Environmental and chemical research has benefited from the continuing development of terahertz-frequency mixers, such as waveguide-balanced components capable of operation above 1 THz. For example, submillimeter-wave and terahertz heterodyne receivers have provided a great deal of insight into many different chemical processes. Analysis is typically performed at frequencies to about 1.6 THz.
Researchers from Sweden, France, and Russia teamed on the design and fabrication of a waveguide-balanced, phonon-cooled NbN hot-electron-bolometer (HEB) mixer on a GaN buffer layer. The mixer was used in a double-sideband (DSB) receiver operating at a local-oscillator (LO) frequency of 1.3 THz with extremely low noise temperatures for analysis of signals across wide bandwidths at terahertz frequencies.
While such NbN-on-GaN mixers have been developed previously for THz applications, compared to mixers on silicon (Si) substrates, little has been documented on the noise performance of such THz mixers. These researchers used the Y-factor technique to characterize mixer integrated circuits (ICs), which were fabricated by photolithography and mounted within a THz waveguide receiver assembly, and subsequently measure the receiver noise temperature.
Measurements were performed by evaluating black-body emitters held at different hot and cold temperatures, such as a hot temperature of 296 K and a cold temperature of 78 K. A commercial spectrum analyzer is then used to gauge the intermediate-frequency (IF) power spectrum of the receiver for hot and cold loads across an IF range of 500 MHz to 8 GHz, utilizing a local oscillator (LO) source tuned from 1.25 to 1.39 THz. The researchers were able to determine excellent gain-bandwidth and noise performance for the mixer, with much credit due to the mixer’s use of a GaN buffer layer mounted atop a thin Si semiconductor wafer.
See “Noise and IF Gain Bandwidth of a Balanced Waveguide NbN/GaN Hot Electron Bolometer Mixer Operating at 1.3 THz,” IEEE Transactions on Terahertz Science and Technology, May 2018, Vol. 8, No. 3, p. 365.