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Diffraction gratings are commonly used in optical instruments such as in spectrometers, monochromators, and wavelength division multiplexing devices. The diffraction efficiency is directly related to the ratio of the grating spacing or pitch to the wavelength of the light. In a sinusoidal grating (or holographic grating) the efficiency is generally lower than a blazed grating with a saw tooth profile, but the spectral coverage can be broader. To address a large spectral range, advanced spectrometers use a number of diffraction gratings with a fixed pitch. In this paper, we show for the first time, the use of a tunable-frequency surface acoustic wave (SAW) delay line as a broad-band sinusoidal diffraction grating. To tune the frequency, we tune the effective spacing of the interdigitated transducers (IDTs) by applying specific phases to each IDT finger. The prototype SAW grating covers frequency range of 86 MHz to 360 MHz corresponding to optical range of $1.3~\mu \text{m}$ to $9~\mu \text{m}$ . [2020-0189]

In this paper, we present ultra-high quality factor ( ${Q}$ ) SAW resonators fabricated on aluminum gallium nitride on gallium nitride on silicon (AlGaN/GaN/Si) heterostructures with ${Q}$ exceeding 6000 at room temperature. We characterize their temperature response in a broad range of temperature (−196°C to 500°C or 77 K to 773 K). The effect of doping on the ${Q}$ and temperature coefficient of frequency (TCF) of the GaN-based SAW resonators is analyzed, for the first time, using un-intentionally doped GaN (UID-GaN), carbon doped GaN (C-doped), and Si doped GaN. The ${Q}$ value for UID-GaN and C-doped GaN is similar and decreases from 2000 to 1000 as the temperature is increased from 77 K to 773 K. The ${Q}$ value of Si-doped GaN is higher than UID and C-doped GaN by a factor of 3 (6622 at 77 K) and decreases with increase of temperature. This value of ${Q}$ at 1.8 GHz is the highest reported amongst aluminum nitride (AlN) or GaN-based SAW resonators. For extreme high temperatures (≥573 K) the TCF value is half the TCF value of UID and C-doped GaN, which shows the possibility of engineering the TCF by tuning the doping concentration. For low temperatures (≤150 K) C-doped and UID-GaN show a turn over point in TCF curve which shows their promise for cold clocks. [2020-0163]

This paper reports on a high quality factor ( $Q$ ) gallium nitride (GaN) thickness-shear (TS) bulk acoustic wave resonator with a reduced temperature coefficient of frequency (TCF). $4.2\mu \mathrm{m}$ -thick AlGaN/GaN-on-Si substrate is used to create high-Q resonators operating in TS mode, as well as high-order width-extensional modes (WEn). The temperature characteristic of TS and WE resonators are measured and compared highlighting the considerable reduction in TCF of the shear mode. GaN resonator prototypes are presented operating in TS mode at 573MHz, with a $Q$ of 2700, and in WE 3 , WE 5 , and WE 7 modes at 239MHz, 407MHz, and 564MHz, with $Q\mathrm{s}$ of 8700, 7200 and 2300, respectively. A TCF of-17.9 ppm/°C is measured for the TS mode, which is significantly lower compared to WE 3,5,7 modes with measured TCF of $-24 \text{ppm}/^{\circ}\mathrm{C}\pm 0.3\ \text{ppm}/^{\circ}\mathrm{C}$ . The large (> 6 ppm/°C) improvement in temperature sensitivity, along with the promise of monolithic integration with HEMT electronics highlights the potential of TS GaN resonators for realization of frequency stable and radiation hard integrated oscillators.

This paper reports on the investigation of anomalous low quality factors ( ${Q}\text{s}$ ) in AlN thin film-based length extensional (LE)-mode resonators using finite element method (FEM), analytical modeling, and experimental techniques. Different heterostructures of LE resonators having Al/AlN/Si, Al/AlN, AlN/Si, and Si are designed and fabricated using standard MEMS processes. Experimental ${Q}\text{s}$ along with resonances are obtained using electrical and optical readout and are compared with the modeled and analytically obtained ${Q}\text{s}$ . The results show that the thermoelastic damping (TED) in metal electrode, anchor loss, and charge redistribution loss are not the dominant loss mechanisms. With the help of Mason’s network modeling, we investigated the dielectric loss, which is also observed to be negligible for these resonators. An internal mechanical loss, originating from the columnar growth of AlN and observed using Mason’s model, is proved to be the dominant loss mechanism. This observation is confirmed and reinforced by the COMOSL’s FEM analysis of TED in AlN, having columnar growth effect and by experimental results. [2018-0258]

