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The expansion of the market for wireless communications and sensors has led to the recent increase in demand for highly integrated MMICs for millimetre-wave wireless applications. These applications require MMICs that offer low cost, high integration, high functionality and high performance as well as simpler, more rapid development. An effective way of meeting these requirements and realising highly integrated MMICs is by employing multilayer three-dimensional (3-D) MMIC technology. The research work described in this thesis presents the modelling and characterisation of newly developed passive components such as coplanar waveguides (CPWs), thin-film microstrips (TFMSs) and transition transmission line structures using 3-D multilayer technology. These structures have been developed with low losses in mind, along with variable characteristic impedances and miniaturised size. With the knowledge obtained from the design and optimisation of CPW and TFMS transmission lines, new and improved compact CPW-to-TFMS transitions have been successfully achieved. Accurate electromagnetic modelling was carried out using the 2.5-dimensional ADS Momentum simulator. Newly improved fabrication techniques were employed to produce reported compact microwave components and circuits, in order to lower cost and simplify the process. Compact MMIC components were fabricated using a seven-layer fabrication procedure on semi-insulating GaAs substrate where pseudomorphic high electron mobility transistors (pHEMTs) pre-fabricated by the manufacturer. High frequency on-wafer RF measurements were carried out using Agilent 8510 series vector network analysers (VNAs). In-depth analysis and comparisons between the simulated and measured results are provided. Analysis of active MMIC components was achieved by developing small-signal equivalent circuits of the GaAs pHEMTs, and knowledge extracted from this analysis was employed in the development of large signal models of the pHEMT devices. Furthermore, the design and characterisation of a few MMIC circuits, such as limiters and amplifiers, demonstrates the integration of multilayer CPW passive components with prefabricated pHEMTs. These components are compatible with RF systems-on-chip sub-systems providing low cost, low loss performance with their ease of fabrication.

With the demand of modern wireless communications, monolithic microwave integrated circuit (MMIC) has become a very promising technique as it is mass-productive, low loss and highly integrated. Microstrip and Coplanar Waveguide (CPW) are both widely used in MMIC. Particularly, CPW has seen a rapid increase on research works recent years due to its unique capability including having less parasitic contribution to the circuit. In this thesis, a novel 3-D multilayer CPW technique is presented. Semi-insulating (S.I.) GaAs substrate, polyimide dielectric layers and Titanium/Gold metal layers are employed in this five-layer structure. The active devices are based on GaAs pHEMTs technology provided by Filtronic Compound Semiconductor Ltd. The fabricated components are simulated and characterized by Agilent Advanced Design System (ADS) and Momentum E.M simulator. A novel Open-short-through de-embedding technique is developed and applied to the passive circuits in order to reduce the impact of pads on probing. A new library of components and circuits are built in this work. Various structures of 3-D CPW transmission lines are designed and characterized to demonstrate the low-loss and highly compact characters. Meanwhile, the influence of various combinations of metal and dielectric layers is studied in order to provide designers with great flexibility for the realization of novel compact transmission lines for 3D MMICs. The effect of temperature on the performance of the transmission lines has also been investigated. Moreover, a set of compact capacitors are designed and proven to have high capacitance density with low parasitics. Finally, based on the extraction of pHEMT parameters from circuit characterization and analysis program (IC-CAP), RF switch and active filter MMICs have been designed and simulated to provide references for further development of 3-D multilayer CPW circuits.

The development of monolithic microwave integrated circuits (MMICs) has enabled the expansion of multiple circuit elements on a single piece of semiconductor, enclosed in a package with connecting leads. Attributable to the widespread use of wireless circuits and sub-systems, MMICs meet stringent demands for smaller chip area, low loss and low cost. These require highly integrated MMICs with compact features. This thesis provides valuable insight into the design of compact multifunctional MMICs using three-dimensional (3-D) multilayer technology. The proposed technology offers compact, hence low-cost solutions, where all active and passive components are fabricated vertically on the same substrate and no expensive via hole or backside processing is required. The substrate used in this work contains pre-fabricated 0.5 µm pseudomorphic High Electron Mobility Transistor (pHEMT) GaAs active devices. The performances of the uncommitted and committed pHEMTs are compared in terms of their DC, small-signal and large-signal RF measurements and modelling results. Committed pHEMT refers to the pHEMT that is connected to multilayer circuit, whereas uncommitted pHEMT is not. The effect of integrating committed pHEMTs with multilayer passive components is studied and the suitability of the multilayer fabrication processing is assessed. Using this technology, two pHEMT Schottky diodes with 120 µm and 200 µm gate widths are designed, fabricated and extensively characterised by I-V, C-V and S-parameter measurements. The information gained from the measurements is then used to extract all unknown equivalent circuit model parameters using high-frequency on-wafer microwave probing. The measured results showed good agreement with the modelled ones over the frequency range up to 40 GHz. Preliminary demonstrations of the use of these pHEMT Schottky diodes in microwave limiter and detector circuit applications are also discussed, showing promising results. Finally, the implementation of 3-D multilayer technology is shown for the first time in single-pole single-throw (SPST) and single-pole double-throw (SPDT) switches design by utilising the pre-fabricated pHEMTs. The design and analysis of the switches are demonstrated first through simulation using TriQuint's Own Model - Level 3 (TOM3). Three optimised SPST and SPDT pHEMT switching circuits which can address applications ranging from L to X bands are successfully fabricated and tested. The performance of the pHEMT switches is comparable to those of the current state-of-the-art, while simultaneously offering compact circuits with the advantages of integration with other MMIC components. All works reported in this thesis should facilitate foundry design engineers towards further development of 3-D multilayer technology.

