Experiments were conducted on current-mirror MOSFETs to examine their suitability for use in radiation environments. These devices, which allow low loss load current sensing (defined by a current-ratio n'), are an important element of many power integrated circuits (PICs). Total-dose testing demonstrated that the current ratio was virtually unaffected for many operating conditions. In all cases, changes were largest when sense resistance was largest and minimal when sense voltage was approximately equal to the load source's voltage. In addition, testing verified the feasibility of using sense-cell MOSFETs for applications which require radiation exposure. A constant-current op-amp circuit showed minimal current shifts, using proper circuit design, following total-dose exposure. Dose-rate testing showed the feasibility of using sense voltage to trigger g&d2; protection through drain-source voltage clamping, providing a relatively inexpensive alternative to voltage derating.

Fast neutron irradiation tests were performed to determine the correlation of change of drain-source resistance and neutron fluence for power MOSFETs. The Objectives of the tests were: (1) to detect and measure the degradation of critical MOSFET device parameters as a function of neutron fluence (2) to compare the experimental results and the theoretical model. In general, the drain-source resistance increased from 1 Ohm to 100 Ohm after exposure to fast neutron fluence of 3 x 1014 neut/cm2, and decreased by a factor of five after high temperature annealing.

Many high performance amplifiers use power MOSFETs in their output stages, especially in operational amplifier applications whenever high current or power is needed. MOSFETs have advantages over bipolar transistors in amplifier output stage because MOSFETs are majority carrier devices. The result is wide frequency response, fast switching and better linearity than power bipolar transistors. But unlike bipolar circuits, which are relatively tolerant of ionizing radiation, MOSFETs may suffer severe parametric degradation at low total-dose levels. The effects of ionizing radiation on MOSFETs are discussed, and the performance of an amplifier circuit that uses a complementary MOSFET source follower in its output stage is simulated to examine the effect of MOSFET radiation damage on amplifier performance. An increase in power dissipation was the most significant degradation caused by ionizing radiation.

The discovery of semi-conducting and conducting organic materials has opened new possibilities for electronic devices and systems. Applications, previously unattainable for conventional electronics, have become possible thanks to the development of conjugated polymers. Conjugated polymers that are both ion- and electron conducting, allow for electrochemical doping and de-doping via reversible processes as long as both forms of conduction remain available. Doping causes rearrangement of the -system along the polymer backbone, and creates new states in the optical band gap, resulting in an increased electronic conductivity and also control of the color (electrochromism). Doping can also occur by charge injection at a metal – semiconducting polymer interface. Electrochemical electronic devices and solid state devices based on these two types of doping are now beginning to enter the market. This thesis deals with organic based-devices whose working mechanism involves electrolytes. After describing the properties of conjugated polymers, fundamentals on electrolytes (ionic conductivity, types, electric double layer and the electric field distribution) are briefly presented. Thereafter, a short review of the field of organic field effect transistors as well as a description of transistors that are gated via an electrolyte will be reviewed. Paper I present a novel technique to visualize the electric field within a two-dimensional electrolyte by applying the electrolyte over an array of electronically isolated islands of electrochromic polymer material on a plastic foil. By observing the color change within each polymer island the direction and the magnitude of the electric field can be measured. This technology has applications in electrolyte evaluation and is also applicable in bio-analytical measurements, including electrophoresis. The focus of paper II lies on gating an organic field effect transistor (OFET) by a polyanionic proton conductor. The large capacitance of the electric double layer (EDL) that is formed at organic semiconductor/polyelectrolyte upon applying a potential to the gate, results in low operation voltages and fast response. This type of transistor that is gated via electric double layer capacitor is called EDLC-OFET. Because an electrolyte is used as a gate insulator, the role of the ionic conductivity of the electrolyte is considered in paper III. The effect on the electronic performance of the transistor is studied as well by varying the humidity level.

