Yes / This work develops a novel dynamic load modulation Power Amplifier (PA) circuity that can provide an optimum compromise between linearity and efficiency while covering multiple cellular frequency bands. Exploiting monolithic microwave integrated circuits (MMIC) technology, a fully integrated 1W Doherty PA architecture is proposed based on 0.1 µm AlGaAs/InGaAs Depletion- Mode (D-Mode) technology provided by the WIN Semiconductors foundry. The proposed wideband DPA incorporates the harmonic tuning Class-J mode of operation, which aims to engineer the voltage waveform via second harmonic capacitive load termination. Moreover, the applied post-matching technique not only reduces the impedance transformation ratio of the conventional DPA, but also restores its proper load modulation. The simulation results indicate that the monolithic drive load modulation PA at 4 V operation voltage delivers 44% PAE at the maximum output power of 30 dBm at the 1 dB compression point, and 34% power-added efficiency (PAE) at 6 dB power back-off (PBO). A power gain flatness of around 14 ± 0.5 dB was achieved over the frequency band of 23 GHz to 27 GHz. The compact MMIC load modulation technique developed for the 5G mobile handset occupies the die area of 3.2. / This research was funded by the European Regional Development Fund (FEDER), through COMPETE 2020, POR ALGARVE 2020, Fundação para a Ciência e a Tecnologia (FCT) under i-Five Project (POCI-01-0145-FEDER-030500). This work is also part of the POSITION-II project funded by the ECSEL joint Undertaking under grant number Ecsel-345 7831132-Postitio-II-2017-IA. This work is supported by FCT/MCTES through national funds and when applicable co-funded EU funds under the project UIDB/50008/2020-UIDP/50008/2020. The authors would like to thank the WIN Semiconductors foundry for providing the MMIC GaAs pHEMT PDKs and technical support. This work is supported by the Project TEC2017-88242-C3-2-R- Spanish Ministerio de Ciencia, Innovación e Universidades and EU-FEDER funding.

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.

Rapid advancement in wireless communications over the years has been the driving force for many novel technologies providing compact and low cost solutions. Recent development of multilayer coplanar waveguide (CPW) MMIC technology promises realization of 3D MMIC in which large area-occupying passive components are translated from horizontal into vertical configuration resulting compact structure. The other main advantages of this technology are elimination of via-holes and wafer-thinning giving alternative performance solution, if not better, from the traditional MMIC. In this thesis, thermal and small-signal characteristics of prefabricated AlGaAs/InGaAs pseudomorphic high electron mobility transistors (pHEMTs) on semi-insulating (S.I.) GaAs substrate incorporated in the 3D MMIC technology have been analysed and modelled for the first time. A comprehensive small-signal parameter extraction procedure has been successfully developed which automatically determines the device small-signal parameters directly from the measured S-parameters. The developed procedure is unique since it provides a great deal of data on measured devices over a wide bias, temperature and frequency range for future incorporation of different active devices for the 3D MMIC technology and provides a first hand knowledge of how the multilayer structure will affect the performance of pre-fabricated pHEMTs. The extracted small-signal models of both pre- and post- multilayer processed pHEMTs have been compared and validated to the RF S-parameters measurements. The main focus was drawn upon the temperature dependent model parameters and how the underlying physics of the transistors behave in response to the change of temperature. These novel insights are especially valuable for devices designed specifically for high power applications like power amplifiers where tremendous heat could be generated. The data can also be interpreted as a way to optimise the multilayer structure, for example, alternative material with different properties can be implemented. The governing physics affecting device performance are also modelled and discussed empirically in details through extracted device parameters. These investigations would assist in the development of reliable, efficient and low cost production of future compact 3D multilayer CPW MMICs.