Passive and active components development for broadband applications

Recently, GaN HEMTs have been proven to have numerous physical properties, resulting in transistors with greatly increased power densities when compared to the other well-established FET technologies. This advancement spurred research and product development towards power-band applications that require both high power and high efficiency over the wide band. Even though the use of multiple narrow band PAs covering the whole band has invariably led to better performance in terms of efficiency and noise, there is an associated increase in cost and in the insertion loss of the switches used to toggle between the different operating bands. The goal, now, of the new technology is to replace the multiple narrow band PAs with one broadband PA that has a comparable efficiency performance. In our study here, we have investigated a variety of wide band power amplifiers, including class AB PAs and their implementation in distributed and feedback PAs.Additionally, our investigation has included switching-mode PAs as they are well-known for achieving a relatively high efficiency. Besides having a higher efficiency, they are also less susceptible to parameter variations and could impose a lower thermal stress on the transistors than the conventional-mode PAs. With GaN HEMTs, we have demonstrated: a higher than 37 dBm output power and a more than 30% drain efficiency over 0.02 to 3 GHz for the distributed power amplifier; a higher than 30 dBm output power with more than a 22% drain efficiency over 0.1 to 5 GHz for the feedback amplifier; and at least a 43 dBm output power with a higher than 63% drain efficiency over 0.05 to 0.55 GHz for the class D PA. In many communication applications, however, achieving both high efficiency and linearity in the PA design is required. Therefore, in our research, we have evaluated several linearization and efficiency enhancement techniques.We selected the LInear amplification with Nonlinear Components (LINC) approach. Highly efficient combiner and novel efficiency enhancement techniques like the power recycling combiner and adaptive bias LINC schemes have been successfully developed and verified to achieve a combined high efficiency with a relatively high linearity

GaN-based HEMTs for Cryogenic Low-Noise Applications

Radio-astronomy deals with signals and radiations of extremely weak intensity. Also, it requires robust and rugged technologies able to sustain and prevent the Radio Frequency Interferences (RFI). Complying with the required high sensitivity, Low Noise Amplifiers (LNAs) operating at cryogenic temperatures are key elements in radio astronomy instrumentation. Thus far, advanced semiconductor technologies but with limited power-handling capabilities have been traditionally employed as LNAs. Over the past decades, Gallium Nitride (GaN)-based high electron mobility transistors (HEMTs) were demonstrated at room temperature to offer a combination of both excellent low-noise operation and a superior high-power handling performance compared to other materials. In addition, a number of studies indicated a promising potential for the GaN technology to operate at cryogenic temperatures. However, the cryogenic noise performance of the GaN-HEMTs remained unexplored so far.This thesis investigates the potential of GaN–based HEMTs for low-noise operation at these cryogenic temperatures. Established characterization and modeling approaches were employed for this purpose. As a main result, this work reveals a first estimation of the noise performance of GaN-HEMTs at cryogenic temperatures of ~10 K which compares to other more advanced technologies in this field. This was achieved through the extraction of a model, based on experimental noise measurements, describing the microwave noise behavior at cryogenic temperatures at the device level. The model predicts the noise contribution of GaN-HEMTs at cryogenic temperatures with respect to the frequency of operation, the dissipated power, and the total periphery of the device. Hence, it constitutes the basis for the design of future GaN-based LNAs which fulfill the different requirements set by the demanding cryogenic applications.The extracted cryogenic noise model was used to identify and analyze the role of the different physical parameters of the device, over which a technological control might be possible in the future in order to improve the assessed noise performance of the cryogenic GaN-HEMTs. From that perspective, GaN-HEMTs featuring superconducting Niobium (Nb)-gates were demonstrated for the first time. The successful integration of superconducting Nb-gates into AlGaN/GaN HEMTs was demonstrated on different samples, showing a suppression of the gate resistance independently of the width and length of the gate below a critical temperature \u1d447\u1d450 < 9.2 K. The superconductivity of the gate leads to the cancellation of the associated noise contribution. Comparing the noise performance of the resulting devices to that of the conventional Gold (Au)-gated GaN-HEMTs, it was concluded that further management of the device’s self-heating is required to enable the full potential of the Nb-gate by maintaining its superconductivity while operating at optimum-noise bias conditions

Caracterização, modelação e compensação de efeitos de memória lenta em amplificadores de potência baseados em GAN HEMTS

