GaN-based devices are becoming a key technology for millimeter-wave applications. Because of outstanding material properties, recent advances in GaN device designs provided a sharp increase in performances including high power, high efficiency, reliability, linearity, and compact size. These capabilities are ideally suited for numerous millimeter-wave power applications such as wireless networks and PAs MMIC for Q-band frequency and beyond. Therefore, GaN will play an important role in advanced RF and millimeter-wave applications including for instance 5G and satellite communications or military oriented applications in harsh environment. However, there is no technology satisfying the whole mm-wave requirement including high DC, RF, large signal, linearity and reliability performances. The aim of this work is to increase the understanding of physical mechanisms involved in mm-wave high performance transistors and their related limitations.