Communication and information systems are subject to rapid and highly so phisticated changes. Currently semiconductor heterostructure devices, such as Heterojunction Bipolar Transistors (HBTs) and High Electron Mobility Transis tors (HEMTs), are among the fastest and most advanced high-frequency devices. They satisfy the requirements for low power consumption, medium integration, low cost in large quantities, and high-speed operation capabilities in circuits. In the very high-frequency range, cut-off frequencies up to 500 GHz [557] have been reported on the device level. HEMTs and HBTs are very suitable for high efficiency power amplifiers at 900 MHz as well as for data rates higher than 100 Gbitfs for long-range communication and thus cover a broad range of appli cations. To cope with explosive development costs and the competition of today's semicon ductor industry, Technology Computer-Aided Design (TCAD) methodologies are used extensively in development and production. As of 2003, III-V semiconductor HEMT and HBT micrometer and millimeter-wave integrated circuits (MICs and MMICs) are available on six-inch GaAs wafers. SiGe HBT circuits, as part of the CMOS technology on eight-inch wafers, are in volume production. Simulation tools for technology, devices, and circuits reduce expensive technological efforts. This book focuses on the application of simulation software to heterostructure devices with respect to industrial applications. In particular, a detailed discussion of physical modeling for a great variety of materials is presented.