Silicon carbide (SiC) has been extensively investigated in the last decade, specifically for applications in harsh environments. However, most SiC sensors require an external power supply, which cannot operate at high temperatures. This letter develops a new sensing technology in a SiC platform based on near field communication to eliminate the requirement for wired power sources. The 3C-SiC temperature sensors were fabricated from a SiC-on-insulator substrate formed by anodic bonding. The sensors functioned based on the thermoresistance of the SiC films with the high TCR of −13 000 ppm/K at 300 K and −3 000 ppm/K at 600 K. The resistance change of the sensors was wirelessly measured using a reading coil placed outside of the heating chamber, showing a significant resonant-frequency-shift (−400 ppm/K at 600 K) of the coupling impedance under temperature variation. The proposed technique is promising for the development of wireless wide-band-gap sensors used in extreme conditions.

We demonstrate coupled bulk and surface acoustic wave (BAW/SAW) resonators with significantly improved coupling efficiency as compared to SAW only devices. Two sets of stacks are investigated: one uses a thin film piezoelectric material with a bottom electrode on GaN substrate (i.e., AlN/Mo/GaN) and another includes a piezoelectric thin film with bottom metal on a Si substrate (i.e., AlN/Mo/Si). A vibrating BAW transducer induces a SAW in the substrate and between two sets of interdigitated transducers improving the coupling efficiency by a factor of ${\geq }\, \mathsf {8}{x}$ as compared to the conventional SAW. The performance of the coupled BAW/SAW resonators using each of the Si and GaN substrates is compared using FEM analysis. By using FEM, coupling efficiency of the hybrid BAW/SAW with AlN/Mo/Si stack is 2.4%, whereas it is 3.2% for the SAW using AlN/Mo/GaN stack.

This letter reports the first results on a new generation of anti-symmetric lamb-wave devices using a simple solidly mounted platform of AlN/Mo/Si and operating at very-high-to-super-high frequency range. AlN/Mo/Si-based solidly mounted resonators (SMRs) with sub-micrometer interdigitated transducers were fabricated using the electron beam lithography and liftoff process. Plasma etching was used to etch vias in ground pads to get access to the bottom electrode for grounding. By comparing the floating, grounded, and no bottom electrode device performances, it has been observed that the presented SMR design with grounded bottom electrode results in enhancement of the coupling efficiency by 500% compared with conventional SAW devices. In addition, these devices are more mechanically robust and can be monolithically integrated with a CMOS platform without requiring complex packaging processes compared with conventional released lamb-wave resonators.

In this work, we present an uncooled infrared (IR) thermal imager featuring a sparse array of resonant pixels that can be monolithically integrated with the readout electronics for long-term operation in high-temperature environments. Each of the detector pixels is made of an IR-sensitive GaN microresonator developed to be operated at up to 500°C. The pixels are employed in sparse array configuration to allow for routing the signal carrying traces and to ease the routing from pixel to circuit, as well as to reduce the power consumption. A 128×128 sparse resonator array with the array density of 37.5% is designed and used in conjunction with the Compressed Sensing method to reconstruct complex test images from highly incomplete data. Capabilities and performance metrics of the recovery technique are explored.

We report integrated, electrically-driven acousto-optic modulators in silicon photonics. Electromechanical transducers in piezoelectric aluminium nitride are harnessed to enable nonlocal phase modulation and non-reciprocal single-sideband modulation in silicon-on-insulator waveguides from 1-5 GHz.

We demonstrate integrated, electrically-driven acousto-optic modulators by fabricating electromechanical transducers in piezoelectric aluminium nitride on a standard silicon-on-insulator waveguide platform. Phase modulation and single-sideband modulation and mode conversion from 1-5 GHz are achieved. © 2020 The Author(s).

