We report on nonresonant detection of terahertz radiation using our original InGaP/InGaAs/GaAs plasmon-resonant high-electron-mobility transistor having a dual grating gate (DGG) structure. The experiments were performed at room temperature using a Gunn diode operating at 0.30 THz as the THz source. Using a device-loading model, the intrinsic responsivity was extracted and was dependent on the polarization of the incident THz wave. The device exhibited highest response when the electric-field vector of the incident THz radiation was directed in the source-drain direction. The 2D spatial distribution image of the transistor responsivity shows a clear beam focus centred on the transistor position, which proves the appropriate coupling of the THz radiation to the device, due to the DGG structure. The device also showed a high intrinsic responsivity of ∼90 V/W and a noise equivalent power (NEP) as low as ∼10-10 WHz-0.5. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)