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Extreme-Temperature and Harsh-Environment ElectronicsPhysics, technology and applications

Vinod Kumar Khanna

Chapter 1

Introduction and overview

1.1 Reasons for moving away from normal practices in electronicsExtreme temperature and harsh environment electronics begins where routineconventional electronics designed for operation in room-temperature friendlyenvironments ends. It breaks away from the traditional treatment of electronics tocover aspects which may, at times, appear less friendly and more antagonistic toelectronic circuit operation, although there are exceptions to this rule, as we shall seebelow. Necessity is the mother of invention. The rationale for taking excursionsfrom routine electronics is that in many applications it becomes imperative to buildelectronic systems that have to perform satisfactorily and reliably for long durationsin unfavorable circumstances (Werner and Fahmer 2001). Such conditions mayprevail when we dig deep into the Earth, when we move out into space, whenelectronic equipment is placed near nuclear reactors and particle accelerators, whenthe operation of heavy machinery creates vibrations in buildings, when equipmenthas to withstand high humidity, rainy and stormy weather, to name just a few of theaggressive situations one can contemplate. So, the need for deviation from conven-tional electronics is primarily driven by the increasing demands from the users andcustomers who work in such hostile conditions (Johnson et al 2004).

But there is a secondary reason as well. This reason originates from the realizationthat many physical phenomena, such as superconductivity, take place only attemperatures which are far below room temperature. To apply such phenomena forhuman use, the temperature has to be deliberately decreased close to absolute zero,or at least to the vaporization temperature of liquid nitrogen. Here, the basicoperational principle of electronic devices and circuits imposes the requirement ofbreaking the norms of working at normal temperatures. This applies not only fromthe viewpoint of superconductivity phenomena; many electrical parameters ofsemiconductor devices show improvement as the temperature is decreased. Thusone should not consider that deviations from the norm will always lead to anaggressive and incompatible situation. It may be a fortunate situation as well.

doi:10.1088/978-0-7503-1155-7ch1 1-1 ª IOP Publishing Ltd 2017

In fact, the properties of semiconductors change over a very wide range as oneincreases the temperature from −273 K to 1000 K. This variation of properties ofsemiconductors is visible in the form of changes in the electrical behavior of theelectronic devices fabricated from them. Some electrical parameters tend to improveat low or high temperatures, while others show deterioration. The comprehensivestudy of these behavioral trends helps scientists take advantage of the changes thatare beneficial in the utilization of electronic circuits when we are operating outsidethe recommended range of temperatures.

In response to both types of situation portrayed above, one stemming fromapplication-specific requirements and the other from phenomenological needs,one has to move away from regular practice and deal with challenging situationsto meet one’s aims.

1.2 Organization of the bookThis book is subdivided into 20 chapters. Chapter 2 elaborates on the motivationsfor departure from treading the established path of electronics. In this chapter, thereader will come across many situations and applications that show the deficienciesof conventional electronics in solving problems. These examples also illustrate theneed to shift away from the normal course to benefit from the advantages of utilizingphenomena that take place only under special conditions.

The remainder of the book (chapters 3–20) is grouped into two parts (seefigure 1.1). Part I, consisting of chapters 3–14, deals with extreme-temperatureelectronics (ETE). It examines countering the harmful effects of very high temper-atures as well as utilizing the beneficial effects of very low temperatures, as insuperconductive electronics, and utilizing the characteristics of semiconductordevices which improve with a fall in temperature, e.g. the leakage currents. Thustemperature effects manifest themselves as both a curse and a boon for electronicsexhibiting hostile as well as friendly behavior. Part II, comprising chapters 15–20,covers only those effects which are detrimental to electronic circuit operation, suchas: highly humid climatic conditions; corrosive environments inside or in theneighborhood of chemical factories; in radiation-contaminated areas, such as nearnuclear power stations, x-ray or gamma-ray equipment in hospitals; and in vibratingbuildings amidst the busy and heavy traffic of metropolitan cities or where heavymachinery is being used, making the surroundings noisy and shaky.

