UMD Theses and Dissertations
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Item DEVELOPMENT OF GALLIUM NITRIDE AND INDIUM GALLIUM PHOSPHIDE BETAVOLTAIC AND ALPHAVOLTAIC DEVICES FOR CONTINUOUS POWER GENERATION(2023) Khan, Muhammad Raziuddin A.; Iliadis, Agis; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Betavoltaic devices are p-n/p-i-n junction diodes that use the kinetic energy of beta (electron) particles emitted by beta isotopes to create electron-hole pairs and generate electrical power in a similar way as photovoltaic devices use photons energy to generate electrical power. Unlike photovoltaic devices (solar cells), betavoltaic devices can generate electrical power day and night continuously for decades (12 years with 3H (tritium) and 100 years with Ni63 (nickel-63)), enabling new capabilities that are not possible with photovoltaic devices or current state of the art chemical batteries. New capabilities include decade-long continuous power for unattended sensor, tagging/tracking devices, and other electronics placed in remote areas (underwater, polar region, space, etc) where change/charge of batteries is highly inconvenient or impossible. It is expected that wide band gap semiconductors like GaN with an energy gap of 3.4 eV may provide a better performance in terms of output power and stability under beta radiations. However, GaN semiconductor technology is still maturing in terms of growth and fabrication techniques. InGaP has a moderately wide bandgap of 1.86 eV, but it is well advanced in terms of crystal growth and fabrication techniques. Therefore, our study and research focused on the development (design, fabrication, evaluation) and comparison of a wide band gap (GaN) and a moderately high band gap InGaP, that are considered very promising in betavoltaic applications. Betavoltaic devices were fabricated on three GaN p-i-n structures with different i-layer thicknesses (600 nm, 700 nm and 1 µm). Two GaN p-i-n structures were grown on top of a sapphire substrate and the third GaN structure was grown on top of a bulk GaN substrate. InGaP devices were fabricated on an InGaP n-i-p structure grown on top of a gallium arsenide (GaAs) substrate. Devices were characterized using current - voltage (IV) measurements in the dark, using a UV light source, and under an electron beam stimulus to mimic their performance under real beta isotopes. Dark IV measurements confirmed good quality diodes with low leakage currents, and IV curves under the UV light (365nm, 3.40 eV) source showed a clear photo-response. IV curves under the electron beam irradiation at 16 KeV (average energy emission of Ni63 beta source at 250 mCi/cm2) resulted in the output powers of 3.01 µW/cm2 with an efficiency of 12.63 % for the InGaP device, and 3.32 µW/cm2 with an efficiency of 13.2% for the GaN device. InGaP and GaN devices were also exposed under a 4.5 MeV alpha beam to determine their suitability for an alphavoltaic power source (Direct energy conversion). Both InGaP and GaN devices showed degradation in their MPPs under the direct alpha beam exposure. We also investigated an indirect alpha-photovoltaic (APV) power source by employing ZnS phosphor as an intermediate layer to limit this degradation. This ZnS layer absorbs all the alpha energy and converts it into photons to create EHPs in the semiconductor device to generate electrical output power via indirect energy conversion. We determined that even though APV approach prevented radiation damage in the semiconductor device but the degradation rate of ZnS phosphor is faster compared to the degradation rate of GaN and InGaP devices under direct alpha beam exposure.Item A GALLIUM NITRIDE INTEGRATED ONBOARD CHARGER(2020) Zou, Shenli; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Compared to Silicon metal–oxide–semiconductor field-effect transistors (MOSFETs), Gallium Nitride (GaN) devices have a significant reduction in gate charge, output capacitance, and zero reverse recovery charge, enabling higher switching frequency operation and efficient power conversion. GaN devices are gaining momentum in power electronic systems such as electric vehicle (EV) charging system, due to their promises to significantly enhance the power density and efficiency. In this dissertation, a GaN-based integrated onboard charger (OBC) and auxiliary power module (APM) is proposed for EVs to ensure high efficiency, high frequency, high power density, and capability of bidirectional operation. The high switching frequency operation enabled by the GaN devices and the integration of OBC and APM bring many unique challenges, which are addressed in this dissertation. An important challenge is the optimal design of high-frequency magnetics for a high-frequency GaN-based power electronic interface. Another challenge is to achieve power flow management among three active ports while minimizing the circulating power. Furthermore, the impact of circuit layout parasitics could significantly deteriorate the