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HIGH-TEMPERATURE ELECTRONIC DESIGN

Part 1: What Are Your Options?
by George Novacek

Start ý A Little Theory ý Size and Type ý Silicon Bipolar Transistors ý Silicon MOSFETs ý Six of Oneý ý SOI Technology ý Other Operation Problems ý Sources and PDF

SILICON BIPOLAR TRANSISTORS

Letýs review how an ordinary, off-the-shelf silicon bipolar transistor works. Figure 3 is a simplified diagram of a PNP transistor. (The NPN device can be understood by simply reversing junction polarities and interchanging the roles of electrons and holes.)

Figure 3ýHere is a PNP transistor in the normal operating mode, showing the principal current components. The width of the base region is shown as W.

The transistor comprises a p⁺-doped emitter and collector, separated by a thin n-conductor base. The base-emitter junction is forward biased, and the base-collector junction is reverse biased. The forward bias drives a large hole current into the base from the emitter. Because the base is thin, all but a fraction of the hole current reaches the base-collector depletion region. Many holes are collected in the collector as a result of the reverse base-collector battery potential. Therefore, I_{e »} I_c, although only a fraction of the holes recombine with the electrons in the base, causing base current I_be. A second current component supplied into the base is the thermal leakage I_oc of the reverse collector-base junction.

Analysis of this simple model points to four major temperature dependencies of the transistor characteristics. First, because the electron recombination time is temperature-dependent, the forward current gain will increase with temperature. Second, I_c dependence on V_be is also a function of temperature. Third, the reverse leakage current I_cb increases with temperature and may affect the base bias point. And finally, the resistivity of silicon is itself temperature-dependent. These dependencies affect all the operational characteristics of the bipolar transistor and are mostly apparent in the design of analog circuits.

A quad op-amp usable from 0ý to 300ýC was built some years ago, but special layouts and processes to minimize leakage had to be used. The lower temperature operating point was not driven by the potential failure of the semiconductor but by the ability to compensate for the temperature effects. Extending compensation of the leakage effects, which varied between five orders of magnitude even with the special care taken in the chip design, was no easy task. With the emphasis on the high-temperature operation, the designers decided to limit the low temperature range. Overall, you have to assume that analog bipolar circuits operating at elevated temperatures are a rare breed.

However, with extensive tests, standard TTL circuits were found to operate reliably at 250ýC. An accompanying problem was lower switching speed, lower noise margin, and lower output fan-out. Tests showed that the failures were caused by a decrease in the high-level output voltage, so although the circuit still toggled, its output was out of tolerance to drive a next TTL input. Dielectric isolated TTL integrated circuits suffered similar performance degradation but worked all the way up to 325ýC.

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