Chemical gas sensors can be used in a variety of applications such as process control, security systems and medical diagnosis. In the research for new functions and new sensing materials a “breadboard” would be useful. A technique that has been investigated for such a purpose is the grid-gate device which is a metal-oxide-semiconductor (MOS) based gas sensor. It is a MOS capacitor consisting of a passive grid-gate with depositions of sensing materials overlapping the grid. The measuring is carried out with a light addressable method called scanning light pulse technique (SLPT) which enables the detection of spatially distributed gas response. A development of the grid-gate sensor would be to separate the sensing materials from the chip. In this thesis the aim was to see if this was possible by depositing the sensing material on a slide of micro pillars which was put on top of a biased grid-gate chip. The test was made with palladium depositions in an ambient of synthetic air and 2500 ppm hydrogen, and the measuring technique was SLPT as for the preceding device. The result of the test was that the new device showed a combined gas response of both charge content shift at flat-band voltage and at inversion voltages. The conclusion is therefore that the sensing material can be separated from the grid-gate chip and that the response will be caused by several mechanisms. The two-dimensional image response utilized for the preceding grid-gate device will instead be a multi-dimensional response consisting of the curve for the charge content shift at every measuring position.

In this thesis, various issues regarding normally-ON silicon carbide (SiC)Junction Field-Effect Transistors (JFETs) are treated. Silicon carbide powersemiconductor devices are able to operate at higher switching frequencies,higher efficiencies, and higher temperatures compared to silicon counterparts.From a system perspective, these three advantages of silicon carbide can determinethe three possible design directions: high efficiency, high switchingfrequency, and high temperature.The structure designs of the commercially-available SiC power transistorsalong with a variety of macroscopic characteristics are presented. Apart fromthe common design and performance problems, each of these devices suffersfrom different issues and challenges which must be dealt with in order to pavethe way for mass production. Moreover, the expected characteristics of thefuture silicon carbide devices are briefly discussed. The presented investigationreveals that, from the system point-of-view, the normally-ON JFET isone of the most challenging silicon carbide devices. There are basically twoJFET designs which were proposed during the last years and they are bothconsidered.The state-of-the-art gate driver for normally-ON SiC JFETs, which wasproposed a few years ago is briefly described. Using this gate driver, theswitching performance of both Junction Field-Effect Transistor designs wasexperimentally investigated.Considering the current development state of the available normally-ONSiC JFETs, the only way to reach higher current rating is to parallel-connecteither single-chip discrete devices or to build multichip modules. Four deviceparameters as well as the stray inductances of the circuit layout might affectthe feasibility of parallel connection. The static and dynamic performance ofvarious combinations of parallel-connected normally-ON JFETs were experimentallyinvestigated using two different gate-driver configurations.A self-powered gate driver for normally-ON SiC JFETs, which is basicallya circuit solution to the “normally-ON problem” is also shown. This gatedriver is both able to turn OFF the shoot-through current during the startupprocess, while it also supplies the steady-state power to the gate-drivecircuit. From experiments, it has been shown that in a half-bridge converterconsisting of normally-ON SiC JFETs, the shoot-through current is turnedOFF within approximately 20 μs.Last but not least, the potential benefits of employing normally-ON SiCJFETs in future power electronics applications is also presented. In particular,it has been shown that using normally-ON JFETs efficiencies equal 99.8% and99.6% might be achieved for a 350 MW modular multilevel converter and a40 kVA three-phase two-level voltage source converter, respectively.Conclusions and suggestions for future work are given in the last chapterof this thesis. / I denna avhandling behandlas olika aspekter av normally–ON junction–field–effect–transistorer (JFETar) baserade på kiselkarbid (SiC). Effekthalvledarkomponenteri SiC kan arbeta vid högre switchfrekvens, högre verkningsgradoch högre temperatur än motsvarigheterna i kisel. Ur ett systemperspektivkan de tre nämnda fördelarna användas i omvandlarkonstruktionen för attuppnå antingen hög verkningsgrad, hög switchfrekvens eller hög temperaturtålighet.Såväl halvledarstrukturen som de makroskopiska egenskaperna för kommersiellttillgängliga SiC–transistorer presenteras. Bortsett från de vanligakonstruktions–och prestandaproblemen lider de olika komponenterna av ettantal tillkortakommanden som måste övervinnas för att bana väg för massproduktion.Även framtida SiC–komponenter diskuteras.Ur ett systemperspektiv är normally-ON JFETen en av de mest utmanandeSiC-komponenterna. De två varianter av denna komponent som varittillgängliga de senaste åren har båda avhandlats.State–of–the–art–drivdonet för normally-ON JFETar som presenteradesför några år sedan beskrivs i korthet. Med detta drivdon undersöks switchegenskapernaför båda JFET-typerna experimentellt.Vid beaktande av det aktuella utvecklingsstadiet av de tillgängliga normally–ON JFETarna i SiC, är det möjligt att uppnå höga märkströmmar endastom ett antal single–chip–komponenter parallellkopplas eller om multichipmodulerbyggs. Fyra komponentparametrar samt strö-induktanser för kretsenkan förutses påverka parallellkopplingen. De statiska och dynamiska egenskapernaför olika kombinationer av parallellkopplade normally-ON JFETarundersöks experimentellt med två olika gate–drivdonskonfigurationer.Ett självdrivande gate-drivdon för normally-ON JFETar presenteras också.Drivdonet är en kretslösning till “normally–ON–problemet”. Detta gatedrivdonkan både stänga av kortslutningsströmmen vid uppstart och tillhandahållaströmförsörjning vid normal drift. Med hjälp av en halvbrygga medkiselkarbidbaserade normally–ON JFETar har det visats att kortslutningsströmmenkan stängas av inom cirka 20 μs.Sist, men inte minst, presenteras de potentiella fördelarna med användningenav SiC-baserade normally-ON JFETar i framtida effektelektroniskatillämpningar. Speciellt visas att verkningsgrader av 99.8% respektive 99.5%kan uppnås i fallet av en 350 MW modular multilevel converter och i en40 kVA tvånivåväxelriktare. Sista kaplitet beskriver slutsatser och föreslagetframtida arbete. / <p>QC 20130527</p>