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have emerged as the most compelling technology for the transmission of highpower radio-frequency (RF) signals for cellular mobile communications and radar applications. However, despite their remarkable power capabilities, the deployment of GaN HEMT-based RF power amplifiers (PAs) in the mobile communications infrastructure is often ruled out in favor of alternative siliconbased technologies. One of the main reasons for this is the pervasiveness of nonlinear long-term memory effects in GaN HEMT technology caused by thermal and charge-trapping phenomena. While these effects can be compensated for using sophisticated digital predistortion algorithms, their implementation and model-extraction complexity—as well as the power necessary for their real-time execution—make them unsuitable for modern small cells and large-scale multiple-input multiple-output transceivers, where the power necessary for the linearization of each amplification element is of great concern. In order to address these issues and further the deployment of high-powerdensity high-efficiency GaN HEMT-based RF PAs in next-generation communications and radar applications, in this thesis we propose novel methods for the characterization, modeling, and compensation of long-term memory effects in GaN HEMT-based RF PAs. More specifically, we propose a method for the characterization of the dynamic self-biasing behavior of GaN HEMTbased RF PAs; multiple behavioral models of charge trapping and their implementation as analog electronic circuits for the accurate real-time prediction of the dynamic variation of the threshold voltage of GaN HEMTs; a method for the compensation of the pulse-to-pulse instability of GaN HEMT-based RF PAs for radar applications; and a hybrid analog/digital scheme for the linearization of GaN HEMT-based RF PAs for next-generation communications applications.Os transístores de alta mobilidade eletrónica de nitreto de gálio (GaN HEMTs) são considerados a tecnologia mais atrativa para a transmissão de sinais de radiofrequência de alta potência para comunicações móveis celulares e aplicações de radar. No entanto, apesar das suas notáveis capacidades de transmissão de potência, a utilização de amplificadores de potência (PAs) baseados em GaN HEMTs é frequentemente desconsiderada em favor de tecnologias alternativas baseadas em transístores de silício. Uma das principais razões disto acontecer é a existência pervasiva na tecnologia GaN HEMT de efeitos de memória lenta causados por fenómenos térmicos e de captura eletrónica. Apesar destes efeitos poderem ser compensados através de algoritmos sofisticados de predistorção digital, estes algoritmos não são adequados para transmissores modernos de células pequenas e interfaces massivas de múltipla entrada e múltipla saída devido à sua complexidade de implementação e extração de modelo, assim como a elevada potência necessária para a sua execução em tempo real. De forma a promover a utilização de PAs de alta densidade de potência e elevada eficiência baseados em GaN HEMTs em aplicações de comunicação e radar de nova geração, nesta tese propomos novos métodos de caracterização, modelação, e compensação de efeitos de memória lenta em PAs baseados em GaN HEMTs. Mais especificamente, nesta tese propomos um método de caracterização do comportamento dinâmico de autopolarização de PAs baseados em GaN HEMTs; vários modelos comportamentais de fenómenos de captura eletrónica e a sua implementação como circuitos eletrónicos analógicos para a previsão em tempo real da variação dinâmica da tensão de limiar de condução de GaN HEMTs; um método de compensação da instabilidade entre pulsos de PAs baseados em GaN HEMTs para aplicações de radar; e um esquema híbrido analógico/digital de linearização de PAs baseados em GaN HEMTs para comunicações de nova geração.Programa Doutoral em Telecomunicaçõe

Distributed Circuit Analysis and Design for Ultra-wideband Communication and sub-mm Wave Applications

This thesis explores research into new distributed circuit design techniques and topologies, developed to extend the bandwidth of amplifiers operating in the mm and sub-mm wave regimes, and in optical and visible light communication systems. Theoretical, mathematical modelling and simulation-based studies are presented, with detailed designs of new circuits based on distributed amplifier (DA) principles, and constructed using a double heterojunction bipolar transistor (DHBT) indium phosphide (InP) process with fT =fmax of 350/600 GHz. A single stage DA (SSDA) with bandwidth of 345 GHz and 8 dB gain, based on novel techniques developed in this work, shows 140% bandwidth improvement over the conventional DA design. Furthermore, the matrix-single stage DA (M-SSDA) is proposed for higher gain than both the conventional DA and matrix amplifier. A two-tier M-SSDA with 14 dB gain at 300 GHz bandwidth, and a three-tier M-SSDA with a gain of 20 dB at 324 GHz bandwidth, based on a cascode gain cell and optimized for bandwidth and gain flatness, are presented based on full foundry simulation tests. Analytical and simulation-based studies of the noise performance peculiarities of the SSDA and its multiplicative derivatives are also presented. The newly proposed circuits are fabricated as monolithic microwave integrated circuits (MMICs), with measurements showing 7.1 dB gain and 200 GHz bandwidth for the SSDA and 12 dB gain at 170 GHz bandwidth for the three-tier M-SSDA. Details of layout, fabrication and testing; and discussion of performance limiting factors and layout optimization considerations are presented. Drawing on the concept of artificial transmission line synthesis in distributed amplification, a new technique to achieve up to three-fold improvement in the modulation bandwidth of light emitting diodes (LEDs) for visible light communication (VLC) is introduced. The thesis also describes the design and application of analogue pre-emphasis to improve signal-to-noise ratio in bandwidth limited optical transceivers

