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Silicon Carbide devices are proving to be most promising for high power and high-temperature application in recent times. Efficient and accurate characterization of the device characteristics is key to the fabrication of high quality devices and reproduction of the quality of the devices fabricated. Capacitance-Voltage profiling is one of the most commonly used techniques to measure the doping profiles of semiconductors. However, interpretation of C-V profiling in the presence of traps in the material becomes complicated. Various complications arising from compensation between donors and acceptors, partial ionization of dopants and presence of deep level impurities could yield anomalous measured profile. Silicon Carbide being a wide bandgap semiconductor, many impurities commonly found such as Boron and Aluminum are not completely ionized at Room temperature. This leads to complications in calculating doping profiles when the trap levels are deeper. Other complications arising due to series resistance effect and diode edge effect may also affect the measured profile. Accounting for these complications may be difficult by mere observation of the measured profile. Simulation can be an excellent tool to extract parameters of interest from experimental results that are influenced by non-ideal effects. Fitting of the experimentally obtained data with simulated profile using specific models may be a useful technique to quantitatively account for the deviations from the actual profiles.

4H-SiC n-channel power MESFETs with nitrogen-doped epitaxially grown channel and nitrogen n+-implanted source/drain ohmic contact regions, with and without p-buffer layer fabricated on semi-insulating substrates exhibited hysteresis in the drain I-V characteristics of both types of devices at 300 K and 480 K due to traps. However, thermal spectroscopic measurements could detect the traps only in the devices without p-buffer. In this study the two-dimensional device simulator, MediciTM, and optical admittance spectroscopy (OAS) measurements are used to help resolve the discrepancy in the initial experimental characterization results and interpret the results. Device simulations also showed hysteresis in the drain I-V curves of both types of devices at 300 K and 480 K. Simulations suggest that, in addition to the SI substrate traps, which are known to be major cause of hysteresis in MESFET drain I-V characteristics, acceptor traps due to source/drain residual implant lattice damage could also contribute to the hysteresis observed in the drain I-V characteristics of the experimental MESFETs. Although surface traps are known to cause hysteresis in the I-V curves of MESFETs, their presence was not observed in the experimental devices. The results of the OAS measurements showed several peaks in the spectra of the devices without p-buffer, while in the spectra of the devices with p-buffer the peaks were generally non-existent or reduced. This demonstrates that the peaks observed in the OAS spectra are largely due to substrate traps and that the p-buffer layer is effective in isolating the channel from the substrate. A peak centered around 1.51 eV below the conduction band, which has also been observed in the literature after He+-implantation, is consistently observed in the spectra of both types of devices although it appears reduced in the spectra of the devices with buffer. In this dissertation it is shown that it is likely the traps responsible for this peak could contribute to the hysteresis observed at 300 K and could be solely responsible for the hysteresis observed at high temperatures such as 480 K, since simulations suggest that hysteresis due to semi-insulating substrate traps disappear at high temperatures such as 480 K.

Die vorliegende Arbeit befasst sich mit der Entwicklung, Herstellung und Untersuchung von ZnO-basierten Feldeffekttransistoren (FET). Dabei werden im ersten Teil Eigenschaften von ein- und mehrschichtigen Isolatoren mit hohen Dielektrizitätskonstanten betrachtet, die mittels gepulster Laserabscheidung (PLD) dargestellt wurden. Die elektrischen und kapazitiven Eigenschaften dieser Isolatoren innerhalb von Metall-Isolator-Metall (MIM) bzw. Metall-Isolator-Halbleiter (MIS) Übergängen wurden untersucht. Letzterer wurde schließlich als Gate-Struktur in Metall-Isolator-Halbleiter-FET (MISFET) mit unten (backgate) bzw. oben liegendem Gate (topgate) genutzt. Der zweite Teil konzentriert sich auf Metal-Halbleiter-FET (MESFET), die einen Schottky-Kontakt alsGate nutzen. Dieser wurde mittels reaktiver Kathodenzerstäubung (Sputtern) von Ag, Pt, Pd oder Au unter Einflußvon Sauerstoff hergestellt. ZnO-MESFET stellen eine vielversprechende Alternative zu den bisher in der Oxid-basierten Elektronik verwendeten MISFET dar. Durch die Variation des verwendeten Gate-Metalls, Dotierung, Dicke und Struktur des Kanals und Kontakstruktur, wurde ein Herstellungsstandard gefunden, der zu weiteren Untersuchungen herangezogen wurde. So wurde die Degradation der MESFET unter Belastung durch dauerhaft angelegte Spannung, Einfluss von Licht und erhöhten Temperaturen sowie lange Lagerung getestet. Weiterhin wurden ZnO-MESFET auf industriell genutztem Glasssubstrat hergestellt und untersucht, um die Möglichkeit einer großflächigen Anwendung in Anzeigeelementen aufzuzeigen. Einfache integrierte Schaltungen, wie Inverter und ein NOR-Gatter, wurden realisiert. Dazu wurden Inverter mit sogenannten Pegelschiebern verwendet, welche die Ausgangsspannung des Inverters so verschieben, dass eine logische Aneinanderreihungvon Invertern möglich wird. Schließlich wurden volltransparente MESFET und Inverter, basierend auf neuartigen transparenten gleichrichtenden Kontakten demonstriert.

