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Transition-metal dichalcogenide materials have attracted considerable attention due to their outstanding properties, and however, the carrier collection efficiency remains a major problem for achieving high-performance electronic and optoelectronic devices. In this research, we compared the carrier transport properties of graphite and Au in contact with MoS2 (WS2) materials. The use of graphite for the drain–source electrodes significantly improves carrier collection efficiency, thus enhancing the overall electronic and optoelectronic performance of MoS2 and WS2 FETs. The device’s ON/ OFF current ratio with two graphite contact electrodes increased by about two orders of magnitude (120 and 207 for MoS2 and WS2 FETs, respectively) than that with conventional Au electrodes. Furthermore, the Gr-MoS2-Gr FET shows ultrahigh responsivity (3869 A/W), detectivity ($4.6\times10$ 12 Jones), and extraordinary short photoresponse fall time ($60 ~\mu \text{s}$). Compared with the Au-MoS2-Au FET, the fall time is reduced by about six orders of magnitude. Interestingly, the Gr-MoS2-Gr and Gr-WS2-Gr FETs demonstrate different detection mechanisms. The Gr-MoS2-Gr photodetector shows a photoconduction mechanism under the gate voltage of −10 to −6 V and a photogate mechanism under −4 to 2 V, whereas the Gr-WS2-Gr photodetector is only driven by a photoconduction mechanism. The Gr-TMDs-Gr’s outstanding electronic and photoelectrical properties hold great potential for various applications in advanced optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this article, we present a novel nonlinear vector network analyzer (NVNA) setup for charactering the in-band full-duplex (IBFD) performance of millimeter-wave antenna systems. To achieve millimeter-wave modulated measurements on dense spectral grids, this NVNA setup utilizes user-defined: 1) digitally-modulated test stimuli; 2) multisine phase reference; and 3) pulsed-radio frequency (RF) phase calibration standards for test bench establishment and calibration. To create the IBFD test scenario, a special design of test stimuli is developed for antenna characterization, where the spectral components of two periodical transmitting signals (digitally modulated within the same frequency band) do not overlap with each other. In this way, the desired magnitude and phase information of transmitted and received signals can be easily separated and extracted from the NVNA measurements, to support various frequency-, time-, and digital-domain analysis. The validity of the proposed NVNA setup is experimentally verified at 28 GHz, where the error vector magnitude (EVM) results of 100 MSymbol/s 64-QAM test signals are from 2.0% to 2.3%. From the application perspective, this measurement setup and test method can be widely used for fifth-generation (5G)-oriented over-the-air test of IBFD antenna systems. [ABSTRACT FROM AUTHOR]

Au nanoparticles (NPs) were drop-casted onto germanium (Ge) devices to investigate the influence of Au NPs on the electronic transport and photodetection properties of Ge metal–semiconductor–metal photodetectors. A significant hysteresis behavior was observed in the current–voltage curves of the Au NP-decorated Ge photodetector, which was ascribed to the energy band bending below the Au NPs and the electrostatic effect of image charges in the Au NPs. More interestingly, at the optical communication wavelength of $1.55~\mu \text{m}$ , the introduction of Au NPs effectively improved the responsivity of the device from 0.18 to 0.92 A/W. Rather than the localized surface plasmon resonance effect, the increase of responsivity in the Au NP-decorated Ge photodetector was caused by a type-II liked energy band alignment, which enhanced the spatial electron–hole separation effect. These results provide inspiration for the development of novel optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this article, we present a vector network analyzer (VNA)-based measurement scheme and setup to obtain repeatable phase spectrum measurements of 75–110-GHz modulated radio frequency (RF) signals. In order to remove the random phase contribution of sweeping local oscillator (LO) in harmonic downconversion, the LO signal is coupled out into an auxiliary measurement channel where its random phase at the fundamental can be derived after additional modulation and downconversion treatments. Afterward, a user-defined math equation is used inside the VNA for LO phase cancellation, based on the traces of RF- and auxiliary-channel raw measurements. The key point of the proposed LO phase cancellation technique is measuring the 75–110-GHz test signal and the auxiliary-channel signal at the same GHz intermediate frequency so that the receivers of VNA can measure both of them simultaneously when working on the peculiar continuous-wave or zero-span mode. According to the method validation at 78, 93, and 108 GHz, a phase fluctuation of ±2° for single-run measurements is found reachable in practice. [ABSTRACT FROM AUTHOR]

