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This work presents a novel LiNbO3/SiO2/SiC piezoelectric-on-insulator (POI) platform for monolithic synthesis of high-performance, multiband, and compact surface acoustic wave (SAW) filters on a single die. By means of the frequency tunability of SAW technology, simultaneous excitation of shear horizontal (SH)-SAW and longitudinal leaky SAW (LLSAW) modes that possess different velocities makes possible envisioning scalable operation frequencies (1–6 GHz). After systematical simulation, optimized 32° Y-LiNbO3 (300 nm)/SiO2 (200 nm)/SiC structure exhibits outstanding capabilities in delivering wideband, low-loss, and thermally stable devices. The implemented resonators show scalable resonance from 1.15 to 5.71 GHz, where a typical SH-SAW resonator exhibits a large electromechanical coupling factor ( $k_{\textbf {eff}}^{\textbf {2}}$ ) of 24.7%, a maximum quality factor (Bode- ${Q}_{\text {max}}$ ) of 1300, and an excellent temperature coefficient of frequency (TCF) of −14 ppm/°C, while the LLSAW resonator demonstrates a decent $k_{\textbf {eff}}^{\textbf {2}}$ of 14.0%, a Bode- ${Q} _{\textbf {max}}$ of 1000, and a figure of merit of 138. The fabricated filters show diverse center frequencies ( ${f}_{\textbf {c}}$ ) ranging from 1.77 to 5.6 GHz, 3-dB fractional bandwidths (FBWs) between 5.68% and 16.41%, a minimum insertion loss (IL) between 0.37 and 1.79 dB, an out-of-band (OoB) rejection above 25 dB, and a TCF of ${f}_{\textbf {c}}$ between −10 and −58 ppm/°C. This work potentially provides a cost-effective multiband filtering solution for 5G applications.

Featuring a pronounced leakage suppression of acoustic energy, the construction of multilayer stacks offers an effective avenue for surface acoustic wave (SAW) devices to remain competitive in fifth-generation (5G) applications. While the target modes experience significant enhancement due to the waveguide effect, transverse spurious responses are also confined and amplified, leading to severe in-band ripples and limited rejection in the corresponding filter. Guided by the slowness dispersion analysis, commercially available AT-Quartz was identified as the optimal substrate that inherently suppresses these unwanted modes without additional countermeasures. The mechanism of slowness manipulation by combining several substrates including AT-Quartz, Si, SiC, and Sapphire with LiTaO3 was studied through the finite-element method (FEM), validating the superior potential of AT-Quartz for transverse mode suppression. Furthermore, the curvature dispersion characteristic of the slowness curve on the $42^{\circ }{Y}$ – ${X}$ LiTaO3 (LT)/AT-Quartz platform as functions of ${h}_{\text {LT}}/\lambda $ and ${h}_{\text {Al}}/\lambda $ were investigated, which revealed a substantial parameter space devoid of spurious responses. Under the guidance of the suppression window, a series of spurious-free resonators with pure spectrum were successfully implemented without any treatment of interdigital transducer (IDT), with a typical one maintaining a remarkable maximum Bode- ${Q}$ value of 2891 and a large figure of merit (FoM) of 334. The demonstrated LT/AT-Quartz platform has shown strong potential for high-performance, spurious-free, and compact acoustic components for front-end applications.

Surface acoustic wave (SAW) filters with improved frequency and bandwidth are highly demanded in the 5G system to cater for the high-throughput wireless communication. This work demonstrates a low-loss SAW filter using shear horizontal (SH) mode in 15° Y-X LiNbO3 (LN)/SiC heterostructure, successfully covering full n79 band. Resonators were investigated theoretically and experimentally to optimize the designing parameters of LN film. The measured resonators with varying LN thickness ( ${h}_{\text {LN}}\text {)}$ exhibit scalable resonant frequencies from 4478 to 5239 MHz with high phase velocity ( ${V}_{\text {p}}\text {)}$ over 4200 m/s, large coupling coefficient ( ${K}^{{2}}\text {)}$ between 16.0% and 28.3% and near spurious-free response. The fabricated filter shows a center frequency ( ${f}_{\text {c}}\text {)}$ of 4693 MHz, a 3-dB fractional bandwidth (FBW) of 16.0%, a minimum insertion loss ( ${\mathrm {IL}}_{\min }$ ) of 0.70 dB and an out-of-band rejection of 17.8 dB. This is the first report of a SAW filter covering n79 full band, which can provide a valuable reference for advancing SAW application in 5G new radio (NR).

