This work presents the design, fabrication, and characterization of a dual-frequency square piezoelectric micromachined ultrasonic transducer (PMUT) featuring a novel dual top electrode architecture that, for the first time, combines a circular inner and square outer ring electrode, enabling enhanced mode-selective excitation. The electrode topology is strategically aligned with the spatial strain distributions of both the fundamental (1.7 MHz) and higher order (6.4 MHz) flexural modes, enabling efficient and selective excitation of each mode within a single PMUT structure. Unlike conventional single-electrode PMUTs, which are typically limited to strong performance at the fundamental resonance, the proposed design achieves enhanced transmission efficiency at both operating modes. Finite element simulations and experimental validation demonstrate significant improvements in electromechanical coupling, with a twofold increase at the higher order mode, as well as enhancements in transmit sensitivity by 36% at the fundamental mode and by a factor of five at the higher order mode, compared to a conventional single top electrode PMUT. Underwater acoustic characterization further confirms dual-mode operation of the proposed SqC PMUT, demonstrating a 68% increase in acoustic pressure at the higher order mode compared to the reference single-electrode PMUT. This dual-mode enhancement allows the higher order resonance to serve as an additional operating frequency, facilitating dual-frequency operation without increasing device complexity or fabrication steps. The proposed PMUT architecture offers a compact and high-performance solution for ultrasonic imaging and sensing applications.

Organic field-effect transistors (OFETs) represent a versatile device class for emerging chemical and biological sensors, offering advantages such as structural flexibility, cost-effective fabrication, and suitability for bio-interfacing applications. While most performance improvements have come from material development, this work focuses on electrode topology as a design lever for enhancing sensor-relevant figures of merit, specifically the saturation drain current. Using a high-mobility diketopyrrolopyrrole-thiophene (DPP-T) copolymer, three radial OFET geometries, namely Ring, Star and Gear, are designed and fabricated under identical material stacks and processing conditions. Devices are characterized in two biasing configurations, termed Inside-Drain and Inside-Source, in which the inner electrode operates as the drain or as the source, respectively. The Ring Inside-Drain configuration yields a saturation drain current of −5.74 μA and is used as the reference for comparison. Under the same bias, the Star Inside-Drain and Gear Inside-Drain layouts yield saturation currents of −6.76 μA and −8.12 μA, corresponding to enhancements of 18% and 41%. When biased in Inside-Source configuration, the Ring, Star and Gear topologies yield saturation currents of −7.45 μA, −8.51 μA and −10.98 μA. These values represent 30%, 48% and 91% higher current than the Ring Inside-Drain reference. These results demonstrate that symmetric, field‑enhancing electrode shapes provide the primary route to higher drain current, and that appropriate choice of bias polarity offers an additional, secondary boost via field‑assisted injection. Overall, the study establishes electrode geometry as a scalable, material‑independent strategy for improving OFET sensitivity in chemical and biosensing applications.

Organic field-effect transistors (OFETs) offer flexible platforms for chemical and biological sensing. However, they face a fundamental trade-off in conventional rectangular geometries where achieving high drain current requires shorter channel lengths, which reduces active sensing area and limits device sensitivity. To address this trade-off, we developed a cross-channel OFET topology consisting of four orthogonal branches intersecting at a central junction. Two device implementations were characterized: a cross-channel design with identical channel lengths across branches and a hybrid cross-channel (HCC) design with two distinct channel lengths alternating across the four branches. To systematically evaluate the geometric benefits of these designs, we fabricated conventional rectangular reference devices: rectangular single-branch reference matching one branch dimension and rectangular area References with the same footprint as the cross-channel devices. All devices were fabricated for two channel lengths of 50 and 100 mu m. Electrical characterization of cross-channel topology designs was performed in multiple biasing modes of principal, parallel, diagonal, and single-branch configurations across both channel lengths. For the cross-channel design at 50 mu m, the principal-mode configuration achieved a mean drain current of -26.1 mu A, corresponding to 308% of the single-branch reference and 146% of the rectangular area reference. At 100 mu m, the cross-channel configuration delivered -10.6 mu A, representing 270% of the single-branch reference and 130% of the rectangular area reference. The HCC design demonstrated tunable current responses across biasing modes, with the principal configuration reaching -19.2 mu A. These results establish that cross-channel topologies offer a scalable, fabrication-compatible approach to simultaneously achieving high drain current and expanded sensing area in field-effect transistor-based platforms without requiring modifications to semiconductor materials or processing conditions.