This paper reports, for the first time, on the engineering of silicon-doped gallium nitride (GaN) based surface acoustic wave (SAW) devices for ultra-stable clocks with frequency stability of better than 20 ppb/°C over the temperature range of −196°C to 27°C. Such exceptional temperature stability of GaN acoustic resonators at low temperatures prove their applicability for emerging space explorations to planetary bodies with extreme low temperature swings. In this work, we have exploited silicon doping of GaN to enable further stabilization of the SAW resonator by self-ovenization using the Joule heating generated from a DC current that is passed through the resonator body. Furthermore, we demonstrate the effect of doping on the temperature coefficient of frequency (TCF) of GaN SAW resonators.

This letter reports on first-time demonstration of non-reciprocal acoustic transmission in a gallium nitride (GaN) delay line structure. The split of forward (S21) and backward (S12) transmissions is observed by applying a dc electric field through the active area with a two-dimensional electron gas (2DEG) sheet. The non-reciprocity (~20.7 dB/mm max. in this letter) varies by tuning the gate voltage to deplete the 2DEG sheet carrier density, which agrees with model prediction for the interaction between 2DEG and surface acoustic waves. Our preliminary results prove the feasibility of implementing chip-scale non-reciprocal acoustic devices in a GaN platform through acoustoelectric amplification.

This paper reports on tunable 2- and 3-pole bandpass filters with a wide frequency tuning range (tuning ratio >3) and a constant bandwidth using switchable varactor-tuned resonators. The wide center frequency tuning range is obtained using p-i-n diodes to switch in and out quarter-wavelength ( $\lambda $ /4) or half-wavelength ( $\lambda $ /2) resonators for low-band or high-band modes, without increasing the tuning capacitance range of the varactors. A combination of electric and magnetic coupling is utilized to realize a near constant absolute bandwidth across the tuning range. A switchable feed line with a fixed matching capacitance is used to realize the external coupling. Two filters are designed and fabricated on a Duroid substrate with $\varepsilon _{r} = 2.2$ and $h =0.787$ mm. For the 2-pole filter, the center frequency is tuned from 550 to 1900 MHz while maintaining a 3-dB bandwidth of 92 ± 6 MHz, insertion loss of 3.2~4.4 dB, and return loss of better than 15 dB. For the 3-pole filter, the center frequency is tuned from 540 to 1800 MHz while maintaining a 3-dB bandwidth of 89 ± 4 MHz, insertion loss of 4~5.4 dB, and return loss of better than 12 dB. For both filter types, the third-order intercept point and 1-dB compression point ( $P_{\mathrm {1~dB}})$ are 11 and 7 dBm, respectively. The rejection level at 200-MHz offset frequency from the passband center frequency is better than 25 and 41 dB for 2- and 3-pole filters, respectively, across the entire tuning range. To the best of our knowledge, this planar bandpass filter exhibits the widest tuning range with a near-constant bandwidth.

The 20th in the series of Solid-State Sensors, Actuators, and Microsystems Hilton Head Workshops was scheduled for May 31–June 4, 2020, at the Sonesta Resort on Hilton Head Island, SC, USA. Unfortunately, like most academic conferences in 2020, it was canceled due to the COVID-19 pandemic. The Hilton Head 2020 Workshop ( https://www.hh2020.org/ ) was designed to continue to represent the science and engineering excellence of the MEMS community while introducing a number of innovations to the structure of this historic meeting. Some of these changes included the workshop becoming an international meeting for the first time while remaining committed to its key mission of excellence in showcasing recent innovative work and offering strong networking opportunities. We would have had new representatives from multiple regions in North America on our Technical Program Committee (TPC) and a strong focus on cultivating the impact of MEMS on global issues and grand challenges (including a full-day short course on “Microsystems Frontiers in the IoT Era”).

There is an increasingly high demand for compact, low-loss reconfigurable filters in various advanced wireless applications: cognitive radios, modern transceivers, and antijamming communication systems, to name a few [1]. With reconfigurable filters, wireless communication systems that require multiple operating frequency bands can be implemented in a smaller size and at a reduced cost compared to conventional systems using multiple single-frequency filters. Bandpass filters provide a better RF performance compared to low-pass or high-pass filters, resulting in improved signal-to-noise ratio and out-of-band performance [2].

