Increasing demand on higher efficiency, lower volume and lower cost power converters forces especially the power electronics engineers to conduct research on different kinds of semiconductor other than Si MOSFETs. In fact, the technological improvements push the Si MOSFETs to their theoretical bound that limit power density of the power converters. The GaN High Electron Mobility Transistor (HEMT) is one of the most promising alternative to the conventional Si MOSFETs. In this paper, GaN HEMT devices are analyzed and compared to Si MOSFET devices. The design considerations and complexities are covered by indicating the electrical characteristics of GaN HEMT devices. Furthermore, in order to evaluate the benefits and disadvantages of the GaN HEMT; an active clamp forward converter is designed. Based on simulation results, converter state of art and advantages and disadvantages of using GaN HEMT are presented.