A quasi-optical (QO) test bench was designed, simulated, and calibrated for characterizing all four S-parameters of devices in the 220-330 GHz (WR3.4) frequency range, from room temperature down to 4.8 K. Quasioptical calibration methods were applied to de-embed the impact of cryostat and optical elements on device under test measurements. The devices were measured through vacuum windows via focused beam radiation. A de-embedding method employing line-reflect-match (LRM) calibration was established to account for the effects of optical components and vacuum windows. Such a method does not require multiple line standards inside the cryostat and mechanical translation of quasioptics. System validation was performed with measurements of cryogenically cooled devices, such as bare silicon wafers and stainless-steel frequency-selective surface (FSS) bandpass filters, and superconducting bandpass FSS fabricated in niobium. A permittivity reduction of Si based on a 4 GHz resonance shift was observed concomitant with a drop in temperature from 296 to 4.8 K. The stainless steel FSS measurements revealed a relatively temperature invariant center frequency and return loss level of 263 GHz and 35 dB on average, respectively. Finally, a center frequency of 257 GHz was measured with the superconducting filters, with return loss improved by 11 dB on average at 4.8 K. To the best of our knowledge, this is the first reported attempt to scale LRM calibration to 330 GHz and use it to de-embed the impact of optics and cryostat from cryogenically cooled device S-parameters.

The superconducting transistor, or Josephson Field Effect Transistor (JoFET), is a versatile building block for ultra-low-power and high-energy-efficiency classical and quantum electronics. In a JoFET, the superconducting current is controlled by electrostatic gate voltage, which enables the scalability of solid-state quantum computers and the creation of next-generation superconducting integrated circuits (ICs) for energy-efficient computing. The development of JoFETs has so far been limited to single devices or a few-transistor circuits due to the lack of a reproducible technological platform. Here, we first report on technological progress of wafer-scale JoFET manufacturing, demonstrating a 98% JoFET device yield on our 150 mm pilot-line platform. Then, we present behavioral and physics-based transistor models - an important step toward designing of novel JoFET-based superconducting ICs. Our models have been implemented in both commercial and freeware circuit simulators. A Verilog-A model implementation is provided with this paper. Using the proposed models, we obtain a good description of JoFET experimental data, thus paving the way for the large-scale design of next-generation superconducting ICs.

A quasi-optical (QO) test bench was designed, simulated, and calibrated for characterizing S-parameters of devices in the 220-330 GHz (WR-3.4) frequency range, from room temperature down to 4.8 K. The devices were measured through vacuum windows via focused beam radiation. A de-embedding method employing line-reflect-match (LRM) calibration was established to account for the effects of optical components and vacuum windows. The setup provides all four S-parameters with the reference plane located inside the cryostat, and achieves a return loss of 30 dB with an empty holder. System validation was performed with measurements of cryogenically cooled devices, such as bare silicon wafers and stainless-steel frequency-selective surface (FSS) bandpass filters, and superconducting bandpass FSS fabricated in niobium. A permittivity reduction of Si based on 4-GHz resonance shift was observed concomitant with a drop in temperature from 296 K to 4.8 K. The stainless steel FSS measurements revealed a relatively temperature invariant center frequency and return loss level of 263 GHz and 35 dB on average, respectively. Finally, a center frequency of 257 GHz was measured with the superconducting filters, with return loss improved by 7 dB on average at 4.8 K. To the best of our knowledge, this is the first reported attempt to scale LRM calibration to 330 GHz and use it to de-embed the impact of optics and cryostat from cryogenically cooled device S-parameters.

The superconducting transistor, or Josephson Field Effect Transistor (JoFET), is a versatile building block for ultra-low-power and high-energy-efficiency classical and quantum electronics. In a JoFET, the superconducting (zero-resistance) current is controlled by electrostatic gate voltage, which enables the scalability of solid-state quantum computers and the creation of next-generation superconducting integrated circuits (ICs). The development of JoFETs has so far been limited to single devices or a few-transistor circuits due to the lack of reproducible technological platform. Here, we report on technological progress of wafer-scale JoFET fabrication, achieving a 98% yield on a 150 mm wafer platform. Now that the large-scale and reproducible fabrication JoFETs has become feasible, we present behavioral and physics models as the required steps toward the design of novel JoFET-based ICs. Using the presented models, we obtain a good description of experimental data, thus paving the way for the design of next-generation superconducting ICs.

