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BEAM From the Ground Up is a BEAM Reference Library site.
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:
- Single-junction
devices -- semiconductor devices with just one
P-N
junction; diodes
and all their variants (Zener diodes, Light Emitting
Diodes, Photodiodes, solar cells, etc.)
- Multi-junction devices -- semiconductor devices with two or more P-N junctions; namely, transistors (bipolar, field-effect, phototransistors, and the like)
I've managed to gather up enough information on each that these two classes of devices now have their own pages.
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