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Materials for High-Temperature Semiconductor Devices Committee on Materials for High-Temperature Semiconductor Devices National Materials Advisory Board Commission on Engineering and Technical Systems National Research Council NMAB-474 National Academy Press Washington, D.C. 1995
NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. This study by the National Materials Advisory Board was conducted under ARPA Order No. 8475 issued by DARPA/CMO under Contract No. MDA 972-92-C-0028 with the U.S. Department of Defense and the National Aeronautics and Space Administration. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the U.S. Government. Library of Congress Catalog Card Number 95-70760 International Standard Book Number 0-309-05335-8 Available in limited supply from: National Materials Advisory Board 2101 Constitution Avenue, NW Washington, D.C. 20418 202-334-3505 nmab~nas.edu Additional copies are available for sale from: National Academy Press 2101 Constitution Avenue, NW Box 285 Washington, D.C. 20055 800-624-6242 202-334-3313 (in the Washington Metropolitan Area) Copyright 1995 by the National Academy of Sciences. All rights reserved. Printed in the United States of America.
Abstract Major benefits to system architecture would result if cooling systems for components could be eliminated without compromising performance (e.g., power, efficiency, and speed). The existence of commercially available high- temperature semiconductor devices would be an enabling technology in such areas as sensors and controls for aircraft, high-power switching devices for the electric power industry, and control electronics for the nuclear power industry. This report surveys the state of the art for the three major wide bandgap materials for high-temperature semiconductor devices (i.e., silicon carbide, the nitrides, and diamond); assesses the national and international efforts to develop high-temperature semiconductors; identifies the technical barriers to their development and manufacture; determines the criteria for successfully packaging and integrating new high-temperature semiconductors into existing systems; recommends future research priorities; and suggests additional possible applications and advantages. · · ~
The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Harold Liebowitz is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an advisor to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce Alberts and Dr. Harold Liebowitz are chairman and vice chairman, respectively, of the National Research Council. 1V
Committee on Materials for High-Temperature Semiconductor Devices WOLFGANG J. CHOYKE, Chair, Professor, Department of Physics, University of Pittsburgh, Pennsylvania MICHAEL G. ADLERSTEIN, Consulting Scientist, Raytheon Research Division, Lexington, Massachusetts JEROME J. CUOMO, Professor. Materials Science and Engineering, North Carolina State University, Raleigh ARTHUR G. FOYT, Jr., Manager, Electronics Research, United Technologies Research Center, East Hartford, Connecticut EVELYN L. HU, Chair, Department of Electrical and Computer Engineering, University of California, Santa Barbara LIONEL C. KIMERLING, Professor, Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge MARK R. PINTO, Department Head, ULSI, AT&T Bell Laboratories, Murray Hill, New Jersey MICHAEL A. TAMOR, Staff Scientist, Ford Motor Company, Dearborn, Michigan IWONA TURLIK, Vice-President, Corporate Manufacturing Research Center, Motorola, Schaumburg, Illinois LIAISON REPRESENTATIVES JANE A. ALEXANDER, ARPA/MTO, Arlington, Virginia T.J. ALLARD, Sandia National Laboratories, Albuquerque, New Mexico DON KING, Sandia National Laboratories, Albuquerque, New Mexico WILLIAM C. M:ITCHEL, U.S. Air Force, Wright Patterson Air Force Base, Ohio YOON SOO PARK, Office of Naval Research, Arlington, Virginia J. ANTHONY POWELL, NASA Lewis Center, Cleveland, Ohio JOHN PRATER, Army Research Office, Research Triangle Park, North Carolina MAX YODER, Office of Naval Research, Arlington, Virginia NMAB STAFF ROBERT M. EHRENREICH, Senior Program Manager PAT WILLIAMS, Senior Secretary v
