No description available.

Due to the small concentration of impurities normally present in semiconductors these impurity concentrations cannot' be measured by ordinary chemical analysis. Electrical methods are the common way by which semiconductor impurity concentrations are measured. The specific problem investigated was to determine the concentration Nd of donors and the concentration Na of acceptors at various points in a crystal of silicon. A description of the crystal structure and the energy band scheme of silicon is given. The density of states in the conduction and valence bands is calculated. The population of the conduction and valence bands by electrons and holes as a function of impurity concentration and temperature is derived. A method is given from which the values of Nd and Na may be found from Hall constant measurements at liquid nitrogen and room temperatures. The experimental apparatus used includes a magnet, cryostat, sample holder, and Hall measuring circuit. The results obtained from the measurements are in agreement with those expected from the available knowledge of the method of crystal growth. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An observable e.m.f. exists in a semiconductor when a non-equilibrium carrier concentration is present in a region of electrostatic potential gradient. These two conditions may arise in a variety of ways, and the e.m.f.'s associated with various combinations of conditions are listed. One of these e.m.f.'s arises from a photo-generated non-equilibrium carrier concentration in regions of electrostatic potential gradient due to an inhomogeneous impurity distribution. The thesis is chiefly concerned with the extension of the theory of this bulk photo e.m.f. and its comparison with experiment. Previous work on the subject is reviewed and the theory is developed to cover all conditions of illumination in the region of an arbitrary impurity density gradient. An expression for the bulk photo e.m.f. is derived by two approaches, integration of the electrostatic potential gradient, and integration of the carrier quasi Fermi levels, over the illuminated region. The latter derivation is shown to be more general in its application, and is used to obtain an expression for the photo e.m.f. both in an illuminated p-n junction and in an illuminated bulk inhomogeneity. Using the general result, expressions for the e.m.f. are written for the extreme cases of weak and strong illumination in extrinsic and nearly intrinsic semiconductors. The relation between bulk photo e.m.f. and photoconductive resistance decrease is examined. Measurements were made of the bulk photo e.m.f. as a function of light intensity. The proportionality of the effect at low levels of illumination was verified. Observations of the photo e.m.f. patterns showed a maximum of e.m.f. at positions of maximum conductivity gradient. Since the bulk photo e.m.f. is a function of conductivity gradient, light probe measurements give a sensitive technique for the detection of inhomogeneous impurity distributions. With weak illumination, the measured ratio of photo e.m.f. to photoconductive resistance decrease was a constant independent of light intensity. These observations verified the theory and suggested a use for this ratio in quantitative measurements of conductivity gradient. The photo e.m.f. at strong illumination was shown to be dependent on the impurity distribution outside the region of incident light. The conditions under which the ratio of photo e.m.f. to photoconductive resistance decrease is constant at strong illumination are shown to be in agreement with the theoretical treatment. Measurements of bulk photo e.m.f. as a function of temperature show a qualitative agreement with theory at low temperatures (extrinsic range). At high temperatures (intrinsic range) the results show a close agreement with the theoretically predicted behaviour. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The thesis is concerned principally with Gunn-effect oscillations, low-frequency oscillations due to field-enhanced trapping (FET), and the interaction of the Gunn effect with traps. In the first part of the thesis, the theory of these phenomena is developed in a form which is unified in that it applies to all three independently of their physical cause i.e. field-enhanced interband transfer or FET. The theory of the quasi-steady-state is treated, in which interlevel electron transfer is assumed to be "fast" compared with electric field changes. The theory of the Gunn effect and of FET instabilities is then reviewed in this framework, as well as experimental observations. Some complements to previous theoretical knowledge are included, such as a discussion of stationary electric distributions, a rigorous derivation of the smalls-signal equivalent circuit of a Gunn diode and its application to a domain-stability criterion. Various computational simulations were performed, of which the following seem not to have been reported previously. Gunn-domains oscillation in a diode with doping increasing linearly from cathode to