Time domain methods for the global simulation of millimeter-wave transistors and circuits

Global modeling of microwave and millimeter-wave devices and circuits is important concept that addresses the challenges posed by modern-day Microwave Monolithically Integrated Circuit (MMIC) technology. Because of the strong coupling between the different parts of the circuit, Computer Aided Design (CAD) tools must evolve from traditional circuit-oriented tools into modern system-oriented tools. This implies the need for nonlinear capabilities and the ability to deal with large size problems. In addition, the model should be able to provide a link between physical and process data on one hand and the electrical performance on the other hand. These facts call for the use of physics based models not only as device design tools but also as circuit design tools. There are several modeling and computational challenges that must be resolved before this goal can be fully realized. In this dissertation we try to provide solutions to some of these challenges.; In the first part of this dissertation we deal with the problem of physical modeling and simulation of submicron field effect transistors. The problem is first approached from quasi-electrostatic point of view. A full-hydrodynamic model is used to study the device performance characteristics as the device dimensions are reduced into submicron ranges. The model allows us to investigate the inertia effects in the device performance by allowing the carrier momentum and effective mass to be time and space dependent. The results indicated that these effects become more and more important as the device gate-length is reduced. This study showed the limits up to which energy transport models can be used without significant toss of accuracy. As the operating frequency increases several electromagnetic effects. such as distributed effects and particle-wave interaction, become dominant in the operation of the device. In order to explore these effects a full-wave physical model is developed. The model couples a 3D time-domain solution of Maxwell's equations to the charge carrier transport model. The model is successfully used to investigate the Radio Frequency (RF) characteristics of a conventional planar Metal-Semiconductor Field-Effect Transistor (MESFET) and an air-bridged gate MESFET.; The second part of the dissertation deals with the problem of simulating large electromagnetic structures. Two different approaches are presented; the directional-field approach and the time-domain impedance approach. Both approaches are based on the principle of time-domain diakoptics. Using this principle complex systems are simulated by breaking them down into smaller parts that can be individually simulated and characterized by an equivalent impulse response matrix. The solution to the overall system is then computed by subsequently interconnecting the different impulse response matrices with appropriate interfaces.