Several applications require the detection of terrestrial UV-C signatures. Efficiency, compactness, environmentally friendly and cost-effective requirements for UV-C–detectors resulted in a research interest in wide-bandgap (WBG) semiconductor-based photovoltaic diodes with a 280 nm cut-off wavelength. Advances in producing group-III-nitride materials allowed the growth of high quality Al[x]Ga[1−x]N, a direct-WBG ternary semiconductor in which the Al mole fraction (x) could be varied, allowing for a tuneable bandgap that made the semiconductor intrinsically 'blind' to longer wavelengths and responsive to selected wavelengths shorter than 360 nm. This dissertation reports on the development and characterization of a tuneable AlGaN-based solar-blind UV-sensitive Schottky photodiode. A fabrication procedure was established using optimized metallization techniques derived from literature. This included metallization schemes, metal thicknesses and annealing methods for metallization of both the ohmic and Schottky contacts for a front-irradiated photodiode. Characterization was done with a newly constructed optoelectronic characterization system. Electrical characterization was performed inside a light-tight shielded enclosure and a software routine aided in applying current–voltage and capacitance–voltage measurements on a Schottky diode. Spectral characterization made use of either a UV source or a visible-to-near-infrared source that was coupled to a monochromator that allowed for wavelength selection. The monochromatic electromagnetic radiation was guided by an optical fibre from the monochromator into the enclosure where the photodiode was irradiated. An additional software routine was developed that allowed for the automation of the spectral characterization. The system was calibrated against standards traceable to the National Institute of Standards and Technology (NIST) by following the photodetector substitution method. The study concluded with the manufacturing of an epoxy wire-bonded front-irradiated four-quadrant detector that was mounted on a commercial microchip carrier. Metal depositions were done through physical contact masks. The quadrants were surrounded with optimized layered ohmic contacts and a quadrant consisted of a thin-film iridium(IV) oxide (IrO₂) as Schottky contact that is UV transmissive with a Au contact on top to which a wire was bonded. Optoelectronic characterization verified that the four-quadrant detector was intrinsically solar-blind and showed good uniformity across the quadrants. Electrical parameters obtained included an average ideality factor of 1.97 ± 0.09, a Schottky barrier height of (1.22 ± 0.08) eV, a reverse leakage current density of (2.1 ± 3.3) nA/cm² and a series resistance of (120 ± 30) Ω. Spectral parameters obtained included a (275 ± 5) nm cut-off wavelength, an average current responsivity at 250 nm of (28 ± 1.0) mA/W with a quantum efficiency of (14 ± 0.5) % and an UV-to-visible and near-infrared rejection ratio between 10³ and 10⁵ for 400 nm to 1100 nm wavelengths. These characteristics allowed for the detector to be used in demonstrating a working solar-blind UV-sensitive electro-optic device. / Dissertation (MSc)--University of Pretoria, 2015. / Physics / Unrestricted

Gallium nitride (GaN) is a semiconductor material highly regarded for visible light generation since it provides the most efficient platform for compact violet, blue, and green light emitters, and in turn, high-quality and ubiquitous white lighting. Despite this fact, the potential of the GaN platform has not been fully exploited. This potential must enable the precise control in the various properties of light, realizing functions beyond the conventional. Simultaneously, the field of the telecommunications is looking for candidate technologies fit for wireless transmission in the next generations of communication. Visible light communication (VLC) may play a significant role in the future of the last mile of the network by providing both a fast internet connection and a high-quality illumination. Hence, a variety of optoelectronic platforms, including distributed-feedback (DFB) lasers, superluminescent diodes (SLDs), and multi-section lasers, can be used to exploit the full potential of GaN while offering unprecedented solutions for VLC and other applications, such as atomic clocks, high-resolution fluorescence microscopy, and on-chip nonlinear processing at visible wavelengths. This dissertation demonstrates green and sky-blue DFB lasers based on GaN, with resolution-limited single-mode emission at wavelengths around 514 nm and 480 nm, side-mode suppression ratio as large as 42.4 dB, and application to up to 10.5 Gbit/s data transmission. Preliminary observations of DFB lasers with emission close to the Fraunhofer lines are presented, offering a pathway for low-background noise applications. Blue-emitting SLDs are used to demonstrate a 3.8 Gbit/s transmitter while achieving spectral efficiency of up 118.2 (mW・nm)/(kA/cm2) in continuous-wave operation. Visual quality is confirmed by coherence length and white light generation. Short-wavelength SLDs have the potential for higher resolution and fluorescence excitation in classical optical coherence tomography and fiber gyroscopes. The demonstration of a two-section green laser diode is presented, achieving coupled-cavity lasing at wavelengths of 514 nm based on an integrated green laser–absorber in self-colliding pulse configuration, operated in continuous-wave electrical injection. The integrated laser offer potential for mode- locked and Q-switched lasing. The integrated laser is suitable for reconfiguration where laser–modulator, laser–absorber, and laser–amplifier are proposed and investigated at green wavelengths.