In this work, superconducting niobium-film (Nb) filters have been designed and simulated based on Mattis-Bardeen theory in CST Microwave Studio. Proposed structures have been fabricated and measured in a WR-3.4 (220-330 GHz) quasioptical setup coupled to a cryostat. To de-embed measurements from the quasioptics, a Line-Reflect-Match calibration approach has been used. The measurements correspond well with numerical simulations, giving improved matching at 261 GHz versus 257 GHz in numerical simulations, however, with lower quality factors.

Field-effect transistors coupled to integrated antennas [terahertz field-effect transistors (TeraFETs)] are photodetectors being actively developed for the terahertz (THz) frequency range (similar to 100 GHz-10 THz). Among them, graphene TeraFETs (G-TeraFETs) have demonstrated distinctive photoresponse features compared to those made from elementary semiconductors. For instance, previous studies have shown that the G-TeraFETs exhibit a THz response that comprises two components: the resistive self-mixing (RSM) and photothermoelectric effect (PTE). The RSM and PTE arise from carrier density oscillations and carrier heating, respectively. In this work, we confirm that the photoresponse can be considered a combination of RSM and PTE, with PTE being the dominant rectification mechanism at higher frequencies. For our chemical vapor deposited (CVD) G-TeraFETs with asymmetric antenna coupling, the PTE response dominates over the RSM at frequencies above 100 GHz. We find that the relative contribution of the RSM and PTE to the photoresponse is strongly frequency-dependent. Electromagnetic wave simulations show that this behavior is due to the relative change in the total dissipated power between the gated and ungated channel regions of the G-TeraFET as the frequency increases. The simulations also indicate that the channel length over which the PTE contributes to the photoresponse below the gate electrode is approximately the same as the electronic cooling length. Finally, we identify a PTE contribution that can be attributed to the contact doping effect in graphene close to the metal contacts. Our detectors achieve a minimum optical noise-equivalent power of 101 (114) pW/root Hz for asymmetric (symmetric) THz antenna coupling conditions at 400 GHz. This work demonstrates how the PTE response can be used to optimize the THz responsivity of the G-TeraFETs.

Electrostatically tunable Josephson field-effect transistors (JoFETs) are one of the most desired building blocks of quantum electronics. Applications of JoFETs range from parametric amplifiers and superconducting qubits to a variety of integrated superconducting circuits. Here, we report on graphene JoFET devices fabricated with wafer-scale complementary metal-oxide-semiconductor (CMOS)-compatible processing based on chemical-vapor-deposited monolayer graphene encapsulated with atomic-layer-deposited Al2O3 gate oxide, lithographically defined top gate, and evaporated superconducting Ti/Al source, drain, and gate contacts. By optimizing the contact resistance down to ∼170 Ω μm, we observe proximity-induced superconductivity in the JoFET channels with different gate lengths of 150–350 nm. The Josephson junction devices show reproducible critical current Ic tunablity with the local top gate. Our JoFETs are in the short diffusive limit with the Ic reaching up to ∼3 µA for a 50 µm channel width. Overall, our demonstration of CMOS-compatible two-dimensional (2D) material-based JoFET fabrication process is an important step toward graphene-based integrated quantum circuits.

A quasioptical setup based on a Gaussian beam telescope system has been created to analyze the room-temperature and cryogenic millimeter-wave S-parameters of materials and devices for astronomical instrumentation, simulated and tested. The room temperature tests were performed in the WR-3.4 (220-330 GHz) frequency range after completing the thru-reflect-line calibration with a set of custom calibration standards. The system capabilities have been tested during the characterization of the planar devices and materials.

