A simulation program has been developed to facilitate the investigation and analysis of power semiconductor devices under the reverse-bias condition. The electrostatic potential distribution is solved by using Poisson's equation alone, with particular attention to the neighborhood of avalanche breakdown. Because of its generality and efficiency, the program emerges as a powerful engineering tool for the design of power devices incorporating special junction termination techniques. Results are presented for a DMOS structure to illustrate the improvement in breakdown voltage when a field plate is applied. Numerical solution techniques for solving elliptic partial differential equations in a multi-material domain are discussed. The discretization of this domain is nonuniform in general due to its highly nonuniform physical parameters. By careful selection of grid lines near interfaces, the difference equation coefficients are considerably simplified. The resultant matrix of coefficients is symmetric even though Neumann boundary conditions are specified.

The Jain and Balk analytic model for two-dimensional effects in short channel MOSFETS is investigated. The effects considered are Drain Induced Barrier Lowering, DIBL, and the maximum electric field, Emax, which influences Drain Induced High Field, DIHF. A scaled short channel design is used as the basis for the investigation. Cases are numerically simulated using the MINIMOS program. DIBL and Emax are calculated using the Jain and Balk model. Model values are compared to numerical simulation values. Results show the model consistently overestimates DIBL. Also, the range for which the model closely estimates Emax is found. Variation in Emax with change of junction depth Xj is investigated. The electric field, Ex, as it varies with depth in the channel is investigated, and compared to the Jain and Balk approximation. The deviations suggest that the model must break down for short channels.

Application of power MOSFETs in spaceborne power converters was simulated by exposing devices to low-dose-rate ionizing radiation. Both radiation-hardened and nonhardened devices were tested with constant and switched gate biases during irradiation. In addition, some of the devices were under load. The threshold-voltage shifts were strongly bias dependent. The threshold-voltage shift of the nonhardened parts was approximately dose-rate independent, while the hardened parts exhibited significant dose-rate dependence. A pre-anneal dose-rate dependence was found for the interface-state buildup of the switched and positively biased devices, but the results for the switched devices were qualitatively different than those for the positively biased devices. The buildup of interface trapped charge was found to be the primary contributor to mobility degradation, which results in reduced drive capability and slower operation of the devices. These results indicate that new methods need to be utilized to accurately predict the performance of power MOSFETs in space environments.

The effects of ionizing radiation on the breakdown-voltage degradation of power-MOSFET termination structures were examined through two-dimensional simulation. A wide variety of sensitivity to surface-charge density was found for various devices employing floating field rings and/or equipotential field plates. Termination structures that were both insensitive to surface charge and possessed a high breakdown voltage were identified. The results were compared with measurements made on selected structures. The principal ionizing radiation damaging mechanisms in MOS devices are discussed. Modifications made to an existing simulation program in order to simulate these complex field ring and field plate structures are described. Background information into how these termination structures improve the breakdown voltage and their sensitivities to positive interface charge buildup is investigated.

No description available.

Organic π-conjugated Donor-Acceptor copolymers are emerging as potential candidate materials for organic field effect transistor (OFET) and organic photovoltaic (OPV) applications. The electron-deficient benzothiadiazole group coupled with an electron-rich oligothiophene to form donor-acceptor copolymers has attracted significant attention. These low optical band gap materials absorb photons in the range of 400-800 nm and exhibit good thermal stability. In particular, poly(benzothiadiazole-sexithiophene) (PBT6) exhibits excellent performance in optoelectronic devices and high thermal stability. Here, we present the chemical synthesis and characterization of the polymer, PBT6. Three samples of PBT-6 with differing molecular weights in the range of Mn 18000-45000 Da were synthesized. Each polymer was characterized with respect to its photophysical, thermal properties and field-effected mobility was determined. Devices were prepared by drop-casting polymer solutions in 1,2-dichlorobenzene (DCB) onto an OFET (bottom gate/bottom contact) substrate and the devices were used to examine the charge transport properties of each polymer system. The optimal solvent to be used for processing technique was determined and surface techniques using OTS-8 and OTS-18 were compared through contact angle measurements. The measured charge carrier mobilities were in the range of 0.45-0.6 cm² / V.s. Polymer films prepared via drop-casting and which were thermal annealed exhibit mobilities as high as 0.825 cm² / V.s. This work examines the effect of molecular weight on the charge carrier transport properties and demonstrates the correlation of performance with molecular ordering. Drop-casted films of PBT-6 exhibit highly ordered crystalline lamellar structure with high degree of π- π stacking with edge-on orientation on the substrate. The longer conjugation lengths promote intrachain charge transfer. This high degree of molecular ordering in high MW samples of PBT6 improves the interchain and intrachain charge transfer leading to enhanced mobilities. The increased molecular weight (MW) facilitates in forming more uniform thin films which is vital in processing and application of polymer thin film technologies. These results and observations clearly demonstrate the potential of PBT-6 as a semiconducting material for Optoelectronic devices.

