Die elektronische Zustandsdichte ist die grundlegende Größe zum Verständnis von Ladungstransportprozessen in Materialien. In der vorliegenden Arbeit wird eine Methode zur Bestimmung dieser Dichte, die für organische Feldeffekttransistoren (OFET) vorgeschlagen wurde, für organischen Transistoren mit durchlässiger Basis (OPBT) adaptiert. Dabei wird aus temperaturabhängigen Transferkurven die Aktivierungsenergie für den Emitterstrom bestimmt und aus deren Spannungsabhängigkeit die Zustandsdichte abgeleitet. Es wird gezeigt, dass die Anwendung der Methode bei OPBTs unter bestimmten Voraussetzungen möglich ist. Die Qualität der Resultate hängt vom jeweiligen Transistor ab. Ein homogeneres und dickeres Basisoxid, wie es durch Anodisierung der Basiselektrode entsteht, wirkt sich positiv auf die Qualität aus. Die bestimmten Zustandsdichten liegen abhängig vom konkreten Transistor im Bereich 10^18 eV^−1 cm^−3…10^20 eV^−1 cm^−3 in einem Energiebereich von 200 meV. Die Form der Verteilung ist näherungsweise exponentiell mit einer Breite von etwa 4 meV.:1 Einleitung 2 Grundlagen 2.1 Organische Halbleiter 2.1.1 Organische Moleküle und Festkörper 2.1.2 Ladungstransport 2.1.3 Modelle und Konzepte 2.1.4 C60 2.2 Organische Transistoren 2.3 Organischer Transistor mit durchlässiger Basis 2.3.1 Geometrie 2.3.2 Zustände und Transferkurvenabschnitte 2.3.3 Energiediagramm 2.3.4 Anodisierung 2.3.5 Chemische und elektrische Belastung 2.4 Zustandsdichte 2.4.1 Modelle 2.4.2 Zustandsdichtebestimmung 3 Experiment 3.1 Transistoren 3.1.1 Herstellung 3.1.2 Anodisierung 3.2 Temperaturabhängige Transferkurven 3.2.1 Wahl der Parameter 3.2.2 Auswertung der Daten 3.3 Belastungstests und Parametervariation 3.4 Kapazitätsmessung 4 Auswertung 4.1 Bestimmung der Aktivierungsenergie 4.2 Bestimmung der Zustandsdichte 4.2.1 Anwendbarkeit und Zuverlässigkeit 4.2.2 Wahl der Kanaldicke 4.2.3 Kapazitätsbestimmung 4.3 Auswirkung der Anodisierung 4.4 Auswirkung von elektrischer und chemischer Belastung 4.5 Einfluss der Halbleiterschichtdicke 5 Diskussion 5.1 Schlussfolgerungen 5.2 Ausblick A Messdaten / The electronic density of states (DOS) is a fundamental quantity which allows a deeper understanding of charge transport processes in materials. In this thesis, a method proposed for organic field-effect transistors (OFET) will be adapted to organic permeable-base transistors (OPBT). The DOS is extracted from temperature dependent transfer curves by determining the activation energy and calculating its derivative with respect to the applied voltage. It is shown that the application of this method to OPBTs is possible under certain circumstances. The quality of the results depends on the transistor. A more homogeneous and thicker base oxide created by anodization results in a better quality. The resulting DOS lies in the range of 10^18 eV^−1 cm^−3…10^20 eV^−1 cm^−3 in an energy interval of 200 meV. The shape of the DOS is approximately exponential with a width of 4 meV.:1 Einleitung 2 Grundlagen 2.1 Organische Halbleiter 2.1.1 Organische Moleküle und Festkörper 2.1.2 Ladungstransport 2.1.3 Modelle und Konzepte 2.1.4 C60 2.2 Organische Transistoren 2.3 Organischer Transistor mit durchlässiger Basis 2.3.1 Geometrie 2.3.2 Zustände und Transferkurvenabschnitte 2.3.3 Energiediagramm 2.3.4 Anodisierung 2.3.5 Chemische und elektrische Belastung 2.4 Zustandsdichte 2.4.1 Modelle 2.4.2 Zustandsdichtebestimmung 3 Experiment 3.1 Transistoren 3.1.1 Herstellung 3.1.2 Anodisierung 3.2 Temperaturabhängige Transferkurven 3.2.1 Wahl der Parameter 3.2.2 Auswertung der Daten 3.3 Belastungstests und Parametervariation 3.4 Kapazitätsmessung 4 Auswertung 4.1 Bestimmung der Aktivierungsenergie 4.2 Bestimmung der Zustandsdichte 4.2.1 Anwendbarkeit und Zuverlässigkeit 4.2.2 Wahl der Kanaldicke 4.2.3 Kapazitätsbestimmung 4.3 Auswirkung der Anodisierung 4.4 Auswirkung von elektrischer und chemischer Belastung 4.5 Einfluss der Halbleiterschichtdicke 5 Diskussion 5.1 Schlussfolgerungen 5.2 Ausblick A Messdaten

