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Basics of semiconductor components
The power of ambivalence

This discussion really ought to start with a bit about semiconductors as materials.

Semiconductors are crystals that, in their pure state, are resistive (that is, their electrical properties lie between those of conductors and insulators) -- but when the proper impurities are added (this process is called doping) in trace amounts (often measured in parts per billion), display interesting and useful properties.

A bit of history
The oldest ancestor of semiconductor devices was the crystal detector, used in early wireless radios. This device (patented by a German scientist, Ferdinand Braun, in 1899) was made of a single metal wire (fondly called a "cat's whisker") touching against a semiconductor crystal. The result was a "rectifying diode" (so called because it has two terminals), which lets current through easily one way, but hinders flow the other way. By 1930, though, vacuum-tube diodes had all but replaced the smaller but much quirkier crystal detector. The crystal and "cat's whisker" were left to languish as a kids' toy in the form of "crystal radios."

The development of radar during World War II did much to revive the fortunes of crystal detectors (and, as a result, that of semiconductors) -- although temperamental, crystals were better than vacuum-tube diodes at rectifying the high frequencies used by radar. So, during the war, much effort was put into improving the semiconductors, mostly silicon and germanium, used in crystal detectors. At about the same time, Russell Ohl at Bell Laboratories discovered that these materials could be "doped" with small amounts of foreign atoms to create interesting new properties.

Depending on the selection of impurities (often called dopants) added, semiconductor material of two electricallly-different types can be created -- one that is electron-rich (called N-type, where N stands for Negative), or one that is electron-poor (called P-type, where P stands for Positive). Most of the "magic" of semiconductor devices occurs at the boundary between P-type and N-type semiconductor material -- such a boundary is called a P-N junction. Ohl and his colleagues found that such a P-N junction made an effective diode (but more on that later).

For BEAM devices, we'll be concerned with two broad types of semiconductor devices:

I've managed to gather up enough information on each that these two classes of devices now have their own pages.

For more information...

I've gathered up even more related information in the Encyclobeamia. See the articles there on

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