This PhD thesis sets out to investigate novel Silicon Carbide (SiC) based field effect devices (Schottky and transistor structures), with gas sensitive layers for monitoring hydrogen and propene gases at high temperatures. The devices developed by the author were shown to exhibit sensitivities at least 1~2 orders of magnitude (voltage shift, ƒ¢V) higher than those reported in literature. Not only did the author seek to investigate the gas sensing potential of such devices, but also he set out to study, analyse and establish the gas interaction mechanism of these novel sensors. High temperature tolerant hydrogen and hydrocarbon sensors are required in numerous applications such as: aerospace, nuclear power plant, space exploration and exhaust monitoring in automobiles. Monitoring these gases in a reliable and efficient manner is of great value in these applications, not only from a safety point of view but also for economical reasons. Hence there is an absolute necessity for simple, efficient and high performance sensors not only for monitoring and leak detection but also to function as part of a safety device to prevent accidents. The proposed sensor structure of combining SiC with gas sensitive oxide layers allow them to be operated at high temperatures, making them extremely appealing for direct or in-situ monitoring applications. The microstructural analysis performed using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Rutherford Backscattering Spectroscopy (RBS) provides no evidence of inter-diffusion between different layers, in spite of the sensors being annealing at 650‹ in O2, H2 and C3H6 atmospheres for approximately 50hrs. Samples in different conditions (as deposited, annealed and tested) were compared. The electrical properties of the MROSiC (current-voltage, I-V and capacitance-voltage, C-V characteristics) and MESFET (drain current-source drain voltage (ID-VSD) and transfer, (ãID-H2 concentration) characteristics) devices were measured in the presence and absence of H2 and C3H6. Several parameters such as barrier height, saturation currents, pinch-off voltages and channel conductance were determined from the electrical characteristics, and their influence on the device performance was studied. The authorfs proposed gas interaction model based on energy band diagram is well supported by the experimental data obtained.

Silicon Carbide (SiC) is a wide band-gap semiconductor material with excel-lent material properties for high frequency, high power and high temperature elec-tronics. In this work different SiC field-effect transistors have been studied using theoretical methods, with the focus on both the devices and the methods used. The rapid miniaturization of commercial devices demands better physical models than the drift-diffusion and hydrodynamic models most commonly used at present. The Monte Carlo method is the most accurate physical methods available and has been used in this work to study the performance in short-channel SiC field-effect devices. The drawback of the Monte-Carlo method is the computational power required and it is thus not well suited for device design where the layout requires to be optimized for best device performance. One approach to reduce the simulation time in the Monte Carlo method is to use a time-domain drift-diffusion model in contact and bulk regions of the device. In this work, a time-domain drift-diffusion model is implemented and verified against commercial tools and would be suitable for inclusion in the Monte-Carlo device simulator framework. Device optimization is traditionally performed by hand, changing device pa-rameters until sufficient performance is achieved. This is very time consuming work without any guarantee of achieving an optimal layout. In this work a tool is developed, which automatically changes device layout until optimal device per-formance is achieved. Device optimization requires hundreds of device simulations and thus it is essential that computationally efficient methods are used. One impor-tant physical process for RF power devices is self heating. Self heating can be fairly accurately modeled in two dimensions but this will greatly reduce the computa-tional speed. For realistic influence self heating must be studied in three dimensions and a method is developed using a combination of 2D electrical and 3D thermal simulations. The accuracy is much improved by using the proposed method in comparison to a 2D coupled electro/thermal simulation and at the same time offers greater efficiency. Linearity is another very important issue for RF power devices for telecommunication applications. A method to predict the linearity is imple-mented using nonlinear circuit simulation of the active device and neighboring passive elements.