This paper demonstrates a tunable color filter in an optical waveguide grating architecture consisting of a phase change material, Germanium Telluride (GeTe). The waveguide and grating resonances were used to enhance the color dynamic when GeTe is transitioned between its amorphous and crystalline states. The optical refractive index of the GeTe grating changes in response to an applied heat, which results in different reflected colors. Red, yellow, and green colors were achieved at room temperature for different grating periods in the amorphous phase. When GeTe is transitioned to the crystalline state, these colors change to orange, green, and blue, respectively.

This paper reports for the first time a high-contrast, low-power reflective color filter employing a phase change material in a waveguide grating with different periods. Refractive index modulation is produced by crystallographic phase transition of Germanium Telluride (GeTe) using an external stimulus, such as heat. Nanostrips of GeTe are used due to their reliable, fast, and reversible phase transitions. These thin nanostrips reduce the total light absorption and yield a vivid, bright reflected color. Moreover, a buried optical waveguide consisting of a silicon nitride (Si3N4) core, which was grown between two layers of silicon dioxide (SiO2) cladding, is placed under these gratings to enhance the color contrast. The waveguide resonance is enhanced using a bottom palladium reflector and GeTe gratings above with different periods. Selective absorption within visible-NIR region that depend on the period of the grating and the phase of the GeTe is introduced within these devices for the first time. A unique anti-reflective coating was also re-introduced to suppress the surface reflection, thus enhancing the color contrast. Vivid reflected red and green colors have been shown for a device with active area of 400 μm2. These colors were electrically transitioned to blue and yellow, respectively, for several cycles. We further report that GeTe nanostrips with different cross-sectional areas demonstrate different phase transition behaviors. Thus, several colors were achieved within the same area of the device employing GeTe nanostrips with different periods.

The electromechanical properties of an acoustic wave propagating in a piezoelectric media can be tuned by modulation of the resistivity of the piezoelectric material. This is readily available in piezoelectric semiconductor materials, wherein acoustic phonons and charge carriers can interact. In this work, we employ epitaxially grown AlGaN/GaN heterostructures in bulk acoustic wave resonators with Schottky interdigitated transducers biased in the depletion region to study the interaction between piezoelectric strain and depletion charges. By modulating the impedance of the depletion layer upon application of DC voltages, we tune the acoustic properties of bulk-mode resonators and show significant Q enhancement as the result of a depletion force added to the piezoelectric actuation force. Furthermore, we compare the performance of such resonators with pure GaN piezoelectric resonators that have the same geometry but with the AlGaN layer removed. When integrated with AlGaN/GaN HEMTs (located on the acoustic cavity or next to the resonator), such resonators can be used as frequency references in oscillator circuits in radio frequency (RF) blocks or utilized in harsh environment sensing applications.

In this paper, design, analysis, and implementation of a piezoelectric microelectromechanical systems (MEMS) oscillator on an ovenized microplatform is presented. An oxide-refill process is used to compensate the first-order temperature coefficient of frequency of MEMS resonators, as well as to realize thermal isolation structures. The technology enables fabrication of low-power ovenized device fusion platforms using standard silicon on insulator wafers. Utilizing the intrinsic frequency-temperature characteristic of two MEMS resonators, temperature sensing and closed-loop oven-control is realized by phase-locking two MEMS oscillators at an oven-set temperature. The design of the phase-lock control loop is studied using multidomain linear models. Control loop dynamics, noise properties, and nonideal effects are analyzed. Low-power and low-noise phase-locked loop-based control circuitry is designed in 0.18- $\mu \text{m}$ CMOS to interface with the MEMS resonators. Using the developed technology, an oven controlled MEMS oscillator exhibits an overall frequency drift of

In this letter, we present ultra-high frequency coupled ring resonators realized in a suspended AlGaN/GaN heterostructure. The piezoelectric transducers consist of back-to-back nickel (Ni) Schottky diodes deposited on a 20-nm-thick AlGaN layer and a floating 2-D electron gas (2-DEG) sheet at the AlGaN/GaN interface. The dispersion characteristics of symmetric and anti-symmetric Lamb waves in ring resonators are characterized. Higher order resonance harmonics are targeted with very high mechanical quality factors ( $Q\text{s}$ ) through optimized design of the coupling beam. Such a scheme minimizes the acoustic energy leakage to the substrate, and thus increases anchor $Q$ . We report on a high-order breathing resonance mode at 900.7 MHz with very high mechanical $Q$ of 8350 measured at room temperature and ambient pressure, achieving a record high frequency $\times Q$ value of $7.5\times 10^{12}$ . [2017-0064]

