Active microwave devices have proved to be a promising approach to compact, reliable, and efficient sources of radiation for many emerging systems application at terahertz (THz) frequencies. These devices consist of a tunnelling diode with a negative-differential-resistance region (NOR) to overcome the losses in a passive tuning circuit. Experimental results from different types of devices in a wide range of material systems have been reported. To date, no such devices were reported on Gallium nitride (GaN). GaN is well known as an ideal material for high-power electronics due to its large direct band gap and for high-frequency electronics, due to its expected high peak velocity. Therefore, a novel transit-time device is proposed to utilize the GaN/AIGaN heterostructure for high radio frequency (RF) power generation at frequenc ies above 300 GHz. The main purpose of these studies is to design and fabricate this proposed device structure and achieve as high an RP output power as possible at the highest possible frequency. Direct current (DC) devices were fabricated to investigate the tunnelling properties of the GaN/AlGaN interface and also its Schottky contacts. Simulations based on injector characteristics measured from these dc devices were done with promising results. A reliable device fabrication process has been established. RP devices with integral heat sinks have been designed and fabricated to operate around 180 GHz. DC measurements were made on these RF devices. Later, they will be tested with a passive tuning circuit for RF power generation. RP devices on GaN substrates have also been designed and fabricated.