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OCR for page 7
1
Background
Trying to enumerate systematically all the possible
applications for new high-temperature electronics would
be a futile endeavor. Rarely are all of the conceivable
uses for any new technology obvious. The committee was
able to identify only a few eager potential users that
currently have active programs that require higher-
temperature electronics. Several more applications are
under consideration but are not in active development.
Any group of technologists could generate a much larger
list of plausible applications. However, while these
applications might seem reasonable to enthusiasts of high-
temperature electronics, they may not be realistic options
to the prospective customers for this new technology.
Furthermore, just as for the microprocessor (or any
number of other new technologies, such as the laser), a
much larger array of "enabled" applications is likely to
evolve if, and when, a proven off-the-shelf technology
becomes a viable option in engineering new products and
systems. Rather than generating one more speculative list,
a few of the better-defined applications are described in
this chapter, supplemented with more generic descriptions
of environments and applications for high-temperature
electronics.
The largest possible range of applications can be
anticipated by means of three surveys. The first survey is
a traditional list describing applications ranging from
programs in progress, through speculative system designs,
to what amounts to a few responses to the question: What
might be done differently if cost-effective high-temperature
electronics were available? A systematic estimate of the
economic value of high-temperature electronics was not
attempted by this committee, but expert estimates that are
available are included in this survey. The second survey
classifies the types of environment that might be
encountered by electronics and then associates some of the
previously identified applications with each environmental
7
type. In principle, a general classification of all possible
operating environments would automatically describe the
environments associated with all possible applications,
including those not yet conceived. The third survey again
uses the list of identified applications to give a sense of
the capabilities that might be needed as a function of
temperature. Although the first survey is, by definition,
incomplete and the second and third are hardly more than
intellectual exercises, together they give a strong sense of
the potential industrial and economic importance of high-
temperature electronics.
SURVEY I: APPLICATIONS OF ~GH
TEMPERATURE ELECTRONICS BY INDUSTRY
Automotive
The automotive industry is often cited as the primary
near-term market for high-temperature electronics. While
the automotive environment is stressful to electronic
systems, the stress is rarely in the form of simple heat.
Conventional vehicle architectures with an open-bottomed
front engine compartment, generous underhood and
underbody airflows, a metal heat-dissipating body and
frame structure, and access to a water-cooling circuit
leave very few locations within a vehicle that regularly
achieve temperatures significantly above 100 °C. These
locations are mainly near the exhaust system or brakes
and can usually be avoided. Occasional problems with
reliability due to high temperatures (as high as 150 °C)
have been addressed by combinations of heat shielding,
redirected airflow, blowers, or simple component
relocation. Except for rare cases of architectural errors,
the major challenge to reliability of automotive electronics
is the combination of rapidly changing environmental
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Materials for High-Temperature Semiconductor Devices
stresses (temperature and humidity cycling), exposure to
corrosives and solvents, and an economic mandate for
low-cost packaging. With careful attention to device and
circuit layout, wire-bond and lead-frame integrity, choice
and use of polymer packaging materials, and strict process
control, automotive electronics actually meet or exceed
military specifications at a small fraction of the cost and
in huge volumes (Motz and Vincent, 1984; Dell 'Acqua
and Marelli, 1990; Frank and Valentine, 1990~.
Despite the illusion of a comfortable status quo, four
trends are forcing major changes in the approach to
automotive electronic component and system design. First,
even with current vehicle architectures, customer
expectations of reliability continue to rise. Flawless
performance for 10 years or 150,000 miles will soon be
standard. Second, the electronics content of modern
automobiles is rising rapidly, both in convenience features
(e.g., heads-up display and navigation systems) and in the
management of powertrain and suspension systems.
Figure 1-1 is a diagram of a hypothetical drive-by-wire
system with computerized traction control, steering, and
suspension. The amount of sensing, signal processing,
data transfer, system control, and power actuation is very
large. A few elements of this system (e.g., semi-active
suspension, antilock brakes, and traction control) are
currently in the marketplace. Multiplex wiring will soon
be standard in motor vehicles. While easing the transfer
ll
. ~
Driver
Inputs
/ Steering /
/Powertrain / ~ Rr, ,
,I Comouter/ TIC - I Comouter/
. J f t
I Suspension k,z'
~:''H~ ~
Reprinted with permission from SAE Paper 861027
0 1986 Society of Automotive Engineers, Inc.