Single electron transistors (SET) have attracted significant attention as a potential building block for post CMOS nanoelectronic devices. However, lack of reproducible and parallel fabrication approach and room temperature operation are the two major bottlenecks for practical realization of SET based devices. In this thesis, I demonstrate large scale single electron transistors fabrication techniques using solution processed single wall carbon nanotubes (SWNTs) and studied their electron transport properties. The approach is based on the assembly of individual SWNTs via dielectrophoresis (DEP) at the selected position of the circuit and formation of tunnel barriers on SWNT. Two different techniques: i) metal-SWNT Schottky contact, and ii) mechanical templating of SWNTs were used for tunnel barrier creation. Low temperature (4.2K) transport measurement of 100 nm long metal-SWNT Schottky contact devices show that 93% of the devices with contact resistance (RT) > 100 K? show SET behavior. Majority (90%) of the devices with 100 K? < RT < 1 M?, show periodic, well-de?ned Coulomb diamonds with a charging energy ~ 15 meV, represents single electron tunnelling through a single quantum dot (QD), defined by the top contact. For high RT (> 1M?), devices show multiple QDs behaviors, while QD was not formed for low RT ( < 100 K?) devices. From the transport study of 50 SWNT devices, a total of 38 devices show SET behavior giving an yield of 76%. I also demonstrate room temperature operating SET by using mechanical template technique. In mechanical template method individual SWNT is placed on top of a Al/Al2O3 local gate which bends the SWNT at the edge and tunnel barriers are created. SET devices fabricated with a template width of ~20 nm shows room temperature operation with a charging energy of ~150 meV. I also discussed the detailed transport spectroscopy of the devices.

[pt] Este trabalho trata dos efeitos das não-linearidades de transistores de efeito de campo utilizados em amplificadores de microondas. Para tanto, o transistor é modelado por um circuito não- linear equivalente, cujos elementos são determinados através da medição dos parâmetros espalhamento do mesmo, na faixa de 3 GHz a 9 GHz, e com o auxílio de um programa de otimização de circuitos e outro de ajuste de curvas. O método de análise utilizado é o da expansão em série de Volterra, para o qual foi desenvolvido um programa computacional que permite a determinação dos ganhos de transdução e das potências de saída na freqüência fundamental e no terceiro produto de intermodulação, bem como do ponto de 1dB de compressão de ganho, da taxa de distorção de intermodulação de terceira ordem. Esse programa permite, ainda, a verificação da influência das impedâncias de fechamento fora da faixa, nas características de distorção de intermodulação. Através dessa análise pôde-se verificar que as terminações fora da faixa exercem pouca ou nenhuma influência nas características de distorção de intermodulação, com exceção das terminações na freqüência diferença, (freqüência de diferença = freqüência 2 - freqüência 1), onde pôde-se constatar uma redução de até 8dB no nível do terceiro produto de intermodulação, para uma escolha apropriada das impedâncias de fechamento nessa freqüência. Esses resultados, contudo, não podem ser considerados definitivos, uma vez que o modelo adotado não levou em consideração o fato do FET utilizado ser pré-adaptado. Também, devido ao transistor ter-se danificado durante as medições de intermodulação, tais resultados