3. MOSFET Split CV Simulation

3.1. Objectives

• learn the basic flows of running device TCAD
• perform split CV simulation
• vary parameters of MOSFET structure to see impact on CV
• using split CV to understand poly depletion effect
• get a feel of computing via command lines
• get to know various scripts

3.2. Required Programs

• sde
• sdevice
• tecplot_sv
• inspect

3.3. Overview

See lecture notes on mos capacitor and mosfet capacitors.

3.4. Files You Need

• the structure specification and mesh generation command file. There are two versions. You have used the version with graphical user interface to input structural parameters, which can also be downloaded here mosfet.scm

  The other version does not use the GUI. Instead, you will use a text editor to change the parameters. See the tutorial below. That file can be downloaded here mosfet_fixed.scm.

• In the mosfet_fixed.scm file, an inspection of the program will quickly show that the variables have the following meanings

  • w1: “Substrate Width”
  • w2: “Gate Width”
  • w3 “Spacer Width”
  • h1 “Substrate Height”
  • h2 “Oxide Thickness”
  • h3 “Spacer Height”
  • Nsub “Substrate Doping”
  • xj “S/D Junction Depth”

• the device simulation command file split_cv_des.cmd, which can also be downloaded here split_cv_des.cmd. It needs to be placed in the same directory as your other files, presumably, ~/mosfetcv, which was created earlier.

• the inspect command file for plotting AC simulated CV curves, cv_ins.cmd, which can also be downloaded cv_ins.cmd.

3.5. Video Tutorial

First download all the files above, place them in the mosfetcv folder.

Watch a 15 minute tutorial I prepared tonight. Do not have my good mic with me. So bare with the default laptop mic. It is audible.

Follow the tutorial. You should see CV curves pop up.

3.6. Steps

3.6.1. Meshing

To generate the mesh, first edit structural parameters in the mosfet_fixed.scm file. If you are on command line, working inside the mosfetcv folder already, just run

gedit mosfet_fixed.scm&

After editing, save the file. You do not have to close the editor as it is running in background.

Next, run

sde -l mosfet_fixed.scm

You will see sde working to generate the MOSFET mesh, and tecplot displaying the end result. You can examine the structure or doping etc inside tecplot. No need to save anything when asked. It has all been saved with commands inside the command file.

You can then close tecplot and sde programs.

3.6.2. Simulating

Examine split_cv_des.cmd file first. Set all voltages to desired values. For vp.dc, make it stop at 3V if you are using thin oxide just a few nm thick. You could modify the start gate voltage to say -3V too.

Then run

sdevice split_cv_des.cmd

You should see reporting of solution process. It should take about 1 minute cpu time to finish.

3.6.3. Plotting

Examine cv_ins.cmd file first. You do not have to completely understand it, but just a quick look will give you some useful idea about what it does. You may modify it for your purpose. You are not required to write such programs. See our notes about how and why we need to take the absolution values of certain capacitances. We talked about this in class too.

For thin gate oxide, set the vdd parameter to a value consistent with the vp.dc set above in the split_cv_des.cmd file. Then, run

inspect -f cv_ins.cmd

You should see some nice split CV plots.