[Truncated abstract] Over the past decade, the group III-nitride semiconducting compounds (GaN, AlN, InN, and their alloys) have attracted tremendous research efforts due to their unique electronic and optical properties. Their low thermal carrier generation rates and large breakdown fields make them attractive for the development of robust electronic devices capable of reliable operation in extreme conditions, i.e. at high power/voltage levels, high temperatures and in radiation environments. For device applications in radiation environments, such as space electronics, GaN-based devices are expected to manifest superior radiation hardness and reliability without the need for cumber- some and expensive cooling systems and/or radiation shielding. The principle aim of this Thesis is to ascertain the level of susceptibility of current GaN-based elec- tron devices to radiation-induced degradation, by undertaking a detailed study of 60Co gamma-irradiation-induced defects and defect-related effects on the electrical characteristics of n-type GaN-based materials and devices . . . While the irradiation-induced effects on device threshold voltage could be regarded as relatively benign (taking into account that the irradiation levels employed in this study are equivalent to more than 60 years exposure at the average ionising dose rate levels present in space missions), the observed device instabilities and the degradation of gate current characteristics are deleterious effects which will have a significant impact on the performance of AlGaN/GaN HEMTs operating in radiation environments at low temperatures, a combination of conditions which are found in spaceborne electronic systems.