The design and optimization of electronic systems often requires appraisal an of the electrical noise generated by active devices, and, at a technological level, the ability to properly design active elements in order to minimize, when possible, their noise. Examples of critical applications are, of course, receiver front-ends in RF and optoelectronic transmission systems, but also front-end stages in sensors and, in a completely different context, nonlinear circuits such as oscillators, mixers, and frequency multipliers. The rapid de­ velopment of silicon RF applications has recently fostered the interest toward low-noise silicon devices for the lower microwave band, such as low-noise MOS transistors; at the same time, the RF and microwave ranges are be­ coming increasingly important in fast optical communication systems. Thus, high-frequency noise modeling and simulation of both silicon and compound­ semiconductor based bipolar and field-effect transistors can be considered as an important and timely topic. This does not exclude, of course, low­ frequency noise, which is relevant also in the RF and microwave ranges when­ ever it is up-converted within a nonlinear system, either autonomous (as an oscillator) or non-autonomous (as a mixer or frequency multiplier). The aim of the present book is to provide a thorough introduction to the physics-based numerical modeling of semiconductor devices operating both in small-signal and in large-signal conditions. In the latter instance, only the non-autonomous case was considered, and thus the present treatment does not directly extend to oscillators.