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Since the seminal discovery of conductive polymers four decades ago, organic electronics has grown from an exploratory field to an industry offering novel consumer products. Research has led to the synthesis of new organic molecules and polymers and their applications: organic field-effect transistors (OFETs), organic light-emitting diodes, and organic photovoltaics. The goal for research as well as for industry is producing low-cost, flexible, and, ultimately, sustainable, electronics. Although on the rise, organic electronics faces several challenges: air instability, reliability, and scaling, to name a few. And despite that organic devices and larger systems have been demonstrated, there remains a gap in understanding underlying mechanisms behind light absorption, photoconduction, charge transport and conduction in them. The primary purpose of this thesis is to use a relatively under utilized technique, photocurrent microscopy (PCM), to directly probe charge carriers in pentacene and 6,13-Bis(triisopropylsilylethynyl) (TIPS) pentacene FETs to learn about charge injection and transport. The latter part of the thesis focuses on the use of thiols to modify electrode properties to both increase charge injection efficiency and to provide passivation to low-work function metal electrodes. It is demonstrated for the first time experimentally by directly probing the OFET channel that top-contact geometry OFETs suffer minimally from a charge injection barrier, and that trap filling and altering of trap density-of-states in the channel is directly observable with PCM. PCM was used to investigate grains and grain boundaries in TIPS-pentacene devices. By varying gate bias, it was shown that the PCM maps of grains are not simply a result of varying absorption on the surface of the film; rather, it is an artefact of charge transport between grains and grain boundaries. Through this study, PCM was shown to be a useful, large-area scanning technique, for observing transport in devices with large (on the order of 50 $\mu$m) grains. This is particularly relevant as solution-proccessable films are likely to dominate the flexible electronics industry. The thiol portion of this thesis compares the impact of two distinct thiols on bottom-contact pentacene FETs: perfluorodecanethiol (PFDT) and pentafluorobenzenethiol (PFBT). Using X-ray photoelectron spectroscopy to measure metal oxidation, it was determined that short aromatic thiols are poor choices for low work-function metal passivation. In addition, both passivation and charge injection enhancement can be achieved with long fluorinated alkanethiols. However, there is a trade-off between passivation and on-current. The enhancement of on-current in thiol-treated Cu-electrode pentacene devices is most likely not morphology related, due to the fact that PFDT was found to be in a standing-up orientation on the metal surface. Additionally, it was demonstrated that although highly electronegative atoms such as fluorine can beneficially modify metal work function, too many fluorine atoms in thiols can lead to too high a work function and a large mismatch between the pentacene highest-occupied-molecular-level and metal work function.

The dependence of the substrate resistance, R[subscript sub], for MOS transistor RF modeling on transistor biasing and layout is studied from device simulations and measurements. Though R[subscript sub] is found to be bias dependent, the error incurred by assuming a constant value equal to the DC resistance is not significant. A scalable model for R[subscript sub] of multiple gate fingers is developed. This model is simple to extract and gives good agreement for the output admittance of a MOSFET. The model is validated by measurements on DC test structures fabricated in a TSMC 0.35 ��m CMOS process. The dependence of Rb on transistor dimensions and the location of substrate contacts with respect to device active area is also presented. A low noise amplifier (LNA) is designed and fabricated in the 0.35 ��m TSMC process to show the effect of R[subscript sub] on the performance of a LNA. / Graduation date: 2003

The electron, gamma and neutron radiation degradation of III-V semiconductors and heterojunction bipolar transistors (HBTs) is investigated in this thesis. Particular attention is paid to InP and InGaAs materials and InP/InGaAs abrupt single HBTs (SHBTs). Complete process sequences for fabrication of InP/InGaAs HBTs are developed and subsequently employed to produce the devices, which are then electrically characterized and irradiated with the different types of radiation. A comprehensive analytical HBT model is developed and radiation damage calculations are performed to model the observed radiation-induced degradation of SHBTs. The most pronounced radiation effects found in SHBTs include reduction of the common-emitter DC current gain, shift of the collector-emitter (CE) offset voltage and increase of the emitter, base and collector parasitic resistances. Quantitative analysis performed using the developed model demonstrates that increase of the neutral bulk and base-emitter (BE) space charge region (SCR) components of the base current are responsible for the observed current gain degradation. The rise of the neutral bulk recombination is attributed to decrease in a Shockley-Read-Hall (SRH) carrier lifetime, while the SCR current increase is caused by rising SCR SRH recombination and activation of a tunneling-recombination mechanism. On the material level these effects are explained by displacement defects produced in a semiconductor by the incident radiation. The second primary change of the SHBT characteristics, CE offset voltage shift, is induced by degradation of the base-collector (BC) junction. The observed rise of the BC current is brought on by diffusion and recombination currents which increase as more defects are introduced in a semiconductor. Finally, the resistance degradation is attributed to deterioration of low-doped layers of a transistor, and to degradation of the device metal contacts. / Graduation date: 2001

Complementary delta-doped AlGaAs/GaAs Heterojunction Field Effect Transistor (CHFET) devices and circuits were fabricated using MBE and a 2�� non-planar gate recess process. Several schemes were used in an attempt to improve the performance of the p-channel HFETs. These included delta-doping, carbon-doping and dipole-doping. Circuits and individual n- and p- channel devices were fabricated on a stacked delta-doped complementary structure. The circuits failed to perform due to complications with adjusting the threshold voltage. However, Individual devices were successfully characterized, p-channel devices with extrinsic transconductances up to 14 mS/mm, n-channel devices with extrinsic transconductances up to 120 mS/mm and a unity power gain bandwidth of 5.5 GHz. / Graduation date: 1995

Graduation date: 1994