Thesis (Ph.D.)--University of Nebraska-Lincoln, 2007. / Title from title screen (site viewed Feb. 22, 2008). PDF text: ix, 104 p. : ill. (some col.) ; 6 Mb. UMI publication number: AAT 3274755. Includes bibliographical references. Also available in microfilm and microfiche formats.

Thesis (Ph. D.)--Washington State University, December 2008. / Title from PDF title page (viewed on Apr. 15, 2009). "School of Molecular Biosciences." Includes bibliographical references.

Thesis (M.Sc.). / Written for the Dept. of Physics. Title from title page of PDF (viewed 2009/08/07). Includes bibliographical references.

Thesis (M. Phil.)--University of Hong Kong, 2005. / Title proper from title frame. Also available in printed format.

Thesis (M.S.)--Tennessee Technological University, 2008. / Title from title page screen (viewed on May 13, 2010). Bibliography: leaves 86-94.

Th.--Sci. phys.--Toulouse 3, 1978. N°: 839. / Rés. en fr. et angl.

The purpose of this thesis is to investigate the operation of GaAs field-effect transistors with particular attention to the existence of negative resistance regions in the current-voltage characteristics, velocity overshoot effects, the role of substrate, and the role of heterojunctions. The approach used is to solve the electron transport equation using the Monte Carlo method which accounts for non-local effects in electron transport. Arguments are presented to support the contention that the negative resistance regions in the current-voltage characteristics observed in some experimental devices and produced by other researchers' computer simulations are attributed, in part, to the negative differential mobility of GaAs. The main reason of the existence of this negative resistance is related to the active layer thickness and it will be explained in terms of the rotation of the velocity vector. Electron velocity overshoot, a consequence of non-local effects, is examined in terms of gate length. The velocity overshoot becomes significant for FET structures with gates less than a micron in length and has many significant effects on the device performance. It is found also that velocity overshoot accounts for the undesirable saturation characteristics of submicron gate length GaAs FET which are observed in practical devices. However, it was also found that the presence of a low-doped n-type GaAs substrate below the active layer removes the negative resistance regions in the current-voltage characteristics. This is attributed to the effect of carrier injection from the active layer into the substrate which leads to the decrease of the effective channel thickness. This then will decrease the transconductance of the device, increase the gate pinchoff voltage and lower the device frequency response. This degradation of device's performance depends entirely on the purity and properties of the substrate. The performance of substrated FETs can be improved by preventing electron penetration into the substrate. This situation can be reached by using AlGaAs substrate whose energy band gap is higher than that of GaAs which then leads to electron confinement in the active layer. The use of AlGaAs in FETs can be in different forms. These will also be demonstrated in this thesis.

Hall effect and resistivity measurements have been carried out as a function of hydrostatic pressure and temperature on a number of samples of indium phosphide ranging from exceptionally pure to highly doped. In the case of pure and lightly doped InP an iterative solution of the Boltzmann Equation has been used successfully to describe the temperature and pressure dependence of mobility over the helium temperature range. Measurements on the highest mobility samples of InP ever grown suggest that the conduction band deformation potential is 6. 7eV. For the case of highly doped material it was found that a theory of scattering from a correlated distribution of impurities describes both the temperature and pressure dependence of mobility well. Pressure dependent mobility measurements on a sample having an impurity density close to the Mott transition suggest that the inclusion of impurity band conduction in the analysis is necessary even at nitrogen temperatures and above. Such an analysis is used successfully to describe the temperature and pressure dependence of both mobility and Hall carrier concentration.

Various techniques to synthesize nanowires and nanotubes as a function of growth temperature and time were investigated. These include growth of nanowires by a chemical vapor deposition (CVD) system using vapor-liquid-solid (VLS) growth mechanism and electro-chemical synthesis of nanowires and nanotubes. Narrow band gap InSb Eg = 0.17 eV at room temp) nanowires were successively synthesized. Using a phase diagram, the transition of the nanowire from metallic- semiconducting- semi-metallic phase was investigated. A thermodynamic model is developed to show that the occurrence of native defects in InSb nanowires influenced by the nanowire growth kinetics and thermodynamics of defect formation. Wide band gap ZnO (Eg = 3.34 eV) and In2O3 (3.7 eV) were also synthesized. ZnO nanowires and nanotubes were successfully doped with a transition metal Fe, making it a Dilute Magnetic Semiconductor of great technological relevance. Structural and electronic characterizations of nanowires were studied for different semiconducting, metallic and semi-metallic nanowires. Electron transport measurements were used to estimate intrinsic material parameters like carrier concentration and mobility. An efficient gas sensing device using a single In2O3 nanowire was studied and which showed sensitivity to reducing gas like NH3 and oxidizing gas like O2 gas at room temperature. The efficiency of the gas sensing device was found to be sensitive to the nature of contacts as well as the presence of surface states on the nanowire.

Dissertation (Ph.D.) --University of Texas Graduate School of Biomedical Sciences at San Antonio, 2008. / Vita. Includes bibliographical references.