Doctor of Philosophy

dissertationThe present dissertation is the result of our studies of the optical and electrical properties of self-assembled monolayer (SAM) diodes and bulk heterojunction organic photovoltaic (BOPV) devices. In our studies of SAM diodes, we fabricated solid-state mixtures of two dierent kinds of molecules; 1,4 benzene-dimethane-thiol (MeBDT) and 1-pentanethiol (PT). By varying the concentration r of MeBDT with respect to PT, we can go from a regime of isolated molecular wires (10ˉ8 10ˉ3). For r = 0, we found that a potential barrier dominated the transport properties of the device. In the isolated molecules regime, the conductance of MeBDT dominates the transport. In this regime, because of the linearity of the conductance with respect to r, we were able to obtain a \single molecule resistance" at V = 0:1V of RM = 6 10ˉ9. In the aggregated molecules regime, an ohmic response in the current-voltage (I-V) characteristics was observed for bias voltages 0:5V with the appearance of a new band in the dierential conductance around V = 0 along with a new double band in the optical gap at 2:4eV resulting in yellow/red photoluminescence emission. Opto-electrical studies of BOPV devices reveal that there are very few similarities between these types of solar cells and conventional solar cells. From simulations and experiemental measurements of the I-V characteristics, we found that while the open voltage circuit (Voc) is important for engineers, it carries no intrinsic information of the device. It cannot exceed the built-in potential of the device (Vbuiltˉin ). The later origin was found to be dependent on electrode work function dierence for a non-Ohmic contact conguration and on the active layer's blend in an Ohmic contact conguration. In a bid to improve BOPV device performance, we added to the blend spin 1=2 radical molecules. At concentration ( 2%), an increase in device performance was observed. The principal cause for this increase was the increase in the carrier's mobility as a function of the concentration of radicals

Space programs summary 37-64. Volume 3 - Supporting research and advanced development, 1 June - 31 July 1970

Interplanetary flight missions and systems development for thermoelectric outer planet spacecraf

Ionic Conductive Membranes for Fuel Cells

This book, titled “Ionic Conductive Membranes for Fuel Cells”, from the journal Membranes, discusses the state of the art and future developments in the field of polymer electrolyte membranes for fuel cells, an efficient and clean system for converting fuel into energy

Doctor of Philosophy

dissertationThis dissertation describes the design, fabrication, testing, reliability, and harsh environment performance of single-device Micro-electro-mechanical-system (MEMS)- based digital logic gates, such as XOR and AND, for applications in ultra-low-power computation in unforgiving settings such as high ionizing radiation and high temperatures. Within the scope of this dissertation are several significant contributions. First, this work was the first ever to report the evolution in logic design architecture from a CMOS-paradigm to a MEMS architecture utilizing a single functional device per logic, as opposed to multiple relays per logic. This novel approach reduces the number of devices needed to implement a logic function by approximately 10X, leading to better reliability, yield, speed, and overall better characteristics (subthreshold characteristics, smaller turn-on/off voltage variations, etc.) and it simplifies implementation of MEMSbased circuits. The logic gates illustrate ~1.5V turn-on voltage at 5MHz with >109 cycles of reliable operations and low operational power consumption (leakage current and power <10-9A, <1^W). Second, this work is the first ever to report an intensive study on the cycle-bycycle evolution of contact resistance (Rc) up to 100,000 cycles, on materials such as, Ir, Pt, W, Ni, Cr, Ti, Cu, Al, and graphite, which are materials commonly used in MEMS switches. Adhesion forces between contacts were also studied using a contact-modeAFM, force vs. displacement, experiment. Results show that materials with high Young's modulus, high melting temperatures, and high density show low initial contact resistances and low adhesion forces (such as Ir, Pt, and W). Third, the devices were interrogated separately in harsh environments where they were exposed to high doses of ionizing radiation (90kW) in a nuclear reactor for a prolonged time (120 min) and, separately, at high temperatures (409K). Here, results show that solid-state devices begin to deteriorate almost immediately to a point where their gate can no longer control the drain-to-source current, whereas MEMS switches survive such ionizing radiation and temperatures portraying clear ON and OFF states for far longer. In terms of the applications empowered and the breadth of topics covered to accomplish these results, the work presented here demonstrates significant contributions to an important and developing branch of engineering