National Materials Advisory Board JAMES C. WILLIAMS, Chair, General Electric Company, Cincinnati, Ohio JAN D. ACHENBACH, Northwestern University, Evanston, Illinois BILL R. APPLETON, Oak Ridge National Laboratory, Oak Ridge, Tennessee ROBERT R. BEEBE, Tucson, Arizona I. MELVIN BERNSTEIN, Tufts University, Medford, Massachusetts J. KEITH BRIMACOMBE, University of British Columbia, Vancouver, Canada JOHN V. BUSCH, IBIS Associates, Inc., Wellesley, Massachusetts HARRY E. COOK, University of Illinois, Urbana ROBERT EAGAN, Sandia National Laboratories, Albuquerque, New Mexico CAROLYN HANSSON, Queen's University, Kingston, Ontario, Canada KRISTINA M. JOHNSON, University of Colorado, Boulder LIONEL C. KIMERLING, Massachusetts Institute of Technology, Cambridge JAMES E. MCGRATH, Virginia Polytechnic Institute and State University, Blacksburg RICHARD S. MULLER, University of California, Berkeley ELSA REICHMANIS, AT&T Bell Laboratories, Murray Hill, New Jersey EDGAR A. STARKE, University of Virginia, Charlottesville JOHN STRINGER, Electric Power Research Institute, Palo Alto, California KATHLEEN C. TAYLOR, General Motors Corporation, Warren, Michigan JAMES WAGNER, Johns Hopkins University, Baltimore, Maryland JOSEPH WIRTH, Raychem Corporation, Menlo Park, California ROBERT E. SCHAFRIK, Director V1
Preface Just as human operators must be protected from extreme environments, so must the electronics that operate and control a functional system. When the environment proves too warm, the electronics must be insulated, refrigerated, or simply moved to cooler locations. This last option is sometimes very difficult, or impossible, and the perceived fragility of electron- ics must then be reconsidered. Vacuum-tube technol- ogy provides a historical example of this process. Although vacuum tubes may be considered mechani- cally fragile, tube-based radio proximity fuses were nevertheless incorporated into artillery shells over 50 years ago! More recently, well-logging electronics derived from available semiconductor technology have been forced to operate for prolonged periods at 300 °C, far exceeding the "standard" limit of 125 °C that appears on uncounted specification documents. When the requirement is unavoidable and the motiva- tion is high (e.g., commercial or military advantage), "accepted" temperature limits need not be accepted. There is a huge difference between what can be done in principle and what should be undertaken in practice, however. If the question were to be asked "if a family of proven high-temperature electronics functions (for the moment meaning anything higher than 125 °C) were suddenly to become available, would its ultimate economic value justify the cost of its development," the answer is likely to be YES. This position is further strengthened by the fact that the shared virtues of radiation hardness, power han- dling, aru] blue-light emission represent an important leverage for the development of high-temperature semiconductors. However, if the question were "is · . V11 there already a market for high-temperature electron- ics sufficient to justify development of all or part of the family function," the answer may not be so clear. In all the processes of our economy, there are cur- rently few in which insertion of electronics into such environments is absolutely required to achieve accept- able functionality. Recognizing that a human operator can usually be protected and that a central controlling computer is easier still to protect, the determination of whether the benefits of high-temperature electron- ics will justify the cost requires the examination of how products and processes might be improved, or even enabled, by high-temperature electronics. The use of distributed control network architec- tures and embedded processors is rapidly growing. In a crude biological analogy, an animal is more agile, efficient, and durable when its nervous system (sen- sor signal processing), skeleton (physical structure), and muscular system (actuator operation) are integrat- ed. Electronics are integrated into systems for several reasons: (1) to simplify control paths, thereby simpli- fying wiring complexity, reducing weight, and im- proving reliability; (2) to distribute control, allowing robust system reliability and system architecture simplification; (3) to permit operational information to be gathered and processed with greater speed, accuracy, and reliability; and (4) to control actuators. For systems that encounter or generate high tempera- tures, this integration, or entwining, demands that electronics work at, or near, their functional tempera- ture limit; the more intimate the integration, the greater the environmental stress.
