This video is taken from the CHARGE Learning Track on Ansys Innovation Courses.
Transcript
In this unit, we will set up the project for a steady-state charge transport simulation
in a p-n diode as our first simulation with the CHARGE solver.
As the first step in simulation workflow, we need to add the materials needed in our
simulation. Let’s open the electrical and thermal material database. This database contains
more than 40 common semiconductor, conductor, insulator and alloy materials. There is also
a material type called "Fluid" which applies to any liquid or gas (e.g. Air) present in
the simulation region. In this example, we will use the material "Si (Silicon)" available
in the material database. The material properties window for silicon has three tabs; Electronic
Properties, Recombination, and Thermal Properties. The first two tabs contain electrical properties
of the material which are used by the CHARGE solver. The third tab contains thermal properties
of the material used by the HEAT solver. For this example, we will focus on the electrical
properties of the materials only. Select the "Si (Silicon)" material from the
material list. Click on the "Electronic Properties" tab which contains the electronic properties
of the material used in the simulation. For the purpose of this example, we will keep
all the properties in their default values and click create to add the material to our
material list in the objects tree. Note that both electrical and thermal properties of
the material will be added to the simulation but when using the CHARGE solver, only the
electrical properties which are marked with the icon “CT” will be considered in the
simulation. Next, we need to add aluminium as the second
material used in our simulation serving as metal contacts for our diode. To do so, select
“Al (Aluminium)” from the list and leave the electrical properties to their default
values. Again, clicking create will add the material to the simulation. Click OK to close
the material database. The Structures section of the Design tab can
be used to introduce various types of structures into the simulation volume. To create a silicon
substrate, we will add a rectangle object to the simulation region (also shown under
"geometry" in the "Objects Tree"). Select the rectangle and click the edit button to
edit its properties. We will name it “substrate” and set its x and y span to 2 , z, -30 and
z span 60 microns. We will also choose its material to be silicon which was previously
added to our material list. Then we add another rectangle to represent the diode’s anode
contact in our simulation. Lets name it “anode” and choose the same x and y span as the substrate,
1 micron for z and a z span of 2 microns. Select the material to be aluminum. Another
rectangle is also needed to define the cathode contact of the diode. Lets add another rectangle,
name it “cathode” with the same x and y span as other rectangles, z value of -61
and z span of 2 microns. The material would be aluminum as well.
After defining the geometry of the structure to be simulated, we can define our simulation
region. We leave its dimension to be 2D Y-normal and keep all boundaries closed. Under the
geometry tab, lets set the x span to 1 , z to -30 and the z span to 61 microns to ensure
that both the contacts and the substrate are included in the simulation region.
Next, we need to add the CHARGE solver. Note that once the solver is added, all the simulation
objects belonging to the CHARGE solver become available under a new tab named CHARGE. Lets
set the CHARGE solver properties. Since we are performing an iso-thermal, steady-state
simulation, the solver physics and mode should be set accordingly. Set the normalization
length to 10000 microns. Since we are setting up a two-dimensional simulation, this parameter
will determine the length of our structure (which is the width of the diode in this example)
in the third dimension. This is needed by the solver to calculate volume dependent parameters
such as contact currents. Under the mesh tab, we set the minimum and maximum edge length
values to 0.01 and 4 microns respectively to ensure an adequately refined mesh based
on the dimensions of our device. Lets leave the rest of the settings to their default
values and close the window. To ensure that the simulation materials and
geometry are defined correctly, we can switch to the partitioned volume mode. We can confirm
that the simulation is formed of three domains with a silicon layer in the middle surrounded
by aluminum contacts on the top and bottom. Now is the time to define the doping profile
of our device. In this example, to create a pn junction in the diode, we will dope the
entire substrate with a light n-type doping and then apply a heavy p-type doping on one
end. This can be done using the constant type doping objects. Lets add a constant doping
region and name it “nepi”. Then set the x and y span to 2, z to -30 and z span to
80 microns. You will notice that we set the span of the doping object to be larger than
that of the silicon substrate. This is to ensure that the entire substrate is doped
with this type of doping. The dopant type should be n-type and doping concentration
should be 1e15 per cubic centimeter. For the p side of the junction, we add another constant
doping object, name it “pwell” with same x and y span as the other doping object, z
value of -7.5 and z span of 25 microns. The dopant type should be p with a concentration
of 1e17 per cubic centimeter. Next, we need to add a bandstructure monitor
to visualize the bandstructure across the pn junction. From the Monitors section of
the CHARGE tab, click on the bandstructure monitor to place the monitor in the simulation.
We will set the type to be linear z to have a one-dimensional monitor along the z direction
and set its z to -30 and z span to 80 microns. This is larger than the z span of the simulation
region. In such cases, the span of the saved data will be determined by the span of the
simulation region and will ensure that the data is recorded exactly up to the edge of
the simulation region. Note that the CHARGE solver will only report the bandstructure
inside semiconductor materials (such as the Si substrate here) and not inside metal contacts.
Finally, we need to define the boundary conditions of our simulation. Here we will use an electrical
boundary condition to set the bias voltage of the anode contact. Click the "Electrical"
button from the Boundary Conditions section of the CHARGE tab. Name it “anode” and
set its mode to steady state. Since we need to apply a forward bias to the diode and obtain
its current over a range of bias voltages, sweep type should be range and set the voltage
to sweep from 0 to 0.8V in 17 points. Also make sure that the force ohmic option is set
to true so that the metal contact is forced to be an ohmic type. Under the geometry tab,
set the surface type to “solid” and select “anode” as the solid to apply the boundary
condition to the anode contact. We also need to define a voltage for the cathode contact
by adding another electrical boundary condition named “cathode” this time with a single
sweep type and 0 as voltage to define the contact as grounded. To assign this condition
to the cathode, select the surface type solid and choose cathode as the solid. If you select
the boundary condition object while still in partitioned volume mode, you can observe
where the boundary condition is applied to. The simulation setup is now complete and you
can save the file. Later in this section, we will learn how to run and analyze this
simulation.