MOS Transistors

In this module we are dealing with phenomena that occur when the transistor dimensions are made small; this is important knowledge to have when one is dealing with modern devices.

This module we will finish with small dimension effects, and then talk about models for circuit simulation (the circuit designers among you are probably already using such models). We discuss what it takes to make a good model for circuit simulation (among other things: a lot of caution and care, and about 20,000 lines of code!), and how you can spot possible problems with such models. Although models for circuit simulation include capabilities we have not yet discussed, such as charge modeling, we think you know enough about models at this point to be able to follow this general discussion (which is out of sequence – it’s from Chap. 10 in the book, where you can find much more on the subject).

In this module there is a significant departure from what we have done up to this point: we will allow the terminal voltages of the transistor to vary with time. We will determine the resulting terminal currents, which will now include a “charging” component. We will do this both for moderate and for very high “speeds”. The transient response of circuits involves similar calculations, only done by computer for an ensemble of transistors and other devices.

The material for this module deals with small-signal modeling, and constitutes an important part of the description of MOS transistor behavior. It relates small changes in terminal voltages to the resulting small changes in currents. Small-signal modeling is key to analog circuit design, but not only; it is also used in high-performance digital circuit design (e.g., in designing memory sense amplifiers).

This material concludes our discussion of small-signal modeling; it shows how we can model the transistor at frequencies at which the simple model we derived last week is not valid. Such modeling is important in high-frequency applications, e.g. transceiver design.

The exam is open book, open notes, open videos. It covers the material of the first nine weeks of videos in the course. You are NOT allowed to discuss the exam, directly or indirectly, with anyone. Anybody found in violation of this rule will be removed from the course. You should, of course, feel free to report technical issues with the exam and its submission.

Although your knowledge of this material will not be tested in the final exam, we urge you to go through it, as it deals with important topics, namely noise, ion implantation and substrate nonuniformity, and statistical variability; these topics will help complete your understanding of MOS transistors.