não puderam ser comprovados experimentalmente. / [en] This work deals with the effects of non-linear ities of field-effect transistors used in microwave amplifiers. To do so, the transistor is modeled by a non-linear equivalent circuit, with its components determined through the measurement of its scattering parameters, in the range of 3 GHz to 9GHz, and with the aid of a circuit optimization program and another for curve fitting. The method of analysis used is the Volterra series expansion, for which a computer program was developed, permitting the determination of the transducer gains and output powers in the fundamental frequency, and in the third-order intermodulation product, as well as the 1 dB compression point, the third-order intermodulation distortion ratio, and the third-order intercept point. This program also allows for the verification of the influence of out-of-band terminating impedances on the intermodulation distortion characteristics. Through this analysis it was possible to verify that the out-of-band terminations have little or no influence on the intermodulation distortion characteristics, with the exception of the terminations in the difference frequency, (difference frequency = frequency 2 - frequency 1), for which it was found a decrease of up to 8 dB in the third- order intermodulation product level, for the appropriate choice of these impedances. These results, however, cannot be said to represent the real behavior of the FET since the model used did not account for the internal matching of the device. Also, due to the fact that the transistor was damaged during the intermodulation measurements, such results could not be verified experimentally.

Detta arbete syftar till att identifiera och undersöka metoder för att designa och optimera transistorer i effektförstärkare av typen A, AB, B och F utifrån parametrar som linjäritet, arbetsfrekvens, uteffekt och verkningsgrad. Arbetsfrekvensen ska vara 1 GHz och effektförstärkaren ska ha hög linjäritet samt verkningsgrad. Det resultat som förväntas efter avslutat arbete är en optimeringsteknik som steg för steg kan användas i simulerad miljö för att optimera transistorer i effektförstärkare. En grundtopologi för alla effektförstärkare konstrueras och endast en transistor används vid design av effektförstärkarna. Olika metoder undersöks för att optimera förstärkarna. De metoder som används vid optimering av effektförstärkarna är två typer av Loadpull-analys, spänning och strömkvot, DCIV-karaktäristik och stabilitetsanalys. Dessa metoder används och demonstreras i arbetet. En optimerad förstärkare av varje förstärkarklass presenteras. De metoder som används evalueras och en optimeringsteknik presenteras. / This work aims to design and optimize transistors in power amplifiers of types A, B, AB, and F based on parameters such as linearity, operating frequency, output power and efficiency. The operating frequency is set to 1 GHz, and the power amplifier shall have high linearity and high efficiency. The expected result after completing the work is an optimization technique that can be used step by step in a simulated environment to optimize transistors in power amplifiers. A basic topology, for all power amplifiers which were analysed, is constructed and only one transistor is used in the design of the power amplifiers. Various methods are investigated to optimize the amplifiers. The methods used in the optimization of the power amplifiers are two types of Loadpull-analysis, voltage and current ratio, DCIV-characteristics and stability analysis. These methods are used and demonstrated in the work. Optimized amplifiers of the different amplifier classes are presented. The methods used has been evaluated and an optimization technique is presented.

