We report on a broadband terahertz emission from a doubly interdigitated grating gates high electron mobility transistor. The observed emission was explained as due to the excitation of multi mode of plasmons: thermally excited incoherent modes and instability-driven coherent modes. The experiment was performed using Fourier spectrometer system coupled with high sensitive 4K Silicon bolometer under the vacuum. To enhance the efficiency, the device was subjected, from the backside, to a CW 1.5 μm laser beam. Dependence of the emission on the gate bias was observed and interpreted as due to the self-oscillation of the plasma waves.