  We studied the effect of the NbN/Au contact on the sensitivities of a NbN hot
electron bolometer (HEB) mixer by measuring the double sideband (DSB) receiver
noise temperature (T_rec_DSB) at three local oscillator frequencies of 1.6, 2.5
and 5.3 THz. The HEB has cleaned contact structures with a thick Au layer. We
demonstrated low mixer noise temperatures (T_mixer_DSB) of 240 K and 290 K at
1.6 and 2.5 THz, respectively. The latter reach roughly 3 times the quantum
noise at their frequencies. The mixer is developed for the proposed OASIS and
SALTUS (concept) missions. The enhanced T_mixer_DSB are more than 30 % better
in comparison with published NbN HEB mixers. The improvement can reduce the
integration time of a heterodyne instrument roughly by a factor of 2. The
T_mixer^DSB of the same HEB has shown limited improvement at 5.3 THz, which is
partly due to non-optimized antenna geometry. Besides, the results also help to
understand device physics of a wide HEB (4 um) at high frequencies.