At present the field of molecular electronics - also known as molecular semiconductors, organic semiconductors, plastic electronics or organic electronics - is dominated by organic materials, both polymeric and molecular, with much less attention being focused on transition metal based complexes despite the advantages they can offer. Such advantages include tuneable frontier orbitals through the ligand/metal interaction and the ability to generate stable paramagnetic species. Devices containing radical materials are particularly interesting in order to examine the interplay between conduction and spin - an effect which is not yet properly understood but can give rise to exotic behaviour. A series of homoleptic, bis-ligand Ni(II) and Cu(II) complexes were prepared using three structurally related phenolic oxime ligands, 2-hydroxy-5-t-octylacetophenone oxime (t-OctsaoH), 2-hydroxy-5-n-propylacetophenone oxime (n-PrsaoH) and 2- hydroxyacetophenone oxime (HsaoH). The complexes were characterised by single-crystal X-ray diffraction, cyclic voltammetry, UV/Vis spectroscopy, field-effect-transistor measurements, DFT/TD-DFT calculations and in the case of the paramagnetic species, EPR and magnetic susceptibility. Variation of the substituent on the ligand from t-octyl to n-propyl to H enabled electronic isolation of the complexes in the crystal structures of M(t-OctsaoH)2, which contrasted with π-stacking interactions observed in the crystal packing of M(n-PrsaoH)2 and of M(HsaoH) (M = Ni, Cu). This was further evidenced by comparing the antiferromagnetic interactions observed in samples of Cu(n-PrsaoH)2 and Cu(HsaoH)2 with the ideal paramagnetic behaviour for Cu(t-OctsaoH)2 down to 1.8 K. Despite isostructural single crystal structures for M(n-PrsaoH)2, thin-film X-ray diffraction and SEM revealed different morphologies depending on the metal and the deposition method employed. However, the complexes of M(n-PrsaoH)2 and M(HsaoH) failed to demonstrate significant charge transport in an FET device despite displaying the ability to form π- stacking structures. A series of planar Ni(II), Cu(II) and Co(II) dibenzotetraaza[14]annulenes (dbtaa) and dinapthotetraaza[14]annulenes (dntaa) were synthesised and studied crystallographically, optically, electrochemically and magnetically. Thin films of each of these complexes have been prepared by vacuum deposition to evaluate the field-effect transistor (FET) performance as well as the morphology and crystallinity of the film formed. Single crystal data revealed that Ni(dbtaa) and Cu(dbtaa) are isomorphous to each other, with Co(dbtaa) displaying a different crystallographic packing. The electrochemistry and UV/Vis absorption studies indicate the materials are redox active and highly coloured, with molar extinction coefficients as large as 80,000 M-1cm-1 in the visible region. The paramagnetic Cu(II) and Co(II) complexes display weak 1-dimensional antiferromagnetic interactions and were fit to the Bonner-Fisher chain model. The data revealed that the Co(II) species possesses much stronger magnetic exchange interactions compared with the Cu(II) complex. Each of the materials formed polycrystalline films when vacuum deposited and all showed ptype field-effect transistor behaviour, with modest charge carrier mobilities in the range of 10-5 to 10-9 cm2 V-1 s-1 . SEM imaging of the substrates indicates that the central metal ion, and its sublimation temperature, has a crucial role in defining the morphology of the resulting film. Structurally related Cu(II) and Ni(II) dithiadiazoletetraaza[14]annulene (dttaa) macrocycles were synthesised and studied in the context of their thin film electrochemical, conducting and morphological properties. Both the Ni(II) and Cu(II) complexes were found to be volatile under reduced pressure, which allowed crystals of both materials to be grown and the single crystal structures solved. Interestingly, the crystal packing of these heterocyclic macrocycles varies depending on whether the central metal ion is Cu(II) or Ni(II), which is in contrast to the analogous dibenzotetrazaannulenes complexes. Soluble Ni(II) analogues containing benzoyl groups on the meso- positions of the macrocycle (dttaaBzOR) were also prepared and contrasted with the insoluble Ni(dttaa) complexes in terms of their solution optical and electrochemical properties. Thin film electrochemical studies of Cu(dttaa) and Ni(dttaa) showed chemically reversible oxidative processes but on scanning to reductive potentials the films disintegrated almost immediately as the bulky counter tetrabutylammonium cation entered the thin film. FET studies undertaken on polycrystalline films of both complexes, using various device configurations and surface treatments, failed to realise any gate effect. Thin film XRD measurements indicate that films of both complexes formed by vacuum deposition are crystalline and contain a mixture of molecular alignments, with molecules aligning “edge on” and “face down” to the substrate. SEM imaging failed to effectively resolve the morphology of the films implying the sizes of the crystallites are small, which may help to explain the lack of FET effect. A series of bis-ligand diimine Ni, Cu and Pd complexes have been synthesised from the ligand 4,5-bis(dodecyloxy)benzene-1,2-diamine (dbdaH2). The same ligand was also used to prepare a series of soluble Cu(II) and Ni(II) tetraaza[14]annulene macrocycles. All the bis-ligand diimine complexes were found to suffer from instability in air due to the ease at which the complexes are oxidised. The Ni complex, Ni(dbda)2, was found to display a NIR transition in the region of 971 to 1024 nm depending on the polarity of the solvent that the molecule is dissolved in. Solution electrochemistry studies of Ni(dbda)2 reaffirmed the facile nature of the first oxidative process, with the HOMO energy calculated at -4 eV by hybrid-DFT. This compound failed to yield semiconducting behaviour in an FET device despite the use of surface treatments aimed at promoting suitable molecular alignment across the conducting channel.