Materials for High-Temperature Semiconductor Devices If the economic value of extended-temperature electronics justify its cost, a natural question arises: "since the possibility of high-temperature electronics has been known for decades and the need is so great, why wasn't this done some time ago?" Although there can be no definitive answer to this question, there have been two historical barriers to the develop- ment of high-temperature electronics. First, the functions and performance goals of most familiar complex electronic systems (e.g., tele- communications and computers) are defined and measured in purely electronic terms. Thus, although it can be elaborate and expensive, the need for heat protection is viewed as an unavoidable element of system design, rather than of function. Second, nonelectronic systems (e.g., turbine engines, nuclear reactors, chemical refineries, and metallurgical mills) are operable without embedded electronic systems. Since the electronic function is not the defining element of these systems and extend- ed-temperature electronics are not available as a robust off-the-shelf technology, many prospective customers will not usually consider such systems. Thus, although cognizant of the architectural advan- tages of high-temperature electronics, prospective developers have not perceived a general commercial market sufficient to justify aggressive development. Even with these barriers, however, considerable international resources are currently being devoted to developing electronic technologies either tailored for or supportive of high-temperature operation. There is a divergence in the central emphases of these efforts. United States-Much of the focus is on high-temperature electronics. One manufac- turer markets a family of silicon-based integrated circuits suitable for prolonged operation at 250 °C, derived in part from radiation-hardened technologies developed for military applications. Silicon-carbide- based devices are being developed for some control applications and rudimentary dia- mond-based devices have been demonstrat- ed. Radiation-hardened electronics for reac · · - vail tor control and waste monitoring are avidly sought in both the United States and Europe. The large bandgap and smaller neutron cross sections of the lighter elements in high- temperature semiconductors also translate to radiation damage resistance. There are approximately seven industry, three universi- ty, and two national laboratory programs currently active in the high-temperature semiconductor field. The committee was briefed by representatives of most of these programs, which are listed on pages vi and vii. There is also some funding of wide bandgap semiconductors for use in high- power devices (e.g., the Semiconductor Research Corporation program at Purdue University). · Europ~Effort is mainly focused on power electronics. This is synergistic with high temperature because the generation of inter- nal heat is a limiting factor in power devices and is mitigated by larger bandgap and higher thermal conductivity materials. A collaborative organization, HITEN, was formed in 1992 to coordinate European nascent efforts in high-temperature electron iCS. Sweden Approximately 55 people are engaged in research at Linkoping Uni- versity and Kista in Stockholm. This is a joint government-ABB industries effort on power electronics, the first goal of which is a 12 kV thyristor. Germany The Deutsche Forschungs Gemeinschaft (DFG) sponsors several universities with Interdisciplinary Re- search Grants for silicon carbide (SiC). Primary among these are the University of Erlangen-Nurnberg and the Friedrich Schiller University in Jena, which are concentrating on novel growth tech- niques and electrical and optical mea- surements. Siemens Research Laborato ries in Erlangen, however, are concen