system interface, due to the sensitivity of GaN device switching characteristics. In this work, the aforementioned challenges have been addressed. First, a comprehensive analysis of the front-end AC-DC power factor correction stage is presented, covering a detailed magnetic modeling technique to address the high-frequency magnetics challenge. Second, the modeling and control of a three-port DC-DC converter, interfacing the AC-DC stage, high-voltage traction battery and low-voltage battery, are discussed to address the power flow challenge. Advanced control methodologies are developed to realize power flow management while maintaining minimum circulating power and soft switching. Furthermore, a new three-winding high-frequency transformer design with improved power density and efficiency is achieved using a genetic-algorithm-based optimization approach. Finally, a GaN-based integrated charger prototype is developed to validate the proposed theoretical hypothesis. The experimental results showed that the GaN-based charging system has the capability of achieving simultaneous charging (G2B) of both HV and LV batteries with a peak efficiency of 95%.Item GALLIUM NITRIDE BASED ONBOARD CHARGER FOR ELECTRIC VEHICLES(2019) Zhang, Zeyu; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Next generation of electric vehicles will be equipped with high power density and high efficiency onboard charging systems with bi-directional power flow. These benefits can be achieved by utilizing the emerging Wide Bandgap devices, planar magnetic solutions, innovative circuit topologies, and advanced control methods to enable MHz switching frequencies without sacrificing efficiency. However, the advantage of higher switching speed is gained at the expense of higher switching losses in both the semiconductors and the magnetics. Conventional circuit topologies, operation modes and control algorithms would no longer be effective in such conditions. Furthermore, the practical implementation of the system has shown more stringent requirements on the controller speed, layout parasitic and the thermal management. In this Ph.D. dissertation work, aforementioned challenges have been addressed, and the proposed innovations have been validated through design and development of a new bi-directional onboard charger using Gallium Nitride devices. The first part of this work has been focused on a thorough characterization of the front-end AC-DC power factor correction and rectification stage of an onboard charger, utilizing advanced planar magnetics and newly proposed soft-switching control methods. The second part of this work is focused on developing a CLLC DC-DC converter, to interface the AC-DC stage and the high-voltage traction battery. Extended Harmonic Approximation method has been developed and a novel “f-φ” control scheme is proposed to enhance the efficiency at high switching speed. This system allows insights into the practical implementation and evaluation of utilizing Wide Bandgap semiconductors to achieve high power density and efficiency for the transportation industry.Item Modeling Key Issues in Post Silicon Semiconductors: Germanium and Gallium Nitride(2018) Xiao, Ziyang; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We are rapidly approaching the end of the semiconductor roadmap with respect to silicon. To continue its growth, the semiconductor industry is therefore looking into new materials. Two primary materials that are of interest for continuing semiconductor development are germanium (Ge) and gallium nitride (GaN). Ge is of interest as a replacement for silicon, in an effort to improve electronics performances because of its high mobility and its ability to grow a native oxide. In addition, Ge is of interest because of its potential use for economical CMOS-based short wave infrared (SWIR) imaging systems. GaN is a nascent wide bandgap semiconductor and has many potential applications in high power electronics and ultraviolet imaging systems. In this thesis, the key material properties and applications of these two ”end of the roadmap” semiconductors are explored. Ge is a semiconductor material with an indirect bandgap of 0.66eV. This bandgap value corresponds to a wavelength of 1.88μm, which lies in the infrared range. The Ge material itself is also compatible with the standard Si CMOS process technology. Because of these advantages, Ge is considered a candidate for the application of photo detecting in the SWIR range. Apart from the indirect bandgap of 0.66eV, Ge also has a direct bandgap of 0.8eV. From early research, the relatively small offset between the indirect and direct bandgaps can be inverted either by applying strain[1, 9, 10, 11] or alloying with tin. GaN is a binary direct wide bandgap material with a direct bandgap of 3.4eV. It has a high breakdown field, and relatively high saturation velocity and carrier mobility. These properties give GaN an advantage in the realm of high power application. GaN can also form a