There is less “clearance” for the antenna for full-screen mobile phones, which is a higher challenge to antenna miniaturization in radio frequency. Magnetoelectric (ME) antenna stands out as a promising solution to antenna miniaturization in the fifth generation (5G) wireless communication technology for its superior miniaturization capability compared with conventional electrical antennas. Although many structures of ME antennas have been proposed, their resonant frequency stays below 3 GHz, which cannot satisfy 5G communication. Here, we report a magnetoelectric antenna with 445 nm AlN coupled with FeGaB/SiO2 multilayers based on the solidly mounted resonator (SMR) structure with Bragg reflectors of SiO2/W/SiO2/W/SiO2, which resonates at 4.97 GHz consistent with theoretical analysis of the reflection performance of the ME antenna. The far-field radiation performance with different FeGaB thicknesses and near-field polarization of the ME antenna were tested. The experimental data demonstrate that the SMR-based ME antenna with 270 nm thick FeGaB film displays the highest far-field performance with a gain of −25.1 dBi, quality factor of 1207, and radiation efficiency of 0.53% with a circular resonating disk of 300 µm in diameter. Such a compact and high-frequency ME antenna is potentially suitable for the 5G wireless communication systems.

Two ultracompact bandpass (UCB) filters have been proposed. To the best of the authors’ knowledge, the filter I is the minimum size microwave bandpass filter ever reported. The proposed circuits successfully integrate an integrated passive device (IPD) technique-based high-Q capacitor and energy coupling circuit with a printed circuit board (PCB) substrate. By creating both separate electric and magnetic coupling path (SEMCP) and source-load coupling path, two transmission zeros (TZs) on the stopband are generated. A wide stopband suppression and good passband selectivity are achieved by this technique. New energy coupling circuits are designed to reduce energy coupling losses of the compact filters; therefore, both compact size and low insertion loss (IL) can be achieved by the filters. Two particular examples of the filter are implemented and experimentally demonstrated. The results show a filter with a size of $0.55\times 1.38$ mm2 ( $5.2\times 10^{-4}~\lambda _{g}^{2}$ at 3.7 GHz), an IL of 2.2 dB at center frequency and two TZs on the upper stopband. The other filter demonstrated with a size of $2.1\times 1.32$ mm2 ( $2.7\times 10^{-3}~\lambda _{g}^{2}$ at 4.5 GHz), an IL of 1.81 dB at the center frequency and two controllable TZs on the stopband.

As a promising material alternative, lithium niobate (LN)-based heterostructure has recently received growing recognition in the field of radio frequency (RF) wideband filtering. However, the corresponding surface acoustic wave (SAW) devices still encounter spurious transverse modes interference and deficient thermal stability, which hinder their capability to process the already crowded spectrums. This work presents spurious-free and near-zero drift shear horizontal SAW (SH-SAW) resonators utilizing an LN-on-quartz (Qz) platform. Guided by the finite element method, we first analyzed the slowness dispersion for various SAW materials. The proposed LN/SiO2/69°Y-90°X Qz heterostructure exhibits superiority in suppressing spurious transverse modes, which has been theoretically and experimentally evidenced by comparing its passband behavior with the performances of SH-SAW resonators employing typical silicon and silicon carbide substrates. Furthermore, with 69°Y-90°X Qz, a near-zero temperature coefficient of antiresonance frequency of -1.42-ppm/°C was successfully realized in LN heterostructure-based devices for the first time. The demonstrated LN/SiO2/Qz platform herein can facilitate gigahertz, spurious-free, and stable RF wideband applications.

The 5G communication system has experienced a substantial expansion of the spectrum, which poses higher requirements to radio frequency (RF) filters in enhancing their operating frequencies and bandwidths. To this end, this work focused on solving the filtering scheme for challenging 5G n77 and n78 bands and successfully implemented the corresponding spurious-free surface acoustic wave (SAW) filters exploiting large-coupling shear horizontal (SH) modes based on X-cut LiNbO3 (LN)/silicon carbide (SiC) heterostructure. Here, we initially investigated the suppression methods for spurious modes theoretically and experimentally and summarized an effective normalized LN thickness ( ${h}_{\text {LN}}/\lambda {)}$ range of 0.15–0.30 for mitigating Rayleigh modes and higher order modes, as well as tilted interdigital transducers (IDT) by about 24° for eliminating transverse modes. Resonators with wavelengths ( $\lambda {)}$ from 0.95 to $2 ~\mu \text{m}$ were also fabricated, showing a scalable resonance from 2.48 to 4.21 GHz without any in-band ripple. Two filters completely meeting 5G n77 and n78 full bands were finally constructed, showing center frequencies ( ${f}_{c}{)}$ of 3763 and 3560 MHz, 3-dB fractional bandwidths (FBW) of 24.8% and 15.6%, and out-of-band (OoB) rejections of 18.7 and 28.1 dB, respectively. This work reveals that X-LN/SiC heterostructure is a promising underpinning material for SAW filters in 5G commercial applications.