With fast growing of the photonics and power electronic systems, the need for high power- high frequency
semiconductor devices is sensed tremendously. GaN provides the highest electron saturation velocity, breakdown
voltage and operation temperature, and thus combined frequency-power performance among commonly used
semiconductors. With achieving the first THz image in just two decades ago, generation and detection of terahertz (THz)
radiation is one of the most emerging photonic areas. The industrial needs for compact, economical, high resolution and
high power THz imaging and spectroscopy systems are fueling the utilization of GaN for the realizing of the next
generation of THz systems. As it is reviewed in this paper, the mentioned characteristics of GaN together with its
capabilities of providing high 2-dimentional election densities and large longitudinal-optical phonon of ~90 meV, make
it one of the most promising semiconductor materials for the future of the THz generation, detection, mixing, and
frequency multiplication. GaN- based devices have shown capabilities of operating in the upper THz frequency band of
5- 12 THz with relatively high photon densities and in room temperature. As a result, THz imaging and spectroscopy
systems with high resolutions and depths of penetrations can be realized via utilizing GaN- based devices. In this paper,
a comprehensive review on the history and state of the art of the GaN- based electronic devices, including plasma
HFETs, NDRs, HDSDs, IMPATTs, QCLs, HEMTs, Gunn diodes and TeraFETs together with their impact on the future
of THz imaging and spectroscopy systems is provided.
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