A MOS Transistor Model for Mixed Analog-digital Circuit Design and Simulation

Abstract

In the design cycle of complex integrated circuits, the compact device simulation models are the privileged vehicle of information between the foundry and the designer. Effective circuit design, particularly in the context of analog and mixed analog-digital circuits using silicon CMOS technology, requires a MOS transistor (MOST) circuit simulation model well adapted both to the technology and to the designer's needs. The MOST model itself should also help portable design, since design-reuse becomes a major advantage in the fast development of new products. Clearly, the MOST model must be based on sound physical concepts, and be parameterized in such a way that it allows easy adaptation to very different CMOS technologies, and provides the designer with information on important parameters for design. This chapter describes an analytical, scalable compact MOST model, called 'EKV' MOST model, which is built on fundamental physical properties of the MOS transistor. Among the original concepts used in this model are the normalization of the channel current, and taking the substrate as a reference instead of the source. The basic long-channel model is formulated in symmetric terms of the source-to-bulk and drain-to-bulk voltages. In particular, the transconductance-to-current ratio is accurately described for all levels of current from weak inversion through moderate and to strong inversion. This characteristic is almost invariant with respect to process parameters and technology scaling; therefore, the model can be adjusted to a large range of different technologies. Short-channel effects have been included in the model for the simulation of deep submicron technologies. A full charge-based dynamic model as well as the thermal noise model are derived within the same approach. The continuity of the model characteristics is based on the use of a single equation, enhancing circuit convergence. The relative simplicity of the model and its low number of parameters also ease the process of parameter extraction, for which an original method is proposed. This MOST model is used in the context of low-voltage, low-current analog and analog-digital circuit design using deep submicron technologies. A version of this model based on the same fundamental concepts, is also available as a public-domain model in various commercially available simulators.