anode was simulated. It was known experimentally that such a device has an interesting voltage-tunability, and this was confirmed and explored. The simulation of a subcritical diode showed that its negative differential conductance is associated with the launching of accumulation layers in synchronism with the voltage modulation, which was not previously made clear. High-field domains due to field-enhanced trapping were found to grow exponentially with time; some relations between their dynamic behaviour and the trapping parameters are inferred from the results. Field-independent trapping was found to have little effect on steadily-propagating Gunn domains; in a domain, traps are depleted. The result show, however, that cumulative trapping near the cathode, and at the anode, might alter the oscillation. Field-enhanced trapping can result in the trapping of the Gunn domain at the anode, These two results are important in connection with the performance of practical Gunn oscillators in that the material should not contain too many traps; impurities causing such traps should not enter the material by contamination from the electrodes at the surface. For the experimental part of the thesis, diodes were cleaved, from slices of N-type, high-resistivity gallium arsenide onto which ohmic contacts had been applied. Above a threshold field of about 0.3 MV/m, some of these diodes exhibited small current oscillations due to high-field domains propagating at some 1 m/s. The diode-current transients revealed the existence of a trap level just below the Fermi level. Illumination strongly enhanced the field-enhanced trapping. The use of the electro-optic effect to probe the electric-field distribution in the diode was investigated. The-experiments showed that, in a non-ohmic diode biased in the high-resistance direction, there exists a high-field region near the cathode. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Since a report that the complex of TTF.TCNQ showed novel solid state properties including high room temperature conductivity, there has been considerable interest in the study of these so called "organic metals." This thesis describes the synthesis of new acceptors derived from TCNQ. First the synthesis of 2-methoxycarbonyl-7,7,8,8-tetracyanoquino-dimethane (56) is presented. Bromination of methyl 2,5-dimethylbenzoate followed by treatment with sodium cyanide yielded the dicyano compound 60 which was converted to the dihydro-TCNQ 62 via the corresponding phenylenetetracyanodiacetate intermediate. Oxidation of 62 afforded 2-methoxycarbonyl-7,7,8,8-tetracyanoquinodimethane (56), which was found to be unstable. The synthesis of 2,3-dimethoxycarbonyl-7,7,8,8-tetra-cyanoquinodimethane was attempted in the following manner: the Diels-Alder cyclization of trans,trans-2,4-hexadiene and dimethyl acetylenedi-carboxylate followed by dehydrogenation of the adduct gave dimethyl 3,6-dimethylphthalate. Benzylic bromination of this material followed by displacement of the bromines by cyanide yielded the dicyano compound 73. However, several attempts to convert 73 to the corresponding tetraester 78 were unsuccessful. The synthesis of 2,3-dimethyl-7,7,8,8-tetracyanoquinodimethane is presented. The intermediate 2,3-dimethylcyclohexane-l,4-dione (87) was prepared from 2,3-dimethylphenol by standard means. It was then condensed with malononitrile, and the product was treated with bromine/pyridine to give the dihydro-DMTCNQ 89. When this compound was treated with a mixture of palladium and sulfur at 180°C the desired 2,3-dimethyl-TCNQ 83 was obtained in low yield. The second part of this thesis describes the preparation of 11,11, 12 ,12-tetracyano-4,5,9,10-tetrahydropyreno-2,7-quinodimethane (TCNTP) from mesitylene. This compound was bisbrominated to give 3,5-bis-(bromomethyl)toluene, which was then coupled with 3,5-bis(mercaptomethyl)-toluene to produce the dithia[3.3jmetacyclophane 98. This was converted to the dimethyl[2.2]metacyclophane 96 via Wittig rearrangement followed by reduction with lithium in liquid ammonia. This cyclophane was then brominated and the resulting dibromo compound was converted to the corresponding dicyano-94. The latter compound was treated with bromine in the presence of iron powder to produce 2,7-bis(cyanomethyl)-4,5,9,10-tetrahydropyrene. Introduction of the remaining two nitrile groups was accomplished via use of the tetracyanodiacetate intermediate 92. Hydrolysis and decarboxylation of 92 followed by oxidation with NBS/triethylamine at -78°C furnished the desired TCNTP (90). / Science, Faculty of / Chemistry, Department of / Graduate

No description available.

Boston University. University Professors Program Senior theses. / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / 2999-01-02

Thesis (Ph. D.)--University of Washington, 2007. / Includes bibliographical references (leaves 181-186).

Thesis (Ph.D.)--University of Texas at Arlington, 2008.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisors: Sanjay K. Banerjee and Amitabh Jain. Vita. Includes bibliographical references. Available also from UMI Company.