Abstract: The gate-to-source/drain admittance and the detection responsivity of the high-electron-mobility transistor（HEMT） driven by the terahertz（THz） radiation was calculated.The results indica that the peaks of the admittances and the responsivities show redshift,and the heights of the peaks decrease with the length of channel beneath the gate increasing.The peaks of the admittances and the responsivities show blueshift,and the heights of the peaks increase with the gate voltage increasing.The present analysis may be helpful to the design of THz plasma detectors.