arXiv subjects

Andrey Generalov

Publications and source records attributed to Andrey Generalov.

5 recordsLinked to original sources

arXiv2025

Quasioptic, Calibrated, Full 2-port Measurements of Cryogenic Devices under Vacuum in the 220-330 GHz Band

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.

physics.ins-det↗

arXiv2023

Terahertz detection with graphene FETs: photothermoelectric and resistive self-mixing contributions to the detector response

Field-effect transistors coupled to integrated antennas (TeraFETs) are photodetectors being actively developed for the THz frequency range ($\sim$ 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 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 CVD G-TeraFETs with asymmetric antenna coupling, the PTE response dominates over the RSM at frequencies above 100 GHz. We find that relative contribution of 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/$\sqrt{Hz}$ 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 G-TeraFETs.

cond-mat.mes-hall↗

arXiv2022

Geometrical magnetoresistance effect and mobility in graphene field-effect transistors

Further development of the 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 the 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.

cond-mat.mes-hall↗

arXiv2019

Does carrier velocity saturation help to enhance fmax in graphene field-effect transistors?

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.

physics.app-ph↗

arXiv2017

Coherently Enhanced Wireless Power Transfer

Extraction of electromagnetic energy by an antenna from impinging external radiation is at the basis of wireless communications and power transfer (WPT). The maximum of transferred energy is ensured when the antenna is conjugately matched, i.e., when it is resonant and it has an equal coupling with free space and its load, which is not easily implemented in near-field WPT. Here, we introduce the concept of coherently enhanced wireless power transfer. We show that a principle similar to the one underlying the operation of coherent perfect absorbers can be employed to improve the overall performance of WPT and potentially achieve its dynamic control. The concept relies on coherent excitation of the waveguide connected to the antenna load with a backward propagating signal of specific amplitude and phase. This signal creates a suitable interference pattern at the load resulting in a modification of the local wave impedance, which in turn enables conjugate matching and a largely increased amount of energy extracted to the waveguide. We develop an illustrative theoretical model describing this concept, demonstrate it with full-wave numerical simulations for the canonical example of a dipole antenna, and verify it experimentally in both near-field and far-field regimes.

physics.app-ph↗