Noise and IF Gain Bandwidth of a Balanced Waveguide NbN/GaN Hot Electron Bolometer Mixer Operating at 1.3 THz

In this paper, we present the comprehensive characterization of a waveguide balanced phonon-cooled NbN hot electron bolometer (HEB) mixer on a GaN buffer-layer operating at approximately 1.3 terahertz (THz). The measured uncorrected double sideband noise temperature was as low as 750 K at 1 GHz intermediate frequency (IF) and 900 K at 4 GHz IF, respectively, and suggests a noise bandwidth of 7 GHz. Moreover, the IF gain bandwidth of the HEB itself was deduced from a mixing experiment with a second monochromatic THz signal source and has shown a 3 dB roll-off at 5.5 GHz. The contribution of the HEB mixer on the overall receiver noise temperature was determined to be in the order of 300 K or 5 hf/k considering losses in the RF transmission path and the waveguide components as well as accounting for the receiver conversion loss, which was deduced from the U-factor method. The achieved performance sets a new benchmark for future THz instruments and emphasizes the technological readiness of waveguide-based NbN HEB mixers employing a GaN buffer-layer featuring significantly improved IF bandwidth without compromising on the receiver's noise temperature.