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OCR for page 1
Executive Summary
Electronics that operate and control functional systems
must currently be protected from extreme environments.
Major benefits to system architecture would result if
cooling systems for electronic components could be
eliminated without compromising system performance
(e.g., power, efficiency, speed). The existence of
commercially available high-temperature semiconductor
devices would provide significant benefits in such areas
as:
· sensors and controls for automobiles and aircraft;
· high-power switching devices for the electric
power industry, electric vehicles, etc.; and
· control electronics for the nuclear power industry.
With the possible exception of light-emitting diodes
(LEDs), however, present commercial demand for wide
bandgap semiconductor materials is limited. While there
are few pressing applications that cannot be achieved
without wide bandgap materials, the vast array of
applications, and hence, the value, will only be realized
once these materials have evolved to such an extent that
off-the-shelf devices are available.
At the request of the U.S. Department of Defense and
the National Aeronautics and Space Administration, the
National Materials Advisory Board of the National
Research Council convened the Committee on Materials
for High-Temperature Semiconductor Devices to assess
the national and international efforts to develop high-
temperature semiconductors; to identify the technical
barriers to their development and manufacture; to deter-
mine the criteria for successfully packaging and
integrating new high-temperature semiconductors into
existing systems; to recommend future research priorities;
and to suggest additional, possible applications and
advantages.
1
This Executive Summary is divided into two sections.
The first section presents general conclusions and
recommendations about future research priorities to
accelerate the acceptance of high-temperature semi-
conductor materials. This section discusses the
temperature ranges for the different materials to be used,
the competitiveness of U.S. research versus foreign
competition, the systems in which high-temperature
electronic materials should initially be introduced, and the
government/industry/university collaborations required to
forward the development of high-temperature
semiconductor materials. The second section discusses the
barriers to the successful development, manufacture,
packaging, and integration of wide bandgap materials into
existing systems and presents the key research and
development priorities to overcome these barriers.
GENERAL CONCLUSIONS AND
RECOMMENDATIONS
Temperature Ranges
Silicon and silicon-on-insulator electronics may be
sufficient for some applications for temperatures up to
300 °C. Such applications include digital logic, some
memory technologies, and some aerated analog and power
applications. Silicon-based technology will not be
sufficient for many applications operating in the 200-
300 °C range, however, such as power-conditioning
devices in higher-temperature control systems. These
devices will have to be produced from another material
system. Based on the evidence presented in this report,
silicon-carbide-based devices are currently in the best
position to meet this need, particularly e-channel
enhan cement-mode metal-oxide semiconductor field elect
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Materials for High-Temperature Semiconductor Devices
transistors (MOSFETsJ. However, significant technological
barriers, such as micropipes, oxide quality, contacts,
metallization, packaging, and reliability evaluation still
need to be further addressed.
As a result of fundamental limitations, silicon-based
technologies will not be useful at temperatures above
300 °C. Other materials must be used for these
temperature ranges, but the choices are somewhat less
clear. Technology based on gallium arsenide (GaAs)
might be used for systems operating up to 400 °C. Just
working at elevated temperatures is not the only concern,
however. It is also essential that the devices reliably
function over a wide range from very cold (i.e., -20 °C)
to very hot (i.e., 400 °C). Based on the evidence
presented in this report, devices based on e-type silicon
carbide (SIC) are the only type that currently appear to
meet the temperature-range and reliability requirements,
but additional development is needed. Eventually, high-
temperature electronic technology could be developed for
reliable operation even for temperatures above 600 °C.
U.S. Competitiveness
As described in the Preface, considerable international
resources are currently being devoted to developing
electronic technologies either tailored for or supportive of
high-temperature operation. The United States is focusing
most of its efforts on high-temperature applications and
currently has a slight lead in SiC research.
Europe appears to be increasing its effort in wide
bandgap materials, especially for power electronics. This
research area is synergistic with high-temperature
applications because the generation of internal heat is a
limiting factor in power devices and can be mitigated by
larger bandgap and higher thermal conductivity materials.
The dedication of European resources to this area is seen
in the founding of the collaborative organization HITEN,
which was established in 1992 to coordinate nascent
European efforts in high-temperature electronics.
Japan is emphasizing the use of wide bandgap
materials for opto-electronics and leads in the use of
nitrides for light sources. Japan is also becoming
interested in power and high-temperature applications.
Unfortunately, the closed nature of Japanese industry
made it difficult for the committee to determine the true
level of interest in wide bandgap materials research. The
increased interest in high-power, high-temperature
2
applications is evident in Japan's annual domestic SiC
conference, however. The Third Domestic (Japan) SiC
Conference convened in Osaka on October 27-28, 1994,
with approximately 160 experts in attendance. Contrary to
Japan's previous two conferences, there was a greater
emphasis at the Osaka conference on high-power, high-
temperature applications than on LEDs.