Since the first isolation of graphene in 2004 interest in superconductivity and the superconducting proximity effect in monolayer or few-layer crystals has grown rapidly. This thesis describes studies of both the proximity effect in single and fewlayer graphene flakes, as well as the superconducting transition in few unit cell chalcogenide flakes. Optical and atomic force microscopy and Raman spectroscopy have been used to characterise the quality and number of molecular layers present in these flakes. Graphene structures with superconducting Al electrodes have been realised by micromechanical cleavage techniques on Si/SiO2 substrates. Devices show good normal state transport characteristics, efficient back-gating of the longitudinal resistivity, and low contact resistances. Several trials have been made to investigate proximity-induced critical currents in devices with junction lengths in the range 250-750 nm. Unfortunately, no sign of proximity supercurrents was observed in any of these devices. Nevertheless the same devices have been used to carefully characterise proximity doping, (due to the deposited electrode), and weak localisation/anti-localisation contributions to the conductivity in them. In addition this work has been extended to investigations of the superconducting transition in few unit-cell dichalcogenide flakes. Four-terminal devices have been realised by micromechanical cleavage from a 2H-NbSe2 single crystal onto Si/SiO2 substrates followed by the deposition of Cr/Au contacts. While very thin NbSe2 flakes do not appear to conduct, slightly thicker flakes are superconducting with an onset ܶ௖ that is only slightly depressed from the bulk value (7.2K). The resistance typically shows a small, sharp, high temperature transition followed by one or more broader transitions, which end in a wide tail to zero resistance at low temperatures. These multiple transitions appear to be related to disorder in the layer stacking rather than lateral inhomogeneity. The behaviour of several flakes has been characterised as a function of temperature, applied field and back-gate voltage. The resistance and transition temperatures are found to depend weakly on the gate voltage. Results have been analysed in terms of available theories for these phenomena.

Two-dimensional materials with atomic thickness have attracted a lot of attention from researchers worldwide due to their excellent electronic and optical properties. As the silicon technology is approaching its limit, graphene with ultrahigh carrier mobility and ultralow resistivity shows the potential as channel material for novel high speed transistor beyond silicon. This thesis summarises my Ph.D. work including the theory and modelling of graphene field-effect transistors (GFETs) as well as their potential RF applications. The introduction and review of existing graphene transistors are presented. Multiscale modelling approaches for graphene devices are also introduced. A novel analytical GFET model based on the drift-diffusion transport theory is then developed for RF/microwave circuit analysis. Since the electrons and holes have different mobility variations against the channel potential in graphene, the ambipolar GFET cannot be modelled with constant carrier mobility. A new carrier mobility function, which enables the accurate modelling of the ambipolar property of GFET, is hence developed for this purpose. The new model takes into account the carrier mobility variation against the bias voltage as well as the mobility difference between electrons and holes. It is proved to be more accurate for the DC current calculation. The model has been written in Verilog-A language and can be import into commercial software such as Keysight ADS for circuit simulation. In addition, based on the proposed model two GFET non-Foster circuits (NFCs) are conducted. As a negative impedance element, NFCs find their applications in impedance matching of electrically small antennas and bandwidth improvement of metasurfaces. One of the NFCs studied in this thesis is based on the Linvill's technique in which a pair of identical GFETs is used while the other circuit utilises the negative resistance of a single GFET. The stability analysis of NFCs is also presented. Finally, a high impedance surface loaded with proposed NFCs is also studied, demonstrating significant bandwidth enhancement.

Graphene, first isolated in 2004 by Andre Geim and Konstantin Novoselov, is an atomically thin two-dimensional layer of hexagonal carbon that has been extensively studied due to its unique electronic, mechanical, thermal and optical properties. Its vast potential has led to the development of a wide variety of novel devices such as, transistors, solar cells, batteries and sensors that offer significant advantages over the conventional microelectronic ones. Although graphene-based devices show very promising performance characteristics, limited has been done in order to evaluate how these devices operate in a radiation harsh environment. Undesirable phenomena such as total dose effects, single event upsets, displacement damage and soft errors that silicon-based devices are prone to, can have a detrimental impact on performance and reliability. Similarly, the significant effects of irradiation on carbon nanotubes indicate the potential for related radiation induced defects in carbon-based materials, such as graphene. In this work, we fabricate graphene field effect transistors (GFETs) and systematically study the various effects of ionizing radiation on the material and device level. Graphene grown by chemical vapor deposition (CVD) along with standard lithographic and shadow masking techniques, was used for the transistor fabrication. GFETs were subjected to different radiation sources, such as, beta particles (electron radiation), gamma (photons) and ions (alpha, protons and Fe particles) under various radiation doses and energies. The effects on graphene’s crystal structure, transport properties and doping profile were examined by using a variety of characterization tools and techniques. We demonstrate not only the mechanisms of ionized charge build up in the substrate and displacement damage effects on GFET performance, but also that atmospheric adsorbents from the surrounding environment can have a significant impact on the radiation hardness of graphene. We developed different transistor structures that mitigate these effects and performed computer simulations to enhance even further our understanding of radiation damage. Our results show that devices using a passivation layer and a shielded gate structure were less prone to irradiation effects when compared to the standard back-gate GFETs, offering less performance degradation and enhanced stability over prolonged irradiation periods. This is an important step towards the development of radiation hard graphene-based devices, enabling operation in space, military, or other radiation sensitive environments.