Graphene-based detectors of THz radiation – despite their improvement over the years – continue to exhibit a lower sensitivity than detectors made from other material systems. In order to gain a deeper understanding of some of the detection processes and their constraints, we analyze here experimentally and by simulations the operation of graphene TeraFETs, detectors based on rectification in antenna-coupled field effect transistors, over the frequency range 0.1-1.2 THz. The devices were fabricated with an advanced wafer-scale process technology. It is well-known that two detection mechanisms contribute, resistive self-mixing and the photothermoelectric effect. We determine the respective contributions to the responsivity and their frequency dependencies, put the results into perspective with competing technologies, and suggest ways to improve detector performance.

We investigate the dependence of the responsivity of antenna-coupled graphene field-effect transistors (graphene TeraFETs) on the transistor's channel geometry. The graphene TeraFETs are fabricated using a wafer-scale CVD process, and the measurements of the detectors of various geometries are performed at 0.1-1.2 THz. The measured data are compared with existing theoretical predictions and help to improve the accuracy of graphene TeraFET responsivity models.

Further development of graphene field-effect transistors (GFETs) for high-frequency electronics requires accurate evaluation and study of the mobility of charge carriers in a specific device. Here, we demonstrate that the mobility in the GFETs can be directly characterized and studied using the geometrical magnetoresistance (gMR) effect. The method is free from limitations of other approaches since it does not require an assumption of the constant mobility and the knowledge of the gate capacitance. Studies of a few sets of GFETs in the wide range of transverse magnetic fields indicate that the gMR effect dominates up to approximately 0.55 T. In higher fields, the physical magnetoresistance effect starts to contribute. The advantages of the gMR approach allowed us to interpret the measured dependencies of mobility on the gate voltage, i.e., carrier concentration, and identify the corresponding scattering mechanisms. In particular, the range of the fairly constant mobility is associated with the dominating Coulomb scattering. The decrease in mobility at higher carrier concentrations is associated with the contribution of the phonon scattering. Analysis shows that the gMR mobility is typically 2–3 times higher than that found via the commonly used drain resistance model. The latter underestimates the mobility since it does not take the interfacial capacitance into account.

This contribution presents the results of investigations performed on monolayer graphene field effect transistor- based (GFET-based) terahertz detectors. We have implemented three different types of planar antennae: a bowtie, a bow-tie with transmission lines and a slot-disc, allowing us to realize different conditions for high-frequency impedance matching. We present a semi-empirical model which uses physical parameters derived from electrical characterization results of devices and electrodynamic characteristics of antennae, allowing us to predict THz responsivity. Model predictions have been compared with the responsivity measurements performed at room temperature in a frequency range from 50 to 1250 GHz. Good agreement between the model predictions and experimental results implies the eligibility of a distributed resistive mixing approximation for GFET. In addition, the device stability, the temperature dependence and the origin of noise in the transistor channel have been investigated. Finally, to the best of our knowledge, we demonstrate the record performance values for room temperature graphene-based terahertz detectors: 80V/W optical responsivity without the normalization to the antenna effective area and a noise equivalent power of 111 pW/√Hz at 336 GHz.

It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get the highest possible maximum oscillation frequency (fmax). This paper numerically investigates whether velocity saturation can help to get better current saturation and if that correlates with enhanced fmax. For such a purpose, we used a drift-diffusion simulator that includes several factors that influence output conductance, especially at short channel lengths and-or large drain bias: short-channel electrostatics, saturation velocity, graphene-dielectric interface traps, and self-heating effects. As a testbed for our investigation, we analyzed fabricated GFETs with high extrinsin cutoff frequency fT,x (34 GHz) and fmax (37 GHz). Our simulations allow for a microscopic (local) analysis of the channel parameteres such as carrier concentration, drift and saturation velocities. For biases far away from the Dirac voltage, where the channel behaves as unipolar, we confirmed that the higher is the drift velocity, as close as possible to the saturation velocity, the greater fmax is. However, the largest fmax is recorded at biases near the crossover between unipolar and bipolar behavior, where it does not hold that the highest drift velocity maximizes fmax. In fact, the position and magnitude of the largest fmax depend on the complex interplay between the carrier concentration and total velocity which, in turn, are impacted by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the maximum fmax from around 60 to 40 GHz.