1.3 Temperature effects1.3.1 Silicon-based electronics

Chapter 3 explains how the properties of semiconductors change as a function oftemperature. Since variation of these properties is reflected in the thermal behaviorof devices, a thorough understanding of this chapter lays a firm foundation forunderstanding the contents of ensuing chapters. The upper temperature limit of asemiconductor material is fixed by its bandgap energy. Device operation is governedby the concentration of free carriers in a pure semiconductor, known as the intrinsiccarrier concentration. The intrinsic carrier concentration is an exponential function

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of temperature. An increase in temperature augments the energy of electrons in thevalence band of a semiconductor material. At a particular temperature called theintrinsic temperature, the thermal energy of electrons exceeds the bandgap energy ofthe semiconductor. Then the electrons are promoted from the valence band to theconduction band. The number of thermally generated carriers becomes equal to thenumber of free carriers due to impurity doping, either n- or p-type. Then there are no

Figure 1.1. Organization of the book.

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longer any distinguishable n- or p-type regions. The p–n junction reduces to aresistor and its function is paralyzed.

On the opposite side, as the temperature of a semiconductor decreases towardsabsolute zero, the ionization of impurity atoms to release free carriers ceases. Thenthere are no, or very few, carriers available for conduction. In this carrier freeze-outregime as well, the normal operation of a semiconductor device stops. In fact, asemiconductor is an insulator at zero kelvin.

Apart from the liberation of electrons from their bonds rendering possible theiravailability for conduction of electrical current, other noteworthy phenomenainclude the scattering of carriers by lattice atoms and impurity ions, which affectthe ease of movement of carriers through the crystal lattice, i.e. the carrier mobility.Mobility is a strong function of temperature. As lattice atoms vibrate with greateramplitudes with rising temperature, the electrons undergo more collisions on theirpaths and mobility decreases. Impurity scattering limited mobility varies in theopposite way, because the increased vibration of impurity ions makes them lesseffective in influencing electron motion. The temperature dependence of carriermobilities is also affected by whether a semiconductor is non-degenerately ordegenerately doped.

From chapter 4 onwards up to chapter 9 we move towards discussion, assessmentof the capabilities and appraisal of the critical issues concerning electronic devicesand circuits made from semiconductor materials of progressively increasing energybandgap, and hence intrinsic temperature. In this sequence, the starting material ofinterest is silicon, which has been the favorite of electronic engineers for a very longtime and has reigned as the king of electronic materials. Silicon electronics forms thecontents of chapters 4 and 5. In chapter 4, bipolar silicon devices are addressed.Chapter 5 focuses on MOS silicon devices. Silicon electronics has two forms: bulksilicon and silicon-on-insulator (SOI) technologies. The objectives of these chaptersare to present simple analytical formulae for the temperature coefficients (TCs)of silicon bipolar and MOS devices. These derivations help in appreciating thedegradation or upgradation in electrical characteristics of bipolar/MOS silicondevices and circuits subjected to constantly rising temperatures.

The forward voltage drop across a p–n diode or Schottky diode decreases withtemperature, the current gain of a bipolar transistor increases with temperature andthe breakdown voltage of a diode increases with temperature. In almost all circuitapplications, the leakage currents of p–n junctions should be kept infinitesimallysmall with respect to the signal currents. These leakage currents increase exponen-tially with temperature. In complementary metal–oxide semiconductor (CMOS)structures, junction leakage occurs at the junctions between source/substrate anddrain/substrate due to the minority carrier diffusion current near the depletionregion, together with electron–hole pair (EHP) generation inside the depletionregion. Gate leakage increases with thinning of the gate oxide as MOSFETs arescaled towards smaller dimensions. The threshold voltage decreases linearly with risein temperature and hot carrier effects become less pronounced with increasingtemperature. High-cost SOI technology considerably helps in obviating the leakagecurrent issues in bulk silicon devices at high temperatures.

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Silicon and germanium have been two key materials forming the backbone ofelectronics. Silicon–germanium, an alloy of silicon with germanium, combines thebest properties of both materials. Silicon/silicon–germanium heterojunction bipolartransistors (HBTs) overcome the problem of a rapid fall in the current gain andswitching frequency of silicon bipolar temperatures at cryogenic temperatures.Chapter 6 treats the mathematical theory of the HBT and shows how this deviceserves as a replacement for bipolar transistors, showing much better performance atthese temperatures.

Silicon was the first-generation material of the twentieth century. It played aleading role in ushering in the microelectronics revolution. Time and again, it hasappeared that silicon electronics has hit a wall, prompting the search for alternativematerials. Silicon was followed by gallium arsenide in the second generation ofsemiconductors. Together with silicon, gallium arsenide set off the informationtechnology and wireless revolution around the turn of the twenty-first century.Chapter 7 deals with gallium arsenide, which is superior to silicon in the fabricationof ultra-fast radio-frequency (RF) devices, and is also suitable for making optoelec-tronic devices such as LEDs and laser diodes. In contrast to silicon technology, wherethe primary devices are bipolar transistors andMOSFETs, GaAs relies onMESFETsand HBTs.