The Commonwealth of Independent States had a
number of major programs in SiC development, but the
current financial difficulties of most of the Common-
wealth's institutions are preventing many laboratories
from continuing their research. There is a wealth of
expertise and information available for leveraging by
other countries, however. For instance, the European
Community is planning on supporting a SiC growth effort
in St. Petersburg (Y.M. Tairov and V.E. Chelnekov,
personal communication, 1994~.
The committee believes that the U.S. wide bandgap
materials research community is currently very
competitive in the international research community. To
remain competitive in the international research
community, the committee recommends that demonstration
technologies be pursued to motivate further research and
increase interest in high-temperature semiconductor
applications.
Demonstration Technologies
To increase interest and motivate further research in
wide bandgap materials, a realistic, inspiring application
focus must be found that can make system designers aware
of the benefits of high-temperature electronics. A wide
bandgap transistor that operates at 150 °C will not drive
the technology because it will be in direct competition
with the more economically efficient silicon technologies.
The demonstration technologies must be system circuits
(i.e., not an individual device) that can be inserted into
essentially nonelectronic systems (e.g., turbine engine,
nuclear reactor, chemical refinery, or metallurgical mill)
with the goal of measurably increasing system
performance.
As discussed in Chapter 1, the committee believes
that there eventually will be a niche market for
semiconductors with temperature capabilities higher than
that of silicon, and that this market will be sufficiently
large to justify the cost of development. However, this
belief is tempered by the recognition that because such
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Executive Summary
electronics will be used in new ways there is little
immediate demand. The market will grow only in synergy
with the availability of components. This suggests that
development of high-temperature electronics not be
undertaken in isolation. Instead, such development can
and should be leveraged from development of other
technologies with more immediate applications, thus
reducing the costs and risks of both. Three suitable
application areas are high-power electronics, nuclear
reactor electronics, and opto-electronics.
Power switching devices, for example, would be a
good demonstration technology for high-temperature
semiconductor materials. High-voltage, high-power
electronics, while not necessarily used as high-temperature
devices, nevertheless need wide bandgap semiconductors
because of their superior breakdown voltages and high
thermal conductivities. There is already considerable
research being pursued in this area because (1) improved
high-power switching devices could save an estimated $6
billion in the cost of construction of additional
transmission lines; and (2) the smoother, more efficient
use of the transmission system would reduce the need for
new generating capacity, which the Electric Power
Research Institute estimates would be a savings of $50
billion in North America alone over the next 25 years
(Spitznagel, 1994~.
The pursuit of demonstration technologies would not
only increase interest in wide bandgap materials, it would
also provide significant test beds for the application of the
technology and enhance our understanding of the generic
technologies required to further high-temperature-device
operation (e.g., materials etching and implantation;
degradation modes of metallic gates, contacts, and
interconnects at high temperatures; packaging behavior at
high temperatures; and accelerated-testing and reliability-
testing methodologies to ensure proper functioning). The
ability to grow a reasonably defect-free material is not the
only requirement for the realization of a successful
technology. The development of demonstration
technologies would also help identify other factors that
must be resolved for high-temperature electronics to be
incorporated into existing systems.
Funding Strategy
The need for new development funds for
demonstration technologies and future wide bandgap
3
materials is not necessary in the committee's opinion.
Government funding currently exists for long-range
research in wide bandgap materials, although additional
funding would certainly allow more options to be
evaluated within a shorter period of time. Industry has
also demonstrated a willingness to commercialize new
developments if the projected payback to their investments
can occur within the short term (NRC, 1993~. The
committee believes that the high-temperature research
community should leverage the research funding for wide
bandgap materials that is currently being provided by the
high-power and optics markets, where no viable
alternatives to wide bandgap materials currently exist.
Building on the funding for other areas dependent on wide
bandgap materials reduces the need for potential users of
high-temperature devices to fund the required materials
development exclusively and, thus, may render it cost
effective.
The committee recommends the following strategy for
the development of wide bandgap materials:
develop precompetitive alliances and integrated
programs (national laboratories, universities, and
industries) for coordinating research, technical
skills, and capabilities to expedite research in the
most efficient manner;
direct research at a technology demonstrator that
has definite applications (i.e., is a product) and
addresses the usually neglected areas of
packaging, assembly, testing, and reliability (e.g.,
high-power switches; integrated motor control;
power phase shifter);
concurrently develop materials, design, testing,
and packaging; and
build and test the demonstration component on a
cost-share basis that encourages teaming, ensures
adequate funds, and requires periodic deliveries.
The committee believes that the founding of a
newsletter that provides a summary of published
worldwide developments in high-temperature semi-
conductor research would assist the establishment,
development, and maintenance of (1) a fundamental long-
term materials effort, (2) an infrastructure within the
industry, (3) a group to monitor international
development, and (4) a U.S. information group for
highlighting advances.
OCR for page 4
Materials for High-Temperature Semiconductor Devices
MATERIALS-SPECIFIC CONCLUSIONS
AND RECOMMENDATIONS
The first three parts of this section concentrate on the
major wide bandgap materials discussed in this report:
SiC, nitrides, and diamond. The final part of this section
concerns the generic problems in packaging that will
affect the production of all high-temperature electronic
devices.
