Many emerging applications in the terahertz (THz) frequency range demand highly sensitive, broadband detectors for room-temperature operation. Field-effect transistors with integrated antennas for THz detection (TeraFETs) have proven to meet these requirements, at the same time offering great potential for scalability, high-speed operation, and functional integrability.

We present broadband high sensitivity THz detectors based on 90 nm CMOS technology with state-of-the-art performance. The devices are based on bow-tie and log-spiral antennacoupled field-effect transistors (FETs) for the detection of freespace terahertz (THz) radiation (TeraFETs). We report on optimized performance, which was achieved by employing an in-house developed physics-based model during detector design and thorough device characterization under THz illumination. The implemented detector with bow-tie antenna design exhibits a nearly flat frequency response characteristic up to 2.2 THz with an optical responsivity of 45 mA/W (or 220 V/W). We have determined a minimum optical noise-equivalent power (NEP) as low as 48 pW/√Hz at 0.6 THz and 70 pW/√Hz at 1.5 THz. The results obtained at 1.5 THz are better than the best narrowband TeraFETs reported in the literature at this frequency and only up to a factor of four inferior to the best narrowband devices at 0.6 THz.

We report on circuit simulation, modeling and characterization of field-effect transistor-based terahertz (THz) detectors (TeraFETs) with integrated patch antennas for discrete frequencies from 1.3 THz to 5.7 THz. The devices have been fabricated using a standard 90 nm CMOS technology. Here, we focus in particular on a device showing the highest sensitivity to 4.75 THz radiation and its prospect to be employed for power monitoring of a THz quantum cascade laser used in a heterodyne spectrometer GREAT (German REceiver for Astronomy at Terahertz frequencies). We show, that a distributed transmission line-based detector model can predict the detector's performance better than a device model provided by the manufacturer. The integrated patch antenna of the TeraFET designed for 4.75 THz has an area of 13×13 μm $^2$ and a distance of 2.2 μm to the ground-plane. The modeled radiation efficiency at the target frequency is 76% with a maximum directivity of 5.5 resulting in an effective area of 1750 μm $^2$ . The detector exhibits an area-normalized minimal noise-equivalent power (NEP) of 404 p $W/\sqrt{H\mathrm{z}}$ and a maximum responsivity of 75 V/W. These values represent the state-of-the-art for electronic detectors operating at room-temperature and in this frequency range.

This work presents, to our knowledge, the first completely passive imaging with human-body-emitted radiation in the lower THz frequency range using a broadband uncooled detector. The sensor consists of a Si CMOS field-effect transistor with an integrated log-spiral THz antenna. This THz sensor was measured to exhibit a rather flat responsivity over the 0.1–1.5-THz frequency range, with values of the optical responsivity and noise-equivalent power of around 40 mA/W and 42 pW/ $\sqrt{\mathrm{Hz}}$ , respectively. These values are in good agreement with simulations which suggest an even broader flat responsivity range exceeding 2.0 THz. The successful imaging demonstrates the impressive thermal sensitivity which can be achieved with such a sensor. Recording of a 2.3 × 7.5-cm ${}^2$ -sized image of the fingers of a hand with a pixel size of 1 mm ${}^2$ at a scanning speed of 1 mm/s leads to a signal-to-noise ratio of 2 and a noise-equivalent temperature difference of 4.4 K. This approach shows a new sensing approach with field-effect transistors as THz detectors which are usually used for active THz detection.

We report on the design and characterization of a novel backside-radiating antenna-coupled direct terahertz detector fabricated in 65 nm CMOS technology. The novelty of the design lies in the low-metal coverage of the biquad antenna geometry, which adapts well to the particular challenging conditions of on-chip antenna integration in silicon and allows optimization for a versatility of operation conditions. The biquad antenna was modified here to achieve wideband radiation and matching to a gate-coupled single-finger field-effect transistor with ac open condition at the drain terminal. The successful detector performance was the result of a careful treatment of transistor, antenna, and optics from a codesign perspective, since the beginning of the design. This included the frequency-dependent complex impedance for optimum matching, the technology restrictions to ensure proper chip fabrication, and the overall detection efficiency after backing the device with a silicon lens. Calibrated detector measurements for 7777 Hz modulation frequency yielded minimum optical noise-equivalent power (NEP) of 25 pW/ √ Hz at 1 THz, with NEP values below 50 pW/√Hz in the 0.84-1.29 THz frequency range. These figures achieve state-of-the-art of wideband CMOS-based detectors and are only a factor of ~2 inferior to the best reported narrowband devices close to 1 THz.

We propose to exploit rectification in field-effect transistors as an electrically controllable higher-order nonlinear phenomenon for the convenient monitoring of the temporal characteristics of THz pulses, for example, by autocorrelation measurements. This option arises because of the existence of a gate-bias-controlled super-linear response at sub-threshold operation conditions when the devices are subjected to THz radiation. We present measurements for different antenna-coupled transistor-based THz detectors (TeraFETs) employing (i) AlGaN/GaN high-electron-mobility and (ii) silicon CMOS field-effect transistors and show that the super-linear behavior in the sub-threshold bias regime is a universal phenomenon to be expected if the amplitude of the high-frequency voltage oscillations exceeds the thermal voltage. The effect is also employed as a tool for the direct determination of the speed of the intrinsic TeraFET response which allows us to avoid limitations set by the read-out circuitry. In particular, we show that the build-up time of the intrinsic rectification signal of a patch-antenna-coupled CMOS detector changes from 20 ps in the deep sub-threshold voltage regime to below 12 ps in the vicinity of the threshold voltage.