This manuscript describes the design, development, and implementation of a linear high efficiency power amplifier. The symmetrical Doherty power amplifier utilizes TriQuint's 2nd Generation Gallium Nitride (GaN) on Silicon Carbide (SiC) High Electron Mobility Transistor (HEMT) devices (T1G6001032-SM) for a specified design frequency of 3.6 GHz and saturated output power of 40 dBm. Advanced Design Systems (ADS) simulation software, in conjunction with Modelithic's active and passive device models, were used during the design process and will be evaluated against the final measured results. The use of these device models demonstrate a successful first-pass design, putting less dependence on classical load pull analysis, thereby decreasing the design-cycle time. The Doherty power amplifier is a load modulated amplifier containing two individual amplifiers and a combiner network which provides an impedance inversion on the path between the two amplifiers. The carrier amplifier is biased for Class-AB operation and works as a conventional linear amplifier. The second amplifier is biased for Class-C operation, and acts as the peaking amplifier that turns on after a certain instantaneous power has been reached. When this power transition is met the carrier amplifier's drain voltage is already approaching saturation. If the input power is further increased, the peaking amplifier modulates the load seen by the carrier amplifier, such that the output power can increase while maintaining a constant drain voltage on the carrier amplifier. The Doherty power amplifier can improve the efficiency of a power amplifier when the input power is backed-off, making this architecture particularly attractive for high peak-to-average ratio (PAR) environments. The design presented in this manuscript is tuned to achieve maximum linearity at the compromise of the 6dB back-off efficiency in order to maintain a carrier-to- intermodulation ratio greater than 30 dB under a two-tone intermodulation distortion test with 5 MHz tone spacing. Other key figures of merit (FOM) used to evaluate the performance of this design include the power added efficiency (PAE), transducer power gain, scattering parameters, and stability. The final design is tested with a 20 MHz LTE waveform without digital pre-distortion (DPD) to evaluate its linearity reported by its adjacent channel leakage ratio (ACLR). The dielectric substrate selected for this design is 15 mil Taconic RF35A2 and was selected based on its low losses and performance at microwave frequencies. The dielectric substrate and printed circuit board (PCB) design were also modeled using ADS simulation software, to accurately predict the performance of the Doherty power amplifier. The PCB layout was designed so that it can be mounted to an existing 4" x 4" aluminum heat sink to dissipate the heat generated by the transistors while the part is being driven. The performance of the 3.6 GHz symmetrical Doherty power amplifier was measured in the lab and reported a maximum PAE of 55.1%, and a PAE of 48.5% with the input power backed-off by 6dB. These measured results closely match those reported by design simulations and demonstrate the models' effectiveness for creating a first-pass functional design.