Brakes ~ ~
FIGURE 1-1 Schematic of a hypothetical dr~ve-by-w~re system for an
automobile with computerized traction control, steering, and
suspension. SOURCE: Rivard (1986).
8
of information and reducing wiring weight and
complexity, multiplex wiring dictates the location of quite
complex nodes in many hostile locations. Third, the
physical architecture of the vehicle itself is changing.
Improved aerodynamics dictates more compact flowing
shapes with less internal airflow, which forces denser
packaging of the powertrain and exhaust systems. Serious
consideration is being given to sealing the engine
compartment and moving the radiator to the rear of the
vehicle. Fourth, replacement of metal body and frame
components with composites of much lower thermal
conductivity will eliminate many safe havens for
electronics. This is not so much a trend to higher
temperature as a trend toward more uniform temperature;
locations near 100 °C may disappear while those between
150 °C and 200 °C will remain plentiful. Nevertheless,
solutions that evolved for the current, more open, steel-
based architecture may not serve in the hotter
environments of future vehicles.
Power electronics are also rapidly proliferating in
automobiles (Thornton, 1992; Bose, 1993~. Figure 1-1
indicates several systems that include high-power
actuators. Full, active suspension requires several tens of
kilowatts. Electric and hybrid-electric vehicles are totally
dependent on power electronics for efficient operation of
motor and braking systems. There are two types of high-
temperature issues for power electronics.
First, in conventional combustion-powered vehicles,
the electronics must be placed somewhere that is
preferably near or within the device they control. Safe,
cool locations have become scarce, however. For
example, the drivers for electric, active front-suspension
components share the underhood environment, while the
flywheel-mounted motor and alternator for torque leveling
are cooled only by the engine oil and may reach 300 °C!
For electric and hybrid vehicles, the wiring weight,
resistive losses, and radio frequency emissions are
minimized by placing the power electronics within the
motor housing. To minimize weight, these motors are
sized such that they may produce several times their
continuous-service power for periods of several seconds.
This translates to a rapid temperature rise that is currently
[Limited to 180 °C only by the magnetic properties of the
permanent magnet rotor. Integral power electronics must
survive repeated and rapid excursions to at least this
temperature.
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Background
Second, power devices generate considerable internal
heat. This heat must be dissipated to prevent thermal
runaway (i.e., when heat generated increases with
temperature) and destructive failure. In many locations, a
cool-sink for large amounts of heat is unavailable.
Although active cooling is always an option, a power-
device technology immune to thermal runaway is highly
desirable. The smaller package size afforded by a higher-
temperature technology is of considerable value on its
own in terms of thermal management.
In summary, two clear needs can be identified for
automotive electronics. First is the need for a low-cost,
highly reliable technology for operation at an intermediate
temperature Perhaps 200 °C). This need might be served
by modification of current silicon-based technology.
Second is the need for power electronics able toff'nction
in elevated ambient temperatures with restricted heat-
s~nk~ng. As current silicon-based power technology is
largely limited by internal heat generation, a switch to a
wide bandgap semiconductor is dictated.
Aerospace
Gas Turbine Engines
High-temperature electronics are essential to the
development of multiplexed systems for gas turbine engine
control (Nieberding and Powell, 19821. In present control
systems, all electronics are centralized in a protected area
that is cooled with ambient air or fuel. This architecture
has proven satisfactory for some time, but, as the
requirements for engine control become increasingly
complex, the wire harness and connectors associated with
point-to-point architecture have become major weight and
reliability issues. Some wire harnesses weigh over 150
pounds, and every connector is a point of system
vulnerability. A solution to this problem is to introduce a
multiplexed architecture in which wire harnesses are
replaced with common busses, a change that demands
high-temperature electronics.