Multifrequency operation in micromachined ultrasonic transducers, enabled by targeted excitation of specific vibrational modes, has emerged as an attractive approach for achieving tunable performance and configurability, well-suited for advanced ultrasound imaging and therapeutic applications. This paper presents a dual-electrode rectangular piezoelectric micromachined ultrasonic transducer (PMUT) designed for efficient dual-frequency operation through mode selective actuation. The proposed architecture employs segmented electrodes that are spatially aligned with the strain distributions of two distinct flexural modes, enabling selective excitation of Mode 1 (fundamental) and Mode 3 (higher order) through appropriate electrode actuation. Finite element simulations and impedance analysis were used to guide the electrode configuration and validate the mode-selective behavior. The dual-mode PMUT was fabricated alongside a conventional single-electrode PMUT using identical membrane dimensions and material stack for direct comparison. Comprehensive electrical and underwater acoustic characterization confirmed that the conventional PMUT is limited to single-frequency operation at the fundamental resonance. In contrast, the proposed design achieved a substantial improvement in higher-order performance, with a threefold increase in acoustic pressure at Mode 3 compared to the conventional device. These results demonstrate that mode-aligned electrode segmentation enables efficient dual-mode operation without added fabrication complexity, making the design highly suitable for multifrequency ultrasonic applications such as biomedical imaging and sensing.

Organic Field-Effect Transistors (OFETs) have emerged as powerful platforms for next-generation sensors due to their flexibility, low fabrication cost and compatibility with biological environments. While conventional approaches focus on material innovation to improve performance, this study emphasizes the impact of electrode geometry on device sensitivity, specifically analyzing how layout influences the saturation drain current, a key figure of merit for biomedical and chemical sensing applications. Using the high-mobility diketopyrrolopyrrole-thiophene (DPPT) polymer as the active layer, multiple OFET topologies including Ring, Gear and Pointed Petal-based configurations are designed and fabricated under identical material and process conditions. Comparative analysis revealed that the proposed Gear design delivered the highest drain current ($-20.1 \mu \mathrm{A}$), about 10% greater than that of the Ring (Inside Drain) reference, affirming the critical role of symmetric and continuous field distribution in maximizing current output. Other proposed geometries, Pointed Petal and Ring (Inside Source) showed varied performance, underscoring how design topology can significantly enhance signal strength without modifying the material system. These findings establish device geometry as a scalable, material-independent lever for enhancing OFET sensitivity, offering practical pathways for optimizing organic transistor-based sensor platforms.

Time Series Classification (TSC) is a key problem in several domains. One of the most popular approaches for TSC is to encode time series as images and use Computer Vision (CV) models for classification. The success of this approach has led to the emergence of several image transformation functions, each offering distinct insights valuable for different classification tasks. However, the significant difference between pre-existing image transformation functions makes it challenging to determine the most suitable transformation for a given TSC task. This work combines insights from various image transformations by concatenating DL-extracted black-box features obtained from several image representations of a given time series. These features are obtained from DL models that are trained on a large family of TSC datasets that have been transformed into one of several image transformations. As a result, each model is specialized in extracting insights from image representations of a specific type. To evaluate the generalizability of these insights, unseen TSC datasets are transformed into feature vector datasets by concatenating the features extracted from several specialized models, transforming a given TSC dataset into a dataset of feature vectors. This feature vector dataset is then learned using an ensemble model, whose initial results are improved using a Genetic Algorithm (GA). The approach is evaluated on the UCR archive and compared to several existing methods. The results show that the insights gained from image representations of time series are both useful and generalizable.

Piezoelectric micromachined ultrasonic transducers (PMUTs) traditionally operate at a single frequency, requiring physical modifications to alter their operational frequency. In ultrasonic applications such as diagnostic imaging, the working frequency is critical in determining resolution and detection range. Dual-electrode PMUTs present a promising alternative to conventional designs, enabling multi-frequency operation by activating distinct resonant modes through tailored electrode configurations and actuations. This study details the design, fabrication, and characterization of three dual-electrode PMUT geometries: circular, square, and rectangular membranes. Fabricated using the PiezoMUMPs process, these geometries feature identical material properties and membrane areas. Their electromechanical performance was evaluated through finite element simulations and experimental methods, focusing on metrics such as electromechanical coupling coefficient (EMCC), resonant frequencies, and membrane displacement. A comparative performance analysis across the geometries is presented. Multi-frequency operation was achieved using a dual-electrode configuration with an outer ring electrode and an inner disk electrode, allowing independent activation of two resonant modes. These findings highlight the potential of dual-electrode PMUTs to enhance multi-frequency operation, especially in ultrasonic imaging and related applications.