Silicon based semiconductor devices are rapidly approaching the theoretical limit of operation and are becoming unsuitable for future military requirements. The scope of semiconductor devices has been expanded by wide bandgap devices such as gallium nitride (GaN) to include the possibility for high power and high frequency operation. A new generation of high speed â high frequency devices is required to meet current and future military needs. The Gallium Nitride High Electron Mobility Transistor (HEMT) is showing great promise as the enabling technology in the development of military radar systems, electronic surveillance systems, communications systems and high voltage power systems. Typically, sapphire or silicon carbide is utilized as the substrate material in most HEMT designs. This thesis explores the possibility of utilizing a diamond substrate to increase the power handling capability of the AlGaN/GaN HEMT. Diamond offers increased thermal property parameters that can be simulated in the commercially available Silvaco software package. A complete electrical and thermal analysis of the model was conducted and compared to actual device characteristics. The results of the software simulation and measurements on the test devices indicate diamond substrates will enable the HEMT to be operated at a higher power than traditional sapphire substrate HEMTS.

In the course of the PhD thesis large area homogeneous strictly monolayer graphene films were successfully synthesized with chemical vapor deposition over both Cu and Si (with surface oxide) substrates. These synthetic graphene films were characterized with thorough microscopic and spectrometric tools and also in terms of electrical device performance. Graphene growth with a simple chemo thermal route was also explored for understanding the growth mechanisms. The formation of homogeneous graphene film over Cu requires a clean substrate. For this reason, a study has been conducted to determine the extent to which various pre-treatments may be used to clean the substrate. Four type of pre-treatments on Cu substrates are investigated, including wiping with organic solvents, etching with ferric chloride solution, annealing in air for oxidation, and air annealing with post hydrogen reduction. Of all the pretreatments, air oxidation with post hydrogen annealing is found to be most efficient at cleaning surface contaminants and thus allowing for the formation of large area homogeneous strictly monolayer graphene film over Cu substrate. Chemical vapor deposition is the most generally used method for graphene mass production and integration. There is also interest in growing graphene directly from organic molecular adsorbents on a substrate. Few studies exist. These procedures require multiple step reactions, and the graphene quality is limited due to small grain sizes. Therefore, a significantly simple route has been demonstrated. This involves organic solvent molecules adsorbed on a Cu surface, which is then annealed in a hydrogen atmosphere in order to ensure direct formation of graphene on a clean Cu substrate. The influence of temperature, pressure and gas flow rate on the one-step chemo thermal synthesis route has been investigated systematically. The temperature-dependent study provides an insight into the growth kinetics, and supplies thermodynamic information such as the activation energy, Ea, for graphene synthesis from acetone, isopropanol and ethanol. Also, these studies highlight the role of hydrogen radicals for graphene formation. In addition, an improved understanding of the role of hydrogen is also provided in terms of graphene formation from adsorbed organic solvents (e.g., in comparison to conventional thermal chemical vapor deposition). Graphene synthesis with chemical vapor deposition directly over Si wafer with surface oxide (Si/SiOx ) has proven challenging in terms of large area and uniform layer number. The direct growth of graphene over Si/SiO x substrate becomes attractive because it is free of an undesirable transfer procedure, necessity for synthesis over metal substrate, which causes breakage, contamination and time consumption. To obtain homogeneous graphene growth, a local equilibrium chemical environment has been established with a facile confinement CVD approach, inwhich two Si wafers with their oxide faces in contact to form uniform monolayer graphene. A thorough examination of the material reveals it comprises facetted grains despite initially nucleating as round islands. Upon clustering these grains facet to minimize their energy, which leads to faceting in polygonal forms because the system tends to ideally form hexagons (the lowest energy form). This is much like the hexagonal cells in a beehive honeycomb which require the minimum wax. This process also results in a near minimal total grain boundary length per unit area. This fact, along with the high quality of the resultant graphene is reflected in its electrical performance which is highly comparable with graphene formed over other substrates, including Cu. In addition the graphene growth is self-terminating, which enables the wide