DESCRIPTION DES PHENOMENES CITES DANS LES TYPES DE TRANSISTORS SUIVANTS: TRANSISTOR VMOS, TRANSISTOR UMOS, TRANSISTOR HEXFET

There is a compelling need to explore different material options as well as device structures to facilitate smooth transistor scaling for higher speed, higher density and lower power. The enormous potential of nanoelectronics, and nanotechnology in general, offers us the possibility of designing devices with added functionality. However, at the same time, the new materials come with their own challenges that need to be overcome. In this work, we have addressed some of these challenges in the context of quasi-2D Silicon, III-V semiconductor and graphene. Bulk Si is the most widely used semiconductor with an indirect bandgap of about 1.1 eV. However, when Si is thinned down to sub-10nm regime, the quasi-2D nature of the system changes the electronic properties of the material significantly due to the strong geometrical confinement. Using a tight-binding study, we show that in addition to the increase in bandgap due to quantization, it is possible to transform the original in direct bandgap to a direct one. The effective masses at different valleys are also shown to vary uniquely in an anisotropic way. This ultra-thin Si, when used as a channel in a double gate MOSFET structure, creates so called “volume in version” which is extensively investigated in this work. It has been found that the both the quantum confinement as well as the gating effect play a significant role in determining the spatial distribution of the charge, which in turn has an important role in the characteristics of transistor. Compound III-V semiconductors, like Inx Ga1-xAs, provide low effective mass and low density of states. This, when coupled with strong confinement in a nanowire channel transistor, leads to the “Ultimate Quantum Capacitance Limit” (UQCL) regime of operation, where only the lowest subband is occupied. In this regime, the channel capacitance is much smaller than the oxide capacitance and hence dominates in the total gate capacitance. It is found that the gate capacitance change qualitatively in the UQCL regime, allowing multi-peak, non-monotonic capacitance-voltage characteristics. It is also shown that in an ideal condition, UQCL provides improved current saturation, on-off ratio and energy-delay product, but a degraded intrinsic gate delay. UQCL shows better immunity towards series resistance effect due to increased channel resistance, but is more prone to interfacial traps. A careful design can provide a better on-off ratio at a given gate delay in UQCL compared to conventional MOSFET scenario. To achieve the full advantages of both FinFET and HEMT in III-V domain, a hybrid structure, called “HFinFET” is proposed which provides excellent on performance like HEMT with good gate control like FinFET. During on state, the carriers in the channel are provided using a delta-doped layer(like HEMT) from the top of a fin-like non-planar channel, and during off state, the gates along the side of the fin(like FinFET) help to pull-off the carriers from the channel. Using an effective mass based coupled Poisson-Schrodinger simulation, the proposed structure is found to outperform the state of the art planar and non-planar MOSFETs. By careful optimization of the gate to source-drain underlap, it is shown that the design window of the device can be increased to meet ITRS projections at similar gate length. In addition, the performance degradation of HFinFET in presence of interface traps has been found to be significantly mitigated by tuning the underlap parameter. Graphene is a popular 2D hexagonal carbon crystal with extraordinary electronic, mechani-cal and chemical properties. However, the zero band gap of grapheme has limited its application in digital electronics. One could create a bandgap in grapheme by making quasi-1D strips, called nanoribbon. However, the bandgap of these nanoribbons depends on the the type of the edge, depending on which, one can obtain either semiconducting or metallic nanoribbon. It has been shown that by the application of an external transverse field along the sides of a nanoribbon, one could not only modulate the magnitude of the bandgap, but also change it from direct to indirect. This could open up interesting possibilities for novel electronic and optoelectronic applications. The asymmetric potential distribution inside the nanoribbon is found to result in such direct to indirect bandgap transition. The corresponding carrier masses are also found to be modulated by the external field, following a transition from a“slow”electron to a“fast” electron and vice-versa. Experimentally, it is difficult to control the bandgap in nanoribbons as precise edge control at nanometer scale is nontrivial. One could also open a bandgap in a bilayer graphene, by the application of vertical electric field, which has raised a lot of interest for digital applications. Using a self-consistent tight binding theory, it is found that, inspite of this bandgap opening, the intrinsic bias dependent electronic structure and the screening effect limit the subthreshold slope of a metal source drain bilayer grapheme transistor at a relatively higher value-much above the Boltzmann limit. This in turn reduces the on-off ratio of the transistor significantly. To overcome this poor on-off ratio problem, a semiconductor source-drain structure has been proposed, where the minority carrier injection from the drain is largely switched off due to the bandgap of the drain. Using a self-consistent Non-Equilibrium Green’s Function(NEGF) approach, the proposed device is found to be extremely promising providing unipolar grapheme devices with large on-off ratio, improved subthreshold slope and better current saturation. At high drain bias, the transport properties of grapheme is extremely intriguing with a number of nontrivial effects. Optical phonons in monolayer grapheme couple with carriers in a much stronger way as compared to a bilayer due to selection rules. However, it is difficult to experimentally probe this through transport measurements in substrate supported grapheme as the surface polar phonons with typical low activation energy dominates the total scattering. However, at large drain field, the carriers obtain sufficient energy to interact with the optical phonons, and create so called ‘hot phonons’ which we have experimentally found to result in a negative differential conductance(NDC). The magnitude of this NDC is found to be much stronger in monolayer than in bilayer, which agrees with theoretical calculations. This NDC has also been shown to be compensated by extra minority carrier injection from drain at large bias resulting in an excellent current saturation through a fundamentally different mechanism as compared to velocity saturation. A transport model has been proposed based on the theory, and the experimental observations are found to be in agreement with the model.