Multi-frequency operation in ultrasonic transducers balances the trade-off between resolution and penetration depth in ultrasonic imaging. Single top-electrode PMUTs couple strongly to the fundamental mode but weakly to higher-order modes because uniform electrode coverage does not conform to the higher-mode strain profile. Dual concentric ring-and-disk (C-C) electrodes access multiple modes yet reduce fundamental-mode coupling due to misalignment between the segmented coverage and the fundamental strain profile. In this work a circular square (C-Sq) dual top electrode is introduced to strengthen coupling in both the fundamental and higher order modes for multifrequency operation. The configuration pairs a circular outer electrode with a square inner electrode on a circular membrane. The topology aligns with the strain profiles of the fundamental and a selected higher mode enabling mode selective excitation by electrode switching. The proposed C-Sq dual electrode design and reference single electrode and dual electrode concentric ring and disk (C-C) devices were fabricated using the PiezoMUMPs multi-user MEMS process for fair comparison. Electrical impedance analysis, laser doppler vibrometry, and acoustic measurements show a 37% increase in fundamental mode electromechanical coupling relative to C-C dual electrode PMUT and a 27% improvement for the higher mode relative to single electrode PMUTs. Membrane displacement increases by 67% and acoustic pressure at the fundamental resonance is enhanced by 13%. The proposed C-Sq PMUT supports efficient, mode selective multifrequency operation for biomedical imaging and sensing.

Ultrasonic imaging is widely used in medical diagnostics and non-destructive testing but remains limited by the trade-off between resolution and imaging depth. This work presents the electromechanical characterization of dualelectrode circular piezoelectric micromachined ultrasonic transducers (PMUTs) for multifrequency operation to address this challenge. Three top electrode configurations: single electrode, dual concentric rings, and disk-ring were designed, fabricated and characterized for mode-selective excitation. Finite element analysis (FEA) guides electrode design by evaluating mode shapes and strain distributions. Devices are fabricated using the PiezoMUMPs process with identical membrane dimensions and materials for fair comparison. Impedance analysis and laser doppler vibrometry (LDV) measurements show strong agreement with simulations. Dual top electrode configuration enables real-time switching between resonant modes, favoring the fundamental mode with dualelectrode excitation and higher-order mode with outer-electrode actuation. The concentric ring configuration demonstrates the strongest overall performance, highlighting the importance of electrode configuration for tunable frequency PMUTs in nextgeneration ultrasound systems.

Detecting volatile organic compounds (VOCs) is essential for health, environmental protection, and industrial safety. VOCs contribute to air pollution, pose health risks, and can indicate leaks or contamination in industries. Applications include air quality monitoring, disease diagnosis, and food safety. This paper focuses on polymer-based hybrid sensor arrays (HSAs) utilizing interdigitated electrode (IDE) geometries for VOC detection. Achieving high selectivity and sensitivity in gas sensing remains a challenge, particularly in complex environments. To address this, we propose HSAs as an innovative solution to enhance sensor performance. IDE-based sensors are designed and fabricated using the Polysilicon Multi-User MEMS process (PolyMUMPs). Experimental evaluations are performed by exposing sensors to VOCs under controlled conditions. Traditional multi-sensor arrays (MSAs) achieve 82% prediction accuracy, while virtual sensor arrays (VSAs) leveraging frequency dependence improve performance: PMMA-VSA and PVP-VSA predict compounds with 100% and 98% accuracy, respectively. The proposed HSA, integrating these VSAs, consistently achieves 100% accuracy in compound identification and concentration estimation, surpassing MSA and VSA performance. These findings demonstrate that proposed polymer-based HSAs and VSAs, particularly with advanced IDE geometries, significantly enhance selectivity and sensitivity, advancing e-Nose technology for more accurate and reliable VOC detection across diverse applications.

Capacitive resonator performance, including sensitivity, is determined by its capacitive change and resonant frequency shift in response to an external perturbation, such as added mass. Conventional designs are inherently defined by fully clamped boundaries around the deflectable plate. However, recent advances suggest that bilateral and quadrilateral concentric boundary resonators can offer improved performance through enhanced deflection compared to conventional fully clamped resonators. This letter analyzes how the spatial distribution of clamped boundaries under identical total clamped angles affects key resonator metrics, including sensitivity, which is manifested through a change in capacitance and frequency shift in electrical characterization. Resonators with bilateral and quadrilateral clamped boundary configurations are the focus of this letter to demonstrate the idea. In order to do this, resonators with total clamped angles of 120 degrees, 180 degrees, and 240 degrees are fabricated and characterized using electrical impedance analysis, with results in agreement with the conducted finite element analysis. The quadrilateral configurations outperformed bilateral ones in both frequency shift and capacitance change, indicating that the clamped boundary distribution serves as a critical design parameter. These findings offer new insight into structural optimization strategies for capacitive resonators beyond conventional clamping schemes.

