Contributing Towards Improved Communication Systems for Future Cellular Networks

The rapid growth of wireless communications and upcoming requirements of 5G networks are driving interest in the areas from wireless transceivers to sensor nodes. One of the most vital components of the wireless transmitter is the radio frequency power amplifier. A large-signal device model of the transistor is an essential part of the power amplifier design process. Despite the significant developments in large-signal modelling, the models for commercially available devices from the manufacturers are still under continuous development and often lack accuracy. One of the main objectives of this thesis is the validation and extension of an analytic approach as an alternative to conventional large-signal modelling for power amplifier designing. The first contribution is the derivation of new analytical expressions based on the equivalent circuit model, including the extrinsic parasitic elements introduced by the package, to calculate the optimum source and load impedances and to predict the performance of a radio frequency power amplifier. These expressions allow to evaluate the effects of a package on the optimum impedance values and performance. The second contribution is establishing the accuracy of the analytic approach. Harmonic balance simulation is used as the first benchmark to evaluate the method at various bias points and frequencies. The validity of the analytic approach is demonstrated at a frequency of 3.25 GHz for gallium nitride based high power devices with measurement of prototype radio frequency power amplifier designed for the impedance values obtained from the analytic expressions. The third contribution is extending the analytic approach to determine the optimum impedance values for different criteria of maximum gain, linearity and efficiency. The analytic expressions are utilized to gain an understanding of the relationship among the device performance, the elements of devices and package models and I-V characteristics. The wireless sensor networks are essential elements for the realization of the Internet of Things. Sensor nodes, which are the fundamental building blocks of these networks, have to be energy efficient and able to produce energy to reduce the maintenance cost and to prolong their lifetime. The second main aim of the thesis is designing and implementing an ultra-low power autonomous wireless sensor node that harvests the indoor light energy. The forth contribution of this thesis includes a comprehensive comparison of six different solar cell technologies under a controlled light intensity, carried out to determine the best option for indoor light energy harvesting. The power consumption of the node is reduced by selecting the appropriate hardware and implementing a wake-up receiver to reduce the active and idle mode currents. The low power consumption coupled with light energy harvesting significantly prolong the operating lifetime of the node

Composite power semiconductor switches for high-power applications

It is predicted that 80 % of the world’s electricity will flow through power electronic based converters by 2030, with a growing demand for renewable technolo gies and the highest levels of efficiency at every stage from generation to load. At the heart of a power electronic converter is the power semiconductor switch which is responsible for controlling and modulating the flow of power from the input to the output. The requirements for these power semiconductor switches are vast, and include: having an extremely low level of conduction and switching losses; being a low source of electromagnetic noise, and not being susceptible to external Electromagnetic Interference (EMI); and having a good level of ruggedness and reliability. These high-performance switches must also be economically viable and not have an unnecessarily large manufacturing related carbon footprint. This thesis investigates the switching performance of the two main semiconductor switches used in high-power applications — the well-established Silicon (Si)-Insulated-Gate Bipolar Transistor (IGBT) and the state-of-the-art Wide-Bandgap (WBG) Silicon-Carbide (SiC)-Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET). The SiC-MOSFET is ostensibly a better device than the Si-IGBT due to the lower level of losses, however the cost of the device is far greater and there are characteristics which can be troublesome, such as the high levels of oscillatory behaviour at the switching edges which can cause serious Electromagnetic Compatibility (EMC) issues. The operating mechanism of these devices, the materials which are used to make them, and their auxiliary components are critically analysed and discussed. This includes a head-to-head comparison of the two high-capacity devices in terms of their losses and switching characteristics. The design of a high-power Double-Pulse Test Rig (DPTR) and the associated high-bandwidth measurement platform is presented. This test rig is then extensively used throughout this thesis to experimentally characterise the switching performance of the aforementioned high-capacity power semiconductor devices. A hybrid switch concept — termed “The Diverter” — is investigated, with the motivation of achieving improved switching performance without the high-cost of a full SiC solution. This comprises a fully rated Si-IGBT as the main conduction device and a part-rated SiC-MOSFET which is used at the turn-off. The coordinated switching scheme for the Si/SiC-Diverter is experimentally examined to determine the required timings which yield the lowest turn-off loss and the lowest level of oscillatory behaviour and other EMI precursors. The thermal stress imposed on the part-rated SiC-MOSFET is considered in a junction temperature simulation and determined to be negligible. This concept is then analysed in a grid-tied converter simulation and compared to a fully rated SiC-MOSFET and Si-IGBT. A conduction assistance operating mode, which solely uses the part-rated SiC-MOSFET when within its rating, is also investigated. Results show that the Diverter achieves a significantly lower level of losses compared to a Si-IGBT and only marginally higher than a full SiC solution. This is achieved at a much lower cost than a full SiC solution and may also provide a better method of achieving high-current SiC switche

Study and design of topologies and components for high power density DC-DC converters

Size reduction of low power electronic DC–DC converters is a topic of major interest for power electronics which requires the study and design of circuits and components working under redefined requirements. For this purpose, novel circuital topologies provide advantages in terms of power density increment, especially where a single chip design is feasible. These concepts have been applied to design and implement an integrated high step-down multiphase buck converter and to study the miniaturization of a stackable fiflyback architecture. Particular attention has been dedicated to power inductors, focusing on the modeling and measurement of magnetic materials’ hysteresis and core losses

Application of waveform engineering to GaN HFET characterisation and class F design

