TUTORIAL  -  PART  1


1) What is Extreme-Temperature Electronics (ETE)?

The term extreme-temperature electronics is used here to mean electronics operating outside the "traditional" temperature range of −55/−65°C to +125°C. ETE covers both the very low temperatures, down to essentially absolute zero (0 K or −273°C), and the high temperatures from +125°C up to as high as electronics can be made to operate. The acronyms LTE, HTE, ETE will be used for low-temperature electronics, high-temperature electronics, and extreme-temperature electronics.

Note: The usual definition of cryogenic temperatures is that they are temperatures of 120 K (about −150°C) and lower.


2) What are the temperature limits of electronics?

At the low end, operation of semiconductor-based devices and circuits has often been reported down to temperatures as low as a few degrees above absolute zero, in other words as low as about −270°C. This includes devices based on Si, Ge, GaAs and other semiconductor materials. Moreover, there is no reason to believe that operation should not extend all the way down to absolute zero.

On the high end, "laboratory" operation of discrete semiconductor devices has been reported at temperatures as high as about +700°C (for a diamond Schottky diode) and 650°C (for a SiC MOSFET). Integrated circuits based on Si and GaAs have operated to +400−500°C. Si ICs have been reported to operate at +300°C for 1000 hours or longer.

Covering both extremes, there are reports of the same transistor working from about −270°C to about +350 to +400°C, an operating temperature span of over +600°C!

Also, many passive components are useable to the lowest temperatures or up to several hundred degrees Celsius.

Bear in mind, however, that operation at extreme temperatures is not automatically true for every semiconductor device or passive component; operation at extreme temperatures depends on a number of materials and design factors.


3) Are these temperature limits attainable for practical applications?

At the low-temperature end, practical operation of devices and circuits is reasonably achievable to as low a temperature as desired, bearing in mind that materials and designs appropriate to the temperature must be used. The various characteristics of a device may improve or degrade. In particular, below about 40 K (about −230°C) Si devices often exhibit significant changes in characteristics.

The high-temperature end presents more difficulty. The practical upper temperature limit is determined by many factors and for semiconductor devices often does not reflect the inherent temperature limit of the semiconductor material. The limit is frequently determined by the interconnections and packaging, both for active devices and passive components. As an indication of the practical upper limit, circuits have been offered commercially for operation up to +300°C [for example see http://www.ssec.honeywell.com/hightemp/].

Parts availability is a major obstacle to practical ETE; there are few components specified for either low- or high-temperature use. Persons needing to construct ETE hardware often have to select and adapt from the available "traditional" temperature range component base. Those with greater resources and time have sometimes followed the path of custom fabrication.


4) Why would anyone want to operate electronics outside "normal" temperatures?

Generally speaking, the usual motivation for operating electronics at extreme temperatures is to improve the overall performance of a system, by having part of the system—an electronic subsystem—operate outside the usual temperature range. The subsystem itself may or may not have improved performance from operating at an extreme temperature.

One of the primary reasons is that measurement or control must be done in a cold or hot environment. For example, a major use of HTE is in well-logging. Exploration and monitoring for petroleum and geothermal wells uses probes that are sent deep into bore holes, where the temperature may reach +200 to +300°C. Since signals from sensors in these probes must be sent to surface instruments via long cables, it is desirable to co-locate some signal-processing electronics in the probes to avoid signal degradation and to multiplex signals. One approach is to allow the electronics to be subjected to the high-temperature environment seen by the probe. This "hot" electronics in the probe will likely have degraded performance; however, the overall system of sensor(s), probe electronics, and surface instrumentation will have improved performance compared to a system that has no electronics in the probe.

There is also interest in applying HTE to aircraft, both commercial and military, particularly with the trend towards "fly-by-wire" and distributed systems. Electronic interface circuitry would be co-located with monitoring and control transducers for engines, actuators, and other aircraft system elements. Many of these locations involve a hot environment, possibly up to +300°C.

A similar situation arises for low temperatures. A thriving application is for scientific and military spacecraft. Many sensors, such as infrared or X-ray detectors, used for astronomical observations or surveillance must operate at very low temperatures. In order to extract the ultimate performance from these sensors it is necessary to co-locate the initial signal-processing electronics with the sensors in the cold environment. Generally this environment is in the cryogenic temperature range. Scientific spacecraft incorporating such cold electronics have included IRAS (Infrared Astronomical Satellite, 1983), COBE (Cosmic Background Explorer, 1988), IRTS (Infrared Telescope in Space, 1995), ISO (Infrared Space Observatory, 1995), and others.

Moreover, since very little of the Solar System falls within the conventional electronics temperature range, ETE might be a key technology for exploration of the Solar System.


5) Isn't it very cold in space—about 3 kelvins?