The Air Force Integrated High-Performance Turbine
Engine Technology Program is a multiphase project aimed
at achieving increased thrust, 50 percent weight reduction,
fault-tolerant control, and system integration of military
aircraft engines. A key element in this program is the
development of higher-temperature electronics. The
environments for electronics in an aircraft engine cover a
wide range for some potential sensors: 175-800 °C. Early
phases of the project call for electronics and optics for
operation at 175 °C, while an intermediate phase calls for
250 °C. The final phase of the program anticipates heat-
sink temperatures as high as 350 °C. Specifications for
commercial engines are not yet available but are likely to
be similar (Skira and Agnello, 1992; Tillman and Ikeler,
1992~. Some of these temperatures are suitable for devices
based on silicon technology, while others lie beyond that
currently anticipated for any electronics technology. While
it is neither desirable nor cost effective (and maybe
impossible) to construct the whole system to survive the
highest temperatures, any increase in operating
temperature offers a corresponding increase in design
flexibility.
Other Aerospace Applications
Engines demand the highest temperature requirements
for current aircraft, but temperature requirements will rise
in many other critical areas as vehicle speed increases. A
recent example is the control of the engine inlet guide
vane for the high-speed civil transport, which requires
that the moderately complex electronics driving the guide
vane actuators operate for prolonged periods at 200 °C.
In high-performance or heavily electronics-laden
aircraft (today almost the same thing), a generic problem
appears: as speed and therefore heat generation and
altitude increase, the ability to dissipate waste heat into
the atmosphere decreases (Christenson, 1991~. Locations
in the aircraft that remain below 125 °C or that can be
conveniently reached by the cooling system cannot be
found. Many electronics systems, including avionics,
radars, and communications equipment, must be aerated
in performance to maintain even the minimal acceptable
reliability at the margins of their operating ranges. Fuel
is often used as the medium for heat transfer within the
aircraft, but some fuel must then be kept in reserve as
essentially dead weight and, when cooling to outside air
is insufficient, the fuel tanks become a limited heat-sink.
The cooling techniques currently in use force tradeoffs
between speed, altitude, and systems shutdown.
In one example of a supersonic fighter plane, 90
percent of the cooling capacity of the environmental
control system (ECS) is devoted to electronics and only
10 percent to the pilot! The single ECS unit weighs
roughly 2,000 lbs and consumes 50 kW of power. Its
9
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Materials for High-Temperature Semiconductor Devices
excessive weight precludes the addition of a backup, so its
failure aborts the mission. A smaller ECS, possibly with
~ hackun would reduce weight and power while
increasing overall system reliability. Higher-temperature
electronics will enhance reliability and enable major
changes in the electronics architecture of aircraft.
Space Vehicles and Exploration
Problems directly related to high temperature are rare
once in space; space is cold and intense sunlight may be
reflected with high efficiency. There are several situations
in which high temperature may be an issue, however.
First, sensing and control of rocket boosters and thrusters
may require proximity to the hot plumbing associated with
combustion. Such problems and issues are very similar to
those for aircraft jet engines, with the notable exception
that maintainability and long-term reliability are less
important. Second, some space exploration vehicles must
enter hot environments. A proposed balloon-borne probe
of Venus' atmosphere must operate at 325 °C, while a
Venus lander must endure 460 °C. Closer approaches to
Mercury or the sun would also require higher-temperature
electronics. Third, material and design factors that support
high-temperature electronics operation would also enhance
radiation hardness and increase resistance to upsets and
damage from the unavoidable flux of cosmic radiation
(Jurgens, 19821.
Nuclear Power
There are two types of nuclear power applications for
wide bandgap semiconductors: those associated with
reactor operation and these as.~nciated with handling
processing, and storing of radioactive waste. It has been
reported that material and devices in high-temperature
operations tend to be resistant to radiation damage (Knoll,
19891. The highest temperature reached in a properly
operating pressurized water reactor (POOR) is nearly 300
°C. Although this temperature is actually somewhat lower
than that used in combustion power plants, accessibility is
much more limited and difficulty of repair or replacement
demands much higher reliability.
High-temperature, radiation-hard electronics can
improve PWR operation by improving reactor control and
reducing expensive and occasionally hazardous repairs.
The most important areas relate to monitoring and control
10
over the distribution of power generation in the core of
the reactor. At present, a three-dimensional map of the
core is developed from an array of thermocouples and
neutron-flux detectors distributed through the reactor core.
