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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 , . .. ..
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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 ~
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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.
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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: