published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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Molecular self-assembly has recently attracted significant attention for possible application in organic electronic and optoelectronic devices, such as self-assembled monolayer field-effect transistors (SAMFETs) and functional self-assembled integrated circuits. Self-assembly combines the advantages of low temperature solution processability, regio-selective monolayer adsorption and nano-scale control of film thickness. Much progress has been made in improving device performance using self-assembled monolayers (SAMs). However, most SAMFET devices reported to date showed current modulation only with submicrometre channels, with low device yields and poor reproducibility as a result of limited lateral interconnection of the semiconducting layer.In an attempt to address these issues, this thesis presents an investigation of the synthesis and properties of conjugated SAM molecules for use as the charge transporting layer in SAMFETs. Chapter 1 gives a comprehensive introduction to SAM-based surface systems, organic semiconductors and their use in OFETs and SAMFETs. Chapter 2 discusses attempts to design and synthesise p-type conjugated molecules capable of self-assembly on oxide surfaces based on a phenylene-bithiophene semiconducting core. The optical and electrochemical properties, as well as the thermal behaviour of these molecules are studied in detail. This theme is carried over to Chapter 3, which describes the synthesis, chemical and physical characterisation of two families of n-type SAM molecules. These molecules consist of NTCDI cores with hexyl or cyclohexyl chains as end-capping groups. Incorporation of a selection of materials as the active layer in OFETs or SAMFETs to evaluate the charge transport is demonstrated in Chapter 4. Monolayer films based on p-type monochlorosilane-terminated SAM molecules are made using the solution assembly technique and characterised by contact angle and AFM. OFETs made from DH-PTTP by both thermal evaporation and spin coating show high mobilities comparable to the best values reported in the literature. Top-contact SAMFETs show a hole mobility of 1.1 × 10-3 cm2V-1s-1 in air, consistent with those of solution processed DH-PTTP based OFETs. Finally, an overview of the project and some suggestions for future work are presented in Chapter 5.

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The process of designing high power RF amplifiers has in the past relied heavily on measurements, in conjunction with simple linear theory. With the advent of the harmonic balance method and increasingly faster computers, CAD techniques can be of great value in designing these nonlinear circuits. Relatively little work has been done in modelling RF power MOSFETs. The methods described in numerous papers for the nonlinear modelling of microwave GaAsFETs cannot be applied easily to these high power devices. This thesis describes a modelling procedure applicable to RF MOSFETs rated at over 100 W. This is achieved by the use of cold S parameters and pulsed drain current measurements taken at controlled temperatures. A method of determining the required device thermal impedance is given. A complete nonlinear equivalent circuit model is extracted for an MRF136 MOSFET, a 28 V, 15 W device. This includes two nonlinear capacitors. An equation is developed to describe accurately the drain current as a function of the internal gate and drain voltages. The model parameters are found by computer optimisation with measured data. Techniques for modelling the passive components in RF power amplifiers are given. These include resistors, inductors, capacitors, and ferrite transformers. Although linear ferrite transformer models are used, nonlinear forms are also investigated. The accuracy of the MOSFET model is verified by comparison to large signal measurements in a 50 0 system. A complete power amplifier using the MRF136, operating from 118 MHz to 175 MHz is built and analysed. The accuracy of predictions is generally within 10 % for output power and DC supply current, and around 30 % for input impedance. An amplifier is designed using the CAD package, and then built, requiring only a small final adjustment of the input matching circuit. The computer based methods described lead quickly to a near-optimal design and reduce the need for extensive high power measurements. The use of nonlinear analysis programs is thus established as a valuable design tool for engineers working with RF power amplifiers. / Thesis (Ph.D.)-University of Natal, Durban, 1992.

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Reliability of sub-micron analog circuits is directly related to impact ionization and the subsequent changes in threshold voltage and drain current of n-MOSFET devices. This thesis presents theory of the hot-electron effects on the device characteristics and circuit performance, explores several approaches to improve performance at both the device and circuit level, and finally shows a new composite n-MOSFET device which significantly suppresses substrate current - an indication of hot-electron degradation. By using the composite device in the output gain stage of a CMOS differential amplifier with 1p.m technology, the normalized substrate current of the n-channel device is reduced by eight orders of magnitude for a sloping input waveform. The reduction in device substrate current is achieved at the cost of increased area and reduced frequency response. Replacing conventional n-channel devices with composite n-MOSFETs provides a simple way to improve device and circuit reliability without modification of the device structure and/or fabrication process. / Graduation date: 1993

Thesis (MEng)--University of South Australia, 1997