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This paper gives a precise insight into the frequency response of self-actuating cantilevers, extended to high-order modes. An original approach is presented, based on a mixed experimental scheme regarding both impedance and laser Doppler interferometry measurements, performed with an impedance analyser and a vibrometer, respectively, and supported by some computational analysis as well. The experimental results obtained by the two different techniques agree reasonably well, as for the location of the resonances in the spectrum is concerned. However, certain particular modes, torsionally shaped and detected by the vibrometer, show no impedance change due to symmetry reasons. A computational model has been successfully employed and utilized to shed some light on this phenomenon. The model demonstrates that the induced reaction charge redistribution cancels out only for those particular torsional modes featuring an anti-symmetrical nature; as a consequence, no impedance change is to be expected in the resonances. This explanation perfectly fits the impedance measurements carried out, including the missing torsional modes.Corrections were made to this article on 10 April 2007. The greyscale figures were replaced with colour versions.

Numerical 2D transient simulations of surface trap effects in AlGaN/GaN HEMT and GaN MESFET devices have been performed. The influence of donor- and acceptor-type traps on drain current characteristics has been studied, when the gate voltage is pulsed. Opposite behaviour in the response mechanism of both devices has been found. The current collapse and related dispersion effects are due to acceptor-type traps, acting as electron traps, in MESFET devices, whereas for HEMT devices, donor-type traps, acting as hole traps, are the origin of these effects. Free hole accumulation on the top surface plays a decisive role in the behaviour of the HEMT. A detailed study about density, ionization, energy level and spatial distribution of traps reveals conclusive results in the two devices analysed.

Two-dimensional simulations of surface charge effects in AlGaN/GaN HEMT and GaN MESFET devices are performed. The influence of charges of different magnitude, sign and spatial distribution at the ungated surface on drain current characteristics is studied. We have found that positive and negative surface charges of the same magnitude produce very different modifications of drain current. The relative influence of polarization fields, surface charges and free hole accumulation at the top surface is considered and gives rise to the opposite behaviour in HEMT and MESFET devices. The implications of our study in current collapse and related dispersion effects in GaN FETs are discussed.

We report on a novel voltage-tunable stacked bound to quasi-continuum quantum well infrared detector designed to work in two of the atmospheric windows (3-5 and 8-12 µm). In order to improve the growth process by molecular beam epitaxy, the abrupt interfaces between different Al-content layers were eliminated. A different choice based on Al-graded triangular confinement barriers was used instead. This device can be voltage controlled due to the formation of field domains in the stacked structure when an external bias is applied. This results in a voltage drop across the detectors proportional to their series resistance. Besides, a significant photovoltaic response was achieved in the 3-5 µm region and attributed to an unintentional asymmetry in the potential profile for the detector operating in this range.

Room-temperature time-resolved photocurrent measurements have been performed on novel InGaAs/GaAs multiple-quantum-well (MQW) P-I-N diodes on (111)B GaAs substrates, having an average electric field of opposite sign to the built-in field. Direct evidence for dipole formation, displacement photocurrents and out-of-well screening by photocarriers is found. In these piezoelectric structures the sign of the internal field can be controlled by an external modulating voltage and the optically pumped charges stored at the extremes of the diode MQW active region can be extracted by appropriate pulsed biasing. The generated dipole moment and its time evolution have been determined by an optical pump and electrical probe technique: the spatial separation of electrons and holes accounts for the observation of slow recombination processes. The piezoelectric constant is evaluated for InxGa1-xAs (x=17% In) from the zero-average-field condition.

