Transistor Modeling: Robust Small-Signal Equivalent Circuit Extraction in Various HEMT Technologies

Small-signal equivalent circuit (SSEC) models prove indispensable to a broad range of activities, ranging from the understanding of device physics, the analysis of device performance, the characterization and comparison of fabrication processes, the bottom-up construction of large-signal models, the extraction of intrinsic noise parameters, and the design of monolithic microwave integrated circuits (MMICs). Because the SSEC model links the physical structure of the device to its circuit behavior, it allows analysis of the microwave performance as a function of the device geometry. A physically representative model can therefore be used for frequencies extending beyond those of the measurement setup. One must keep in mind that a model is only a physical approximation of a given device, and the more we demand of a model, the more likely we are to expose its various shortcomings. For example, one can stress the limits of a model by extending its frequency range or by applying it to dissimilar technologies; experience shows that with newer material systems, models tend to provide poorer fits to the measured data. In the course of our work, we specifically investigated differences brought about by different materials for devices implemented with a given mask set.