heterostructure with AlGaN, which can give rise to a 2D electron gas (2DEG) layer at the interface without intentionally doping either material. The 2DEG layer has an even higher mobility when compared to the mobility of the bulk GaN, which allows the heterostructure to be utilized for the design of high electron mobility transistors (HEMTs). The formation of the 2DEG layer also gives rise to potential well confinement at the heterostructure interface. The width of the potential well is only a few nanometers, making the interface electron gas subject to quantum confinement along the direction perpendicular to the interface. The detailed shape of the potential well is determined by the configuration of the heterostructure, as well as the applied voltage across the heterostructure. The first set of goals for this research is to investigate how the bandstructure of Ge changes: Part (1) with the applied strain, and Part (2) with alloyed tin (Sn). The empirical pseudopotential method (EPM) was utilized for the band structure calculation, together with the rules for strain translation for the investigation of Part (1). In Part (1), simulation results give the optimal orientation for different types of applied strain and also thoroughly map the influence of strain applied on any arbitrary orientations. It also reveals that for biaxial strain, there exists another orientation that is more robust against misalignment with respect to the originally desired orientation than the optimal plane, with little compromise of bandgap and slightly higher requirements for the sufficient strain. For Part (2), EPM is combined with perturbation theory for the inclusion of the influence of the Sn atoms in the Ge lattice. A new and computationally inexpensive method is developed during the research. Simulation results agree significantly when compared to reported experimental measurements, indicating the capability of the method. The second set of goals is to investigate the electron transport properties of the 2DEG layer at the interface of GaN HEMT and related power transistors. The potential well is approximated and quantified by a triangular potential well and the carrier sheet density is kept the same during the approximation. Thorough simulations are conducted by calculating the band alignment of the heterostructure with different structural configurations. A fixed correlation between the carrier sheet density and the shape of the potential well (slope of the triangular potential well and the height of the well) is revealed. This correlation is used as an input for the Monte Carlo (MC) simulation. The changes to the mobility of the electrons at the 2DEG layer with changing interface potential well shape are investigated and statis- tics of drift velocity, electron energy, and valley occupation are collected. Mobility information is also extracted and compares favorably with reported experimental measurements. The simulation results are used in the device simulations, which compares the performances of two GaN/AlGaN heterostructure based devices: a lateral HEMT and a current aperture vertical electron transistor (CAVET).Item Design and Characterization of p-i-n Devices for Betavoltaic Microbatteries on Gallium Nitride(2015) Khan, Muhammad Raziuddin A.; Iliadis, Agis A.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Betavoltaic microbatteries convert nuclear energy released as beta particles directly into electrical energy. These batteries are well suited for electrical applications such as micro-electro-mechanical systems (MEMS), implantable medical devices and sensors. Such devices are often located in hard to access places where long life, micro-size and lightweight are required. The working principle of a betavoltaic device is similar to a photovoltaic device; they differ only in that the electron hole pairs (EHPs) are generated in the device by electrons instead of photons. In this study, the performance of a betavoltaic device fabricated from gallium nitride (GaN) is investigated for beta particle energies equivalent to Tritium (3H) and Nickel-63 (N63) beta sources. GaN is an attractive choice for fabricating betavoltaic devices due to its wide band gap and radiation resistance. Another advantage GaN has is that it can be alloyed with aluminum (Al) to further increase the bandgap, resulting in a higher output power and increased efficiency. Betavoltaic devices were fabricated on p-i-n GaN structures grown by metalorganic chemical vapor deposition (MOCVD). The devices were characterized using current - voltage (IV) measurements without illumination (light or beta), using a laser driven light source, and under an electron beam. Dark IV measurements showed a turn on-voltage of ~ 3.4 V, specific-on-resistance of 15.1 m Ω-cm2, and a leakage current of 0.5 mA at – 10 V. A clear photo-response was observed when IV curves were measured for these devices under a light source at a wavelength of 310 nm (4.0 eV). These devices were tested under an