The traditional procedure followed by winemakers for monitoring grape must fermentation is not automated, has not enough accuracy or has only been tested in discrete must samples. In order to contribute to the automation and improvement of the wine fermentation process, we have designed an AlN-based piezoelectric microresonator, serving as a density sensor and being excited in the 4th-order roof tile-shaped vibration mode. Furthermore, conditioning circuits were designed to convert the one-port impedance of the resonator into a resonant two-port transfer function. This allowed us to design a Phase Locked Loop-based oscillator circuit, implemented with a commercial lock-in amplifier with an oscillation frequency determined by the vibrating mode. We were capable of measuring the fermentation kinetics by both tracking the resonance frequency and by determining the quality factor measurements of the microresonator. Moreover, the resonator was calibrated with an artificial model solution of grape must and then applied for the monitoring of real grape must fermentation. Our results demonstrate the high potential of MEMS resonators to detect the decrease in sugar and the increase in ethanol concentrations during the grape must fermentation with a resolution of 100 μg/ml and a sensitivity of 0.16 Hz/μg/ml as upper limits.

Expressions for the field distribution in novel clamped or free-standing piezoelectric strained InGaAs/GaAs superlattices are given. The voltage for miniband formation can be selected by incorporating these structures into p-i-n diodes, without requiring strain compensation between wells and barriers. This feature is evidenced by the photocurrent spectra and photocapacitance versus voltage characteristics obtained from high quality 40-period (111)-oriented superlattices. Wannier-Stark localization and resonant transport of carriers have been observed in the piezoelectric superlattices. The In concentration, thickness of the superlattice period and strain relaxation are deduced from X-ray measurements.

We report the use of two AlN-based piezoelectric microresonators for the monitoring of density and viscosity of liquids and its application to detect lubricant oil dilution with diesel fuel. Two devices designed to resonate in the 4th-order roof tile-shaped vibration mode, but with two different anchor schemes, were fabricated and characterized. Interface circuits were designed to convert the one-port impedance into a resonant two-port transfer function. This allowed us to implement a phase locked loop (PLL)-based oscillator circuit based on the resonators, the interface circuit and a commercial lock-in amplifier. Our results demonstrate the performance of the resonators in fluids having viscosities up to 500 mPa s. The performance of the sensors in terms of sensitivity and resolution are compared for both anchor configurations.

In this work, roof tile-shaped modes of MEMS (micro electro-mechanical systems) cantilever resonators with various geometries and mode orders are analysed. These modes can be efficiently excited by a thin piezoelectric film and a properly designed top electrode. The electrical and optical characterization of the resonators are performed in liquid media and the device performance is evaluated in terms of quality factor, resonant frequency and motional conductance. A quality factor as high as 165 was measured in isopropanol for a cantilever oscillating in the seventh order roof tile-shaped mode at 2 MHz. To support the results of the experimental characterization, a 2D finite element method simulation model is presented and studied. An analytical model for the estimation of the motional conductance was also developed and validated with the experimental measurements.

It is the objective of this paper to report on the performance of piezoelectric MEMS resonators for viscosity and density measurements at elevated temperatures. A custom-built temperature controlled measurement setup is designed for fluid temperatures up to 100 °C. Piezoelectric single-side clamped resonators are fabricated, excited in 2nd order of the roof tile-shaped mode (13-mode) and exposed to several liquids (i.e. D5, N10, N35, PAO8, olive oil, ester oil and N100). At the next step, these results are analysed applying a straightforward evaluation model, thus demonstrating that with piezoelectric MEMS resonators the density (i.e. from kg m−3 to kg m−3) and viscosity (i.e. from mPa s to mPa s) values of liquids can be precisely determined in a wide range. Compared to standard measurement techniques, the results show for the first parameter a mean deviation of about 1.04% at 100 °C for all the liquids investigated. For the second parameter, the standard evaluation model implies a systematic deviation in viscosity with respect to the calibration being N35 in this study. This inherent lack of strength has a significant influence on the accuracy, especially at 100 °C due to fluids having a viscosity reduced by a factor of 30 for N100 compared to room temperature. This leads to relative deviations of about 23% at 100 °C and indicates the limits of the evaluation model.