These require numerous penetrations (roughly 60) of the
reactor vessel and must be replaced every three years.
With integrated-drive electronics and multiplexing, a
different detector type would last at least twice as long
and require only four penetrations. By the year 2010, this
alteration would amount to a savings of nearly half a
billion dollars in materials and over $100 million in
avoided costs of radiation exposure for the 100 operating
PWRs in the United States (Spitznagel, 19941.
With the limit on penetrations relieved, more
detectors might be used to provide a more detailed "map"
of the core. In-core measurement of the water level also
enhances this mapping. A more accurate map of the core
allows for safer operation, more efficient consumption of
the fuel, and extension of the period between shutdowns
for refueling. Downtime, whether deliberate or forced,
has been conservatively estimated to cost roughly
$500.000 ner day (NRC, 1993~. At an estimated $50,000
per man-rem of radiation exposure, repairs and
maintenance are very expensive (Spitzoagel, 1994~. If
radiation-hard, high-temperature-electronics control and
monitoring devices could improve the current "nuclear
generating capacity factor" from 65 percent to a
reasonable target of 85 percent, then yearly savings per
plant would be $36.5 million per year. With at least 100
plants in the United States, this constitutes a yearly
savings of $3.6 billion per year. This is an impressive
savings and should be a great encouragement to the
development of high-temperature semiconductors.
Other PWR applications include monitoring of boron
and nitrogen 16 in the water. The thousands of valves and
pipes in a reactor must be monitored for proper valve
positioning, corrosion, and fatigue. Following an accident,
the environment of the reactor containment building can
be hot (420 °C), wet, and radioactive. Actuators and
sensors must survive under these conditions.
High-temperature electronics may also play a role in
radioactive waste storage and handling. The condition of
stored nuclear waste must be monitored. Buildup of
explosive gasses must be prevented, as must leaks of toxic
or radioactive material. This requires sensors in both the
tanks and the surrounding environment. Monitors include
neutron and gamma radiation monitors, temperature
, .
.. ..
OCR for page 11
Background
sensors, chemical sensors (gas and liquid), leak sensors, industrial processes (e.g., refining, annealing, baking, and
and cameras. Currently, conventional television cameras
survive approximately only 30 minutes in a nuclear
storage container (Spitznagel, 19941. A radiation-hard
television camera would be a great asset in reactor
monitoring, repair, and waste handling. Under an accident
condition described above wherein the containment
building may become hot, wet, and radioactive, remote
visual inspection of a damaged reactor is extremely
difficult with current technology. Robust monitoring
equipment will also limit the need for opening the
containers for maintenance.
The case of the orbiting power reactor (e.g., the
curing) involves monitoring the process flow from a fixed
location. While this monitoring may expose some sensors
to temperature extremes and other hazards, the associated
electronics are easily protected and cooled. Some
processes are best observed from inside, however. For
example, careful control of the time-temperature profile
during epoxy curing is a key element to yield and
reliability in the electronics industry. Appropriately
insulated recorders and transmitters are currently sent
through baking and curing ovens, but these devices are
expensive. In this example, cure temperatures do not
exceed 200 °C, and many others do not exceed 300 °C.
Thus, these temperatures are within reach of many high-
temnerature technologies. which would offer the
Russian-designed Topaz) combines all the difficulties of
nuclear power with space electronics. Although space is
indeed cold, heat is only lost by radiation. The size of the
radiator for the cold end of a heat engine (i.e., the
reactor) increases very rapidly as the cold-end temperature
is reduced. Raising this temperature allows a much more
compact design but exposes the control electronics to
higher temperatures. At the same time, radiation shielding
for sensitive electronics is wasted payload weight. High- ~
temperature, radiation-hard electronics would allow a throughout the manufacturing process. Such "smart tags"
smaller, lighter, and simpler design for a space-borne would be useful for process and quality control (Arbab et
reactor. al., 1993~.
