15 February 2001

It's not the humidity; it's the HEAT!

by David Wick

Semiconductor technology offers major improvements in measurement, control, and efficiency of harsh environment systems.

Applying traditional electronics design methodology to high-temperature (HT) applications can be impossible or expensive.

However, with some understanding of what's available in high-temperature electronics (HTE) in the marketplace and a different approach taken to packaging and integration, cost-effective designs happen.

This is not to say there are ways to realize HT designs at equivalent cost to room temperature versions. But with system on a chip becoming a reality, the gap between high and ambient temperature sensing is narrowing.

Unfortunately, one cannot simply grab a distributor catalog of HT-rated components and start ordering like one can with low-temperature parts. Such a catalog does not exist, for one thing, and if it did, it wouldn't be very comprehensive, due to many holes in the universe of available HT components.

The challenge is real

HT sensor-based system design is challenging because of the adverse effects of HT on electronic materials.

Designs must tolerate the following:

Mechanical stresses on all materials used, including semiconductor die, passive components, substrates, leads, and boards.
Chemical reactivity of interfacial contacts, including wire bonds and substrate, board, and cable connections.
Thermal effects on performance.

Above and beyond these challenges, designs must often accommodate performance requirements such as remote communications, battery operation, high precision, and extended life.

Pushing the range limits

A number of recent requirement changes in down-hole instrumentation, turbine engine control architecture, industrial process control, and internal combustion engine control have pushed the upper operating temperature range limit higher, forcing designers to use commercial parts in ways they were never meant to be used.

The applications that need or benefit directly from incorporating HTE are naturally those situations where there is a need to sense and/or control a device that is hot or in a hot environment-specifically, the following:

Down-hole instruments, which need HT capability for oil and gas exploration and production, as well as for geothermal well drilling and monitoring.
Turbine engines used in aircraft and stationary power generation, which need HTE and sensors to implement more modem control techniques and monitoring systems.
Internal combustion engines, which need HTE as engine compartments get hotter and control strategies are refined to meet stricter emissions standards.
Integrated motor controls, which want control and power electronics to be mounted directly on the motor casing and that run at elevated temperatures.
Industrial process controllers such as locating signal conditioning and signal processing electronics directly at the pipe or tank, which contains product at high temperature.
Appliances, furnaces, and ovens operating at HT.
Building, aircraft, or land vehicle fire safety controls, which must operate beyond the normal temperature range to guarantee supervisory shutoff control of critical systems.

By and large, the three industry sectors into which these applications fall provide the main driving forces for developing the technology. These three-the petroleum exploration industry, which has been the leading sponsor for HTE for two decades; the aerospace industry; and the automotive industry-will probably account for more than 90% of demand during the next 10 years.

No mature choices

Standard, commercial off-the-shelf (COTS), bulk, silicon-processed semiconductors and passive components cannot handle the increased temperature demands. Their designed service range is -55° to +70°C for commercial, 85°C for industrial, and 125°C for military temperature range.

Manufacturers don't sell these components for use beyond their intended range. Often, HT designers will derate the component's power rating to prevent significant self-heating from damaging the part.

A recent evaluation comparing HT components and COTS components, which had been screened for HT operation, showed that they failed at the 150-hour mark when operating at 210°C.

Silicon-on-insulator (SOI) HT components, however, continued to operate until power was removed at 700 hours.

The most important physical properties for a good HT semiconductor material are a wide band gap and high thermal conductivity.

Band gap determines the amount of leakage current flowing across the junction, and thermal conductivity determines the semiconductor's ability to dissipate heat to the ambient environment.

Band gap dominates the field

With some modifications, silicon (Si) has very good thermal properties, theoretically extending to temperatures of 400°C. Low-power Si devices have demonstrated operability to 300°C.

Because most applications operate at temperatures lower than 300°C, Si-based technologies dominate the field now and will for many years to come. It is also true that packaging and interconnection problems present some of the biggest challenges for HTE at temperatures above 250°C.

Si is suitable for power devices running at up to 200°C. Above that temperature, the choice is less clear, the three main technologies being SiC, GaN, and diamond. The latter two, though superior in performance due to wider band gap and lower thermal conductivity, are in such an immature development state, it will take several years for their importance to equal that of Si.

Thus, temperature ranges for HTE can be broken into three ranges: degrees up to 200°C, degrees and SiC from 200° to 300°C, and SiC above 300°C.

Isolate transistors trench

Standard silicon-based CMOS integrated circuit (IC) material has intrinsic limitations as temperatures rise. Practical limitations start to set in with traditional CMOS circuits at temperatures higher than 150°C.

Silicon-on-insulator (SOI) technology incorporates transistors fabricated in a thin layer of silicon atop a silicon dioxide buried layer. The transistors are isolated from one another on the surface by a trench layer of oxide, which completely surrounds the transistor.

Recent advances in SOI technology have generated increased interest in its application to high-speed CMOS circuits. The advantages of SOI for these applications result from an insulating layer between the active silicon and the substrate.

SOI has demonstrated HT capability with CMOS circuits. Its HT performance allows the fabrication of signal conditioning circuits on the same chip and near the sensing element.

The marriage of SOI's mechanical properties, with its HT circuit operation, is most important and is the reason SOI provides performance unobtainable with other technologies. IT

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Author Information

David Wick is a manager at Honeywell's solid-state electronics center in Plymouth, Minn. He works with aerospace electronic systems.

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