Improving 8x8 TeraFET array sensitivity through liquid-nitrogen cooling in a compact low-noise cryostat

Abstract: The sensitivity of antenna-coupled field-effect transistors (TeraFETs) to terahertz (THz) radiation has been shown to improve continuously with decreasing temperature. In this work, we first present a quantitative evaluation of the temperature-dependent noise-equivalent power (NEP) of recently developed patch-antenna-coupled TeraFET detectors resonant at 540 GHz, with measurements down to 20 K. Based on these results, we project NEP values approaching 1 to 2 pW/$\sqrt{\textrm{Hz}}$ under efficient power coupling-comparable to state-of-the-art superconducting niobium transition-edge sensors (TESs) operated at 4 K. Building on these findings in the sub-1 THz range, a compact, low-noise, liquid-nitrogen-cooled (77 K) TeraFET power detection system for spectroscopy applications was realized. The system incorporates an 8$\times$8 pixel-binned detector array fabricated in a commercial 65 nm Si-CMOS process, optimized for operation in the 2.85- to 3.4 THz band, where fast, sensitive and spectrally specific detectors that do not require helium cooling remain scarce. Final system characterization was performed in the focal plane of a 2.85-THz quantum-cascade laser delivering approximately 2 mW of optical power. An experimental linear dynamic range exceeding 67 dB was achieved without saturation (for 1 Hz-detection bandwidth). The system provides a -3 dB detection bandwidth of 5 MHz vastly exceeding that of conventional thermal detectors (typically 1-kHz), thus potentially enabling advanced applications such as time-resolved THz spectroscopy down to the sub-$\mu$s scale. Combined with its broad temperature operability and compact design, the system is particularly well suited for space- and payload-constrained platforms such as balloon- and satellite-based missions, where deep cryogenic cooling is impractical.