Die vorliegende Arbeit befasst sich mit der Simulation und Modellierung organischer Feldeffekt-Transistoren (OFETs). Mittels numerischer Simulationen wurden detaillierte Untersuchungen zu mehreren Problemstellungen durchgeführt. So wurde der Einfluss einer exponentiellen Verteilung von Trapzuständen, entsprechend dem sogenannten a-Si- oder TFT-Modell, auf die Transistorkennlinien untersucht. Dieses Modell dient der Beschreibung von Dünnschicht-Transistoren mit amorphen Silizium als aktiver Schicht und wird teils auch für organische Transistoren als zutreffend angesehen. Dieser Sachverhalt wird jedoch erstmals in dieser Arbeit detailliert untersucht und simulierte Kennlinien mit gemessenen Kennlinien von OFETs verglichen. Insbesondere aufgrund der Dominanz von Hysterese-Effekten in experimentellen Kennlinien ist jedoch eine endgültige Aussage über die Gültigkeit des a-Si-Modells schwierig. Neben dem a-Si-Modell werden auch noch andere Modelle diskutiert, z.B. Hopping-Transport zwischen exponentiell verteilten lokalisierten Zuständen (Vissenberg, Matters). Diese Modelle liefern, abhängig von den zu wählenden Modellparametern, zum Teil ähnliche Abhängigkeiten. Möglicherweise müssen die zu wählenden Modellparameter selbst separat gemessen werden, um eindeutige Schlussfolgerungen über den zugrundeliegenden Transportmechanismus ziehen zu können. Unerwünschte Hysterese-Effekte treten dabei sowohl in Transistorkennlinien als auch in Kapazitäts-Spannungs- (CV-) Kennlinien organischer MOS-Kondensatoren auf. Diese Effekte sind bisher weder hinreichend experimentell charakterisiert noch von ihren Ursachen her verstanden. In der Literatur findet man Annahmen, dass die Umladung von Trapzuständen oder bewegliche Ionen ursächlich sein könnten. In einer umfangreichen Studie wurde daher der Einfluß von Trapzuständen auf quasistatische CV-Kennlinien organischer MOS-Kondensatoren untersucht und daraus resultierende Hysterese-Formen vorgestellt. Aus den Ergebnissen läßt sich schlussfolgern, dass allein die Umladung von Trapzuständen nicht Ursache für die experimentell beobachteten Hysteresen in organischen Bauelementen sein kann. Eine mögliche Erklärung für diese Hysterese-Effekte wird vorgeschlagen und diskutiert. In einem weiteren Teil der Arbeit wird im Detail die Arbeitsweise des source-gated Dünnschicht-Transistors (SGT) aufgezeigt, ein Transistortyp, welcher erst kürzlich in der Literatur eingeführt wurde. Dies geschieht am Beispiel eines Transistors auf der Basis von a-Si als aktiver Schicht, die Ergebnisse lassen sich jedoch analog auch auf organische Transistoren übertragen. Es wird geschlussfolgert, dass der SGT ein gewöhnlich betriebener Dünnschicht-Transistor ist, limitiert durch das Sourcegebiet mit großem Widerstand. Die detaillierte Untersuchung des SGT führt somit auf eine Beschreibung, die im Gegensatz zur ursprünglich verbal diskutierten Arbeitsweise steht. Ambipolare organische Feldeffekt-Transistoren sind ein weiterer Gegenstand der Arbeit. Bei der Beschreibung ambipolarer Transistoren vernachlässigen bisherige Modelle sowohl die Kontakteigenschaften als auch die Rekombination von Ladungsträgern. Beides wird hingegen in den vorgestellten numerischen Simulationen erstmalig berücksichtigt. Anhand eines Einschicht-Modellsystems wurde die grundlegende Arbeitsweise von ambipolaren (double-injection) OFETs untersucht. Es wird der entscheidende Einfluß der Kontakte sowie die Abhängigkeit gegenüber Variationen von Materialparametern geklärt. Sowohl der Kontakteinfluß als auch Rekombination sind entscheidend für die Arbeitsweise. Zusätzlich werden Möglichkeiten und Einschränkungen für die Datenanalyse mittels einfacher analytischer Ausdrücke aufgezeigt. Es zeigte sich, dass diese nicht immer zur Auswertung von Kennlinien herangezogen werden dürfen. Weiterhin werden erste Simulationsergebnisse eines ambipolaren organischen Heterostruktur-TFTs mit experimentellen Daten verglichen.

