for the highest operating temperature parts and bulk silicon for high temperature parts that do not require the additional 100^oC afforded by SOI. So what is SOI?

The most common material used in ICs (integrated circuits, semiconductors, computer chips) is silicon. In fact the moniker "Silicon Valley" was given to the area around Santa Clara and San Jose in Northern California because of the extensive use of silicon. The term "integrated circuit" is used because all of the transistors are "integrated" into one chip of silicon.

Putting all the transistors on one piece of material has been one of the keys to miniaturization of electronics. But, like most solutions, it creates some new problems. In its most common configuration, digital logic functions are implemented using transistors as switches that can be turned on and off. In a theoretical world, these transistor switches work quite well. In actual implementation, the transistors forming these switches are in the same bulk silicon material. At room temperature, they generally perform well and can be counted on to act like switches.

While some like it hot, electronics do not. Most of us have noticed that there are fans on many types of electronic equipment to help get the heat out. With bulk semiconductor ICs, the standard methods to electrically isolate the parts of the transistors are increasingly less effective as they get hotter. A transistor switch that is supposed to be open actually leaks a very small amount at room temperature. As the temperature rises, the leakage eventually gets so bad that there is little electrical difference between a switch that is open and a switch that is closed. By the time there is this much leakage, the logic cannot work.

The problem has been finding a better way to electrically isolate the components of these integrated transistors. An approach that is finding wider usage is isolating each transistor and its parts with a thin layer of insulating glass. Since glass is formed from silicon and oxide (silicon dioxide), the addition of oxygen to the silicon in the right way can form silicon dioxide in the right places. Oxygen is implanted into the silicon at the correct depth in such a manner that it forms an insulating glass layer below the surface of the silicon. The result is Silicon On Insulator, SOI. Of course in the real world, this is not a simple procedure. It is also desirable to form an insulating barrier to completely surround the transistors to keep them completely isolated, not just the thin layer of glass below the transistors.

This short overview of SOI obviously leaves out most of the details and usages. The additional processing raises the cost of these parts. The designs are somewhat different than those in bulk silicon, such as the lack of ability to easily connect to the backs of these isolated transistors. In addition to higher temperature operation, SOI processed devices are more tolerant of radiation than the same part without the oxide isolation. Somewhat faster operation, an important characteristic in cutting edge products, is also achieved with SOI since the isolation results in less charge to be moved during switching. Additional and more in-depth information about SOI advantages and disadvantages can be found on the web, as well as information about the design and processing of SOI.

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