Silicon Carbide
SiC is an indirect bandgap semiconductor and has
enjoyed the longest history and greatest development with
regard to both materials growth and device realization. As
such, SiC is currently the most advanced of the wide
bandgap semiconductor materials and in the best position
for near-term commercial application. Its main application
will be in high-power, high-temperature, high-frequency,
and high-radiation environments. It will not be suitable for
blue lasers or ultraviolet light emitters, however, except
as a potential substrate material. The three key research
efforts for the development of commercially viable SiC
devices are:
· Wafer production: The 1- and 2- inch SiC wafers
now in production are rapidly approaching device
quality where they might be used for commercial
production of devices and circuits with acceptable
yield. It could be argued that such small wafers
are entirely sufficient for what will be a relatively
small market (compared with silicon) with a very
high-price premium, and therefore an early
investment in larger wafers is not justified. How-
ever, the entire commercial infrastructure for
electronics manufacture is based on a wafer size
of at least 3 inches, and preferably 4 inches, as a
minimum. Reconstructing a small-wafer infra-
structure that became obsolete over 30 years ago
will be both an expense and an obstacle to the
introduction of commercial SiC electronics. The
committee believes that the development of larger
SiC wafers is viewed as the more cost-effective
approach to commercial development.
· Film growth: Chemical vapor deposition
molecular-beam epitaxy, and other film-growth
4
technologies and chemistries require refinement to
produce epitaxial films with n- and p-type doping
ranges from 10~3 to 102° C~3 for nitrogens
aluminum, boron, gallium, transition metals, and
rare earth elements.
· Manufacturing processes: Lower-cost device-
production methods are required to make the
manufacture of SiC devices more competitive
with the silicon technologies.
c7 ,
Nitrides
Interest in the direct bandgap nitride materials (i.e.,
gallium nitride, aluminum nitride, aluminum gallium
nitride, and indium gallium nitride) has dramatically
increased recently because of their optical properties. The
materials show great promise and are likely to dominate
the visible and ultraviolet opto-electronics market.
Nichia's recent bright blue LEDs have already stimulated
increased industrial effort (e.g., Hewlett Packard, Spectra
Diode Laboratories, Xerox PARC) in materials growth,
contact metallurgy and reliability, and device reliability
and testing, although the materials have defect densities of
greater than 10~°/cm2 and the mechanism of photo
emission is currently unknown. Heterojunctions in the
nitrides also hold promise for higher-speed devices com-
pared with SiC. Their applicability for power development
and high-frequency devices is unproven at this time, and
the technologies for wafer production, doping, and etching
are currently less developed than SiC and require more
longer-term research before they will be competitive with
other electronic materials. However, as development of
photonic applications for wide bandgap materials
progresses, the opto-electronic market may provide an
effective way to leverage the development of these
materials for high-temperature-device applications. The
committee identified the following three research efforts
as being key to the development of nitride devices:
Compatible substrates: Better-matched substrates
are required for nitride wafer production to be
commercially tenable.
Wafer production: Growth of quasi-crystalline
films of gallium nitride, aluminum gallium
nitride, and aluminum nitride should be pursued
on substrates such as SiC to gain thermal
advantages.
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Executive Summary
· Doping: Methods for both n- and p-type doping
of Group III nitrides are required.
Diamond
Diamond is a well-understood material, but its use for
active electronic device applications is not feasible at this
time because of the difficulties associated with its
economical growth and doping. While diamond transistors
have been designed, fabricated, and tested, their perfor-
mance is also orders of magnitude less than that which is
expected from the electrical properties intrinsic to dia-
mond. The poor performance is thought to result from
excessive nitrogen impurities and from as yet not fully
explained surface-depletion effects. The current prognosis
for diamond is primarily as a protective coating, a thermal
management film, and a material for electron-emitting
cathodes.
Packaging
Much more research is required in the area of high-
temperature packaging. For high-temperature electronics
to be commercially viable arut provide true performance
5
advantages, interconnection and packaging technologies
are required that can reliably operate at temperatures up
to 600 °Cfor 104 hours. To attain these goals, innovative
packaging techniques will be required. The three key
research efforts for the development of high-temperature
packages are:
· Metallization: Contacts are required in the 10-6 to
10-7 Q/cm2 range that have long-term durability at
temperatures up to 600 °C. Greater understanding
is needed of the long-term effects of high tem-
peratures on contact and interconnect metallurgy,
degradation and failure modes, reliability, and
interfaces.
· Device reliability and aging testing: Existing
methods of accelerated, environmental-life testing
of packages must be adapted for high-temperature
applications to ensure the accurate assessment of
device reliability and aging.
· Computer-aided design tools: Computer-aided
design tools are required that incorporate
electrical and mechanical simulation of high-
temperature electronic systems.
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Representative terms from entire chapter:
bandgap materials
