The objective of this project is to design and fabricate THz modulators based on the AlGaN/GaN HEMT technology. Therefore, the fundamentals of FETs are described with a focus on AlGaN/GaN-based HEMTs. In this regard, the formation of a two-dimensional electron gas (2DEG) conducting channel and fabrication of proper electrical contacts are discussed. The dispersion relations of plasma waves in the 2DEG conduction channel of a FET are described in order to explain the coupling between them and THz radiation. Different types of AlGaN/GaN-based THz modulators presented in the literature are analysed, in order to determine which design is preferable in regards to available equipment and relevant time frame of the project. Based on this survey, circular grating-gate structures are chosen, designed, and fabricated in collaboration with Twente University in the Netherlands. Comprehensive electrical characterisation of these devices reveals appropriate transistor-like behaviour making it very promising to obtain THz modulation properties. Unfortunately, we did not succeed to examine THz modulation due to difficulties with wire bonding of individual devices into arrays and connection to the external electrical circuit. Consequently, THz spectroscopy measurements could not be performed.