The SPICE FET models: pitfalls and prospects (Are you an educated model consumer?)

Although they are very important for design success, the basic structure of the analytical FET models available in the SPICE circuit simulator have 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 paper reviews the current "state of the art" of analytical FET modeling in SPICE. The target audience is the circuit design user of these models.