Harvesting sustainable energy resources from surrounding environments to power small electronic devices and systems has attracted massive research attention. Herein, we develop a novel technology to harvest light energy to self-power and simultaneously sense mechanical acceleration in a monolithic structure. When the photonic gate is illuminated the operation mode of the device changes from conventional mode to light harvesting and self-powered operation. The light illumination provides a gradient of majority carrier concentration on the top semiconductor layer, generating a lateral photovoltage, which is the output voltage of the sensor. Under acceleration, the mechanical inertial force induces stress in the sensor material leading to the change of mobility of the charge carriers, which shifts their diffusion rate, and hence changes the gradient of the majority carriers and the lateral photovoltage. The sensitivity at 480 lx light illumination was measured to be 107 μ V / g , while it was approximately 30 μ V / g under the ambient light illumination without any electrical power source. In addition, the acceleration sensitivity is tunable by controlling parameters of the photonic gate such as light power, light spot position and light wavelength. The integration of sensing and powering functions into a monolithic platform proposed in this work eliminates the requirement of external power sources and offers potential solutions for wireless, independent, remote, and battery-free sensing devices and systems.

Semiconductor nanowires are widely considered as the building blocks that revolutionized many areas of nanosciences and nanotechnologies. The unique features in nanowires, including high electron transport, excellent mechanical robustness, large surface area, and capability to engineer their intrinsic properties, enable new classes of nanoelectromechanical systems (NEMS). Wide bandgap (WBG) semiconductors in the form of nanowires are a hot spot of research owing to the tremendous possibilities in NEMS, particularly for environmental monitoring and energy harvesting. This article presents a comprehensive overview of the recent progress on the growth, properties and applications of silicon carbide (SiC), group III‐nitrides, and diamond nanowires as the materials of choice for NEMS. It begins with a snapshot on material developments and fabrication technologies, covering both bottom‐up and top‐down approaches. A discussion on the mechanical, electrical, optical, and thermal properties is provided detailing the fundamental physics of WBG nanowires along with their potential for NEMS. A series of sensing and electronic devices particularly for environmental monitoring is reviewed, which further extend the capability in industrial applications. The article concludes with the merits and shortcomings of environmental monitoring applications based on these classes of nanowires, providing a roadmap for future development in this fast‐emerging research field., Wide bandgap semiconductor nanowires with unique properties and advanced fabrication/integration technologies set a new class of nanosensing and electronic applications. Implementation of these smart devices such as gas sensors, photodetectors, wearable devices, strain sensors, and energy harvesters into environmental monitoring systems offers promising solutions for critical environmental challenges.

Tuning the natural frequency of a resonator is an innovative approach for the implementation of mechanical resonators in a broad range of fields such as timing applications, filters or sensors. The conventional electrothermal technique is not favorable towards large tuning range because of its reliance on metallic heating elements. The use of metallic heaters could limit the tuning capability due to the mismatch in thermal expansion coefficients of materials forming the resonator. To solve this drawback, herein, the design, fabrication, and testing of a highly-doped SiC bridge resonator that excludes the use of metallic material as a heating element has been proposed. Instead, free-standing SiC structure functions as the mechanical resonant component as well as the heating element. Through the use of the Joule heating effect, a frequency tuning capability of almost∆f/fo≈80% has been demonstrated. The proposed device also exhibited a wide operating frequency range from 72.3kHz to 14.5kHz. Our SiC device enables the development of highly sensitive resonant-based sensors, especially in harsh environments.

