In this work, an accurate analysis of state-of-the- art high-frequency and high-power High Electron Mobility Transistor (HEMT) devices has been performed by using both commercial simulation tools and a full band Cellular Monte Carlo (CMC) simulator, in order to investigate the most important factors allowing improvements in reliability and manufacturability of AlGaN/GaN HEMTs at mm-wave frequencies. The effects of polarization charge on the electrostatic potential distribution across the heterostructure of a GaN HEMT device was first investigated. Subsequently, an intensive characterization on more than 10 wafers has been performed, in order to identify parasitic effects related to charge trapping phenomena. The effects of threading edge dislocations on the electron transport properties of GaN HEMT devices were then investigated. Finally, several studies have been performed on N-Face and Ga-face structures (both in enhancement and depletion mode). Scaling effects on the RF performance of these devices have also been investigated.