Field-effect transistor technologies have been critical building blocks for satellite systems since their introduction into the microelectronics industry. The extremely high cost of launching payloads into orbit necessitates systems to have small form factor, ultra low-power consumption, and reliable lifetime operation, while satisfying the performance requirements of a given application. Silicon-based complementary metal-oxide-semiconductors (Si CMOS) have traditionally been able to adequately meet these demands when coupled with radiation hardening techniques that have been developed over years of invested research. However, as customer demands increase, pushing the limits of system throughput, noise, and speed, alternative technologies must be employed. Silicon-germanium BiCMOS platforms have been identfied as a technology candidate for meeting the performance criteria of these pioneering satellite systems and deep space applications, contingent on their ability to be hardened to radiation-induced damage. Given that SiGe technology is a relative new- comer to terrestrial and extra-terrestrial applications in radiation-rich environments, the same wealth of knowledge of time-tested radiation hardening methodologies has not been established as it has for Si CMOS. Although SiGe BiCMOS technology has been experimentally proven to be inherently tolerant to total-ionizing dose damage mechanism, the single event susceptibility of this technology remains a primary concern. The objective of this research is to characterize the physical mechanisms that drive the origination of ion-induced transient terminal currents in SiGe HBTs that subsequently lead to a wide range of possible single event phenomena. Building upon this learning, a variety of device-level hardening methodologies are explored and tested for efficacy.