Cette thèse porte sur l'étude d'un diélectrique de type ferroélectrique pour une application aux transistors organiques. La configuration adoptée est de type bottom-gate top- contact. Le matériau semi-conducteur utilisé est un transporteur d'électrons. Dans la première partie de ce projet, nous avons réalisé des transistors organiques à effet de champ (OFETs) avec une couche de PMMA comme diélectrique de grille. Ce matériau, très étudié et connu, permet d'avoir un composant servant de référence. Nous avons également mené une étude sur la longueur de canal, la vitesse de dépôt du semi-conducteur organique et l'épaisseur du diélectrique, en vue d'en déduire l'influence de ces grandeurs sur les performances électriques des OFETs. Après l'optimisation de ces paramètres, nous avons démontré une amélioration de la mobilité des porteurs, une augmentation du rapport Ion/Ioff, une amélioration de la capacité et une diminution des tensions d'alimentation et de seuil. Ces résultats ont été interprétés à l'aide de caractérisations électriques. Dans un second temps, le diélectrique ferroélectrique poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) a été ajouté au composant, afin de réaliser un diélectrique hybride avec le PMMA. Ce dernier permet de combiner les avantages de la haute permittivité relative du P(VDF-TrFE), et de la faible rugosité du film de PMMA en contact avec le semiconducteur. Une étude comparative a été effectuée avec les transistors de référence. Il en ressort, pour une épaisseur identique de diélectrique, une diminution des tensions d'alimentation et de seuil, et une amélioration de la mobilité des charges avec l'OFET implémentant le matériau ferroélectrique. La discussion de ces résultats est appuyée par des caractérisations électriques et morphologiques. / This thesis deals with the study of a ferroelectric material as gate dielectric for organic transistor applications. The configuration adopted is bottom-gate top-contact. The semiconductor used is an electron transport material. In a first part, we made organic field effect transistors (OFETs) with a layer of PMMA as a gate dielectric. This material, very studied and well known, serves as reference. We also carried out a study on the channel length, the organic semiconductor deposition rate and the dielectric thickness, in order to deduce the impact of these parameters on OFETs performances. After optimization, we have demonstrated an improvement of the mobility, on/off current ratio, capacitance and a reduction of supply and threshold voltages. These results have been interpreted using electrical characterizations. In a second step, the poly (vinylidenefluoride-co- trifluoroethylene) (P(VDF-TrFE)) ferroelectric material was added to provide a hybrid dielectric with PMMA. This OFET combine the advantages of high permittivity of P(VDF-TrFE) and low roughness of PMMA. A comparative study was carried out with reference transistors. For same dielectric thickness, a reduction of the supply and threshold voltages and an improvement of the mobility is obtained for the OFET implementing ferroelectric material. The discussion of these results is supported by electrical and morphological characterizations.