1.3.2 Wide bandgap semiconductors

The wide bandgap semiconductors, silicon carbide and gallium nitride, belong to thethird-generation semiconductors. They heralded the optoelectronics and HTE era atthe beginning of the twenty-first century. The capability of silicon carbide andgallium nitride chips to operate at higher temperatures, voltages and frequenciespromoted the research interest in these materials. Approximately three times theenergy used in the case of silicon is needed to transport an electron from theconduction to valence band in SiC and GaN, which makes both ideal candidates forrealizing high-temperature devices with high breakdown strength. Naturally, thepower electronic modules built from these materials are significantly more energyefficient.

The manufacturers of silicon carbide wafers have been able to minimize the defectdensity considerably as well as increase the size of the wafers. Technologicalbreakthroughs in SiC materials and process techniques have led to the realizationof several devices such as p–n diodes, Schottky barrier diodes (SBDs), JFETs,bipolar transistors, etc. SiC JFETs are very attractive for HTE, but SiC MOSFETsstill need a lot of improvement. In SiC, the interface state density is high and thecarrier mobility is very low. Scrupulous efforts are being made in the development ofSiC thyristors, IGBTs and gate turn-off thyristors (GTOs). Chapter 8 providesglimpses into achievements in silicon carbide electronics.

Gallium nitride is an excellent material for the fabrication of light-emitting diodesand power transistors. Advancing from gallium nitride wafers on sapphire substratesto free-standing gallium nitride wafers with low dislocation density is likely to boost

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confidence in GaN technology. GaN MESFETs and HEMTs have been developedand tested at high temperatures. Chapter 9 takes a look at some of the GaN devices.

After silicon carbide and gallium nitride, the superior properties of diamondmake it suitable to catapult another major upheaval in microelectronics. Diamondhas been crowned as the ultimate semiconductor material due to its spectacularcombination of properties, including high thermal conductivity and radiationhardness. Diamond electronics is still immature because the full capabilities ofdiamond have so far evaded utilization. The ability to synthesize diamond from thevapor phase to produce large-area films has generated a lot of interest in diamond-based devices, and the scenario seems to be gradually changing as single-crystalelectronic grade (EG) diamond is becoming commercially available. Chapter 10deals with the synthesis, processing and characteristics of diamond films and relateddevices.

1.3.3 Passive components and packaging

The aforementioned few chapters concentrate on wide-bandgap semiconductors andthe active devices fabricated from them, the aim being to fabricate semiconductordevices that are able to withstand successively higher temperatures. Apart fromsemiconductors, the other important materials used in electronic devices are themetals used for contact electrodes. No electronic circuit can be fabricated withoutpassive components. Furthermore, the devices have to be packaged within safeenclosures to protect them from mechanical damage, as well as atmospheric andweather effects. The electronic circuit will fail at elevated temperatures if the activeor passive components, metallization or packaging are not up to the mark. Fromthese considerations, chapter 11 diverts attention from semiconductors towardsresistors, capacitors, metal interconnections and packaging. Carbon resistors showgood thermal stability. Diamond resistors are fabricated from CVD diamond on analuminum nitride substrate. Teflon capacitors can be used up to 200 °C and micacapacitors up to 260 °C. Diamond capacitors are based on a dielectric film ofdiamond with Au contacts. Low dielectric loss and constant capacitance areobserved up to 450 °C.

In addition to good adhesion with the underlying silicon, the metallization mustbe able to withstand thermal cycling and must not decompose or undergo chemicalreactions at high temperatures. Films of refractory metals and refractory metalsilicides formed by chemical vapor deposition (CVD) serve as a useful metallizationfor high temperatures. This method provides selective deposition over siliconregions, leaving aside oxides and insulating areas. Electrolytic Cr–Ni–Au metal-lization is a robust scheme for wire bonding applications exposed to high temper-atures (250 °C).

During die attachment, care must be taken to match the coefficients of thermalexpansion of the die, die-attach and substrate. This avoids any mechanical stressingor fracturing of the die during thermal cycling. Die-attach materials proven forroom-temperature operation cannot be used owing to their low glass transitiontemperature.

