This work initially compares GaN high electron mobility transistors (HEMTs) based on the established Ga-face technology and the emerging N-face technology. An investigation is then carried out on the short channel effects in ultra-scaled GaN and InP HEMTs. The dielectric effects of the passivation layer in millimeter-wave, high-power GaN HEMTs are also investigated by focusing on the effective gate length, the gate fringing capacitance, and the drain-to-gate feedback capacitance. Lastly, efficient Full Band Monte Carlo particle-based device simulations of the large-signal performance of millimeter-wave transistor power amplifiers with high-Q matching networks are reported for the first time. In particular, a Cellular Monte Carlo code is self-consistently coupled with a Harmonic Balance frequency domain circuit solver. This book provides device engineers with an insight about the link between the nano-scale carrier dynamics and the device performance. It also introduces an efficient tool for the device early-stage design for RF power amplifiers.