Supply modulated GaN HEMT power amplifiers - From transistor to system

Power amplifiers (PAs) for mobile communication applications are required to fulfil stringent requirements concerning linearity while keeping a high efficiency over a wide power range and bandwidth. To achieve this, a number of advanced PA topologies have been developed, mostly based on either load modulation, such as Doherty PAs or load modulation balanced PAs, or on supply modulation such as envelope tracking or envelope elimination and restoration. Supply modulation has an advantage over other topologies as the power range of high efficiency can be realised over arbitrary bandwidths, only limited by the bandwidth of the PA. This does, however, come at the cost of a significantly more complicated voltage supply. Instead of a static supply voltage, the PA needs to be provided with one which is rapidly changing, requiring a supply modulator capable of powering the PA while modulating its supply voltage. This thesis investigates a number of challenges in supply modulated power amplifiers, ranging from the transistor itself to circuit design and system level considerations and focusses on power levels up to 10 W and frequencies between 1 GHz and 4 GHz. Transistors, as the centre-piece of a PA, determine how well the PA reacts to a changing supply voltage. In this work, the traits that make GaN HEMTs suitable for supply modulated PAs were investigated, and gain variation with changing supply voltage was established as an important parameter. This gain variation is described in detail and its impacts on PA performance are discussed. By comparing transistors in literature, gain variation has been demonstrated to be a prevalent characteristic in transistors with GaN HEMTs showing a very wide range of gain variation. Using a small-signal model based on measurements, the voltage dependent behaviour of the feedback capacitance CGD is, for the first time, identified as the origin of small-signal gain variation. This is traced down to the gate field plate which is commonly used to combat surface trapping effects in GaN HEMTs. With this in mind, two different ways of changing the transistor geometry to reduce the impact of gain variation and thus optimise the transistor for operation in supply modulated PAs are discussed and demonstrated using a 250 nm GaN HEMT. As a result of the non-linearity of the feedback and gate-source capacitances, the input impedance of GaN HEMTs changes with supply voltage and drive power. This prevents the transistor from being matched at all supply voltages and input powers and introduces phase distortion. Using simulation and measurement, the impact of input impedance on linearity and efficiency of supply modulated power amplifiers is demonstrated on a 2.9 GHz 10 W PA. Careful selection of the input impedance allows improvement of AM/PM distortion of a supply modulated PA with little cost in terms of AM/AM and PAE. I Supply modulators have a significant impact on efficiency and linearity of the ET system. One supply modulator topology with the potential to generate a supply voltage with a high modulation bandwidth is the RF modulator in which the input DC voltage is turned into an RF signal and rectified, resulting in an output voltage which depends on the excitation of the PA. While PAs are well understood in every detail, there are gaps in the understanding of RF rectifiers. Using active load-pull/source-pull measurements, intrinsic gate and drain waveforms of a GaN HEMT operated as a rectifier are demonstrated for the first time. This allows in-detail evaluation of the impact of the gate termination in self-synchronous rectifiers. It also allows detailed analysis of the loss mechanisms in rectifiers and formulation of the required impedances to realise efficient self-synchronous operation, resulting in efficiencies exceeding 90% over wide power ranges. Using waveform engineering, a new type of RF modulator, with potentially very high bandwidths, based on even harmonic generation/injection is proposed. The necessary operating conditions of the rectifier part of the modulator are emulated using an active load-pull/source-pull system to successfully demonstrate that the rectifier behaves as predicted. Using a simple demonstrator, preliminary measurements were conducted and the RF modulator was shown to work, reaching efficiencies up to 78%. As PA and supply modulator are combined, they present impedances to each other. These impedances have a significant impact on the behaviour of both sub-systems. A simple way to characterise both the impedance presented to the PA by the modulator and the impedance presented to the modulator by the PA is described. Using a state-of-the-art modulator, these impedances are measured, the modulator impedance is demonstrated to be close to the simulated value. These measurements also demonstrate that the impedances change significantly with the operating conditions