This thesis comprises eight chapters in two parts: the first part, chapters 1 to 6, details the design, synthesis, self-assembly and electrical properties of a new class of contorted polyheteroaromatic molecules, and the chapters 7 and 8 in the second part describes the design and fabrication of the first nanoscale field-effect transistor for single-molecule kinetics study. Chapter 1 is an introductory chapter. It first introduces the concept of organic photovoltaics (OPV), including the operation principles, important parameters, device structures, and relevant studied small molecules for the active layer in OPV devices. The second part of the chapter will be an overview of single-molecule biosensors involving various techniques and some important aspects on the design and fabrication. Chapter 2 details the development of a new synthetic methodology for polyheteroaromatic compounds. As one example, contorted dibenzotetrathienocoronenes (c-DBTTC) have been efficiently synthesized in three steps with high yields (>80%). Importantly this class of molecules displays an unusual intermolecular stacking in solid state and intimate interaction with n-type materials (TCNQ and C60) due to their shape-shifting ability. Chapter 3 will describe an unusual molecular conformation in highly fluorinated contorted hexa-cata-hexabenzocoronenes (c-HBC) via the fluorine-fluorine repulsive interaction. Chapter 4 describes the self-assembly properties of a new class of materials, chalcogenide-fused c-DBTTC, investigated by grazing incidence X-ray diffraction (GIXD), fluorescence microscopy and scanning electron microscopy (SEM). In chapter 5 a reticulated heterojunction OPV device applying c-DBTTC as the p-type active layer will be detailed. Combining the excellent self-assembly of c-DBTTC with the patterned graphene electrodes gives improved field-effect mobility in devices and will be described in chapter 6. In chapter 7, a field-effect transistor using a carbon nanotube (CNTFET) will be introduced. DNA hybridization kinetics will be detected using this "label-free" nanoscale device that represents a breakthrough in the field of single-molecule techniques by delivering high sensitivity and bandwidth. In chapter 8, a basic scientific research concerning Debye screening in buffer solution will be demonstrated utilizing above-mentioned DNA devices. Again, this nanoscale device uses its ability of single-molecule detection to correlate Debye length with buffer concentrations and charge distances, respectively; the correlations will serve as important references for the design of nanoscale biosensors using carbon nanotubes.

Single-molecule carbon nanotube-based field-effect transistors are promising all-electronic devices for probing interactions of various biological and chemical molecules at the single- molecule level. Such devices consist of point-functionalized carbon nanotubes which are charge sensitive in the vicinity of a generated defect on the nanotube sidewall. Of particular interest is the characterization of the kinetic rates and thermodynamics of DNA duplex formation through repeated association (hybridization) and dissociation (melting) events on timescales unmatched by conventional single-molecule methods. In this work, we study the kinetics and thermodynamics of DNA duplex formation with two types of single-walled nanotubes: CVD-grown and solution-processed. In both assessments, we are able to extract kinetic and thermodynamic parameters governing the hybridization and melting of DNA oligonucleotides. In the latter case, devices are spun onto a wafer surface from an organic suspension, revealing consistent electrical characteristics. Significant effort is made to expand this work to wafer-level, in an effort to make the fabrication manufacturable.

This thesis explores the RNA folding problem using single-molecule field effect transistors (smFETs) to measure the lifetimes of individual RNA base-pairing rearrangements. In the course of this research, considerable computational, chemical, and engineering contributions were developed so that the single-molecule measurements could be conducted and quantified. These advancements have allowed, on the basis of the smFET data collected herein, the quantification of a kinetic model for RNA stem-loop structures which has been generalized to quantitatively explore the phenomenological observation that an RNA found in the bacillus subtilis strain acts as a metabolite-sensing switch, allowing RNA polymerase to transcribe the messenger RNA when the metabolite is present and preventing transcription when the metabolite is absent. Together, the data presented quantify a simple model for the base pairing rearrangements that underlie RNA folding.