Petroleum Exploration
Well-logging is a strong driver for high-temperature
electronics. Modern petroleum exploration involves
elaborate probing of wells during drilling. For this reason,
oil exploration companies have been some of the earliest
customers for high-temperature electronics. Earlier efforts
have resulted in fairly complex circuits built of discrete
devices that are able to operate for periods of several
hundred hours at temperatures up to 300 °C. Since it is
very expensive to withdraw and replace probes during
drilling, reliability is of extreme importance. An off-the-
shelf family of more sophisticated components would
enable far more reliable and effective logging tools.
Industrial Process Control
Industrial process control is rarely mentioned in the
context of high-temperature electronics, but may prove to
be one of the most important high-temperature electronics
applications. Most monitoring of high-temperature
11
~ ~ 7
possibility of widely available, inexpensive, compact
sensors, memories, and transponders that ride through the
high-temperature process beside, or even buried within,
the product. It may even be possible to report the
temperature and stresses on an integrated circuit while the
package is being formed or to attach coded identifiers to
components that record and report on their history
Power Electronics
The importance of power electronics in vehicles was
discussed earlier. Many of the issues concerning internally
generated heat and in-motor integration also apply to
many other applications. In vehicles, there are three
general areas of application. These include high-torque
induction-motor controllers, high-efficiency voltage
converters and switches, and variable high-voltage ultra-
capacitors (Miller, 19871. The integration of control and
power electronics so-called "smart power" is certainly
an architectural advantage. There are many additional
applications for small, high-torque electric motors besides
motor vehicles. Such small motors will replace hydraulics
in many applications once the reliability issues are settled,
offering considerable design and control advantages and
eliminating the weight of hydraulic fluid and the
complexity of associated plumbing.
A good example of extensive integration of power
electronics is the Air Force's More-Electric Airplane. The
anticipated advantages of all-electric actuation are
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Materials for High-Temperature Semiconductor Devices
considerable: a 20-30 percent reduction in system weight
and cost; fivefold increase in system efficiency; fivefold
reduction in heat generation; faster system response; and
improved maintainability, reliability, and survivability.
The More-Electric Airplane incorporates a starter/gener-
ator as an integral part of each engine. The generator will
provide all auxiliary (nonthrust) power for aircraft
operation. In current aircraft, a smaller generator is
connected by a gear shaft. Temperatures in the new
location already exceed 125 °C and may exceed 200 °C
in future engines. Furthermore, the very high powers
involved (hundreds of kW) dictate that the power-
conditioning electronics be located close to the generator
and the engine. Thus, both cooling and "remoting" are not
attractive options and high-temperature electronics are
highly desirable. There are other military "More-Electric"
programs that relate to armored vehicles, ships,
submarines, and even the individual combat soldier.
Similar civilian "electric-hydraulic" applications include
lighter and more agile industrial robots and more precise
and efficient excavation and earth-moving equipment.
Electric power was once measured simply by its cost
and quantity. Recently, the quality of electricity has
become a serious issue. Disturbances to line voltage and
noise on power lines is disruptive to such sensitive
systems as computers. Utility power conditioning has been
identified as a key area for application of power
electronics on a large scale (Hingorani and Stahlkopf,
1993~. On average, roughly a third of the rated capacity
of the power transmission grid is unused in the United
States. This margin is held to absorb very large
inductance transients from disturbances (e.g., generator
failure, overload cutouts, and broken cables) without
damaging switching and generating equipment. Large-
scale power electronics would allow real-time phase-
shifting of utility power and provide this protection while
allowing nearly 100 percent use of the national power
grid. The Electric Power Research Institute (EPRI)
estimates an available savings of $6 billion compared to
the cost of additional transmission lines of the same
capacity. Smoother and more efficient use of the
· · . ~. ~
these are at best only 80 percent efficient, their
elimination would effectively increase power-generating
capacity at essentially no cost.
SURVEY II: APPLICATIONS BY
THERMAL ENVIRONMENT
Three factors define the thermal environment for
electronics: (1) ambient temperature, to which a quiescent
device will inevitably rise in the absence of any
circulating coolant; (2) external temperature gradients
around the device or module, which are defined by the
details of the nature of the application; and (3) internal
temperature gradients, which are generated by active
devices. When these factors appear singly, high
temperature applications can be classified as immersion
(i.e., no temperature gradients and therefore no cold-sink
to cool the devices), proximity (i.e., the application brings
the electronics close to a hot region but does not dictate
immersion; at least a limited cold-sink is available), and
internal (i.e., where internally generated heat must be
removed to a cold-sink). Temperatures for these
applications are discussed in the next section.
