Throughout the history of power electronics, main driving force of developments is attribute to innovations in power semiconductor technology. With continuous technical improvements in the past 30 years, Si devices, being the most widely used power semiconductor technology, are approaching physical limits e.g. breakdown field, thermal conductivity, etc. of the basic material. Performances of power converters such as efficiency, power density, etc., therefore, has entered into a stage that further improvements are not likely to happen without revolutionary advance in power semiconductor technology. GaN power semiconductor devices, judging from the wide bandgap nature of its material, have the potential of outperforming conventional Si counterparts in measures of high voltage, high temperature and high frequency operations. Capabilities of a GaN device, however, is influenced by more issues, which, apart from material properties, also include die design and fabrication approaches, device packaging technologies, how they are used in an application, etc. In fact, at this stage, available GaN transistors are mostly of lateral structure and, as a consequence, confined to low voltage ones (<1kW) while maximum junction temperature of these products is limited to 175 °C because of the lack of suitable packaging technologies. So far, high frequency operation performance of GaN power semiconductors is unknown and needs to be investigated. This thesis explores high frequency operation potentials of single-die, normally-off GaN power semiconductors that are suited for high voltage, low current applications. The exploration is carried out by means of conducting loss modeling of GaN transistors, uncovering desirable operation conditions of GaN devices for high frequency operations according to analysed results from the model, identifying optimal topologies and operation modes in power converters that can facilitate such conditions for GaN to achieve optimal utilization of the new technology and demonstrating potentials of GaN power semiconductors in an application with all the developed techniques employed.