In this unit, we will perform a simple small signal AC simulation using the CHARGE solver
to simulate the change in reverse bias capacitance of a pin diode as a function of frequency.
The germanium slab and aluminum contacts of the diode are already setup in the simulation
It is important to make sure that the CHARGE solver mode has been set to “ssac” and
under the small signal AC tab, the frequency range has been set correctly.
Lets switch to partitioned volume mode to see how our simulation boundary conditions
and doping profiles are assigned.
The emitter contact has been assigned to the top aluminum layer and the base is applied
to the bottom one.
This has been done by selecting the “solid” surface type in the geometry tab for each
boundary condition and choosing the corresponding contact from the list of solids.
In addition, two diffusion doping profiles are added.
The n-type doping has been applied to an area just above the base contact and the p-type
is located right below the emitter contact.
For this example, the emitter contact is biased at -0.2V by choosing the “single” sweep
type for the emitter voltage boundary condition.
Also, apply ac small signal has been changed to “all” to have the AC signal applied
to this contact.
The base contact will be kept at zero volt.
Lets run the simulation.
Once the simulation is run, right-click on the CHARGE solver and select visualize.
You will notice that compared to a steadystate DC simulation, there are some additional sets
of results with names starting with “ac” which are the results from small signal simulation.
For example, to view the small signal current-voltage results at the base contact of the device,
we can visualize the “ac_base” results.
To obtain the reverse bias capacitance value for the device, we need to obtain its AC admittance
which can be easily calculated by dividing the AC current by the voltage.
Then the capacitance can be calculated from the imaginary part of the admittance.
These calculations should be done at each frequency point to obtain the change in capacitance
as a function of frequency.
All of this can be done using Lumerical’s scripting language and we have prepared a
script file which can be run to perform the necessary calculations.
The script file can be downloaded from the link provided above this video.
To learn more about Lumerical’s scripting language, please check out the scripting 100
course available in Lumerical University.
Lets open the script file in script file editor and click run script.
The script will gather the necessary data, performs capacitance calculations and returns
the results as a plot.
From the plot, it is obvious that the diode has a DC capacitance of around 2.3 femto farads
which is identical to the capacitance of a parallel plate capacitor with a thickness
equal to the width of the intrinsic layer of the pin diode and the capacitance will
decrease as the signal becomes higher in frequency.