Diese Arbeit stellt eine eingehende Studie des sogenannten Vertikalen Organischen Feld-Effekt-Transistors (VOFET) dar, einer neuen Transistor-Geometrie, welche dem stetig wachsenden Bereich der organischen Elektronik entspringt. Dieses neuartige Bauteil hat bereits bewiesen, dass es in der Lage ist, eine der fundamentalen Einschränkungen herkömmlicher organischer Feld-Effekt-Transistoren (OFETs) zu überwinden: Die für Schaltfrequenz und An-Strom wichtige Kanallänge des Transistors kann im VOFET stark reduziert werden, ohne dass teure und komplexe Strukturierungsmethoden genutzt werden müssen. Das genaue Funktionsprinzip des VOFET ist bisher jedoch weitgehend unerforscht. Durch den Vergleich von experimentellen Daten mit Simulationsdaten des erwarteten Bauteil-Verhaltens wird hier ein erstes, grundlegendes Verständnis des VOFETs erarbeitet. Die so gewonnenen Erkenntnisse werden im Folgenden genutzt, um bestimmte Parameter des VOFETs kontrolliert zu manipulieren. So wird beispielsweise gezeigt, dass die Morphologie des organischen Halbleiters, und damit seine Abscheidungsparameter, sowohl für die VOFET-Herstellung als auch für den Ladungsträgertransport im fertigen Bauteil eine wichtige Rolle spielen. Weiterhin wird gezeigt, dass der VOFET, genau wie der konventionelle OFET, durch das Einbringen von Kontaktdotierung deutlich verbessert werden kann. Mit Hilfe dieser Ergebnisse kann gezeigt werden, dass das Funktionsprinzip des VOFETs mit dem eines konventionellen OFETs nahezu identisch ist, wenn man von geringen Abweichungen aufgrund der unterschiedlichen Geometrien absieht. Basierend auf dieser Erkenntnis wird schließlich ein VOFET präsentiert, welcher im Inversionsmodus betrieben werden kann und so die Lücke zur konventionellen MOSFET-Technologie schließt. Dieser Inversions-VOFET stellt folglich einen vielversprechenden Ansatz für leistungsfähige organische Transistoren dar, welche als Grundbausteine für komplexe Elektronikanwendungen auf flexiblen Substraten genutzt werden können.:Zusammenfassung 5 Abstract 6 Publications 13 Introduction 17 Basic Principles of Organic Semiconductors and Related Devices 23 1. The Physics of Organic Semiconductors 25 1.1. Electronic and structural properties of organic semiconductors 28 1.2. Charge carrier transport 34 1.3. Doping of organic semiconductors 43 2. Organic field-effect transistors 47 2.1. Operational principle 50 2.2. Functional interfaces in OFETs 55 2.3. Contact resistance and short-channel effects in OFETs 60 2.4. Applications of OFETs and related devices 65 3. Vertical organic transistors 77 3.1. Organic permeable-base transistors (OPBTs) and organic static induction transistors (OSITs) 81 3.2. Organic Schottky barrier transistors (OSBTs) 85 3.3. Vertical organic field-effect transistors (VOFETs) 90 Study of the Vertical Organic Field-Effect Transistor 97 4. Methods and Materials 99 4.1. Materials 101 4.2. Sample preparation 104 4.3. Sample characterisation 110 5. Material Optimisation for VOFETs 121 5.1. Variation of the source insulator 123 5.2. Effects of the pentacene morphology 133 5.3. Summary 137 6. Charge Transport in the VOFET 139 6.1. Simulating current flow in the VOFET 141 6.2. The vertical channel 154 6.3. Charge transport in pentacene 161 6.4. Effects of mobility and layer thickness in pentacene VOFETs 167 6.5. Summary 175 7. Doping Concepts for VOFETs 177 7.1. Doping of the bulk regions 179 7.2. Selective contact doping 183 7.3.Impact on the understanding of VOFET operation 194 7.4. Summary 198 8. Vertical Organic Inversion Transistors 201 8.1. Discussion of suitable material systems 204 8.2. Realising inversion VOFETs 207 8.3. Summary 212 9. Conclusion and Outlook 215 9.1. Conclusion 217 9.2. Outlook 219 Appendix 221 A. XRD spectra of pentacene films 223 B. Additional simulation data 227 Bibliography 229 Addresses 257 Important Symbols, Constants and Abbreviations 263 List of Figures 271 Acknowledgements 283 / This work represents a comprehensive study of the so-called vertical organic field-effect transistor (VOFET), a novel transistor geometry originating from the fast-growing field of organic electronics. This device has already demonstrated its potential to overcome one of the fundamental limitations met in