In this article, we present a novel nonlinear vector network analyzer (NVNA) setup for generating and measuring digitally modulated signals at millimeter-wave test bands. The first difference between this setup and commercial NVNAs is the use of a user-defined multisine signal as the phase reference, in order to enable more precise synchronization and denser spectral grid. To achieve this, a single 12.5-GHz microwave source is used for generating both the digitally modulated test signal and the phase reference, which not only offers the clock to the arbitrary waveform generator for user-defined intermediate frequency (IF) waveform generation but also provides the local oscillator drive after frequency multiplication. The second novelty of this work is the use of pulsed-radio frequency (RF) signals for NVNA phase calibration. By replacing the digitally modulated IF waveform with pulsed-RF signals, tone-by-tone phase calibration on dense spectral grids can be easily achieved under the same synchronization scheme. The validity of the proposed NVNA setup (as well as its simplified modification) and the phase calibration method is verified at the fifth-generation (5G)-oriented millimeter-wave bands of 28, 39, 43, and 70 GHz in this work, where the EVM measurements of 64-QAM test signals with a wide range of symbol rates from 0.1 to 1.5625 GSymbol/s are demonstrated reachable in practice. [ABSTRACT FROM AUTHOR]

Ge1-x Snx alloy films with Sn contents of 3% and 10% were grown on Si wafers by low-temperature nonequilibrium molecular beam epitaxy. The thermostabilities of the GeSn films and photodetectors containing them were studied. No Sn segregation was observed in Ge0.97Sn0.03 films annealed below 750 °C. Conversely, Sn segregation occurred and Sn nanoparticles formed within the Ge0.90Sn0.10 films annealed above 400 °C. Upon increasing the annealing temperature to 750 °C, Sn particles were thermally driven and segregated on the surface of both Ge0.97Sn0.03 and Ge0.90Sn0.10 films. Photodetectors were fabricated based on the as-grown and annealed Ge0.90Sn0.10 films. Because of the decreased defect content of the Ge0.90Sn0.10 film after annealing, the dark current of the GeSn photodetectors decreased obviously and the responsivity of the devices increased. These results are fundamentally important for the applications of high-performance photodetectors and lasers based on GeSn alloys. [ABSTRACT FROM AUTHOR]

This article presents the evaluation of using pulsed-radio frequency (RF) signals as multitone phase standards for modulated measurements at millimeter-wave (mm-Wave) bands, offering an alternative phase calibration choice for nonlinear vector network analyzers (NVNAs). Based on the NVNA measurements of two pulsed-RF signals generated by square-wave-driven mixers, a high correlation between magnitude and phase errors of unideal pulsed-RF signals is discovered, and an empirical method is developed to accurately estimate the phase errors according to magnitude-only measurements. To confirm its robustness and key preconditions for wide-range applications, theoretical support and explanation to the empirical method is revealed, in which the dominant waveform distortion of an applicable pulsed-RF signal can be decomposed into modulated perturbations with Sinc-like envelopes. The validity of the proposed empirical method, as well as its application for mm-Wave NVNA phase calibration, is verified by both experimental measurements and simulation results, where an estimation error (or phase calibration error) less than ±2° is demonstrated that is reachable in practice. [ABSTRACT FROM AUTHOR]

This paper proposes a measurement strategy to remove the random phase contributions of the sweeping local oscillator (LO) from modulated radio frequency phase measurements based on downconversion. To obtain a stable phase spectrum for each run of tone-by-tone sweeping measurement, the LO used for downconversion is coupled out to an extra path and modulated to be a wideband multisine signal. By monitoring the phase change of multisine signal at a specific and fixed frequency point, random LO phase offsets during the frequency sweep can be measured and compensated. In this paper, two experimental test benches with the same core measurement strategy but different architectures are established for method validation. According to the experimental results using test bench one, the phase stability is ±(0.2°–0.4°) at 1.8 GHz for fundamental mixing-based downconversion and ±(0.6°–1°) for harmonic mixing at 6 GHz. Based on test bench two, relative phase deviations of ±(0.8°–1.2°) at 22 GHz for fundamental mixing and ±(2°–3°) at 66 GHz for harmonic mixing are reachable. [ABSTRACT FROM AUTHOR]

We investigated the measurement techniques of the nonlinear vector network analyzer (NVNA) test bench, to fully characterize the nonlinear behavior of radio frequency (RF) power amplifiers (PAs) driven by complex modulated signals. In order to extend the baseband measurements, two kinds of NVNA phase reference approaches are developed as alternative solutions, so that the baseband phase measurements can be achieved with those of modulated components at multiharmonic RF bands. In the first approach, a modulated baseband signal is combined with another modulated RF one to become the desired NVNA phase reference. While as an alternative solution, the second phase reference design is based on a “stepped” multisine, which is stepped through the baseband and each harmonic following the NVNA swept measurements, to achieve stable phase measurements. To validate these proposed NVNA test bench designs, an RF PA, driven by a large-signal long-term evolution-like multisine, was tested and compared with digital real-time oscilloscope measurements. [ABSTRACT FROM PUBLISHER]

