This dissertation is explored the photonics inter sub band transitions (ISBT) phenomenon in an electronics Gallium Nitride (GaN) high electron mobility transistor (HEMT) device. Conventional photonic devices are operated at cryogenic temperatures to minimize the thermal effect. The reported maximum operating temperature of THz quantum cascade laser (QCL) is in the range of 150-200 K which is too low for general applications. The conduction band tuning through external gate bias makes advantage of HEMT device for room temperature (RT) terahertz applications. The theoretical models for electrically tuneable plasmonic metamaterials assisted ISBT have been developed. Experimental demonstration of electrical tuning of ISBT in a GaN HEMT device at room temperature has not only provided a novel mechanism but also discriminates ISBT from other transitions induced by deep-level traps and defects in the 100 nm GaN HEMT device. It is possible to tune the subband energy level inside triangular quantum well of GaN HEMT by applying gate voltage. The GaN HEMT device responds toward incident terahertz radiation due to inherent advantage of conduction subband tuning through external bias. The presented novel approach for ISBT in GaN HEMT has the potential possibilities in the context of overcome the THz gap in the electromagnetic spectrum at ambient temperature.