Preface bating on power devices, as is Daimler- Benz in Frankfurt for electric cars. These laboratories as well as several collaborating universities (i.e., Regens- burg, Erlangen-Nurnberg, TH Aachen, Ilmenau, and Fraunhofer Institut fur Angenwandte Festkorperphysik in Frei- burg) have large BMFT contracts for the development of SiC power-devices. France At least 10 university labora- tories as well as LETI-Grenoble and Thomson CSF (Paris) have government funding for SiC high-frequency and other devices. Japan The committee was unable to dis- cover critical details about the industrial involvement of Japanese companies in SiC development. However, emphasis appears to be on optoelectronics with occasional men- tion of high-temperature applications for the automotive and aerospace industries. Optical data transmission rates and storage densities are enhanced by the use of shorter wave- length laser light, which is synergistic with high-temperature work because it requires larger bandgap semiconductors. However, the 1994 domestic Japanese SiC conference drew 160 participants, many of whom were interested in power devices. In the nitrides ~ i . e . , g a 1 1 i u m n i t r i d e , g a 1 1 i u m - i n d i u m - n i t r i d e ~ light sources, Nichia Chemical is producing a 3 percent efficient blue light-emitting diode. The interest in Japan in large bandgap semiconductors for opto-electronics purposes is highly visible, but an interest for power electronics is growing. Japanese universities that are active in SiC are the University of Kyoto, the Kyoto Institute of Technology, Osaka University, and the Electrotechnical Laboratory in Tsukuba. Nitrides research is also being pursued at Nagoya University. Against this assessment of the national and inter- national efforts to develop high-temperature semicon o 1X ductors, the goals of this study are to (1) identify the technical barriers to the development and manufacture of high-temperature semiconductor materials; (2) determine the criteria for successfully packaging and integrating new high-temperature semiconductors into existing systems; (3) recommend future research priorities; and (4) suggest additional possible applica- tions and advantages. The report is structured as follows. Chapter 1 discusses the need for high-temperature electronics. Chapter 2 reviews the state of the art of wide band- gap materials. The fundamental limit to high-tempera- ture operation is the energy of the semiconducting bandgap of the host material. By this measure, even silicon with its "small" bandgap (1.1 eV) is not wide- ly used near its limit of 300 °C (silicon as a high- temperature material is discussed in Appendix A). A1- though the technology has not been optimized for high temperature and there are concerns about its chemical stability, gallium arsenide (1.4 eV) does offer the prospect of significantly higher temperature in a mature technology (gallium arsenide is discussed as a high-temperature semiconductor in Appendix B). Alternative materials for yet higher temperatures must be selected with care; larger gap is necessary but not sufficient. Sulfide semiconductors have large band- gaps but decompose at high temperatures. Thus, Chapter 2 reviews the state of the art of materials alternatives for which the prospect of robust high- temperature operation has been confirmed. These include SiC (2.4-3.3 eV depending on polylype), gallium nitride (3.5 eV), aluminum nitride (6.2 eV), boron-nitride (>6.4 eV), and diamond (5.4eV). Chapters 3-6 discuss generic, technological issues related to the design, fabrication, packaging, and testing of high-temperature circuits and devices (spe- cific case-studies are presented in Appendix C). These chapters contain common elements that must be established for any high-temperature electronics technology to be possible. Chapter 7 presents recom- mendations as to how to overcome critical hurdles on the path to a family of robust high-temperature elec- tronic devices.
Acknowledgements The committee expresses its appreciation to the following individuals for their presentations to the committee: Dr. H.M. Hobgood, Westinghouse Science and Technical Center, Pittsburgh; Dr. Calvin Carter, Jr., CREE Research Incorporated, Durham, North Carolina; Professor Peter Barnes, Auburn University; Mr. R.C. Clarke, Westinghouse Science and Technology Center, Pittsburgh; Dr. Joseph S. Shor, Kulite Semiconductor, Leonia, New Jersey; Dr. John Palmour, CREE Research Incorporated, Durham, North Carolina.; Dr. Dale M. Brown, General Electric, Schenectady, New York; Professor Robert J. Trew, Case Western Reserve University, Cleveland; Dr. Terrance Lee Aselage, Sandia National Laboratory, Albuquerque; Dr. Michael W. Gels, Lincoln Laboratory, Massachusetts Institute of Technology; Dr. Jeff Glass, North Carolina State University, Raleigh; Dr. Asif Khan, APA Optics, Inc., Blaine, Minnesota; Dr. Gary