GaN HEMT Low Frequency Noise Characterization for Low Phase Noise Oscillator Design

The thesis presents low frequency noise (LFN) characterization of Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) for low phase noise oscillator design. First, GaN HEMT technology is benchmarked versus other transistor technologies, e.g., GaAs-InGaP Heterojunction Bipolar Transistor (HBT) and GaAs pHEMT, in terms of noise and power. In the comparison, LFN at given frequency normalized to DC power is used as a benchmark parameter. It is verified that InGaP HBT technology provides better performance in terms of both absolute noise level and normalized values compared to other technologies. However, at higher frequencies where flicker noise is less critical, GaN HEMT has an advantage of higher power. For this reason, GaN HEMT is considered to have good potential for design of oscillators for communication systems with large channel bandwidth. Then, some factors which influence the LFN of two types of GaN HEMTs: AlGaN/GaN based HEMT and AlInN/AlN/GaN based HEMT such as surface passivation methods and variations in transistor geometry are studied. It is seen that the surface passivation has a major impact on the noise level while the effect of transistor geometry (e.g. gate length, gate width and source-drain distance) is insignificant. The best surface passivation, with respect to LFN, is Al2O3 deposited with thermal Atomic Layer Deposition (ALD). Finally, two monolithic integrated circuit (MMIC) oscillators based on GaN HEMT technology are demonstrated. A fixed frequency GaN HEMT oscillator is designed at about 10 GHz with the best achieved phase noise of -100 dBc/Hz @ 100 kHz offset. Another GaN HEMT voltage controlled oscillator (VCO) is also designed with medium (15%) tuning range between 6.45-7.55 GHz, high tuning linearity, average output power about 1 dBm and low phase noise. For a bias of Vd /Id = = 6 V/33 mA, the measured phase noise is -98 dBc/Hz @ 100 kHz and -132 dBc/Hz @ 1 MHz offset frequencies, respectively. This is the lowest phase noise reported for a GaN HEMT based VCO with comparable tuning range and oscillation frequency. Its 1 MHz phase noise performance is comparable to state-of-the-art VCOs based on InGaP-HBT technology with similar tuning range

Electrostatic Radio Frequency (RF) Microelectromechanical Systems (MEMS) Switches With Metal Alloy Electric Contacts

RF MEMS switches are paramount in importance for improving current and enabling future USAF RF systems. Electrostatic micro-switches are ideal for RF applications because of their superior performance and low power consumption. The primary failure mechanisms for micro-switches with gold contacts are becoming stuck closed and increased contact resistance with increasing switch cycles. This dissertation reports on the design, fabrication, and testing of micro-switches with sputtered bi-metallic (i.e., gold (Au)-on-Au-(6.3at%)platinum (Pt)), binary alloy (i.e., Au-(3.7at%)palladium (Pd) and Au-(6.3at%)Pt), and ternary alloy (i.e., Au-(5at%)Pt-(0.5at%)copper (Cu)) contact metals. Performance was evaluated, in-part, using measured contact resistance and lifetime results. The micro-switches with bi-metallic and binary alloy contacts exhibited contact resistance between 1 - 2 ohms and, when compared to micro-switches with sputtered gold contacts, showed an increase in lifetime. The micro-switches with tertiary alloy contacts showed contact resistance between 0.2-1.8 and also showed increased lifetime. Overall, the results presented in this dissertation indicate that micro-switches with gold alloy electric contacts exhibit increased lifetimes in exchange for a small increase in contact resistance