parameter window for easy control. This chemical vapor deposition approach is easily scalable and will make graphene formation directly on Si wafers competitive against that from metal substrates which suffer from transfer. Moreover, this growth path shall be applicable for direct synthesis of other two dimensional materials and their Van der Waals hetero-structures. / Im Zuge dieser Doktorarbeit wurden großflächige und homogene Graphen-Monolagen mittels chemischer Gasphasenabscheidung auf Kupfer- (Cu) und Silizium-(Si) Substraten erfolgreich synthetisiert. Solche monolagigen Graphenschichten wurden mithilfe mikroskopischer und spektrometrischer Methoden gründlich charakterisiert. Außerdem wurde der Wachstumsmechanismus von Graphen anhand eines chemo-thermischen Verfahrens untersucht. Die Bildung von homogenen Graphenschichten auf Cu erfordert eine sehr saubere Substratoberfläche, weshalb verschiedene Substratvorbehandlungen und dessen Einfluss auf die Substratoberfläche angestellt wurden. Vier Vorbehandlungsarten von Cu-Substraten wurden untersucht: Abwischen mit organischen Lösungsmitteln, Atzen mit Eisen-(III)-Chloridlösung, Wärmebehandlung an Luft zur Erzeugung von Cu-Oxiden und Wärmebehandlung an Luft mit anschließender Wasserstoffreduktion. Von diesen Vorbehandlungen ist die zuletzt genannte Methode für die anschließende Abscheidung einer großflächigen Graphen-Mono-lage am effektivsten. Die chemische Gasphasenabscheidung ist die am meisten verwendete Methode zur Massenproduktion von Graphen. Es besteht aber auch Interesse an alternativen Methoden, die Graphen direkt aus organischen, auf einem Substrat adsorbierten Molekülen, synthetisieren konnen. Jedoch gibt es derzeit nur wenige Studien zu derartigen alternativen Methoden. Solche Prozessrouten erfordern mehrstufige Reaktionen, welche wiederrum die Qualität der erzeugten Graphenschicht limitieren, da nur kleine Korngrößen erreicht werden konnen. Daher wurde in dieser Arbeit ein deutlich einfacherer Weg entwickelt. Es handelt sich dabei um ein Verfahren, bei dem auf einer Cu-Substratoberfläche adsorbierte, organische Lösungsmittelmoleküle in einer Wasserstoffatmosphäre geglüht werden, um eine direkte Bildung von Graphen auf einem sauberen Cu-Substrat zu gewahrleisten.Der Einfluss von Temperatur, Druck und Gasfluss auf diesen einstufigen chemothermischen Syntheseweg wurde systematisch untersucht. Die temperaturabhängigen Untersuchungen liefern einen Einblick in die Wachstumskinetik und thermodynamische Größen, wie zum Beispiel die Aktivierungsenergie Ea, für die Synthese von Graphen aus Aceton, Isopropanol oder Ethanol. Diese Studien untersuchen außerdem die Rolle von Wasserstoffradikalen auf die Graphensynthese. Weiterhin wurde ein verbessertes Verständnis der Rolle von Wasserstoff auf die Graphen-synthese aus adsorbierten, organischen Lösungsmitteln erlangt (beispielsweise im Vergleich zur konventionellen thermischen Gasphasenabscheidung). Die direkte Graphensynthese mittels chemischer Gasphasenabscheidung auf Si-Substraten mit einer Oxidschicht (Si/SiOx ) ist extrem anspruchsvoll in Bezug auf die großflächige und einheitliche Abscheidung (Lagenanzahl) von Graphen-Monolagen. Das direkte Wachstum von Graphen auf Si/SiOx -Substrat ist interessant, da es frei von unerwünschten Übertragungsverfahren ist und kein Metall-substrat erfordert, welche die erzeugten Graphenschichten brechen lassen können. Um ein homogenes Graphenwachstum zu erzielen wurde durch den Kontakt zweier Si-Wafer, mit ihren Oxidflachen zueinander zeigend, eine lokale Umgebung im chemischen Gleichgewicht erzeugt. Diese Konfiguration der Si-Wafer ist nötig, um eine einheitliche Graphen-Monolage bilden zu können. Eine gründliche Untersuchung des abgeschiedenen Materials zeigt, dass trotz der anfänglichen Keimbildung von runden Inseln facettierte Körner erzeugt werden. Aufgrund der Bestrebung der Graphenkörner ihre (Oberflächen-) Energie zu minimieren, wird eine Facettierung der Körner in polygonaler Form erzeugt, was darin begründet liegt, dass das System idealerweise eine Anordnung von hexagonal geformten Körnern erzeugen würde (niedrigster Energiezustand). Der Prozess ist vergleichbar mit der sechseckigen Zellstruktur einer Bienenstockwabe, welche ein Minimum an Wachs erfordert. Dieser Prozess führt auch zu einer nahezu minimalen Gesamtkorn-grenzlänge pro Flächeneinheit. Diese Tatsache zusammen mit der hohen Qualität der resultierenden Graphenschicht spiegelt sich auch in dessen elektrischer Leistungsfähigkeit wider, die in hohem Maße mit der auf anderen Substraten gebildeten Graphenschichten (inklusive Cu-Substrate) vergleichbar ist. Darüber hinaus ist das Graphenwachstum selbstabschliessend, wodurch ein großes Parameterfenster für eine einfache und kontrollierte Synthese eröffnet wird. Dieser Ansatz zur chemischen Gasphasenabscheidung von Graphen auf Si- Substraten ist leicht skalierbar und gegenüber der Abscheidung auf Metallsubstraten konkurrenzfähig, da keine Substratübertragung notig ist. Darüber hinaus ist dieser Prozess auch für die direkte Synthese anderer zweidimensionalen Materialien und deren Van-der-Waals-Heterostrukturen anwendbar.