In this work, the largely theoretical existing research on class F has been extended to include a measured waveform based analysis. The results demonstrate how optimum class F performance can be achieved using real devices and highlights a number of interesting issues that a designer of a class F amplifier should consider.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Particle-Based Modeling of Reliability for Millimeter-Wave GaN Devices for Power Amplifier Applications

abstract: In this work, an advanced simulation study of reliability in millimeter-wave (mm-wave) GaN Devices for power amplifier (PA) applications is performed by means of a particle-based full band Cellular Monte Carlo device simulator (CMC). The goal of the study is to obtain a systematic characterization of the performance of GaN devices operating in DC, small signal AC and large-signal radio-frequency (RF) conditions emphasizing on the microscopic properties that correlate to degradation of device performance such as generation of hot carriers, presence of material defects and self-heating effects. First, a review of concepts concerning GaN technology, devices, reliability mechanisms and PA design is presented in chapter 2. Then, in chapter 3 a study of non-idealities of AlGaN/GaN heterojunction diodes is performed, demonstrating that mole fraction variations and the presence of unintentional Schottky contacts are the main limiting factor for high current drive of the devices under study. Chapter 4 consists in a study of hot electron generation in GaN HEMTs, in terms of the accurate simulation of the electron energy distribution function (EDF) obtained under DC and RF operation, taking into account frequency and temperature variations. The calculated EDFs suggest that Class AB PAs operating at low frequency (10 GHz) are more robust to hot carrier effects than when operating under DC or high frequency RF (up to 40 GHz). Also, operation under Class A yields higher EDFs than Class AB indicating lower reliability. This study is followed in chapter 5 by the proposal of a novel π-Shaped gate contact for GaN HEMTs which effectively reduces the hot electron generation while preserving device performance. Finally, in chapter 6 the electro-thermal characterization of GaN-on-Si HEMTs is performed by means of an expanded CMC framework, where charge and heat transport are self-consistently coupled. After the electro-thermal model is validated to experimental data, the assessment of self-heating under lateral scaling is considered.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Study of Silicon Carbide Power MOSFETs Behaviour in Out-of-SOA Conditions

The need for efficient conversion and control of electrical power in many application areas has rapidly increased the demand for power devices with better and better performances. In order to go beyond the limit imposed by Silicon devices, there has always been a great interest for new materials. In recent years, Silicon Carbide Power devices, mainly power diodes and MOSFETs, have become commercially available and have begun to replace their Silicon counterpart in many application areas. The reason lays in some superior material properties that allow developing higher efficient power systems. Nevertheless, a wider spread of these devices could not be achieved without a deep analysis of the elements that might affect their reliability. The current work deals with the study of SiC Power MOSFETs reliability, with particular focus on short-circuit operation. To achieve this purpose, wide set of experiments has been carried out on commercially available devices, providing both electrical and thermal characterization. Alongside experimental evidences, TCAD simulations have been used to get a full understanding of the inner physical failure dynamics. Eventually, it has been possible to give explanation about SiC Power MOSFETs failure mechanisms. In particular, two different phenomena might occur and both are related to temperature increase inside the device

High Efficiency IGBTs through Novel Three-Dimensional Modelling and New Architectures

New Insulated Gate Bipolar Transistor (IGBT) designs are reliant on simulation tools, such as Sentaurus technology computer-aided design (TCAD) models, which allow for rapid device development that could not be achieved by manufacturing prototypes due to the cost and time associated with fabrication. These simulations are, though, computationally expensive and typically most design engineers develop these TCAD models only in two dimensions. This leads to inaccuracies in the model output since manufactured transistors are inherently three-dimensional (3D). Based upon a commercial IGBT, this thesis begins by outlining the development of a 3D TCAD model using design details provided by the manufacturer. Large variations between the experimental data from the manufactured device and the simulation model lead to the discovery of widespread birds-beaking within the IGBT – an uncontrollable processing defect that the manufacturer was unaware of. This thesis presents a new simulation technique to account for this processing error while minimising computational effort and investigates the consequence of this birds-beak on the reliability of the device. The verified 3D IGBT model was also used to determine an optimum cell design that considered critical 3D effects omitted from previous studies. An extensive literature review for the Reverse-Conducting IGBT (RC-IGBT) is provided. It is shown that despite the benefits of the RC-IGBT, the device suffers from many undesirable design trade-offs that have prevented its widespread use. The RC-IGBT designs that have currently been proposed in literature, either present a trade-off in performance, an inability to be manufactured, or a requirement for a custom gate drive. This thesis presents a new RC-IGBT concept, the ‘Dual Implant SuperJunction (SJ) RC-IGBT’ that addresses these concerns and is manufacturable using current state of the art techniques. The concept and proposed manufacturing method enables, for the first time, a full SuperJunction structure to be achieved in a 1.2kV device. In addition, an investigation into a coordinated switching scheme using both a silicon IGBT and silicon-carbide MOSFET was undertaken, which aimed to improve turn-off losses within the IGBT without sacrificing on-state losses. Thermal modelling of the power devices switching under inductive load was explored as the system was optimised to use a SiC MOSFET in excess of its nominal ratings, reducing the overall system cost.EPSRC Doctoral Training Partnership scheme (grant RG75686

Application of waveform engineering to GaN HFET characterisation and class F design

In this work, the largely theoretical existing research on class F has been extended to include a measured waveform based analysis. The results demonstrate how optimum class F performance can be achieved using real devices and highlights a number of interesting issues that a designer of a class F amplifier should consider