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In today's industrial power converters, converter reliability is essential, and converter topologies are well-established. Without a doubt, the power electronic industry continues to seek efficient power delivery and high power density. Resonant converters, especially LLC converters, have been intensively studied and applied in DC-DC converters. One of the most demanding applications for LLC converters is data centers. To date, LLC Resonant converters, are deployed in many applications for improved efficiency, density, and reliability. With the introduction of WBG devices coupled with the soft switching feature, the switching frequency can be extended beyond Mega-Hertz. With the significant increase in operating frequency, complicated magnetic components can be broken down into a cellular structure, each with a few number of turns. They can be easily implemented using 4-6 layers of PCB windings. Moreover, integrating the cellular cores using flux cancellation can further improve the power density. The proposed integrated magnetics can be automated in the manufacturing process. The magnetic size is reduced at this frequency, and planar magnetics using PCB winding become more relevant. PCB magnetics feature multiple advantages over Litz wire. The benefits are summarized as follows: The labor-intensive manufacturing process can be automated, thus reduction of cost. There is much reduced CM noise by using the shield layer. They have parasitics with much-improved reproducibility in large quantities. PCB windings feature less leakage between transformer windings because of the flexibility of the winding interleaving and the reduced number of turns. There is better thermal management due to the increased surface-to-body ratio. The design has a low profile and high-power density. However, it is not without its own limitations. There are challenges for high frequency PCB-magnetic magnetic design for the LLC converter. Firstly, With the recently developed high frequency core material, a phenomenon referred to as the dimensional resonant is observed. The effects of dimensional resonance were discussed in the literature when using an unusually large core structure; however, it can be observed more frequently under high excitation frequency, particularly with integrated magnetics. This dissertation discusses the dimensional effects of core loss on a PCB-based magnetics structure. A case study is presented on a 3-kW 400-to-48-V LLC prototype running at 1 MHz. The converter utilizes a low-profile matrix of two integrated transformers with a rectangular and thin cross-section area for reduced core loss. Specific solutions are presented. % Secondly, The matrix transformer is suitable for an LLC converter with high output current. However, the matrix transformer also increases the core size and core losses. The core loss degrades the LLC converter's light load and peak efficiency. In this dissertation, We discuss the design process and implementation of the DC-DC stage of the power supply unit for narrow range 48 V data center bus architecture. The optimization takes into account the number of elemental transformers, number of transformer turns, switching frequency, and transformer dimensions, namely winding width and core cross-section area. The optimization process results in a nearly 99% efficient 400-to-48-V LLC with a very high-power density and low profile fully integrated on PCB. A matrix of four transformers is used to reduce the termination loss of the secondary synchronous rectifier and achieve better thermal management. The number of secondary turns is optimized to achieve the best trade-off between winding loss, core loss, and power density. Another challenge arises for magnetic integration when multiple magnetic components with different characteristics come together. For instance, in the case of a transformer and an inductor on the same PCB. The PCB transformer is designed with perfectly interleaved primary and secondary layers to utilize the full PCB layer thickness. As a rule of thumb, the transformer winding layer is designed within 1 to 2 times the skin depth. On the other hand, the inductor's winding lacks interleaving and suffers from high MMF stress on layers. This makes the inductor prone to high eddy currents and eddy loss. Furthermore, this dissertation addresses the challenges associated with the high winding and core loss in the Integrated Transformer-Inductor (ITL). To overcome these challenges, we propose an improved winding design of the ITL by utilizing idle shielding layers for inductor integration within the matrix transformer. This method offers full printed circuit board (PCB) utilization, where all layers are consumed as winding, resulting in a significant reduction in the winding loss of the ITL. Moreover, we propose an improved core structure of the ITL that offers better flux distribution of the leakage flux within the magnetic core. This method reduces the core loss by more than 50% compared to the conventional core structure. We demonstrate the effectiveness of our proposed concepts by presenting the design of the ITL used in a