<p dir="ltr">Rapid evolution in wireless technology and the increasing demand for high bandwidth communication for 5G/6G and the Internet of Things (IoT) have necessitated a growing number of components in radio front-end modules in an increasingly overcrowded radio frequency (RF) spectrum. Low-cost ad-hoc radios have drawn consumer interest, enabling new devices like microelectromechanical (MEMS) resonators for on-chip clocking, frequency-selective filters, RF signal processing, and spectral sensing for their small footprint and low power consumption. Gallium nitride (GaN) is an attractive electromechanical material due to its high coupling coefficient, acoustic velocity, and low viscoelastic losses. These benefits enable high-Q MEMS resonators in GaN monolithic microwave integrated circuits (MMICs) with scaling capability up to mm-wave frequencies, making this technology platform a contender for high-performance programmable radios in RF/mm-wave, sensors for harsh environments, and information processing in quantum systems.</p><p dir="ltr">The bias-dependent control mechanism of the 2D electron gas (2DEG) in GaN heterostructures can be exploited to design different switchable and tunable devices for reconfigurable MEMS components. This work presents, for the first time, a comprehensive study of the electromechanical performances of different transduction mechanisms in switchable GaN MEMS resonators. A unique OFF-state shunt design, where the 2DEG in an AlN/GaN heterostructure is utilized to control electromechanical transduction in Lamb mode resonators, is also experimentally demonstrated in this work. To make a valid comparison among switchable transducers, equivalent circuit models are developed to extract key parameters from the measurements by fitting them in both ON and OFF states. The switchable transducer with Ohmic interdigitated transducers (IDTs) and Schottky control gate shows superior performance among the designs under consideration with complete suppression of the mechanical mode in the OFF state and a maximum frequency-quality factor product of 5x10¹²s^-1 and a figure-of-merit of 5.18 at 1GHz in the ON state.</p><p dir="ltr">Over the past few years, there have been numerous efforts to scale the frequencies of MEMS devices in the GaN platform towards mm-wave frequencies. However, challenges remain due to the multi-layer thick buffer, typical in the growth of GaN epilayer on a substrate. This work presents the investigation of SweGaN QuanFINEbuffer-free and ultrathin GaN-on-SiC for the performance of surface acoustic wave (SAW) devices beyond 10GHz. Finite element analysis (FEA) is performed to find the range of frequencies for the Sezawa mode in the structure. Transmission lines and resonators are designed, fabricated, and characterized. Modified Mason circuit models are developed for each class of devices to extract critical performance metrics and benchmark with the state-of-the-art and theoretical limits for GaN. Sezawa modes are observed at frequencies up to 14.3GHz, achieving a record high in GaN MEMS to the best of our knowledge. A maximum piezoelectric coupling of 0.61% and frequency-quality factor product of 6x10¹²s^-1 are achieved for Sezawa resonators at 11GHz, with a minimum propagation loss of 0.26dB/λ for the two-port devices. The devices also exhibit high linearity with input third-order intercept points (IIP3) of 65dBm at 9GHz.</p><p dir="ltr">This work also investigates tunable acoustoelectric (AE) devices in the QuanFINE platform, leveraging its inherent 2DEG in the AlGaN/GaN heterostructure. Using 9.7GHz Sezawa mode acoustic delay lines, we report the highest frequency of AE in GaN to date. Active and passive AE devices are designed for voltage-dependent non-reciprocity and propagation loss without modification to the standard process for the High Electron Mobility Transistors (HEMTs) in MMICs. Drain/source Ohmic contacts control the drift velocity of the 2DEG, and the Schottky gate modulates 2DEG carrier concentration, resulting in a 30dB/cm separation between forward and reverse acoustic waves for a 2.56kV/cm lateral DC electric field and a maximum change in propagation loss of 50dB/cm for -5V DC at the control gate, respectively. The QuanFINEtechnology with AlGaN/GaN heterostructure enables a platform for switchable MEMS resonators and tunable acoustoelectric devices in MMICs for reconfigurable front end approaching mm-wave frequencies.</p>