This paper reports on a platform for monolithic integration of piezoelectric and piezoresistive devices on a single chip using the ScAlN/3C-SiC/Si heterostructure. Surface acoustic wave devices with an electromechanical coupling of 3.2% and an out-of-band rejection as high as 18 dB are demonstrated using the excellent piezoelectric properties of ScAlN and low acoustic loss of 3C-SiC. Additionally, a large piezoresistive effect in the low-doped n-type 3C-SiC(100) thin film has been observed, which exceeds the previously reported values in any SiC thin films. The growth of the n-type 3C-SiC thin film was performed using the low pressure chemical vapor deposition technique at 1250 °C and the standard micro-electro-mechanical systems process is used for the fabrication of 3C-SiC piezoresistors. The piezoresistive effect was measured using the bending beam method in different crystallographic orientations. The maximum gauge factor is –47 for the longitudinal [100] orientation. Using the longitudinal and transverse gauge factors for different crystallographic orientations, the fundamental piezoresistive coefficients of the low-doped n-type 3C-SiC thin film are measured to be π 11 = ( − 14.5 ± 1.3 ) × 10 − 11 Pa−1, π 12 = ( 5.5 ± 0.5 ) × 10 − 11 Pa−1, and π 44 = ( − 1.7 ± 0.7 ) × 10 − 11 Pa−1.

This paper introduces tunable color reflectors for three primary colors (red, blue, and green) for use in low power display systems employing a phase change material (PCM) using interference resonance in an optical cavity. Optical index tunability of the PCM sitting on top of the cavity, results in tuning the device reflection spectrum and thus permits vivid color tuning. The phase change material used to achieve these results was Germanium Telluride (GeTe) due to its high stability. Specifically, ultra-thin films of GeTe was grown on top of a SiO 2 cavity with a bottom palladium reflector in a Fabery-Perot type design. This enhances the color tuning when a double-layer anti-reflection coating with high and low refractive indexes are used on top of the GeTe film. Low sensitivity to incident light angle and polarization without the need of sub-micron lithography, provide the potential for this device to be very useful for portable device applications. The devices with different thickness of GeTe were fabricated to demonstrate green, red, blue colors. Electrical pulses with different periods and duty cycles were used to switch the phase of the GeTe locally using joule heating method for several cycles. After transition, darker green, blue, and purple colors were shown for devices with 8 to 20 nm thickness of GeTe films.

The Lunar Flashlight (LF) mission will send a CubeSat to lunar orbit via NASA’s Space Launch System (SLS) test flight. The LF spacecraft will carry a novel instrument to quantify and map water ice harbored in the permanently shadowed craters of the lunar South Pole. The LF instrument, an active multi-band reflectometer which employs four high power diode lasers in the 1-2 μm infrared band, will measure the reflectance of the lunar surface near water ice absorption peaks. We present the detailed instrument design and system engineering required to deploy this instrument within very demanding CubeSat resource allocations.

There is a high demand for high-performance and low-cost uncooled infrared (IR) detectors. In order to meet this need we are investigating the use of MEMS piezoelectric resonator technology using aluminum nitride (AlN) thin films. Recent research has shown that piezoelectric resonators have the potential to be used as a core element for highly sensitive, low-noise, and lowpower uncooled IR detectors. A novel design of an AlN IR sensor based on piezoelectric bending resonator is described and analyzed in this paper. The detector is constructed by using thermally mismatched materials which stress the resonator and shift the resonance frequency. The IR thermal input is sensed by monitoring the frequency shift induced by the in-plan thermal stress. These designs have the potential for very high sensitivity and are compatible with commercially viable CMOS fabrication technology.

With advances in telecommunications, an increasing number of services rely on high data rate spectrum access. These critical services include banking, telemedicine, and exchange of technical information. As a result, spectrum resources are in ever-greater demand and the radio spectrum has become overly crowded. For efficient usage of spectrum, smart or cognitive radios are sought after. However, current wireless phones can only select a few specific bands. In this paper, we discuss the advantages of reconfigurable radios in not only increasing the efficiency of spectrum usage but also in potentially reducing the cost of wireless handsets and the barriers for new wireless service providers to enter the market. We review available technologies that make the implementation of reconfigurable radios possible and discuss technical challenges that need to be overcome before multistandard reconfigurable radios are put into practice. We also evaluate the ability of reconfigurable radios in reducing entry costs for new competitors in wireless service.

