This book discusses the evolution of multigate transistors, the design challenges of transistors for high-frequency applications, and the design and modeling of multigate transistors for high-frequency applications. The contents particularly focus on the cut-off frequency and maximum oscillation frequency of different multigate structures. RF stability modeling for multigate transistors is presented, which can help to understand the relation between the small-signal parameter and the physical parameter of the device for optimization. This is a useful reference to those in academia and industry.