MOSFET Transistor Modeling


A few critical points to remember:

Reading Material

FPAA Reading Material

You might find these papers useful, since they describe the IC, the tools, and the remote testing infrastructure.

Class Lecture Boards

Project Items: Characterizing MOSFET Devices

This section we will use static experimental measurements of MOSFET channel current versus gate voltage, source voltage, and drain voltage, both for nFET and pFET devices. All of these devices are compiled from an FPAA device and are identified as specific transistors, "golden" devices, if you will. When we go through a calibration process, each board measures one "golden" nFET and pFET device for a source, gate, and drain sweep. Making measurements of transistors requires more infrastructure for the current measurements; if you want to watch measured data, those opportunities could be arranged (at a time convienent to the instructors). The goal is that if you have transistors characterized, particularly on the board your group is using, then you can always extract the actual current in a particular circuit. Further, voltage measurements in this system are easier and faster than current measurements.

Initial FPAA setup Data

We want to start the class making sure everyone has a working setup / infrastructure using the FPAA IC. In your summary, you need to mention your FPAA board setup results. including a plot showing remote and in-class measurement results.

What you will do when you are in class:

You need to have a grounding strap to use the equipment. Many of you might already have one. Yes, you would only need one per group, although each person in a group having one would be very helpful. These boards are unique, and although they look like industrial quality boards (and are close), they were developed here at GT, so you must take precautions. It is costly to replace, and I want to spare all of you of having to deal with that concern. Assume this is a requirement.

Getting your Data

Regressing the MOSFET I-V curves uses the chips (3.0) assigned to you in the pdf. Since the remote systems use chip number 01 or 13, I would use those numbers for extracting your data. The data for these I-V curves is read:

Gate voltage sweep

First we took a measurement of source current versus gate voltage swept for a device with the source voltage held at GND / Vdd (nFET / pFET) for the device operating in saturation throughout the sweep. Focus your analysis so you get primarily the subthreshold region and some points just outside near the subthreshold region. You may do additional analysis for above threshold currents if you want.

Fit the theoretical expression in the subthreshold region of operation (an exponential curve fit). Identify "kappa" and threshold voltage and Io. Identify any interesting / unexpected parts of the graph, and explain where it comes from (i.e. do not say it was "measurement error"... what type of measurement error, if that is the case). You should include one figure for this experiment showing Ids versus Vg curve showing subthreshold operation (exponential current). In both plots you should show the experimental data, and the appropriate curve fit; do not have separate plots where you can not compare the results and expect to get any credit.

Source voltage sweep

Next we took a measurement of source current versus source voltage swept for a device with the gate and drain voltage held at Vdd / GND (pFET / nFET) for the device operating in saturation throughout the sweep. Focus your measurements so you get primarily the subthreshold region and some points just outside near the subthreshold region. The data was taken at a temperature nearly equal to 300K.

You should include a figure for this experiment showing Ids versus Vs curve showing subthreshold operation (exponential current). From the subthreshold graph, you should extract a value for UT. Does this value agree with the expected value at room temperature?

Drain voltage sweep

This experiment looks at the resulting drain current due to changes in drain voltage. The dependance of Channel Current on Drain Voltage will illustrate transistors are a Tunable Current Source, as well as the Early Effect shows that deviation from an ideal current source. In this experiment you find the Early voltages from measured data of source current versus drain voltage for an nFET device of length of 2 microns.

For the experimental measurements of the source current as a function of drain voltage from an nFET and pFET for fixed voltages on the gate, drain, and bulk, you should: ( the source voltage is connected to ground or 0V for the nFET and 2.5V for the pFET, and the gate was connected to a fixed voltage for the nFET and pFET)

Finishing up the model

In the presentation, there should be a final table of extracted parameters.

Further, after the parameters you have curve fit, and say a starting value for "sigma" that you got for your drain sweeps, you should modify the existing model in Scilab / Xcos for your transistor data, and should have this simulation data overlapping your obtained measured data.

You should use point markers (e.g. 'o') for data and straight lines for curve fits and sims. Avoid using a legend, but rather identify the graphs directly with the data where possible. You should show a curve fit in a reasonable region (not over the entire graph).

For your presentation, you really only need three plots (if you put it together well), one for gate sweep and source sweep for all of your devices, with curve fits, and simulation data.

Additional Data Examples

We wanted to include examples from previous classes for your reference. These examples come in the form of a report, but remember, you are not turning in a report, but as a team, you turn in a 5-7 minute video of your presentation.

Two video examples for this project are example 1 and example 2 . The grading rubrics for this class is shown here

How we turn in the project has completely changed, as well as we use a different data set, but we use the same IC process. Examples from older classes (where a report was required) are

One should not assume everything is correct in these writeups; I make no comment on what is or is not correct other than I would probably not put up a project that is not reasonably good. The material may or may not correlate to what you need to do for your project, although the fact I have included it means it might be helpful.

Another set of experimental data for this project: You don't have to do anything with this data, but you might find it helpful.