conventional organic transistor architectures (OFETs): In the VOFET, it is possible to reduce the channel length and thus increase On-state current and switching frequency without using expensive and complex structuring methods. Yet the VOFET's operational principles are presently not understood in full detail. By simulating the expected device behaviour and correlating it with experimental findings, a basic understanding of the charge transport in VOFETs is established and this knowledge is subsequently applied in order to manipulate certain parameters and materials in the VOFET. In particular, it is found that the morphology, and thus the deposition parameters, of the organic semiconductor play an important role, both for a successful VOFET fabrication and for the charge transport in the finished device. Furthermore, it is shown that VOFETs, just like their conventional counterparts, are greatly improved by the application of contact doping. This result, in turn, is used to demonstrate that the VOFET essentially works in almost exactly the same way as a conventional OFET, with only minor changes due to the altered contact arrangement. Working from this realisation, a vertical organic transistor is developed which operates in the inversion regime, thus closing the gap to conventional MOSFET technology and providing a truly promising candidate for high-performance organic transistors as the building blocks for advanced, flexible electronics applications.:Zusammenfassung 5 Abstract 6 Publications 13 Introduction 17 Basic Principles of Organic Semiconductors and Related Devices 23 1. The Physics of Organic Semiconductors 25 1.1. Electronic and structural properties of organic semiconductors 28 1.2. Charge carrier transport 34 1.3. Doping of organic semiconductors 43 2. Organic field-effect transistors 47 2.1. Operational principle 50 2.2. Functional interfaces in OFETs 55 2.3. Contact resistance and short-channel effects in OFETs 60 2.4. Applications of OFETs and related devices 65 3. Vertical organic transistors 77 3.1. Organic permeable-base transistors (OPBTs) and organic static induction transistors (OSITs) 81 3.2. Organic Schottky barrier transistors (OSBTs) 85 3.3. Vertical organic field-effect transistors (VOFETs) 90 Study of the Vertical Organic Field-Effect Transistor 97 4. Methods and Materials 99 4.1. Materials 101 4.2. Sample preparation 104 4.3. Sample characterisation 110 5. Material Optimisation for VOFETs 121 5.1. Variation of the source insulator 123 5.2. Effects of the pentacene morphology 133 5.3. Summary 137 6. Charge Transport in the VOFET 139 6.1. Simulating current flow in the VOFET 141 6.2. The vertical channel 154 6.3. Charge transport in pentacene 161 6.4. Effects of mobility and layer thickness in pentacene VOFETs 167 6.5. Summary 175 7. Doping Concepts for VOFETs 177 7.1. Doping of the bulk regions 179 7.2. Selective contact doping 183 7.3.Impact on the understanding of VOFET operation 194 7.4. Summary 198 8. Vertical Organic Inversion Transistors 201 8.1. Discussion of suitable material systems 204 8.2. Realising inversion VOFETs 207 8.3. Summary 212 9. Conclusion and Outlook 215 9.1. Conclusion 217 9.2. Outlook 219 Appendix 221 A. XRD spectra of pentacene films 223 B. Additional simulation data 227 Bibliography 229 Addresses 257 Important Symbols, Constants and Abbreviations 263 List of Figures 271 Acknowledgements 283

Organic transistors with vertical current transport like the Permeable Base Transistor (PBT) show a high performance while allowing for an easy fabrication on the device level. For a simple implementation on a circuit level, ambipolar transistors, providing the functionality of n-type as well as p-type devices, have a benefit for complementary logic. This requires transistors where electrons and holes are present. Here, we investigate a potential concept of bipolar current transport in PBTs. In our device structure, we use the base electrode to control the current flow, but also to investigate the charge carrier transport. The ambipolar organic PBT achieves a charge carrier transmission of 88% and a current density above 200mA=cm². Additionally, we show that recombination near the base is required in an ambipolar PBT for a good performance.