Real-Time and Low-Noise Terahertz Imager Based on CMOS Technology

Abstract

Terahertz (THz) technology has immense applications in imaging, communication, and spectroscopy because electromagnetic waves in the THz band (0.1 - 10 THz) exhibit unique straightness and permeability characteristics. In particular, “THz see-through” imaging using the transmittance properties into non-metallic materials at THz frequency is an impactful application. Nevertheless, few THz imaging systems have been developed considering that THz frequency is higher than the cutoff frequency of Si-based FET, the most widely used device in RF applications. Moreover, it has too low photon energy to be detected by a photo-diode used in CMOS image sensor (CIS). However, FET- based plasmonic THz detectors have received considerable attention considering the multi-pixel integration potentials for real-time imaging. In the plasmonic mode, THz detection mechanism involves detecting the density oscillation of local electrons with DC offset voltage instead of detecting the electron transfer. While plasmonic THz detectors utilizing high-electron-mobility transistor (HEMT) technologies offer exceptional sensitivity, integrating such detectors with signal processing circuits on a single chip presents challenges. To address this issue, CMOS technology has garnered attention due to its advantages in terms of cost-effectiveness and high integration capabilities. In recent developments, THz detectors employing CMOS technology have utilized Schottky diodes, which function as current-driven transit-mode devices, and field-effect transistors (FETs), known for their plasmonic power detection capabilities. Within CMOS FET-based detectors, the advantages of the plasmonic power detection mechanism are evident. Unlike transit-mode devices limited by cutoff frequency, plasmonic detection isn't restricted in the same manner. This feature enables enhanced performance with increasing THz frequencies and imparts robustness against high-input THz power. CMOS technology-based plasmonic THz detectors have attracted much attention for real-time/large- scale multi-pixel THz imagers. Recently, we reported a remarkable THz detector performance enhancement by reconfiguring the boundary conditions in nano-ring FETs. To accelerate the commercialization of THz imagers, pixel-level high-performance and low-noise characteristics should be guaranteed in large-scale array operation with peripheral analog circuitry. Therefore, optimizing the analog buffer design considering the pixel’s ac boundary conditions is essential based on compact simulation model simulation. In addition, high-speed modulation must be performed to reduce flicker noise that increase due to additional circuitry. In this work, we report a record-high performance THz imager based on trantenna (transistor-antenna) integrated with an analog system using a 65-nm CMOS foundry. We investigate the high-speed and low-noise operation of ground (gnd)-out trantenna pixel-based single-chip THz imager array with low- impedance analog buffers and the reset modulation switch.