The unique electrical and thermal properties of GaN are used to improve two different approaches to generate THz radiation. One method is heterodyne photomixing, where two laser beams with slightly different wavelengths illuminate an ultrafast photodetector. The electrical and mainly the thermal limits of the conventionally used LT-GaAs restrict the THz output power generated by this method up to now. In a second approach, ultrafast transistors, e.g. hetero field effect transistors, are applied in high power high frequency oscillator circuits that act as input for a frequency multiplier chain. In this approach we investigate the utilization of GaN based transistors. Devices in this material system are usually used for high power applications at moderate frequencies, but the very high electron saturation velocity of GaN allows the application above 100 GHz as well.