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For the reliability of wire bonds at high temperatures, the metals used for thewire and the metallization of bonding pad must be mutually compatible. Poorcompatibility of metals involved in wire bonding leads to two types of problem.Either an intermetallic compound is formed at the interface between the metalscausing brittleness of the bond and breakability, or voids are formed at the interfaceby diffusion, an effect called the Kirkendall effect, thereby weakening the bond.Inopportunely, the common Au–Al combination between the Au wire and Almetallization pad is prey to such phenomena. These arguments entice us to use thesame metal for the wire bond and the bond pad.

For high-temperature operation, hermetic ceramic packages are far better thanplastic packages. They also serve as moisture and contamination impenetrablebarriers. Limiting the ingress of moisture and dirt prevents corrosion. Regrettablyceramic packages are larger, heavier and costlier than plastic packages, whichcannot be used past 150 °C. High melting point solders with a melting point >250 °Cmust be used.

1.3.4 Superconductivity

The following three chapters (chapters 12–14) are concerned with superconductivity,both low-temperature and high-temperature. Superconducting films exhibit lowresistance even at frequencies of approximately a few hundred GHz, paving the wayfor their utilization in magnetometry using superconducting quantum interferencedevices (SQUIDs), microwave filters, transmission lines, etc. Rapid flux singlequantum (RFSQ) logic electronics uses Josephson junctions (JJs) to perform logicoperations based on the quantization of magnetic flux. HTS-based power trans-mission uses cables comprising hundreds of strands of HTS wire with a cryogeniccooling system to maintain the required low temperature. In dense urban localities,power substations often reach capacity limits. HTS systems bind these substationstogether circumventing expensive transformer upgradation. HTS power delivery isused to load pockets in high-demand metropolitan areas with a saturated grid.

1.4 Harsh environment effects1.4.1 Humidity and corrosion effects

Temperature is an important parameter, and has been the focus of the attention ofelectronic engineers because by increasing/decreasing the temperature, both advan-tageous and disadvantageous results can be produced on electronic equipment.In addition to temperature, humidity is another vital parameter. Humidity-relatedfailures and remedial schemes are discussed in chapters 15 and 16. Throughmoisture condensation, water droplets accumulate on the surfaces of semiconductordevices, producing ionic current flow. The effects of humidity depend on thematerials used, the dimensions of the components and their layout. Humidityaccelerates the corrosion rate. Corrosion is the deterioration of the materialsconstituting the electronic devices and circuits under the influence of reactive gasesin the environment. The principal reason for the vulnerability of electronic productsto corrosion is the large variety of metals and alloys used in the electronics industry.

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A few of these are aluminum, copper and its alloys, silver and its alloys, tin and itsalloys, titanium, chromium, nickel, gold, platinum, palladium, tungsten, etc. Apartfrom humidity, corrosion depends on inorganic and organic contaminants, atmos-pheric pollutants, salt spray, noxious gases, and residues from soldering and otherassembly processes. Corrosion increases the contact resistances between joints andleakage currents between wires and decay products. Corrosion by water, dust andgases causes short circuits and produces ugly surfaces. It can initiate cross-talks. As aconsequence of the reduced spacing between components in the wake of miniatur-ization of circuits, a material loss of a few pictograms due to corrosion is adequate tospawn a fault. Therefore, the impact of corrosion becomes more magnified indamaging electronic circuits. Chapter 17 suggests ways to mitigate corrosionproblems.

1.4.2 Radiation effects

Next to temperature and humidity comes the radiation from space as well asterrestrial sources. These types of radiation have a negative influence on electroniccircuits located on Earth, those placed in orbiting satellites or used in long-distancespace flights. These defects range from performance degradation to complete loss offunctionality. Due to these radiation-induced failures, satellite lifetimes are short-ened and space missions are disrupted. Radiation effects can be one or more of thefollowing types: single event effects (SEEs), displacement damage (DD) and totalionizing dose (TID) in the form of a cumulative effect over a long time span.Radiation-hardened circuits are modified versions of non-hardened equivalents,incorporating revised designs for fault-tolerance and software approaches to dealwith disturbances, along with suitable manufacturing process amendments. BipolarICs are more radiation-hard than CMOS circuits. Radiation-hard circuits arefabricated in SOI or silicon-on-sapphire substrates, instead of the common bulksilicon wafers, because of the higher leakage currents produced by radiation injunction isolated devices. Chapter 18 describes the detrimental effects of radiationexposure on electronic circuits. Feasible ways to thwart the disturbances and damageproduced by radiation in electronic circuits are suggested in chapter 19.

