one optical generation is added in the simulation only for 2 fs, starting at 100fs to 102fs. After simulation I am checking the current vs time profile for cathode, where electron should flow to the cathode. here is the graph

where I is the total current, In:electron current, Ip:hole current, Id: displacement current. as mentined here: https://optics.ansys.com/hc/en-us/articles/360034918633-Solver-CHARGE,

is I=In+Ip ? why the electron current is higher than the total current? and why In is started from 200fs, where the generation term only is upto 102fs? What is the significant of Id?

• Khashayar Ghaffari
Ansys Employee

Hi,

Thanks for reaching out, great question. Actually, the full relation is I = In + Ip + Id. In small signal or transient simulations Id can be important since with the electric field altering, the value may no longer be negligible.  Your results seem to match the mentioned formula if Id is accounted for. More detailed information about displacement current and other components of the current are provided in this article (please refer to small signal section specifically).

Edit: Regarding the results it is likely that the step is not small enough, espically around when the shutter is turned on. This might negatively impact the accuracy of the results where the reported rise (here for In) is slower than the actual response. Please try reducing the time step (min and max in transient tab) and monitor the effects on the results. If the change is drastic it will be worth performing convergence testing on the time step to find the best trade-off between accuracy and simulation time.

Best regards

• sagar
Subscriber

It is expected that I_n will start a few times after the generation term, as the generation term is concentrated only in the middle of the sample (only in nm volume), so I can consider it as a point. So electron will take time to reach the cathode, depending on the electron mobility of the sample.

But the thing I could not understand that why it is decaying, where in simulation, I did not include any recombination term or any lifetime. So only once everything is collected by anode should there not be any electron current (I_n). As I am using a point source, I expect one peak with minimal width rather than a long tail.

Thanking you.