Examples of purely immersion applications include
reactor monitoring, well-logging, ride-through process
monitoring, some nodes in aircraft or motor vehicle
multiplex systems, and the Venus lander. In such
applications, every component of the system must perform
satisfactorily at the nominal operating temperature. An
example is combustion-flame sensing for jet engine
control. The sensor itself must survive a very hot location
with line-of-sight to the combustion chamber while the
associated interface signal circuit is placed as close as
possible. Obviously, there are design and cost tradeoffs in
how much of the system needs to be exposed to the
nominal high-temperature environment. Support
electronics may be removed to cooler locations at the
expense of cabling and reduced signal.
Proximity applications typically appear where some
high-temperature component or process must be
transmission system also reauces the neea Jor spare monitored or where system architecture motivates
generating capacity. EPRIestimates that this efficient use incorporation of control electronics near a very hot
would create a savings of $50 billion in North America component. An example is the engine-mounted control
alone over the next 25 years. With higher-quality power computer for automobiles. While exposing the computer
available directly from the utility grid, the need for to the increased temperatures associated with the large
uninterruptable power supplies will be greatly reduced. As gradients of the exhaust system, moving the computer
12
r ~
OCR for page 13
Background
from the vehicle to the engine realizes two advantages. It
allows calibration of the computer to the specific engine
on which it is mounted (rather than a single-model engine)
for improved performance and reduced emissions. It also
minimizes the number of wires connecting the engine to
the vehicle, simplifying assembly and improving
reliability.
Generally, internal heating is a major issue only for
power electronics. Power electronics must be incorporated
wherever electrical actuation is required. To a first
approximation, heat generated by power devices simply
superimposes an internally generated gradient on the
externally defined thermal environment and raises the
nominal-device operating temperature accordingly. Power
devices appear in both immersion and proximity
applications. Examples of "immersed" power electronics
are the torque-leveling motor and integrated traction
motor described in the previous section. A case of power
devices in proximity to a hot region would appear in any
case where the actuating motor grows extremely hot or
the objects to be actuated are hot themselves. Such
situations will appear in many aircraft and vehicle control
applications (e.g., the inlet guide vane mentioned earlier).
Just as for nonpower proximity, device temperature may
be reduced at the expense of system integration.
One important consideration is that the temperature
rise in a power device can be very large. For silicon
based power devices, junction temperatures in excess of
200 °C are apt to result in catastrophic failure. Current 1°°°°°°°
systems are engineered to sustain the rated power output
with heat-sinking into a 100 °C ambient, which is
adequate to keep the devices below their failure
temperature; in effect, they are designed to operate at the
edge of disaster. The same rate of heat generation and
heat-sinking capacity into a 200 °C ambient (or cold-sink
temperature) would imply a junction temperature of 300 ~1000
°C, beyond the abilities of silicon. This simplistic linear c'
analysis suggests that even a small increase in ambient
temperature for sil~con-hased power electronics will
require a combination of improvements in heat extraction
and aerating of the devices themselves. Both of these
changes increase the size and cost of the systems. The
heat extraction problem is further compounded by the fact
that thermal conductivity of most materials decreases with
increasing temperature. Power electronics based on wider
bandgap semiconductors would address this issue.
13
SURVEY m: ~GH-TEMPERATURE
ELECTRONICS APPLICATIONS
BY COMPLEXITY
The ability to satisfy the need for electronics for a
given temperature is predominantly a function of what is
required for the application. Complexity, as crudely
measured by the number of active devices in the module
or system node, varies by nearly seven orders of
magnitude. Figure 1-2 is a log-log plot of the complexity
of some of the applications identified in Survey I as a
function of their temperature. Because the scales are
logarithmic, large errors in either parameter cannot
eliminate the obvious trend. This figure suggests three
general categories of high-temperature electronics.