This question frequently comes up, and the answer is "yes and no." Far from anything (in intergalactic space, for example) a passive object would cool to a few kelvins (a few degrees above absolute zero). However, spacecraft are not in such an environment during their useful life; most spacecraft are near bodies such as the Earth and also receive energy from the Sun. In addition, spacecraft usually incorporate power sources (chemical batteries, solar cells, or nuclear generators), and the resulting heat must be dissipated. Thus in practice, the temperature of an object in space is a complicated matter, depending on proximity and orientation to other bodies, absorption and emission of energy, and internal heat generation.

Cooling a spacecraft down to a few kelvins passively (without refrigeration or a cryogen) in the inner Solar System is probably impossible. However, quite low temperatures can be attained by using well designed thermal shielding and insulation combined with large heat radiators. For example, major parts of the James Webb Space Telescope (JWST) (Formerly the Next Generation Space Telescope, NGST) are planned to be operated as cold as about 35 K (about −240°C) by these techniques. Temperatures below this (down to a few kelvins) require an active refrigerator or liquid helium, which has been used in many spacecraft such as the scientific spacecraft mentioned above.

Considering only the influence of the Sun on a black body in space, calculated temperatures range from a high of about +175°C at the orbit of Mercury (nearest the Sun) to a low of about −230°C (44 K) at the orbit of Pluto (furthest from the Sun) [http://www.grc.nasa.gov/WWW/RT1996/5000/5480di.htm].

However, when a spacecraft lands on a body in the Solar System, or enters its atmosphere, the situation can be entirely different. In general, bodies nearer the Sun are hotter and those distant from the Sun are colder, but not always. Some of the more interesting places to explore present the following extremes of temperature: the surface of Mercury varies from about −180°C (90 K) to +425°C, Venus has a surface temperature of about +460°C, Mars surface temperatures dip to about −100°C at night, and Europa (a satellite of Jupiter that is of interest in connection with extraterrestrial life) is expected to subject probes to −170°C.


6) Why not use conventional electronics and keep it within its operating temperature range?

Conventional-temperature-range electronics can be used in an extreme-temperature environment by means of insulation and heating (for low-temperature environments) or refrigeration (for high-temperature environments). This can be combined with thermal sinks or thermal sources. For example, the well-logging electronics mentioned earlier may be placed in a dewar (vacuum-insulated vessel) to protect it from the hot environment. In addition, the electronics may be thermally connected to a thermal sink, a material that can absorb a large amount of heat without a substantial temperature increase. This is usually done by employing the material's phase change from solid to liquid, which absorbs a large amount of heat (the latent heat of fusion). For low temperature, the opposite effect may be used to provide a thermal source. The same material may serve as a thermal sink for high temperature or a thermal source for low temperature, and may be as basic as ice/water or less familiar such as a bismuth alloy.

Note: The terms thermal sink and thermal source are used here, but the terms heat sink and heat source are also used.

However, in many situations the techniques described above would be undesirable or impractical. There are trade-offs: the passive techniques may have a limited lifetime that is insufficient for the application. The active techniques require additional power and subsystems. For some applications, active techniques may be too disturbing to the environment because of the additional heat that must be dumped. All the techniques add weight, bulk, and some degree of complexity. These burdens may be less acceptable or less practical compared to using special electronics that can withstand the temperature of the environment. Thus, operating electronics beyond the normal limits is an option worth considering.


7) What about improving performance of electronics by cooling?

Another reason that electronics has been operated at low temperatures is improved performance of the cooled electronics itself. Improvement upon cooling results from a combination of effects: in general, transistors (field-effect types) exhibit increased gain and speed and lower leakage; also parasitic resistances and capacitances in interconnections decrease, heat transfer improves, and many devices exhibit lower noise.

There have been two main application areas. One is to speed up digital electronics. Perhaps the most impressive embodiment of this idea was the ETA10 supercomputer, which had its central processor boards (each with about 240 integrated circuits) immersed in liquid nitrogen (77 K or −196°C). Another example of cooling to enhance computer performance was the Kryotech Athlon-900. There appears to be no "low-temperature" computer offered at present.

Another successful ongoing area is microwave preamplification. The cooling of amplifiers to reduce noise is well established and this has been employed for many years in the scientific community for receivers used in radio astronomy as well as for deep-space communications with distant spacecraft. Cooling transistors greatly reduces their thermal noise, which is the dominant noise at microwave frequencies.


8) Who provides electronic components for extreme temperatures?

A listing of sources of electronic components and subsystems for extreme temperatures is available in a separate listing.




The preceding has been adapted from the Extreme-Temperature Electronics Newsletter Issue #1 (26 April 2001). The coverage is abbreviated and many points are only touched on. Further information may be obtained from the included Internet links and from the items listed separately.


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