This study presents a novel design methodology for microbridge resonators aimed at reducing pull-in voltage while maintaining resonant frequency. Previous studies have primarily focused on adjusting resonator plate geometry, modifying anchor conditions, or altering material properties to control pull-in voltage. In contrast, this work introduces the ratio of the bottom electrode length to the microbridge length as a critical and tunable design parameter, which has remained unexplored in previous studies. An analytical model is developed to capture the effects of this ratio, and its predictions are validated through finite element analysis. To demonstrate the concept, two microbridges are designed and fabricated with bottom electrode lengths of 42 and 82 mu m, corresponding to 35 % and 68 % of the microbridge length, respectively. All other design parameters, such as plate thickness, material properties and cavity height, are kept constant to enable a fair comparison. Electrical characterizations confirm that increasing the bottom electrode-to-microbridge length ratio effectively lowers the pull-in voltage without degrading resonator performance. Results show a 16 % reduction in pull-in voltage when the bottom electrode length is 68 % of the microbridge length, demonstrating the feasibility and advantages of the proposed methodology over existing techniques.

Time Series Classification (TSC) is the task of using a time series to predict the value of a categorical variable-the class of the time series. An interesting approach that has been studied in the TSC literature is converting a given time series into an image, through one of various proposed methods, and classifying the resultant image through a vision model. Furthermore, existing image transformation methods may require the values of one or more parameters. Modifying these parameter values can greatly alter the generated image, thereby impacting the performance of a vision-based classifier. However, guidelines for selecting appropriate parameter values have not been detailed. In fact, due to the diversity of time series used for TSC tasks, establishing such guidelines in a way that ensures optimal results may be impossible. Following this realization, we attempt to build Deep Learning models capable of independently learning appropriate parameter values to maximize classification accuracy. We propose a method wherein these parameters are dynamically determined through a Policy Network, which takes a time series as input. Then, an image is obtained using the resultant parameters and a pre-determined image transformation function. Finally, the image is fed into a vision model to obtain the class of the time series. This approach was tested for three pre-existing image transformation functions on the entirety of the UCR Archive of 128 TSC datasets. The results demonstrate that it may be more effective to allow models to learn the parameters of image transformation functions instead of setting them beforehand.

This work presents the first fabrication and experimental validation of a novel electrode design approach for enhancing the mass sensitivity of Quartz Crystal Microbalances (QCMs). The development of unique QCM electrode configurations includes a study of mass loading area distribution and its impact on resonant frequency shift, a key parameter that defines mass sensing performance. Finite Element Analysis (FEA) is conducted to identify areas of opportunity where localized energy trapping occurs and simulate the sensing performances of the configured electrode topologies compared to the conventional circular design. Theoretical models are experimentally validated through the fabrication of 5 MHz QCM sensors with non-conventional designs and the utilization of an automated controlled environment and sensor readout system. The unique QCMs presented herein exhibit noticeably higher resonant frequency shifts in response to variations in water vapor concentration, where the observed shift in frequency serves as an indicator for sensing performance. Experimental results reveal that unique topologies based on the novel Distribution of Area for Improving mass Sensitivity (DAIS) electrode design approach, featuring patterns of annularly distributed small electrodes, effectively utilize the energy trapping effect and outperform the conventional QCM design.

Piezoelectricity has long been the dominant transduction mechanism to detect and generate ultrasound. Recently, capacitive micromachined ultrasonic transducers (cMUTs) based on electrostatic transduction have emerged as an alternative to piezoelectric transducers. Wide bandwidth in immersion operation, ease of fabricating large arrays, and potential for integration with electronics are some of the advantages offered by cMUT technology. This chapter explains the theory and modeling of this modern type of electrostatic transducers. Both analytical and numerical models are presented. Fabrication processes based on surface and bulk micromachining techniques are explained. Examples of imaging systems based on this new technology are also described.

A new set of resonators is introduced in this work, which utilizes a flexible polysilicon plate symmetrically clamped at two positions across the plate's boundary. The proposed resonators provide larger deflection and hence change in capacitance with respect to the conventional fully clamped device. To demonstrate the concept, three topologies clamped at 60 degrees, 90 degrees, and 120 degrees are designed and fabricated using a poly multiuser microelectromechanical systems (MEMS) process. For a fair comparison, a fully clamped conventional topology is designed and fabricated with the same dimensions and material properties. Finite element analysis along with electrical and optical characterizations are performed. The obtained measurement results demonstrate improved average deflection of 40 nm with 26% decrease in pull-in voltage.

