Silicon Carbide Semiconductors

Research scientists of the Westinghouse Electric Corporation in the United States have recently developed a transistor capable of operating above 650 degrees Fahrenheit, a temperature higher than the melting point of lead. The new transistor is the first to be made from silicon carbide, a hard crystalline material which, in impure form, is used as an abrasive in grinding wheels.The high-temperature capabilities of the new transistor mark it as a significant advancement in the technology of these semiconductor devices. Present-day transistors, manufactured almost exclusively from germanium and silicon, can operate at temperatures no higher than about 200 degrees F (germanium) or 400 degrees F (silicon). Germanium or silicon transistors, however, cannot always meet the high temperature requirements of today’s existing and planned aircraft and space vehicles. Such applications, therefore, have furnished strong motivation for the development of higher-temperature transistors. Because of its great chemical stability and desirable electrical properties which it retains at elevated temperatures, silicon carbide is one of the most promising transistor materials for extremely high temperature applications. Laboratory tests show that the new silicon carbide transistor still amplifies at 670 degrees F, and with further development, an upper operating temperature of more than 925 degrees F should be achievable.

The new device is actually a “unipolar” or “field-effect” transistor, which differs in operating principle from those usually made from germanium and silicon. Such conventional transistors regulate the flow of an electric current through them by injection of electric charge carriers across a junction built into the semiconductor material. The unipolar transistor, on the other hand, acts more like a valve which opens and closes to regulate the electron flow.

The new transistors are made from exceptionally pure crystals about two-thousandths of an inch thick. The necessary junction is build into the material by exposing it to vaporized aluminium at the white hot temperature of 3,900 degrees F. The aluminum atoms diffuse into the silicon carbide crystal, changing its electrical behavior from so-called n-type material to p-type. The junction is formed where the two types meet, and the process is controlled to an accuracy of a few millionths of an inch.

Then, to establish the input and output terminals of the transistor, the wafer is etched at two points in such a way that the silicon carbide is eaten away until the junction within the body of the crystal is reached. Electrical connections at these two points and to the body of the wafer complete the transistor.

A typical finished transistor is about 80-thousandths of an inch long and 40-thousandths of an inch wide, and the “working” area of the crystal surface is smaller than the head of a pin. Electrical measurements of the finished transistors show them to give a power gain of about 60 at room temperatures. [Lorant 1961]

Lorant M 1961 Silicon Carbide Transistor, New High-Temperature Device Wireless World January 1961