high-efficiency, high-power-density 3-kW 400-to-48-V LLC module. The proposed converter achieves a peak efficiency of 98.7% and a power density of 1500 W/in3. This dissertation presents the concept of matrix inductors to solve such problems. A matrix of four resonant inductors is also designed to reduce the proximity effect between inductor windings and reduce inductor PCB winding loss. The matrix inductor provides a solution for high thermal stress in PCB-based inductors and reduces the inter-winding capacitance between inductor layers. This dissertation solves the challenges in magnetic design in high-frequency DC-DC converters in offline power supplies and data centers. This includes the transformer and inductor of the LLC converter. With the academic contribution in this dissertation, Wide-bandgap devices WBG can be successfully utilized in high-frequency DC-DC converters with Mega-Hertz switching frequency to achieve high efficiency, high power density, and automated manufacturing. The cost will be reduced, and the performance will be improved significantly. / Doctor of Philosophy / Industrial power converters need to be reliable and efficient to meet the power industry's demand for efficient power delivery and high power density. Research should focus on improving existing converter designs to improve fabrication, efficiency, and reliability. Resonant converters have been found to be effective in power conversion, especially in data centers where energy consumption is high. Three-element Resonant converters (LLC) are already used to improve efficiency, density, and reliability. By using Wide Bandgap devices and soft switching, the switching frequency can be extended beyond MHz, simplifying magnetic components and improving power density. The proposed integrated magnetics can be automated during the manufacturing process, further improving power density. At higher frequencies, planar magnetic components made with PCB winding are more effective than Litz wire. They are cheaper to make because of automation, have less common-mode noise, and are more reproducible in large quantities. PCB winding also has a low profile, high-power density, and better thermal management. However, it is not without its own limitations. There are challenges for high frequency PCB-magnetic magnetic design for the LLC converter. Firstly, With the recently developed high frequency core material, a phenomenon referred to as the dimensional resonant is observed. The effects of dimensional resonance were discussed in the literature when using an unusually large core structure; however, it can be observed more frequently under high excitation frequency, particularly with integrated magnetics. This dissertation discusses the effects of core loss on a PCB-based magnetics structure and presents solutions, including a case study on a 3-kW 400-to-48 V LLC prototype running at 1 MHz. Another challenge arises for magnetic integration when multiple magnetic components with different characteristics come together. For instance, in the case of a transformer and an inductor on the same PCB. The PCB transformer is designed with perfectly interleaved winding and low Ohmic loss. On the other hand, the inductor's winding lacks interleaving and suffers from a high proximity field. This makes the inductor prone to high eddy currents and eddy loss. This dissertation presents the concept of matrix inductors to solve such problems. A matrix of four resonant inductors is also designed to reduce the proximity effect between inductor windings and reduce inductor PCB winding loss. The matrix inductor provides a solution for high thermal stress in PCB-based inductors and reduces the inter-winding capacitance between inductor layers. Furthermore, this dissertation addresses the challenges associated with the high winding and core loss in the Integrated Transformer-Inductor (ITL). To overcome these challenges, we propose an improved winding design of the ITL by utilizing idle shielding layers for inductor integration within the matrix transformer. This method offers full printed circuit board (PCB) utilization, where all layers are consumed as winding, resulting in a significant reduction in the winding loss of the ITL. Moreover, we propose an improved core structure of the ITL that reduces the core loss by more than 50% compared to the conventional core structure. We demonstrate the effectiveness of our proposed concepts on a high-efficiency, high-power-density 3-kW 400-to-48-V LLC module. The proposed converter achieves a peak efficiency of 98.7% and a power density of 1500 W/in3. This dissertation solves the challenges in magnetic design in high-frequency DC-DC converters in offline power supplies and data centers. This includes the transformer and inductor of the LLC converter. With the academic contribution in this dissertation, Wide-bandgap devices WBG can be successfully utilized in high-frequency DC-DC converters with Mega-Hertz switching frequency to achieve high efficiency, high power density, and automated manufacturing. The cost will be reduced, and the performance will be improved significantly.

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