McGuire, Center for Microelectronic System Technologies, MCNC, Research Triangle Park, North Carolina; Professor Hadis Morkoc, University of Illinois-Urbana; Dr. Nate Newman, University of California, Berkeley; Dr. Harold West, Honeywell, Incorporated, Plymouth, Minnesota; Dr. Gerald Wilt, AFOSR/NE, Boiling Air Force Base, Washington, D.C.; Professor Manijeh Razeghi, Director, Center for Quantum Devices, Northwestern University; Dr. John A. Spitznagel, Westinghouse Science and Technology Center, Pittsburgh; Professor Aris Christou, Chairman, Department of Materials and Nuclear Engineering, University of Maryland, College Park; Dr. Richard Eden, Consultant, Thousand Oaks, California; Professor R. Wayne Johnson, Electrical Engineering Department, Auburn University; and Dr. Philip L. Dreike, Sandia National Laboratory, Albuquerque. The committee acknowledges with thanks the contributions of Robert M. Ehrenreich, Senior Program Manager; Jack Hughes, Research Associate; and Pat Williams, Senior Secretary, to the project. x
Contents EXECUTIVE SUMMARY 1 BACKGROUND Survey I: Applications of High-Temperature Electronics by Industry, 7 Survey II: Applications by Thermal Environment, 12 Survey III: High-Temperature Electronics Applications by Complexity, 13 Summary, 14 STATE OF THE ART OF WIDE BANDGAP MATERIALS Silicon Carbide, 15 Nitride Materials, 24 Diamond, 28 3 DEVICE PHYSICS: BEHAVIOR AT ELEVATED TEMPERATURES High-Temperature Effects: Fundamental, Materials-Related Properties, 31 Predicting High-Temperature-Device Performance: Materials-Related Figures of Merit, 33 4 GENERIC TECHNICAL ISSUES ASSOCIATED WITH MATERIALS FOR HIGH-TEMPERATURE SEMICONDUCTORS Electrical Contacts, 39 Doping and Implantation, 40 Gate Oxides and Insulators, 43 Etching, 45 Defect Engineering and Control, 46 Yield, 47 Device reliability, 48 5 HIGH-TEMPERATURE ELECTRONIC PACKAGING Chip Packaging, 51 Substrates, 53 Thick-Film and Thin-Film Metallization9 53 Component Attachment, 55 Interconnection, 56 Second-Level Packaging, 57 Summary, 58 X1 7 15 31 39 51
6 DEVICE TESTING FOR HIGH-TEMPERATURE ELECTRONIC MATERIALS Short-Term Constant-Temperature Tests, 61 Constant-Temperature Life Tests, 62 Thermal-Cycling Tests, 62 Future Requirements for High-Temperature Testing, 63 CONCLUSIONS AND RECOMMENDATIONS General Conclusions and Recommendations, 65 Materials-Specific Conclusions and Recommendations, 67 References Appendix A: Silicon as a High-Temperature Material Appendix B: Gallium Arsenide as a High-Temperature Material Appendix C: High-Temperature Microwave Devices 61 65 71 81 87 93 Appendix D: Biographical Sketches of Committee Members 119 xii
Figures 1-1 Schematic of a hypothetical drive-by-wire system for an automobile with computerized traction control, steering, and suspension Log-log plot of the complexity of some example applications as a function of temperature Average values of the optical constants of SiC from the vacuum ultraviolet to the middle infrared Calculated band structure of 3C-SiC Calculated band structure of 2H-SiC Summary of the experimentally observed excitor bandgaps and their temperature variation for the different SiC polytypes 2-5 Thermal conductivity of two single crystals of SiC 2-6 Schematic showing the basic elements of the modified sublimation process 2 7 Schematic of a typical SiC CVD growth chamber 2-8 Band structure of hexagonal and cubic modifications of A1N 2-9 Band structure of hexagonal and cubic modifications of GaN 2-10 Band-structure calculation of diamond 2-11 Thermal conductivity of two Type IIa diamonds 3-1 Calculated electron mobility as a function of temperature for undoped 6H-SiC and 3C-SiC 3-2 Calculated electron mobility as a function of temperature for GaN doped e-type, 10~7 cm~3 Intrinsic carrier density for silicon, GaAs, and SiC Decrease in silicon bandgap with increasing temperature Calculated reverse leakage current densities in pen junctions of various materials Variation in threshold voltage versus temperature for n- and p-channel MOSFET devices Operating temperatures for different devices per material Schematic of the device structure for a AlN/AlxGa~ xN SISFET Increase in resistivity of unintentionally doped AlxGa~ xN with increasing aluminum mole fraction Decrease in insulation resistance as a function of temperature 5-2 Reduction from nine to three electrical path segments between two integrated circuits with multichip module technology 6-1 Variations in threshold voltage for p- and e-type silicon MOSFETs with temperature 6-2 Drain characteristics of a SiC inversion-mode MOSFET at 650 °C A-1 Reduction in large junction isolation areas by the use of trenches and SOI A-2 Leakage currents as function of temperature for three types of n-MOS transistors with gate lengths of 2 microns · . ~ x~' 8 13 17 18 18 19 20 22 23 26 27 29 29 32 32 32 33 35 36 37 44 45 52 59 61 61 83 83