In this paper, a design procedure to perform an efficient actuation of in-plane modes in piezoelectric resonators is presented. This procedure is applied to different microplate structures, paying attention to two in-plane mode families: contour modes and flexure-actuated modes. A representative set of devices from both families were used as illustrative examples. These devices were characterized electrically by measuring the impedance and their in-plane modal shapes were measured with a novel technique based on speckle-pattern interferometry. Figures of merit such as the quality factor or the motional resistance were obtained and used to evaluate the different design approaches.

This paper presents a robust Q-control approach based on an all-electrical feedback loop enhancing the quality factor of a resonant microstructure by using the self-sensing capability of a piezoelectric thin film actuator made of aluminium nitride. A lock-in amplifier is used to extract the feedback signal which is proportional to the piezoelectric current. The measured real part is used to replace the originally low-quality and noisy feedback signal to modulate the driving voltage of the piezoelectric thin-film actuator. Since the lock-in amplifier reduces the noise in the feedback signal substantially, the proposed enhancement loop avoids the disadvantage of a constant signal-to-noise ratio, which an analogue feedback circuit usually suffers from. The quality factor was increased from the intrinsic value of 1766 to a maximum of 34?840 in air. These promising results facilitate precise measurements for self-actuated and self-sensing MEMS cantilevers even when operated in static viscous media.

An all-electrical velocity feedback control to enhance the quality factor of piezoelectric aluminium nitride (AlN)-based microcantilevers and microbridges was implemented. Two alternatives to obtain a velocity-proportional signal were demonstrated depending on the top electrode configuration. For a straightforward electrode design in one-port configuration (i.e. self-actuation and self-sensing), a velocity signal, proportional to the piezoelectric current, was used in the feedback loop by cancelling out the dielectric current electronically. For top electrodes allowing a two-port configuration (i.e. one for actuation and one for sensing), the piezoelectric current is directly extracted and its relationship with velocity is analysed taking the symmetry of the modal shape into account. Standard operational amplifier-based configurations for the feedback circuits were implemented on a printed circuit board. Quality factors were determined from the transient electrical response of the devices. Comparable results were obtained from the displacement spectrum applying a laser Doppler vibrometer. Quality factors as high as 2 × 105, corresponding to an enhancement factor of about 200, were achieved in air for the lowest gain margin achievable before the circuit becomes unstable, making this kind of device more competitive for mass sensor applications due to enhanced spectral resolution.

A systematic design procedure to tailor the modal response of micro-resonators based on flexible plates with piezoelectric films is demonstrated. Sensors/actuators were designed by optimizing the surface electrode shapes in the plane dimensions. A numerical finite element procedure, which considers the effective surface electrode covering the piezoelectric film as a binary function on each element, has been implemented. Two design goals are considered: (i) optimized response (actuation or sensing) in a given mode; (ii) implementation of a modal transducer by filtering specific modes. For a given mode in a plate with arbitrary boundary conditions, our calculations allowed us to predict the top electrode layout reaching higher displacement in resonance than any other electrode design for the same structure. Microcantilevers and microbridges were fabricated and their modal response characterized by laser Doppler vibrometry. In comparison to a conventional square-shaped electrode, our experiments show that the implemented designs can increase the response in any desired resonant mode and simultaneously attenuate the contributions from other unwanted modes, by simply shaping the surface electrodes. Enhancement ratios as high as 42 dB, relative to a full-size electrode case, are demonstrated. The limitations imposed by the fabrication are also discussed.

The response of commercial piezoelectric AFM probes for potential applications in the field of chemical or biological sensors operating in liquids is investigated using laser Doppler vibrometry. The present work investigates the roles played in the frequency response by the density and the viscosity of different water–glycerol mixtures, in a frequency range of up to 1 MHz in air. Since the width of the tested probes is relatively large (and hence the aspect ratio remains small), inertial loading effects dominate viscous effects, unlike in cantilevers characterized by larger aspect ratios. Measurements are compared with results provided by a simplified computer model of a probe immersed in an inviscid surrounding fluid.