Improving Terahertz-Detection Sensitivity of $8 \times 8$ FET Arrays Through Liquid-Nitrogen Cooling in a Compact Low-Noise Cryostat

We show that the sensitivity of antennacoupled field-effect transistors (FETs) to terahertz (THz) radiation improves continuously with decreasing temperature. The noise-equivalent power (NEP) of 540 GHz patch-antenna-coupled FETs decreases as temperature reduces to 20 K. We project NEP values approaching$1\rm{--}2\rm{ pW}/\sqrt{\rm{Hz}}$under efficient power coupling conditions (e.g., using a superstrate Si-lens), which is comparable to superconducting niobium transitionedge sensors (TESs) at 4 K. Building on these findings, a compact, low-noise, liquid-nitrogen-cooled (77 K) FETbased direct (incoherent) THz-power sensing system for spectroscopy applications was realized. Here, an$8 \times 8$pixel-binned detector array fabricated in a commercial 65-nm Si-CMOS process, was optimized for operation in the 2.85-3.4 THz band. Characterization was performed in the focal plane of a 2.85-THz quantumcascade laser delivering$\sim 2\rm{ mW}$of THz power. A linear dynamic range exceeding 67 dB was achieved without saturation (for 1 Hz-detection bandwidth). The system provides a -3 dB readout bandwidth of 5 MHz, exceeding that of conventional thermal detectors (typically 1 kHz). Combined with its broad temperature operability (20-300 K) 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.