Improving terahertz-detection sensitivity of 8x8 FET arrays through liquid-nitrogen cooling in a compact low-noise cryostat

TL;DR

This paper demonstrates that cooling FET-based terahertz detectors to liquid nitrogen temperatures significantly enhances their sensitivity, enabling compact, high-performance THz sensing systems suitable for space applications.

Contribution

It introduces a liquid-nitrogen-cooled 8x8 FET array with improved NEP and bandwidth, optimized for THz spectroscopy in a compact, practical design.

Findings

NEP approaches 1-2 pW/√Hz at 20 K

Achieves >67 dB dynamic range without saturation

Provides 5 MHz bandwidth exceeding thermal detectors

Abstract

We show that the sensitivity of antenna-coupled 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 to 2 pW/sqrt(Hz) under efficient power coupling conditions (e.g., using a superstrate Si-lens), which is comparable to superconducting niobium transition-edge sensors (TESs) at 4 K. Building on these findings, a compact, low-noise, liquid-nitrogen-cooled (77 K) FET-based direct (incoherent) THz-power sensing system} for spectroscopy applications was realized. Here, an 8x8 pixel-binned detector array fabricated in a commercial 65-nm Si-CMOS process, was optimized for operation in the 2.85 to 3.4 THz band. Characterization was performed in the focal plane of a 2.85-THz quantum-cascade laser delivering approx. 2~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 K to 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.