This paper presents novel nonlinear vector network analyzer (NVNA)-based techniques for characterizing the harmonic and intermodulation nonlinearity of dual-band radio frequency (RF) power amplifiers (PAs). On the one hand, a wideband Schroeder-phase multisine is used to drive a step recovery diode comb generator, so that a novel multiharmonic-band NVNA phase reference on a dense spectral grid is obtained. On the other hand, a novel phase calibration approach is developed to synchronize the NVNA multiband measurements. To validate the NVNA testbed with novel techniques, an RF PA under two typical kinds of dual-band multisine stimulus is tested, where the NVNA measurement is compared with that of an oscilloscope. This paper can be easily adapted for various multiband conditions. [ABSTRACT FROM PUBLISHER]

This paper presents a novel realization of nonlinear vector network analyzer (NVNA) test-bed with a new phase calibration technique, in order to characterize the multiharmonic intermodulation nonlinearity of a radio frequency (RF) power amplifier (PA) under a large-signal modulated input. The NVNA test-bed uses a step recovery diode comb generator as the phase reference, which is driven by a pulsed-RF signal with a high duty cycle of over 99%. Hence all the multiharmonic intermodulation tones of interest can be measured at once. A multisine phase standard provided by a vector signal analyzer is used to calibrate the intermodulation tones at each harmonic on a dense spectral grid. Two harmonic phase standards with different fundamental frequencies are used to synchronize the phase calibration results at different harmonics. The validity of this paper is verified by comparing NVNA measurements with oscilloscope measurements of the same PA under test. This paper can be used to inspect the nonlinearity of RF amplifiers under large-signal modulated excitations in both the frequency and time domains. [ABSTRACT FROM AUTHOR]

This paper proposes a practical approach for dense-spectral-grid phase measurements based on a nonlinear vector network analyzer (NVNA). With the multitone outputs of commercial VSGs as both phase references and standards, a modified NVNA is presented to effectively obtain the phase spectra of modulated RF signals with spectral resolution of 1–100 kHz. Due to the limited tone number of a VSG multitone standard, a multistep phase calibration method is also proposed in this paper for multiplying the calibratable tone numbers in practice. A phase accuracy of ±2° has been experimentally achieved with both the VSG-based approach and the multistep phase calibration method at 2.4 GHz on a 1–100-kHz spectral grid. [ABSTRACT FROM AUTHOR]

The step-recovery-diode-based impulse generator is a classical picosecond-level impulse-formation technique, whose strong harmonic nonlinearity under sinusoidal stimulus is widely utilized as the phase reference of the nonlinear vector network analyzer (VNA) (NVNA) and large-signal network analyzer for wideband measurements. This paper further exploits its intermodulation nonlinearity under multitone stimulus and proposes a new approach, rather than the traditional “harmonic” way, of the NVNA phase reference for future polyharmonic intermodulation measurements. Experimental results show that this method can provide plentiful intermodulation components around each harmonic available and is stable for NVNA use, with the advantages of flexible local frequency-resolution setup from tens of megahertz to less than hundreds of hertz. [ABSTRACT FROM PUBLISHER]

This paper studies the systematic uncertainties of nonlinear vector network analyzer (NVNA) measurements and presents a covariance-matrix-based method to analyze the uncertainty propagation from calibration standards and hardware to error model terms and effective measurements. Analytic deductions of the Jacobians for frequency-domain quantities are proposed, dealing with the complex NVNA measurements and calibration functions. An NVNA two-port-calibration uncertainty is put forward as a new kind of systematic uncertainty consideration for this vector-correction-based instrument. Experiments on our prototype NVNA show the necessity of using covariance-matrix-based uncertainty analysis, particularly for polyharmonic measurements, and reveal some features of NVNA uncertainty analysis due to the calibration techniques. [ABSTRACT FROM PUBLISHER]

With the unprecedented developments in deep learning, automatic segmentation of main abdominal organs seems to be a solved problem as state-of-the-art (SOTA) methods have achieved comparable results with inter-rater variability on many benchmark datasets. However, most of the existing abdominal datasets only contain single-center, single-phase, single-vendor, or single-disease cases, and it is unclear whether the excellent performance can generalize on diverse datasets. This paper presents a large and diverse abdominal CT organ segmentation dataset, termed AbdomenCT-1K, with more than 1000 (1K) CT scans from 12 medical centers, including multi-phase, multi-vendor, and multi-disease cases. Furthermore, we conduct a large-scale study for liver, kidney, spleen, and pancreas segmentation and reveal the unsolved segmentation problems of the SOTA methods, such as the limited generalization ability on distinct medical centers, phases, and unseen diseases. To advance the unsolved problems, we further build four organ segmentation benchmarks for fully supervised, semi-supervised, weakly supervised, and continual learning, which are currently challenging and active research topics. Accordingly, we develop a simple and effective method for each benchmark, which can be used as out-of-the-box methods and strong baselines. We believe the AbdomenCT-1K dataset will promote future in-depth research towards clinical applicable abdominal organ segmentation methods. [ABSTRACT FROM AUTHOR]