Mighty MOSFET Simulation in COMSOL - Visualizing Its Properties

In this project, I will be simulating a MOSFET using COMSOL simulation software. MOSFET is basically a switch that turns on when a Gate voltage is applied. It has revolutionized the world of electronics. It is faster, efficient and do not require gate current like BJT transistor.

In this simulation I will show how the gate channel is formed inside a MOSFET when a small gate voltage is applied. Moreover, I will plot some graphs relating to its band diagram, electron/hole concentrations.

You will need COMSOL 5.5. You can find the software from this LINK

You can download my MOSFET simulation in COMSOL from this LINK

Open COMSOL and start a "Blank Project".

We are interested in building a 3D structure of MOSFET so open a 3d component by going to Home>Add Component > 3D

From the Model Builder set the length unit to micro meter since most MOSFETs are made in micro or even nano meter!

Right click on "Geometry 1" and create a work plane. This will create a base space where we can build our structure.

Right click on plane geometry and create a rectangle.

From the settings of rectangle set the width and Height to 3 and 1 micro meter respectively. These are arbitrary values, you can set your own.

Press Build all objects to see your design.

Then create two more rectangles under plane geometry to define the source and drain of the MOSFET.

I took their dimension as 0.7 x 0.2 um. You need to set the offset by changing base position of the rectangle (To move the rectangles to their respective positions).

Similarly add two points at top of the body to define the gate.

Press Build all objects to see your design.

Right click on Work plane again and create an extrude. This will extrude the structure in z direction.

Set the distance to 0.2um (logical distance) and press build all objects. And our structure is done!

MOSFETS are semiconductor devices. Semiconductors are material that can conduct under certain conditions that enable it to serve as a foundation for computers and other electronic devices.

Right click on materials and add a materials from library.

COMSOL has a wide range of built-in materials. A new section will appear. Find silicon under semiconductor and double click on it.

It will automatically be applied on the whole structure.

This is a very important step. it defines how the structure will work and what results will be created. Add semiconductor physics and double click on it. It will automatically be applied to whole structure.

Right click on semiconductor to dope the Body, Drain and Source. Choose analytical doping model and apply N-type doping to source and drain. Apply P-type doping to the body as shown.

This step completes the Building of MOSFET

To view the switching property of the MOSFET, we can supply a drain to source voltage Vd. If the voltage across gate is Vg = 0V then, no channel will form and the MOSFET behaves like open circuit. If Vg > 0V then the channel will form and current can flow.

To do this we need to add few metal contacts in COMSOL and add the corresponding Vd and Vg values. See the next step

Add metal contacts as shown in the pictures.

Metal contact 1 is the Source

Metal contact 2 is the Drain

Metal contact 3 is the Body

Thin insulator gate is the Gate. A metal oxide of thickness 120nm is used.

Applying a mesh allows physics to be applied on individual sections. To do this go to Mesh > free tetrahedral. Right click on Free tetrahedral and click on size. Mesh will be applied to the geometry.

To run the simulation add a stationary physics as shown in the picture and hit compute.

COMSOL automatically creates some results for you like electron and hole concentrations.

As you can see the channel is formed and electron can flow. Change the Vg to observe how channel is formed. For Vg value 0V no channel(orange portion) will be seen.

You can tweak different value and run simulation to see the effects. However, keep in mind:

> If you see any error while computing the simulation. change the size of mesh. Fine to Finer or Normal

> Value of the dimension must not be very large. In that case doping concentrations need to be changed.

> Restarting the software sometimes help solve errors.