1.4.3 Vibration and mechanical shock effects

Last but not the least, the damaging effects of vibrations, impacts, kicks, drops andshocks, as caused by acceleration/deceleration and impulsive forces on electroniccircuitry cannot be ignored. The concluding chapter 20 describes the commontechniques that must be adopted for protection from vibrations.

1.5 Discussion and conclusionsThe temperature effects on electronic circuits must be carefully understood, takingdue consideration of their pros and cons. Pernicious effects must be suppressed.Beneficial effects must be gainfully exploited to execute utilitarian functions, as anenhancement to system functionality. The effects of humidity, corrosion, radiationand vibration need to be addressed according to the specific application. They have

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to be dealt with on a case-by-case basis. Some ideas of this chapter are succinctlydescribed in the following poem:

Safeguarding electronics

Extreme temperatures are sometimes hostile, sometimes cozy;Sometimes thorny, sometimes rosy;Away from room temperature, some device parameters downgrade,Other parameters upgrade.But high humidities and aggressive chemicalsare always detrimental.Vibrations too are harmful.So, please be carefulTo make electronic design fault-tolerant,And package environment-resistant.Wide bandgap semiconductors are promising,Superconductive electronics is amazing.To ensure success of mission,Choose electronic design, materials and fabricationAccording to application;Provide proper surface passivationto protect from moisture invasion.Prevent chemical corrosion.Take precautions for radiationAnd cushion electronic product against vibration.Keep in touch with new developments and innovations.

Review exercises1.1. Contemplate two situations, one negative or unfavorable situation and

one positive or favorable situation, which urge electronic engineers todevelop circuits that can work in non-conventional conditions.

1.2. What property of a semiconductor material fixes the upper permissiblelimit of temperature up to which an electronic device made from it may beoperated?

1.3. Why is operation of a semiconductor device not possible at a temperatureexceeding its intrinsic temperature?

1.4. Give some examples illustrating the beneficial effects of low temperaturesfor electronic device operation.

1.5. Explicate, with reference to carrier mobility, the outcomes of scattering ofelectrons by lattice atoms and impurity ions. Elucidate the influence oftemperature on mobility variations caused by the two types of scattering.

1.6. Cite three examples showing the variation of the most important electricalparameters of semiconductor devices with temperature.

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1.7. Does temperature affect the leakage current of a p–n junction? If so, whydoes it happen? Describe the various types of leakage currents that aCMOS structure is susceptible to.

1.8. In what manner is a silicon–germanium heterojunction transistor superiorto a silicon bipolar transistor for operation at cryogenic temperatures?

1.9. Mention two application areas where gallium arsenide devices find wideusage.

1.10. In what ways can silicon carbide and gallium nitride outperform siliconand gallium arsenide devices?

1.11. Is it possible to synthesize diamond artificially?1.12. Give examples of resistors and capacitors developed for high-temperature

applications.1.13. What kind of contact metallization can be used in HTE?1.14. What precautions are necessary for selecting appropriate die-attach

materials to be used at high temperatures?1.15. What problems occur due to poor compatibility of the wire metal and

metallization pad?1.16. What are the advantages and disadvantages of hermetic ceramic

packages?1.17. Give three applications of superconductivity.1.18. What is the physical basis of RFSQ logic?1.19. Why is HTS-based power delivery beneficial in upgrading dense, con-

gested urban power network?1.20. Why are electronic products prone to corrosion effects? How do humid

environments aggravate corrosion effects? How does corrosion impair theperformance of electronic devices?

1.21. What are the three types of radiation effects on electronic circuits? Whatare the approaches followed for countering these effects?

1.22. Which type of ICs are more radiation-hard: bipolar or CMOS?1.23. Which type of silicon wafers are used for the fabrication of radiation-hard

circuits: bulk silicon wafers or SOI wafers?1.24. Mention one adversary of electronic circuits, apart from temperature,

humidity, corrosion and radiation.

ReferencesJohnson R W, Evans J L, Jacobsen P, Thompson J R and Christopher M 2004 The changing

automotive environment: high-temperature electronics IEEE Trans. Electron. Packag.Manuf. 27 164–76

Werner M R and Fahmer W R 2001 Review on materials, microsensors, systems and devices forhigh-temperature and harsh-environment applications IEEE Trans. Ind. Electron. 48 249–57

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