AlGaN/GaN and InAlN/GaN-based heterojunction field effect transistors (HFETs) have demonstrated great high power and high frequency performance. Although AlGaN/GaN HFETs are commercially available, there still remain issues regarding long-term reliability, particularly degradation and ultimately device failure due to the gate-drain region where the electric field peaks. One of the proposed degradation mechanisms is the inverse-piezoelectric effect that results from the vertical electric field and increases the tensile strain. Other proposed mechanisms include hot-electron-induced trap generation, impurity diffusion, surface oxidation, and hot-electron/phonon effects. To investigate the degradation mechanism and its impact on DC, microwave, and noise performance, comprehensive stress experiments were conducted in both un-passivated and passivated AlGaN/GaN HFETs. It was found that degradation of AlGaN/GaN HFETs under reverse-gate-bias stress is dominated by inverse-piezoelectric effect and/or hot-electron injection due to gate leakage. Degradation under on-state-high-field stress is dominated by hot-electron/phonon effects, especially at high drain bias. Both effects are induced by the high electric field present during stress, where the inverse-piezoelectric effect only relates to the vertical electric field and the hot-electron effect relates to the total electric field. InAlN/GaN-based HFETs are expected to have even better performance as power amplifiers due to the large 2DEG density at the InAlN/GaN interface and better lattice-matching. Electrical stress experiments were therefore conducted on InAlN/GaN HFETs with indium compositions ranging from 15.7% to 20.0%. Devices with indium composition of 18.5% were found to give the best compromise between reliability and device performance. For indium compositions of 15.7% and 17.5%, the HFET devices degraded very fast (25 h) under on-state-high-field stress, while the HFET devices with 20.0% indium composition showed very small drain. It was also demonstrated that hot-electron/phonon effects are the major degradation mechanism for InAlN/GaN HFETs due to a large 2DEG density under on-state operations, whereas the inverse-piezoelectric effect is very small due to the small strain for the near lattice-matched InAlN barrier. Compared to lattice-matched InAlN/GaN HFETs, AlGaN/GaN HFETs have much larger strain in the barrier and about half of the drain current level; however, the hot electron/hot phonon effects are still important, especially at high drain bias.