A novel biasing scheme having the potential to dynamically control the effective cavity heights of two fabricated, multiple moving membrane capacitive micromachined ultrasonic transducers (M 3 -CMUTs) has been developed. With this unique approach it is may be possible to avoid design trade-off requirements of the ultrasonic transducers that affect both receiving and transmitting operating modes. Each of these M 3 -CMUTs has two vibrating membranes suspended over a fixed bottom electrode. The two air-coupled M 3 -CMUT devices with the resonant frequencies of 1MHz single cell and 1.5MHz array device were fabricated. Finite Element Analysis (FEA) was carried out to investigate the effect of an additional middle membrane on the effective cavity height and device performance. Experimental validation was then completed with the transducers operating under both positive and negative biasing conditions of middle membrane, while keeping the top membrane at selected positive biasing voltages to realize a dynamically controlled range of effective cavity heights. The new biasing scheme may facilitate dynamic tunning of resonant frequencies of transmitter and receiver transducers through the variation of effective cavity height for improved performance.

An innovative technique is explored, aiming to expand the limit of detection (LOD) and increase the operating resonant frequency of the capacitive micromachined ultrasonic transducer (CMUT) working as a mass sensor through modifying anchor widths. The LOD is associated with the concentration range of target compounds and, therefore, their correspondent mass that can be measured using a CMUT mass sensor. The poly multiuser MEMS process (PolyMUMPs) sacrificial technique is used to fabricate three CMUT sensors with a range of anchor widths while keeping all other sensor design parameters and material properties constant. Experimental analysis is conducted to evaluate the influence of anchor width to resonant frequency and frequency shift at different biasing conditions. The experimental results demonstrate that a larger anchor width can increase CMUT operating resonant frequency and pull-in voltage, allowing for the expanded LOD. These results also agree with analytical expressions presented. In addition, it is shown that through employing a designed anchor width, resonant frequencies can be varied to fulfill specific sensor requirements with minimal fabrication adjustments. This study contributes to the evolving knowledge of the CMUT as a mass sensor, enabling more methods for variability of these devices.

Two sets of multiple moving membrane capacitive micromachined ultrasonic transducers (M-3-CMUTs) are designed and fabricated for a range of middle membrane radii, from 35 - 49 mu m and 34 - 44 mu m. All other design parameters were kept identical within each set of experiments to enable a fair comparison. To study the dynamic control range of the designed M-3-CMUTs, their performance was first evaluated through COMSOL simulation yielding results on the resonant frequency and corresponding cavity height. This was followed by fabrication of the same designs using multiuser microelectromechanical systems processes (MUMPs). The development of these prototypes enabled the comprehensive electrical characterization of the designs with responses that are demonstrated to be in full agreement with the simulation results. The middle membrane radii of the M-3-CMUTs impact the collapse voltage of these devices in addition to affecting the measured resonant frequencies when tested devices within the same measurement conditions. A 20 kHz increase in resonant frequency shift was recorded when the middle membrane radius of these devices was increased from 7 mu m to 10 mu m while operating under a 20V top membrane bias and -3V to -5V middle membrane bias. These observations indicate that the middle membrane radius has influence on the dynamic control of effective cavity height in M3-CMUTs, which enables the expansion of the devices' cavity heights beyond that of conventional capacitive micromachined transducers. This finding has potential in applications with a need for acoustic power where dynamic control of the cavity height could serve as a solution, such as in medical imaging.

Conventional sensor systems employ single-transduction technology where they respond to an input stimulus and transduce the measured parameter into a readable output signal. As such, the technology can only provide limited corresponding data of the detected parameters due to relying on a single transformed output signal for information acquisition. This limitation commonly results in the need for utilizing sensor array technology to detect targeted parameters in complex environments. Multi-transduction-mechanism technology, on the other hand, may combine more than one transduction mechanism into a single structure. By employing this technology, sensors can be designed to simultaneously distinguish between different input signals from complex environments for greater degrees of freedom. This allows a multi-parameter response, which results in an increased range of detection and improved signal-to-noise ratio. In addition, utilizing a multi-transduction-mechanism approach can achieve miniaturization by reducing the number of required sensors in an array, providing further miniaturization and enhanced performance. This paper introduces the concept of multi-transduction-mechanism technology by exploring different candidate combinations of fundamental transduction mechanisms such as piezoresistive, piezoelectric, triboelectric, capacitive, and inductive mechanisms.