Tunable thin film bulk acoustic wave resonators with improved Q-factor

The tunable solidly mounted Ba0.25Sr0.75TiO3 (BSTO) thin film bulk acoustic wave resonators (TFBARs) with improved Q-factor are fabricated and characterized. The BSTO films are grown by magnetron sputtering at temperature 600 degrees C and extremely low sputter gas pressure 2 mTorr using on-axis configuration. The measured TFBARs Q-factor is more than 250 and mechanical Q-factor is more than 350 at 5 GHz resonance frequency. The improvement in the Q-factor is associated with reduction in the BSTO film grain misorientation. The latter is responsible for generation of shear waves leaking through the Bragg reflector and corresponding acoustic loss. (C) 2010 American Institute of Physics

Composite Ferroelectric FBARs That Are Switchable Between the First and Second Harmonics: Experimental Demonstration

Digital switching between the first and second harmonics, in a composite thin-film bulk acoustic wave resonator (FBAR), is demonstrated experimentally. The FBAR consists of two 180-nm-thick paraelectric-phase Ba0.25Sr0.75TiO3 films separated by a 50-nm-thick SrRuO3 conducting layer. The resonant frequency of this composite resonator (with Pt bottom and Al top electrodes) is switched from 3.6 GHz to 7.6 GHz, where the polarity of the 5 V dc bias is reversed on one of the ferroelectric films. The frequency switching ratio (f(2)/f(1) >= 2) depends on the thickness of the electrodes. Some adjustment of f(2)/f(1) is possible by changing the applied dc bias

Ferroelectric film bulk acoustic wave resonators for liquid viscosity sensing

A concept of accurate liquid viscosity sensing, using bulk acoustic wave (BAW) resonators, is proposed. The proposed BAW resonators use thin ferroelectric films with the dc field induced piezoelectric effect allowing for generation of pure longitudinal acoustic waves in the thickness excitation mode. This makes it possible to utilize exclusively shear liquid particle displacement at the resonator side walls and, therefore, accurate viscosity evaluation. The BAW resonators with the dc field induced piezoelectric effect in 0.67BiFeO(3)-0.33BaTiO(3) ferroelectric films are fabricated and their liquid viscosity sensing properties are characterized. The resonator response is analyzed using simple model of a harmonic oscillator damped by a viscous force. It is shown that the resonator Q-factor is inversely proportional to the square root of the viscosity-density product. The viscosity measurement resolution is estimated to be as high as 0.005 mPa.s, which is 0.5% of the water viscosity

Charge carrier transport in graphene field-effect transistor scaled down to submicron gate lengths

We present a preliminary study of charge carrier transport in graphene field-effect transistor with gate lengths ranging from 2 μm down to 0.2 μm applying a model of the quasi-ballistic charge carrier transport. The analysis indicates that, in particular, at the gate length of 0.2 μm the fraction of the ballistic carriers can be up to 60 %. Our finding can be used as a guidance for further development of the graphene field-effect transistors with submicron gate length for variety of the advanced and emerging applications

Charge carrier transport in graphene field-effect transistor scaled down to submicron gate lengths

We present a preliminary study of charge carrier transport in graphene field-effect transistor with gate lengths ranging from 2 μm down to 0.2 μm applying a model of the quasi-ballistic charge carrier transport. The analysis indicates that, in particular, at the gate length of 0.2 μm the fraction of the ballistic carriers can be up to 60 %. Our finding can be used as a guidance for further development of the graphene field-effect transistors with submicron gate length for variety of the advanced and emerging applications

Low-field mobility and high-field velocity of charge carriers in InGaAs/InP high-electron-mobility transistors

Development of transistors for advanced low noise amplifiers requires better understanding of mechanisms governing the charge carrier transport in correlation with the noise performance. In this paper, we report on study of the carrier velocity in InGaAs/InP high-electron-mobility transistors (HEMTs) found via geometrical magnetoresistance in the wide range of the drain fields, up to 2 kV/cm, at cryogenic temperature of 2 K. We observed, for the first time experimentally, the velocity peaks with peak velocity and corresponding field decreasing significantly with the transverse field. The low-field mobility and peak velocity are found to be up to 65000 cm2/Vs and 1.2x106\ua0cm/s, respectively. Extrapolations to the lower transverse fields show that the peak velocity can be as high as 2.7x107\ua0cm/s. The corresponding intrinsic transit frequency can be up to 172 GHz at the gate length of 250 nm. We demonstrated, for the first time, that the low-field mobility and peak velocity reveal opposite dependencies on the transverse field, indicating the difference in carrier transport mechanisms dominating at low- and high-fields. Therefore, the peak velocity is an appropriate parameter for characterization and development of the low noise HEMTs, complementary to the low-field mobility. The results of the research clarify the ways of the further development of the HEMTs for advanced applications

Mobility and quasi-ballistic charge carrier transport in graphene field-effect transistors

The optimization of graphene field-effect transistors (GFETs) for high-frequency applications requires further understanding of the physicalmechanisms concerning charge carrier transport at short channel lengths. Here, we study the charge carrier transport in GFETs with gatelengths ranging from 2 μm down to 0.2 μm by applying a quasi-ballistic transport model. It is found that the carrier mobility, evaluated viathe drain–source resistance model, including the geometrical magnetoresistance effect, is more than halved with decreasing the gate lengthin the studied range. This decrease in mobility is explained by the impact of ballistic charge carrier transport. The analysis allows for evaluationof the characteristic length, a parameter of the order of the mean-free path, which is found to be in the range of 359–374 nm. Themobility term associated with scattering mechanisms is found to be up to 4456 cm2/Vs. Transmission formalism treating the electrons aspurely classical particles allows for the estimation of the probability of charge carrier transport without scattering events. It is shown that atthe gate length of 2 μm, approximately 20% of the charge carriers are moving without scattering, while at the gate length of 0.2 μm, thisnumber increases to above 60%

Dielectric model of point charge defects in insulating paraelectric perovskites

Some point defects (i.e., oxygen vacancies) create deep trapping levels in the bandgap of the paraelectric phase ferroelectric crystals. Under applied DC field the traps release electrons via the Poole-Frenkel mechanism and become charged. The electric field of a point charge polarizes the crystal locally reducing its permittivity. In this paper a simple theory is proposed for calculating the DC field dependent apparent (measureable) permittivity of a paraelectric crystal with point charge defects. It is shown that the apparent permittivity of a paraelectric crystal may be sufficiently lower as compared with the defectless crystal. This reduction is in addition to the possible reduction of the apparent permittivity associated with the interfacial "dead" layers and strain