electron beam in order to evaluate their behavior as betavoltaic microbatteries without using radioactive materials. Output power of 70 nW and 640 nW with overall efficiencies of 1.2% and 4.0% were determined at the average energy emission of 3H (5.6 keV) and 63N (17 keV) respectively.Item INVESTIGATION OF RELIABILITY IN GALLIUM NITRIDE HIGH ELECTRON MOBILITY TRANSISTORS USING EQUIVALENT CIRCUIT MODELS FOR USE IN HIGH POWER, HIGH FREQUENCY MICROWAVE AMPLIFIERS(2010) Huebschman, Benjamin David; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gallium Nitride (GaN) is beginning to emerge as an alternative to the Gallium Arsenide in high power, high frequency microwave communications. Other novel semiconductors show potential at higher frequency applications. The largest obstacles to GaN emerging as the dominant microwave semiconductor are the issue of cost, which could be reduced through volume, and question of reliability. A new approach to the analysis of reliability has been developed based on the periodic generation of equivalent circuit models while a device is stressed in a manner that is similar to performance likely to be seen during commercial operation. Care was made in this research to ensure that the stress measurements used to induce degradation are as close as possible to those that would degrade a device in real world applications. Equivalent circuit models (ECM) can be used to simulate a device in computer aided design (CAD) software, but these models also provide a picture of the physical properties within the device at a specific point in time. The periodic generation of ECMs allows the researcher to understand the physical changes in the device over time by performing non-destructive electronic measurements. By analyzing the changes in device performance, the physical mechanism of device degradation can be determined. A system was developed to induce degradation and perform measurements of sufficient detail to produce a large signal ECM. Software for producing the ECM was also created. The changes in the ECM were analyzed to diagnose the physical changes in the device under test (DUT) and to identify a method of degradation. The information acquired from this system can be used to improve the device manufacturing process at the foundry. It can also be used to incorporate device degradation into the operation of systems.Item Gallium Nitride Nanowire Based Electronic and Optical Devices(2007-07-26) Motayed, Abhishek; Melngailis, John; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gallium nitride nanowires have significant potential for developing nanoscale emitters, detectors, and biological/chemical sensors, as they possess unique material properties such as wide direct bandgap (3.4 eV), high critical breakdown field, radiation hardness, and mechanical/chemical stability. Although few results of individual GaN nanowire devices have been reported so far, most of them often utilize fabrication processes unsuitable for large-scale nanosystems development and do not involve fundamental transport property measurements. Understanding the transport mechanisms and correlating the device properties with the structural characteristics of the nanowires are of great importance for realizing high performance devices. Focused ion beam induced metal deposition was used to make individual GaN nanowire devices, and assessment of their electrical properties was performed. The nanowires were grown by direct reaction of Ga and NH3, with diameters ranging from 80 nm to 250 nm and lengths up to 200 µm. Dielectrophoretic alignment was used to assemble these nanowires from a suspension on to a large area pre-patterned substrate. A fabrication technique, utilizing only conventional microfabrication processes, has been developed for realizing robust nanowire devices including field effect transistors (FETs), light emitting diodes (LEDs), Schottky diodes, and four-terminal structures. Nanowire FETs with different gate geometries were studied, namely bottom gate, omega-backgate, and omega-plane gate structures. Utilizing omega-backgated FETs, transconductance as high as 0.34 103 µS mm-1 has been obtained. Room temperature field effect electron mobility in excess of 300 cm2 V-1 s-1 have been exhibited by a nanowire FET, with a 200 nm diameter nanowire and Si substrate as the backgate. The observed reduction of mobility in the GaN nanowire FETs with decreasing diameter of the nanowire is attributed to the surface scattering. Electron beam backscattered diffraction revealed that the grain boundary scattering is present in some of the nanowires. Temperature dependent mobility measurements indicated that the ionized impurity scattering is the dominant mechanism in the transport in these nanowires. GaN nanoLEDs have been realized by assembling the n-type nanowires on a p-GaN epitaxial layer using dielectrophoresis. The resulting p-n homojunctions exhibited 365 nm electroluminescence with a full width half maximum of 25 nm at 300 K.