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...