Gallium nitride (GaN) is a wide bandgap semiconductor material and is the most popular material after silicon in the semiconductor industry. The prime movers behind this trend are LEDs, microwave, and more recently, power electronics. New areas of research also include spintronics and nanoribbon transistors, which leverage some of the unique properties of GaN. GaN has electron mobility comparable with silicon, but with a bandgap that is three times larger, making it an excellent candidate for high-power applications and high-temperature operation. The ability to form thin-AlGaN/GaN heterostructures, which exhibit the 2-D electron gas phenomenon leads to high-electron mobility transistors, which exhibit high Johnson's figure of merit. Another interesting direction for GaN research, which is largely unexplored, is GaN-based micromechanical devices or GaN microelectromechanical systems (MEMS). To fully unlock the potential of GaN and realize new advanced all-GaN integrated circuits, it is essential to cointegrate passive devices (such as resonators and filters), sensors (such as temperature and gas sensors), and other more than Moore functional devices with GaN active electronics. Therefore, there is a growing interest in the use of GaN as a mechanical material. This paper reviews the electromechanical, thermal, acoustic, and piezoelectric properties of GaN, and describes the working principle of some of the reported high-performance GaN-based microelectromechanical components. It also provides an outlook for possible research directions in GaN MEMS.

This paper presents a high-contrast electro-optical modulator with record-breaking amplitude modulation index of 27 dB and forward loss of < 3 dB at 1.5 μm. The high contrast is achieved by utilizing slit and surface plasmon polariton resonances in an array of gold lines filled with a phase change material (Germanium Telluride-GeTe). Stacking multiple layers of the phase change plasmonic grating enabled development of the high-index modulator in chip scale dimensions at telecommunication wavelength. Coupling the optical modes of multiple layers results in such a high contrast when GeTe goes through crystallographic phase transition. Phase transition through joule-heating is achieved by employing a matching circuit.

This paper reports on the first demonstration of GaN-based switchable Lamb wave delay lines with >40 dB ON/OFF ratio at several gigahertz frequency range. The high-density two-dimensional electron gas (2DEG) at the interface of AlGaN/GaN heterostructure is used to screen out the lateral electric field and switch off the piezoelectric transducer. By properly selecting the thickness-to-wavelength ratio, the proposed Lamb wave delay line only exhibits zero-order symmetric (S0) and anti-symmetric (A0) Lamb wave mode responses, while providing significant spurious mode reduction up to 26.5 GHz. The Lamb wave delay lines show superior performance in terms of insertion loss and out-of-band rejection over conventional surface acoustic wave (SAW) devices and also achieve a good linearity.

The radio spectrum facilitates a wide variety of applications and services but has become overly crowded, especially in the past two decades. Spectrum resources are in ever-greater demand as new applications emerge and existing ones continue to grow. Reconfigurable radios can mitigate the problems associated with limited spectrum availability and limited open bandwidth by tuning to the frequency and standard that is least heavily used at any given time. In this paper, reconfigurable electronic technologies based on microelectromechanical systems (MEMS) as well as phase change materials are discussed with applications in reconfigurable radios.

We describe a systematic study on quantifying the effects of heterostructure layer stacking upon measured dissipation and quality factors (Qs) of AlN-on-Si coupled-ring breathing mode micromechanical resonators. For the first time, we design and fabricate resonators of the same lateral dimensions but with different stacking layers, namely, Si, AlN-on-Si, and Al-on-AlN-on-Si. With both optical and electrical readout techniques, we measure the fundamental radial-extensional mode and its Q values and compare the observed and simulated energy dissipation. For the same device geometry, the results show that the bare Si device has the highest Q (11,304). Adding an AlN layer leads to a reduction in Q for the AlN-on-Si device by a factor of ∼1.46 compared to bare Si device. Top layer metallization further compromises the Q by a factor of ∼1.85. Modeling of the dissipation processes suggests that neither the thermoelastic damping (TED) nor the anchor loss is the limiting loss factor. A piezoelectric-specific loss mechanism known as charge redistribution loss and interface loss are both possible limiting mechanisms.

This paper covers various sensors and microsystems based on gallium nitride (GaN) micro mechanical (acoustic) resonators. III-nitrides have been a subject of extensive research for power electronics as well as photonics applications but their use in micro-electromechanical systems (MEMS) has been limited. In this paper, basic material properties of this group of materials (GaN, AlGaN, and other III-nitrides) are discussed and their current and future applications in MEMS and Microwave circuits are detailed.

