The sensitivity of direct terahertz detectors based on self-mixing of
terahertz electromagnetic wave in field-effect transistors is being improved
with noise-equivalent power close to that of Schottky-barrier-diode detectors.
Here we report such detectors based on AlGaN/GaN two-dimensional electron gas
at 77~K are able to sense broadband and incoherent terahertz radiation. The
measured photocurrent as a function of the gate voltage agrees well with the
self-mixing model and the spectral response is mainly determined by the
antenna. A Fourier-transform spectrometer equipped with detectors designed for
340, 650 and 900~GHz bands allows for terahertz spectroscopy in a frequency
range from 0.1 to 2.0~THz. The 900~GHz detector at 77~K offers an optical
sensitivity about 1pW/Hz being comparable to a commercial
silicon bolometer at 4.2~K. By further improving the sensitivity,
room-temperature detectors would find applications in active/passive terahertz
imaging and terahertz spectroscopy.Comment: 4.5 pages, 5 figure