MOSFET Modeling and SPICE: Are You an Educated Model Consumer? Daniel Foty Gilgamesh Associates Fletcher, Vermont 05444 USA dfoty@sover.net http://www.sover.net/~dfoty Although they are very important for design success, the basic structure of the analytical FET models available in the SPICE circuit simulator has received little attention from the circuit design community. However, a number of factors are forcing a change in this situation. The rapid growth of analog and signal processing applications, along with mixed digital/analog functions on the same integrated circuit, are forcing renewed interest in the details of the FET models. These designs require more stringent model accuracy, and it has been found that FET models which were "good enough" for digital circuit design are inadequate in these new cases. In addition, the increasing use of low power technology has also begun to impose a greater need for accuracy. Finally, with the growth of the fabless design industry, the designers and their fabrication facilities are separated in both the geographic and business senses. It behooves the circuit designer to take a more detailed interest in the models which are provided, as these models serve as the critical "communication vehicle" between a circuit designer and the foundry. This class will review the current "state of the art" of analytical FET modeling in SPICE, and is targetted at the circuit design user of those models. The intention is to improve the level of understanding of the FET models employed in SPICE, and to raise the effectiveness of CMOS circuit design. Attendees will be treated to a very practical discussion of the analytical FET models and related topics. Models currently in use will be carefully examined to note the strengths and weaknesses of each model, along with criteria for determining the applicability of each model for various types of circuit design; methods are also presented to allow a circuit designer to decide if provided models properly represent the underlying fabrication technology and are "the best that they can be" for a given type of model. The overall goal and benefit of this class is to help make the circuit designer an "educated model consumer." Topics to be covered include: o The growth of CMOS technology and the emergence of the fabless design industry. o The formal structure of analytical FET modeling in SPICE. o An examination of the evolution of the FET models presently in widespread use. This will include a discussion of model strengths, weaknesses, applicability, and circuit simulation usage. o Modeling the active gate capacitance. o Methods of accounting for process variations. o Correlating models and hardware. o Emerging new FET model candidates. o A discussion of the future of analytical FET modeling for use in circuit simulation. This topic is particularly important at this time, as new models are being included in various SPICE circuit simulators. However, the increasingly empirical character of these models is causing serious problems for the practicing circuit designer. Instructor bio -------------- Daniel Foty is the President of Gilgamesh Associates, a consulting and research firm specializing in the construction, implementation, and usage of analytical MOS models for circuit simulation. He is the author of "MOSFET Modeling with SPICE: Principles and Practice", which was recently published by Prentice-Hall. Suggested readings ------------------ The course instructor recently published "MOSFET Modeling with SPICE: Principles and Practice" (Prentice-Hall, 1996), a comprehensive treatment of the analytical MOS models available in various implementations of SPICE. Those interested in a very detailed and thorough discussion of the basic device physics issues involved in analytical MOS modeling will enjoy Narain Arora's text "MOSFET Models for VLSI Circuit Simulation" (Springer-Verlag, 1993). Excellent practical discussions of the pitfalls of SPICE and its underlying element models, along with simple explanations of coping with and overcoming these limitations may be found in Ron Kielkowski's two texts, "Inside SPICE: Overcoming the Obstacles of Circuit Simulation" (McGraw-Hill, 1994), and "SPICE: Practical Device Modeling" (McGraw-Hill, 1995).