Schematics of the dielectric isolation material process flow and the bonded wafer material process flow A-4 Open-loop gain as a function of temperature B-1 GaAs MESFET and silicon MOSFET drain leakage currents B-2 MESFET transconductance, am, after three-hour anneals at various temperatures B-3 Diffusion barrier constructed of nine alternating layers of electron-beam evaporated tungsten and silicon B-4 Comparison of conventional MESFET with MESFET using temperature-hard ohmic contacts, buried p-type channel implants, and gate sidewall spacers B-5 MESFET showing on/off current ratio decreasing from 106:1 at room temperature to near 20:1 at 400°C High-temperature MESFET incorporating modifications to standard process Operating characteristics of MESFET structure shown in Figure B-6 Contours of normalized power dissipation on the gain-efficiency plane Enhancement- and depletion-mode MOSFETs Structure of a bipolar junction transistor Simulated microwave performance of SiC BJTs Comparison of SIT with MESFET: (a) potential gate barriers established, (b) resulting current-voltage curves for SIT; (c) generic MESFET I-V curves Structure of the Junction Field Effect Transistor (JFET) Typical current-voltage curves for a JFET at various temperatures Structure of an inverted JFET in SiC C-9 Measured small signal current and unilateral gain for SiC MESFETs C-10 IMPATT diode performance compared with projections for wide bandgap semiconductors C-l l Material structures and electric field profiles possible for IMPATT diodes C-12 A simplified equivalent circuit for an IMPATT diode embedded in a microwave circuit C-13 'a' contours for MESFETs of silicon, GaAs, silicon carbide, and gallium nitride C-14 Calculated locus of drain-current saturation for (a) silicon carbide, (b) silicon, and (c) GaAs (with and without parasitic series resistance) C-15 A simple model for ohmic contact and channel resistance contributions to MESFET source resistance C-16 Contact resistance calculated as a function of contact length for three materials C-17 Contours of constant Z plotted on rc-Rsq plane C-18 Representation of current-voltage curves for a MESFET and typical loadlines for Class A operation C-l9 Small signal equivalent circuit for a MESFET C-20 Contours of constant temperature rise in the GaAs MESFET channel C-21 Contours of constant temperature rise in the SiC MESFET channel C-22 A MODFET transistor with a two-dimensional electron gas at the interface between GaN and AlGaN B-6 B-7 C-1 C-2 C-3 C-4 C-S C-6 C-7 C-8 x~v 84 85 88 88 89 89 90 90 91 94 96 97 98 98 100 101 101 103 104 104 105 106 107 108 109 110 111 112 113 114 115
Tables 2-1 Comparison of Semiconductor Properties 2-2 Notations for Selected SiC Polytypes Exciton Binding, Nitrogen Ionization, and Valley-Orbit Splitting Energies and Effective Mass for SiC Polytypes Comparison of Normalized Figures of Merit of Various Semiconductors for High-Power and High-Frequency Unipolar Devices Selected Ohmic Contacts to e-Type 6H-SiC and Measured Contact Resistivities at Room Temperature Selected Ohmic Contacts to p-Type 6H-SiC and Measured Contact Resistivities at Room Temperature 4-3 Additional Ohmic Contact for SiC 4-4 Ohmic Contacts for GaN 5-1 Properties of Ceramic AlN, Ceramic SiC, Glass + Ceramics as Compared with 90 percent Alumina Metallizations for AlN Substrates Dielectrics for A1N Substrates Summary of Properties of Metallizations for AlN Typical Cofired Metals 6-1 Short-Term Constant-Temperature Tests Constant-Temperature Life Tests Summary of Room-Temperature DC Gain for Various Field Effect Transistors of SiC Assumed and Calculated MESFET Current-Voltage Model Parameters Listing of Several Refractory Metallizations on SiC and their Contact Resistivities Assumed and Calculated MESFET Power Model Parameters xv 16 17 21 34 40 41 42 43 54 55 56 57 58 62 63 102 108 111 112