Modeling and Simulation of Varistors

This thesis treats various problems that arise in the context of varistors and microvaristors, which are are used for the suppression of transient overvoltages, due to their extraordinary nonlinear electrical conductivity. The present work is mainly motivated by the desire to simulate the behavior of high-voltage surge arresters used for lightning protection on the one hand and of microvaristors as materials for future applications in nonlinear resistive stress control on the other hand. The analysis of surge arresters requires the numerical calculation of mutually-dependent electric and thermal fields, whereby the principal difficulty resides in the extreme nonlinearity of the electric problem. For this purpose, the electro-quasistatics equation is solved in time domain by means of the finite-element method. The calculation of the thermally stationary state of a surge arrester and the evaluation of an envelope equation model for simulating the heating and cooling behavior of arresters are discussed in more detail. These simulations depend on sufficiently accurate models that describe the material properties. The estimation of nonlinear conductivity and permittivity of varistor materials is an inherent part of this thesis. Furthermore, nonlinear capacitance and conductance matrices are introduced. The presented approach is based on an equivalent circuit model. Its parameters are determined from field-simulation results

Influence Of Zinc Oxide Particle Size And Surface Properties On The Electrical, Optical And Cytotoxicity Characteristics Of Zinc Oxide Discs

Kesan saiz butiran / partikel yang berbeza dan pengubahsuaian permukaan melalui penyepuhlindapan haba, struktur elektrik, dan sifat optik daripada cakera ZnO tulen dan koposit (varistor) yang difabrikasi daripada mikro atau nanopartikel ZnO dikaji dalam kajian ini berhubung dengan ketoksikan mereka. Empat sampel ZnO dengan saiz berbeza iaitu ZnO-White, ZnO-Pharma, 40mm ZnO dan 20mm-ZnO dicirikan. The effects of different particle/grain sizes and surface modification by thermal annealing process on the structural, electrical, and optical characteristics of pure and composite (varistors) ZnO discs fabricated from ZnO micro or nanoparticles were investigated in this study with respect to their toxicity. Four ZnO powder samples with different particle sizes namely ZnO-White (130 nm), ZnO-Pharma (80 nm), 40nm-ZnO and 20nm-ZnO were characterized

Effect of compaction parameters and sintering configurations on the performance of ZnO varistor

Zinc oxide varistors are electronic ceramic devices processed through conventional ceramic technique. Its primary function is to protect an electrical circuit by limiting transient surges repeatedly without failing thereby enhancing the system reliability. There are various critical parameters of varistors, defined to evaluate the performance characteristics. Investigating the influence o f the compaction and other processing variables and their optimization in terms of the performance characteristics were the primary objectives of this project. Enhanced energy absorption capability of the varistor is highly beneficial either in increasing the reliability of the device and of the system or in reducing its volume providing the same level of protection. Varistor discs capable of absorbing more energy will aid in reducing the cost or be suitable for more demanding applications. Evaluation of the effect of compaction parameters was carried out by varying the pressing load and speed. The influence of holding times in pressing cycle was also studied. In addition to the energy absorption capability, some other important properties of the varistor were also investigated. Statistical approaches such as the response surface methodology and factorial design of experiments were adopted to develop necessary mathematical models on the basis of the experimental data. The reliability of the models were also verified. A correlation of the physical property and the electrical performance of the ZnO varistor with the sintering orientation was established. The analysis by extensive measurement of the microhardness revealed a relationship between the hardness and grindability of varistor. The tensile strength measured on the disc shaped sections of the arrester block was also found to differ significantly. The density gradient of the sintered disc shows a remarkable influence of the sintering orientation of the disc. By microstructural analysis the observed difference was confirmed. A few alternative methods have been suggested to overcome the adverse effects arising from the sintering orientation. In this regard horizontal sintering on Veegroove support was found to be advantageous. Influence of the surface to volume ratio was evaluated by converting the cylindrical discs into hexagonal shape by grinding. A new design with a hexagonal shape has been proposed. The fracture mechanism of the varistor in a high amplitude short duration (HASD) test was studied and a significant role of the stress wave was observed. The celerity measured by a laser beam based technique through the varistor material was found to be in good agreement with the theoretical prediction. An experimental study also revealed the effect of the stress wave. This understanding will lead to a new approach in combating the fracture from the high current pulse