Reference STATE OF THE ART Stan Augarten

ISBN 0-89919-195-9

Photo of
The Most Efficient Way to Make Transistors
The Planar Process


By the late 1950s, transistors had gone through several stages of development. For one thing, they were no longer fashioned out of germanium, but silicon, which offered certain distinct manufacturing and electrical advantages; for another, they were no longer made piece by piece, but in batches, through a simple photolithographic technique known as the mesa process. This process, which led directly to the creation of the commercially viable integrated circuit, is a form of contact printing.

  A cross section of a typical mesa transistor resembles a windswept corner of the Grand Canyon: a plateau, of mesa, of silicon squatting on top of a foundation of silicon. The three essential parts of a transistor are all there: the base is the mesa, the collector is the foundation, and the emitter is a tiny piece of doped silicon embedded in the base. to fabricate a mesa transistor, a flat wafer of silicon was doped with either positive ions or electrons, covered with a photomask (a photographic plate), exposed to ultraviolet light, and then immersed in an acid bath, which etched away the exposed area around the mesa.

  For all the manufacturing benefits brought about by the mesa process, it had two major drawbacks: the mesa was susceptible to both physical harm and contamination, and the process didn't lend itself to the making of resistors. The Jean Hoerni, a Swiss physicist and one of Fairchild's founders, invented an ingenious way around these obstacles by creating a flat, or planar, transistor.

  Instead of mounting the mesa, or base, on top of a foundation of silicon, he diffused it into the foundation, which served as the collector. Next he diffused the emitter into the base. (The base was composed of negatively doped silicon, the collector and emitter of positively doped silicon; the first planar device was thus a pnp transistor.) Then he covered the whole thing with a protective coating of silicon dioxide, an insulator, leaving certain areas in the base and the emitter uncovered. He diffused a thin layer of aluminum into these areas, thereby creating "wires" that hooked the device up to the outside (this was the idea of his colleague and Fairchild co-founder, Robert Noyce). The result was a durable and reliable transistor, and the all-important breakthrough that made commercial production of ICs possible.

The planar transistor can amplify current about thirty times. All of its components lie within a horizontal plane, protected by an insulating layer of silicon dioxide (invisible to the naked eye). The mauve region is the base, the inner green ring the emitter, and the surrounding green sea the collector. The central cream-colored plug and the teardrop-shaped ring are aluminum connectors (shown here without attached wires). Actual size: 0.06 inches in diameter. Photo of


©Copyright Stan Augarten
This book is provided for general reference. The National Museum of American History and the Smithsonian Institution make no claims as to the accuracy or completeness of this work.

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