Yuehang Xu

Radio frequency electrical transduction of graphene mechanical resonators

We report radio frequency (rf) electrical readout of graphene mechanical resonators. The mechanical motion is actuated and detected directly by using a vector network analyzer, employing a local gate to minimize parasitic capacitance. A resist-free doubly clamped sample with resonant frequency ∼34 MHz, quality factor ∼10 000 at 77 K, and signal-to-background ratio of over 20 dB is demonstrated. In addition to being over two orders of magnitude faster than the electrical rf mixing method, this technique paves the way for use of graphene in rf devices such as filters and oscillators.

An Electrothermal Model for Empirical Large- Signal Modeling of AlGaN/GaN HEMTs Including Self-Heating and Ambient Temperature Effects

Accurate modeling of electrothermal effects of GaN electronic devices is critical for reliability design and assessment. In this paper, an electrothermal model for large signal equivalent circuit modeling of AlGaN/GaN HEMTs including self-heating and ambient temperature effects is presented. To accurately describe the effect of ambient temperature, two separate electrothermal networks (Idiss, Rdiss, and Cdiss for self-heating, and Iamb, Ramb, and Camb for ambient temperature effect) are used to describe drain-source current slump due to self-heating and ambient temperature effects, respectively. A temperature-dependent thermal resistance and thermal capacitance model is proposed and implemented in the electrothermal network. The extraction of the thermal parameters is fulfilled by using numerical finite-element method. Single tone on wafer load-pull measurements at two operating frequencies (3 and 14 GHz) are carried out for verification purposes. The results show that good agreements on fundamental output power, the second and third harmonics output power, and power added efficiency have been achieved between simulations and measurements over a wide range of -55 °C to 175 °C.

PERMEABILITY MEASUREMENT OF FERROMAGNETIC MATERIALS IN MICROWAVE FREQUENCY RANGE USING SUPPORT VECTOR MACHINE REGRESSION

A newmethod based on supported vector regression (SVR) approach is proposed for permeability measurement. The microstrip transmission-line is used as measurement cell, and supported vector machine (SVM) is introduced to extract permeability of ferromagnetic materials. Experiment results showthat thanks to SVMs good ability of generalization, permeability of ferromagnetic materials can be extracted accurately and easily.

An Support Vector Regression Based Nonlinear Modeling Method for Sic Mesfet

An approach for the microwave nonlinear device modeling technique based on a combination of the conventional equivalent circuit model and support vector machine (SVM) regression is presented in this paper. The intrinsic nonlinear circuit elements are represented by Taylor series expansions, coefficients of which are predicted by its support vector regression (SVR) model. Example of a SiC MESFET nonlinear model is demonstrated, and good results is achieved.

A scalable GaN HEMT large-signal model for high-efficiency RF power amplifier design

This paper presents a large-signal empirical model for GaN HEMT devices using an improved Angelov drain current formulation with self-heating effect and a modified non-linear capacitance model. The established model for small gate-width GaN HEMTs is validated by on-wafer load-pull measurements up to 14 GHz. Moreover, a scalable large-signal model is presented by adding scalable parameters to drain-source current and non-linear capacitance equations. The scalable model of a 1.25 mm GaN HEMT has been employed to design a class-AB power amplifier for validation purposes. The results show that good agreement has been achieved between the simulated and measured results with 37.2 dBm saturation output power (Psat) and 58% maximum power-added-efficiency at 3 GHz.

A NEW METHOD TO AVOID CROWDING PHENOMENON IN EXTRACTING THE PERMITTIVITY OF FERROELECTRIC THIN FILMS

In this paper, a new method is proposed to avoid crowding phenomenon in extracting the permittivity of ferroelectric thin films. Polynomial curve fitting technique is used to determine the filling factor while the thickness of the thin film is very small. Conformal mapping (CM) combining with partial capacitance approach (PTA) is used to obtain the relationshipbetween the effective p ermittivity of multiplayer coplanar waveguide (CPW) and the permittivity of each layer. A CPW with a thin film layer is simulated and the permittivity of thin film is extracted, the results show that, by using the proposed method, the crowding phenomenon can be avoided successfully and the permittivity of thin films can be extracted accurately.

Modeling of SiC MESFETs by Using Support Vector Machine Regression

In this paper, a support vector machine (SVM) regression approach is introduced for modeling of field effect transistors (FETs). Benefits to the good generalization ability of SVM, a SVM regression (SVR) model is established using a set of training and testing data, which is produced by simulation using an available empirical model of SiC MESFETs. Experimental results show the SVR model has good ability in predicting electrical performance of FETs.

Measuring Complex Permeability of Ferromagnetic Thin Films using Microstrip Transmission Method

A transmission method is proposed to measure the complex permeability of ferromagnetic thin films in the frequency range of 100 MHz–18 GHz. The air microstrip is used as measurement fixture; S-parameters are measured by vector network analyzer (VNA). Ferromagnetic thin film samples deposited on silicon substrate with thickness less than 90 nm are measured. Effective permeability is deduced from the s-parameters loaded with and without thin films, and complex permeability of thin films is extracted from the effective permeability. The experimental results show that, using the method proposed in this paper, the complex permeability of thin films can be determined accurately up to 18 GHz.

An Improved Measurement Configuration for Determining the Permeability of Ferromagnetic Thin Film Materials

An improved measurement configuration is proposed to measure the permeability of ferromagnetic thin film materials. For best consideration of the large conductivity of ferromagnetic materials, the thin film is integrated as under layer substrate instead of upper layer substrate in microstrip configuration. Formulations are deduced and support vector regression (SVR) is used to extract the complex permeability of the ferromagnetic thin film materials. The results show that the error for both real part and imaginary part of the permeability (μ′ and μ″) is less than 1%.

A New Shorted Microstrip Method to Determine the Complex Permeability of Thin Films

A new method is proposed to determine the complex permeability of ferromagnetic thin films from 100 MHz to 15 GHz. In this method, shorted microstrip transmission-line perturbation combined with the conformal mapping method is used. In contrast with previous methods to measure the thin films deposited on rigid substrates, this method neither requires any reference sample for calibration, nor requires the additional measurement to determine saturation magnetization. To assess the validity of this new method, three samples with different thicknesses are measured; comparisons are performed between the theoretical and experimental results, and the repeatability of this method is studied by measuring the samples five times. The results show that the complex permeability of ferromagnetic thin films can be measured within 10% errors.