The holy grail of seamless monolithic integration of MEMS with supporting circuitry has driven the development of electromechanical devices in compound semiconductors. Piezoelectricity manifests itself in the majority of compound semiconductors due to the inherent crystal asymmetry of the two or more atomic species comprising the material. The interaction of acoustic waves with charge carriers in these piezoelectric semiconductors was investigated as early as 1953 [1] and revealed interesting phenomena such as reduced electron effective mass due to phonon drag [2] and wave amplification and velocity shift in the presence of free carriers [3–5]. The unique properties of piezoelectric semiconductors and their implications for resonant devices are explored in this chapter.

This paper presents the analysis, design, fabrication, and the first measured results demonstrating the use of gallium nitride (GaN)-based micromechanical resonator arrays as high-sensitivity, low-noise infrared (IR) detectors. The IR sensing mechanism is based on monitoring the change in the resonance frequency of the resonators upon near IR radiation. The resonators are characterized for their RF and thermal performance and exhibit a radiant responsivity of 1.68%/W, thermal time constant on the order of 556 μs, and an average IR responsivity of -1.5% when compared with a reference resonator, for a 100 mK radiation-induced temperature rise. An analysis of the design of the devices is presented as a path toward better design, specifically, for low thermal noise equivalent temperature difference in the long wavelength IR spectrum.

A multigigahertz AlGaN/GaN resonant body transistor (RBT) is reported, wherein the mechanical resonance and electrical signal modulation are achieved simultaneously. A 175-Å-thick AlGaN layer is used as the piezoelectric transduction layer, and the 2-D electron gas present at the AlGaN/GaN interface is employed as the bottom electrode as well as the transistor conducting channel. The carrier concentration of the channel is modulated when the device undergoes acoustic strain. A quality factor of 250 and acoustic transconductance of 25 μS is achieved at resonance frequency of 4.23 GHz, marking the highest frequency and highest transconductance reported to date for GaN-based RBTs. The frequency×Q of this device is among the best reported for GaN-based resonators.

We report on the characterization of thin-film near and short wavelength infrared absorbers comprised of carbon nanotubes dispersed in a polymer. Charged nanodiamond particles are used to effectively and uniformly disperse the carbon nanotubes in the polymer matrix, leading to a very homogenous film. Using this new technique, we demonstrate an infrared absorption of up to 95% in films with thicknesses . This remarkably high absorption is the result of low reflection off the surface and high absorption across the film thickness. The complex refractive index of the films is extracted using an effective media approximation. Calculations show the film has a wide angle for high absorption and is polarization independent. These films are easy to fabricate, robust and damage-resistant, and are compatible with post-processing techniques. These films can be used as the coating layer to boost the efficiency of uncooled infrared sensors and solar-thermal energy harvesters.

We report on the first time observation of acoustoelectric (AE) effect from the interaction of acoustic Lamb waves and two-dimensional electron gas (2DEG) in an AlGaN/GaN heterostructure. Micro-fabricated Lamb wave delay lines are used to launch and guide travelling acoustic waves through the 2DEG region, resulting in a DC current flow between two ohmic contacts positioned on the delay line. The Lamb wave delay line shows much better acoustic transmission efficiency than the conventional surface acoustic wave (SAW) counterpart. The dependence of AE current on RF power and frequency is also verified.

A GaN bulk acoustic wave resonator is presented in this letter, showing fundamental thickness-mode resonance at 2.18 GHz, with a quality factor ( Q ) of 655 and a coupling coefficient ( \({k_{t}}^{2}\) ) of 1%. The resonator is integrated with an AlGaN/GaN high electron mobility transistor (HEMT); the integrated resonator/HEMT structure is coated with a silicon dioxide (SiO2) passivation layer. It is shown that a 400-nm-thick SiO2 layer reduces the temperature coefficient of frequency (TCF) of the GaN-based resonator by >50%, while improving Q and \({k_{t}}^{2}\) of the fundamental thickness-mode resonance. The effect of SiO2 passivation layer is studied on \({k_{t}}^{2}\) , Q , and TCF of the device. Furthermore, the effects of temperature and input RF power on the resonator performance are characterized.

In this paper, we explore the piezoelectric transduction of in-plane flexural-mode silicon resonators with a center frequency in the range of 1.3-1.6 MHz. A novel technique utilizing oxide-refilled trenches is implemented to achieve efficient temperature compensation. These trenches are encapsulated within the silicon resonator body so as to protect them during the device release process. By using this method, we demonstrate a high-Q (> 19 000) resonator having a low temperature coefficient of frequency of