The low thermal conductivity of gallium arsenide compared to silicon results in self-heating effects in GaAs MESFETs that limit the electrical performance of such devices for high power applications. To date, analytical thermal models of self heating in GaAs MESFETs are based on the assumption of a uniformly heated channel. This thesis presents a two dimensional analysis of the electrothermal effect of this device based on the two dimensional power density distribution in the channel under various bias conditions. The numerical simulation is performed using the finite difference technique. The results of the simulation of an isothermal MESFET without heat effects is compared with various one dimensional analytical models in the literature. Electro thermal effects into the two-dimensional isothermal MESFET model allowed close examination of the temperature profile within the MESFET. The large gradient in power distribution results in a localized heat source within the channel which increases the overall channel temperature, which shows that the assumption of a uniformly heated channel is erroneous, and may lead to an underestimation of the maximum channel temperature. / Graduation date: 1992

Present day data processing technology requires very high speed signal processing and data conversion rates. One such application which requires high speed is switched capacitor circuits used in Sigma-Delta modulators. A major active component of switched capacitor circuits is the monolithic operational amplifier(opamp). Because of the relatively poor speed performance of the currently available silicon based technology, such high speed circuits can not be designed. GaAs technology appears to be a promising alternative technology for high speed switched capacitor circuits. One problem with GaAs is the lack of complementary technology. Until now, most of the design of GaAs analog integrated circuits has been implemented using depletion mode n-MESFETs, where operational amplifiers and switched capacitors have been developed by various groups. This thesis develops the techniques for implementation of analog integrated circuits using complementary GaAs Heterojunction Field Effect Transistors(HFETs). Several operational amplifiers have been designed and their performance studied via simulation. The designs studied predict superior high frequency performance for C-HFETs over conventional GaAs MESFET and Silicon CMOS technology. The opamp designs are currently being implemented at Oregon State University for fabrication in the future. / Graduation date: 1993

Present day data processing technology requires very high speed signal processing and data conversion rates. Traditionally, these circuits have been implemented in silicon MOS technology, whose high speed performance is limited, due to inherent material properties. Though relatively immature compared to silicon technology, GaAs integrated circuit technology appears to be a potential vehicle for realizing high-speed circuits because of its high electron mobility and low parasitic capacitance. One major drawback of GaAs technology has been the lack of complementary technology in contrast to silicon where CMOS technology has greatly facilitated the development of analog ICs. This thesis investigates the suitability of complementary GaAs Heterojunction FET integrated circuit technology for the realization of high sample-rate switched-capacitor circuits. In order to yield an accurate device model for the design work, model parameters of both n and p GaAs Heterojunction FET devices are extracted from measurement results. Based on the extraction results, a set of analog building blocks are presented. These circuits include a high bandwidth operational amplifier and a fast settling switch which are essential for high sample-rate circuits. A second order switched-capacitor low pass filter sampling at a clock rate of 100MHz is designed using the above building blocks. The designs studied predict better high frequency performance for C-HFETs compared to Si CMOS technology. / Graduation date: 1994

Graduation date: 1992

High speed photodetectors are a necessary element in broad band digital and analog optical communication systems. In this thesis easily integrable planar high speed photodetectors made on undoped semi-insulating (SI) GaAs substrates are modeled and tested. The fabrication process of the detectors is fully compatible with GaAs metal-semiconductor field effect transistor (MESFET) processing technology. Interdigitated fingers are used as the contacts to achieve both high sensitivity and large bandwidth. Detectors made with both ohmic and Schottky contacts are fabricated and tested. The equivalent circuit elements of the interdigitated structure are modeled using accurate lumped element circuit models associated with the various discontinuities of the structure. The results of the model agree well with the experimental results as well as with other published results. Numerical simulation of the SI-GaAs metal-semiconductor- metal (MSM) photodetector is performed. The carriers are tracked after an ideal optical pulse is applied and the intrinsic current as a function of time is computed. Then the influence of all the external circuit elements is included and the output current across the load resistor is computed. The simulated response is compared with other published models. The electrical and optical characteristics of the detectors are measured. For ohmic contact detectors, the dark current increases linearly with bias until some critical field is reached beyond which the dark current increases nonlinearly with bias. The time response of the detectors is measured with a 10 ps pulsed laser operating at - 600 nm and also with a pulsed GaAs /AlGaAs semiconductor laser operating at 850 nm. The ohmic and Schottky contact detectors have approximately the same rise time. The fall time of the Schottky contact detector is much smaller than the fall time of ohmic contact detector. The long fall time of the ohmic detector does not depend on the spacing between contacts. This long fall time is due to the large barrier that exists near the ohmic metal/SI-GaAs cathode contact. No such barrier exists for SI-GaAs MSM photodetector. The simulated impulse response of the SI-GaAs MSM photodetector is compared with the measured impulse response. / Graduation date: 1991