This work presents an approach to distinguish the thermoelectric detection mechanism from the resistive mixing or plasma wave rectification in graphene FET THz detectors. Numerical full-wave simulations validate the asymmetric feeding of the existing antenna design and allow for comparison with a reference design of thermoelectric detectors. The experimental results verify quantitively the thermoelectric contribution to the overall rectification, which allows for more accurate modelling of the GFET THz detectors.

Plasma waves play an important role in many solid-state phenomena and devices. They also become significant in electronic device structures as the operation frequencies of these devices increase. A prominent example is field-effect transistors (FETs), that witness increased attention for application as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies, where they exhibit very good sensitivity even high above the cut-off frequency defined by the carrier transit time. Transport theory predicts that the coupling of radiation at THz frequencies into the channel of an antenna-coupled FET leads to the development of a gated plasma wave, collectively involving the charge carriers of both the two-dimensional electron gas and the gate electrode. In this paper, we present the first direct visualization of these waves. Employing graphene FETs containing a buried gate electrode, we utilize near-field THz nanoscopy at room temperature to directly probe the envelope function of the electric field amplitude on the exposed graphene sheet and the neighboring antenna regions. Mapping of the field distribution documents that wave injection is unidirectional from the source side since the oscillating electrical potentials on the gate and drain are equalized by capacitive shunting. The plasma waves, excited at 2 THz, are overdamped, and their decay time lies in the range of 25–70 fs. Despite this short decay time, the decay length is rather long, i.e., 0.3-0.5 μm, because of the rather large propagation speed of the plasma waves, which is found to lie in the range of 3.5–7 × 10 6 m/s, in good agreement with theory. The propagation speed depends only weakly on the gate voltage swing and is consistent with the theoretically predicted 1/4 power law.

In this letter, we report on the performance of graphene field-effect transistors (GFETs) in which the extrinsic transit frequency ( ${f}_{T}$ ) and maximum frequency of oscillation ( ${f}_{\text {max}}$ ) showed improved scaling behavior with respect to the gate length ( ${L}_{g}$ ). This improvement was achieved by the use of high-quality graphene in combination with successful optimization of the GFET technology, where extreme low source/drain contact resistances were obtained together with reduced parasitic pad capacitances. GFETs with gate lengths ranging from $0.5~\mu \text{m}$ to $\text {2}~\mu \text{m}$ have been characterized, and extrinsic ${f}_{T}$ and ${f}_{\text {max}}$ frequencies of up to 34 and 37 GHz, respectively, were obtained for GFETs with the shortest gate lengths. Simulations based on a small-signal equivalent circuit model are in good agreement with the measured data. Extrapolation predicts extrinsic ${f}_{T}$ and ${f}_{\text {max}}$ values of approximately 100 GHz at ${L}_{g}=\text {50}$ nm. Further optimization of the GFET technology enables ${f}_{\text {max}}$ values above 100 GHz, which is suitable for many millimeter wave applications.

A number of electronic devices such as phase shifters, polarizers, modulators, and power splitters are based on tunable materials. These materials often do not meet all the requirements namely low losses, fast response time, and technological compatibility. Novel nanomaterials, such as single-walled carbon nanotubes, are therefore widely studied to fill this technological gap. Here we show how the dielectric constant of single-walled carbon nanotube layers can be substantially modified by illuminating them due to unique light-matter interactions. We relate the optical excitation of the nanotube layers to the illumination wavelength and intensity, by resistance and capacitance measurements. The dielectric constant is modified under laser illumination due to the change of material polarization and free carrier generation, and is shown to not be temperature-related. The findings indicate that SWCNT layers are a prospective tunable optoelectronic material for both high and low frequency applications.