The first category includes all the complexity and
functionality now available in conventional silicon
technology that is functional to roughly 200 °C (e.g.,
memories, microprocessors, analog circuits). These
applications might be served by modifications of current
junction isolation, integrated circuit technology with new
metallization and packaging, or if necessary, by silicon-
on-insulator technology for operation up to 300 °C.
Decreases in device speed and noise margins must be
accepted but might be mitigated by changes in device
geometry and layout rules. In all the categories discussed,
~ oooooo
a_
.O
a, 1 OOooo
,` 1 0000
x
a)
100
10
l ~ Pentium
- ~ · Radiation-Hard Camera
Baking/Curing e ~ ~ SP-100/Topaz
Well-Log
PwR Sensor ~ IHPTET
100
Temperature (°C)
EGO
I 1 1 1 1 1 1 1 ~
1000
EGO: emission gas-oxygen sensor
IHPTET: integrated high-performance turbine engine technology
FIGURE 1-2 Log-log plot of the complexity of some example
applications as a function of temperature.
OCR for page 14
Materials for High-Temperature Semiconductor Devices
it must also be remembered that even seemingly minor
modifications in technology must be carried out with
adequate circuit yield, which is critical at these levels of
integration.
The second group includes applications of
intermediate complexity, perhaps several dozen to several
thousand devices, requiring operation at temperatures of
up to roughly 450 °C. This level of complexity is
sufficient to support the local functionality of sensing and
measurement along with the signal conditioning, basic
signal processing and control, limited memory, and
interface (via wire or radio) to higher-level systems in
cooler environments. Although this definition remains
vague, it does appear that no reasonable application calls
for duplication of all silicon capabilities in a 450 °C
technology. It does appear that a more limited family of
devices, integrated circuits, and circuit-board technology
will be necessary for these applications.
The third group of applications generally involve
sensing of one or more parameters of a very hot
environment. Examples include automobile exhaust-gas
analysis and jet engine flame detection, which are
considered proximity applications; plausible immersion
applications above 500 °C have not been identified. In
these two cases, the sensor design is driven by its function
and the required environment. For the automotive exhaust
gas-oxygen sensor, this is 700-900 °C in a strongly
oxidizing or reducing atmosphere. EGO sensors use TiO2
or ZnO2 as wide bandgap semiconductors in which oxygen
ions behave as holes. Thus, these high-temperature
applications involve only one, or at most a few, active
devices: the sensor itself and the minimal biasing and
correction circuitry. The very large temperature gradients
(several hundred degrees centigrade in a few centimeters)
in most proximity applications could be made to appear
inside the module. This allows use of intermediate-range
electronics in support of the high-temperature sensing
component.
One element omitted from this temperature-
complexity scatter plot is internally generated heat from
power devices. They can be treated as individual "hot"
14
devices in need of lower-temperature support electronics,
analogous to the high-temperature sensing applications.
The critical difference is that the temperatures are much
lower. While silicon power devices may run into
difficulties in ambient temperatures much above 100 °C,
the low-power support electronics could easily be made to
function at much higher temperatures. This strongly
suggests a mixed technology consisting of silicon-based
control electronics from the first category in support of
power devices in a wide bandgap semiconductor
technology.
SUGARY
Although it is impossible to anticipate all possible
applications for high-temperature electronics, it is possible
to categorize them. A real need exists for advanced
microprocessors functional to 200 °C, but system
complexity appears to decrease rapidly with required
appear that suggest
operating temperatures. Thus, some natural groupings
__ directions for technological
development. The low-temperature, high-complexity
applications require a silicon-based technology modified
for reliable operation up to at least 200 °C. with a
- 7
reduced family of functions at 300 °C. Intermediate-
complexity, intermediate-temperature applications require
rudimentary integrated circuit technology (i.e., logic
functions, small memories, and analog signal devices) and
discrete circuit technology for circuits containing several
dozen to several thousand devices operational to 450 °C.
Low-complexity, high-temperature applications are driven
by sensing. A family of devices for such high
temperatures is probably not necessary. The sensors
themselves are per force designed for such environments,
and a slightly more benign environment suitable for
intermediate temperature devices is usually quite near.
The need for reduced package size and weight and higher
operating temperature defines a pressing need for wide
bandgap power devices.
Representative terms from entire chapter:
power devices