In this thesis, the development of a novel experimental technique for measuring the spontaneous, stochastic work function (WF) fluctuations of conducting polymer films, at equilibrium, is discussed. Polyaniline (PANI) is studied as a representative conducting polymer. This technique utilizes an insulated-gate field-effect transistor (IGFET) with PANI gate electrode (PANI-IGFET). The fluctuations of PANI WF are transduced into measurable drain current fluctuations of the device. By analyzing these fluctuations while systematically controlling the temperature, electric field and doping level, a model of WF fluctuations in PANI films is developed. These experiments suggest that the source of WF fluctuations is the hopping of charge carriers, or trapping/detrapping of charge carriers, around the Fermi level of the PANI film at the PANI-insulator interface. This process is thermally activated with a field and doping dependent activation energy in the range of 0.1 to 0.5 eV. Thus, this new technique provides detailed information about charge-carrier dynamics in the space-charge region of the PANI film, at equilibrium. These results have important implications for organic electronics and furthering fundamental understanding of the relationship between doping, disorder and work function in organic semiconductors.

A self-consistent, full-band, electrothermal ensemble Monte Carlo device simulation tool has been developed. It is used to study charge transport in bulk GaN, and to design, analyze, and improve the performance of AlGaN/GaN high electron mobility transistors (HEMTs) and avalanche photodiodes (APDs). Studies of electron transport in bulk GaN show that both peak electron velocity and saturated electron velocity are higher for transport in the basal plane than along the c-axis. Study of the transient electron velocity also shows a clear transit-time advantage for electron devices exploiting charge transport perpendicular to the c-axis. The Monte Carlo simulator also enables unique studies of transport under the influence of high free carrier densities but with low doping density, which is the mode of transport in AlGaN/GaN HEMTs. Studies of isothermal charge transport in AlGaN/GaN HEMTs operating at high gate bias show a drain current droop with increasing drain-source bias. The cause of the droop is investigated and a design utilizing source- or gate-connected field plate is demonstrated to eliminate the drain current droop. Electrothermal aspects of charge transport in AlGaN/GaN HEMTs are also investigated, and the influence of non-equilibrium acoustic and optical phonons is quantified. The calculated spatial distribution of non-equilibrium phonon population reveals a hot spot in the channel that is localized at low drain-source bias, but expands towards the drain at higher bias, significantly degrading channel mobility. Next, Geiger mode operation of wurtzite GaN-based homojunction APDs is investigated. The influences of dopant profile, active region thickness, and optical absorption profile on single photon detection efficiency (SPDE) are quantified. Simulations of linear mode gain as a function of multiplication region thickness and doping profile reveal that weakly n-type active regions may be exploited to achieve higher avalanche gain, without penalty to either applied bias or active region thickness. A separate absorption and multiplication APD (SAM-APD) utilizing a AlGaN/GaN heterojunction is also investigated. The presence of strong piezo-electric and spontaneous polarization charges at the heterojunction enables favorable electric field profile in the device to reduce dark current, improve excess noise factor, improve quantum efficiency, and improve breakdown probability. To maximize SPDE, a new device structure with a buried absorber is proposed and improved SPDE is demonstrated. Lastly, a new approach for the direct generation of self-sustaining millimeter-wave oscillations is proposed. In contrast to Gunn diodes, which exploit a bulk-like active region, periodic oscillation is achieved in the proposed structures through the creation, propagation and collection of traveling dipole domains supported by fixed polarization charge and the associated two-dimensional electron gas along the plane of a polar heterojunction. Numerical simulation of induced oscillations in a simple triode structure commonly used for AlGaN/GaN HEMTs reveals two distinct modes of self-sustaining millimeter-wave oscillation.