The importance of simulation is increasing in the researchon semiconductor devices and materials. Simulations are used toexplore the characteristics of novel devices as well asproperties of the semiconductor materials that are underinvestigation, i.e. generally materials where the knowledge isinsufficient. A wide range of simulation methods exists, andthe method used in each case is selected according to therequirements of the work performed. For simulations of newsemiconductor materials, extremely small devices, or deviceswhere non-equilibrium transport is important, the Monte Carlo(MC) method is advantageous, since it can directly exploit themodels of the important physical processes in the device. One of the semiconductors that have attracted a lot ofattraction during the last decade is silicon carbide (SiC),which exists in a large number of polytypes, among which3C-SiC, 4H-SiC and 6H-SiC are most important. Although SiC hasbeen known for a very long time, it may be considered as a newmaterial due to the relatively small knowledge of the materialproperties. This dissertation is based on a number of MCstudies of both the intrinsic properties of different SiCpolytypes and the qualities of devices fabricated by thesepolytypes. In order to perform these studies a new full-bandensemble device MC simulator, the General Monte CarloSemiconductor (GEMS) simulator was developed. Algorithmsimplemented in the GEMS simulator, necessary when allmaterial-dependent data are numerical, and for the efficientsimulation of a large number of charge carriers in high-dopedareas, are also presented. In addition to the purely MC-relatedstudies, a comparison is made between the MC, drift-diffusion,and energy-balance methods for simulation of verticalMESFETs. The bulk transport properties of electrons in 2H-, 3C-, 4H-and 6H-SiC are studied. For high electric fields the driftvelocity, and carrier mean energy are presented as functions ofthe field. For 4H-SiC impact-ionization coefficients,calculated with a detailed quantum-mechanical model ofband-to-band tunneling, are presented. Additionally, a study oflow-field mobility in 4H-SiC is presented, where the importanceof considering the neutral impurity scattering, also at roomtemperature, is pointed out. The properties of 4H- and 6H-SiC when used in short-channelMOSFETs, assuming a high quality semiconductor-insulatorinterface, are investigated using a simple model for scatteringin the semiconductor-insulator interface. Furthermore, theeffect is studied on the low and high-field surface mobility,of the steps formed by the common off-axis-normal cutting ofthe 4H- and 6H-SiC crystals. In this study an extension of theprevious-mentioned simple model is used.

<p>The importance of simulation is increasing in the researchon semiconductor devices and materials. Simulations are used toexplore the characteristics of novel devices as well asproperties of the semiconductor materials that are underinvestigation, i.e. generally materials where the knowledge isinsufficient. A wide range of simulation methods exists, andthe method used in each case is selected according to therequirements of the work performed. For simulations of newsemiconductor materials, extremely small devices, or deviceswhere non-equilibrium transport is important, the Monte Carlo(MC) method is advantageous, since it can directly exploit themodels of the important physical processes in the device.</p><p>One of the semiconductors that have attracted a lot ofattraction during the last decade is silicon carbide (SiC),which exists in a large number of polytypes, among which3C-SiC, 4H-SiC and 6H-SiC are most important. Although SiC hasbeen known for a very long time, it may be considered as a newmaterial due to the relatively small knowledge of the materialproperties. This dissertation is based on a number of MCstudies of both the intrinsic properties of different SiCpolytypes and the qualities of devices fabricated by thesepolytypes. In order to perform these studies a new full-bandensemble device MC simulator, the General Monte CarloSemiconductor (GEMS) simulator was developed. Algorithmsimplemented in the GEMS simulator, necessary when allmaterial-dependent data are numerical, and for the efficientsimulation of a large number of charge carriers in high-dopedareas, are also presented. In addition to the purely MC-relatedstudies, a comparison is made between the MC, drift-diffusion,and energy-balance methods for simulation of verticalMESFETs.</p><p>The bulk transport properties of electrons in 2H-, 3C-, 4H-and 6H-SiC are studied. For high electric fields the driftvelocity, and carrier mean energy are presented as functions ofthe field. For 4H-SiC impact-ionization coefficients,calculated with a detailed quantum-mechanical model ofband-to-band tunneling, are presented. Additionally, a study oflow-field mobility in 4H-SiC is presented, where the importanceof considering the neutral impurity scattering, also at roomtemperature, is pointed out.</p><p>The properties of 4H- and 6H-SiC when used in short-channelMOSFETs, assuming a high quality semiconductor-insulatorinterface, are investigated using a simple model for scatteringin the semiconductor-insulator interface. Furthermore, theeffect is studied on the low and high-field surface mobility,of the steps formed by the common off-axis-normal cutting ofthe 4H- and 6H-SiC crystals. In this study an extension of theprevious-mentioned simple model is used.</p>