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Metal conducts electricity; rubber doesn't. Gold conducts electricity; Styrofoam doesn't. Most materials fall easily into one category or the other. Everyone knows, for example, that if you want a good wire you're going to make it out of copper, not plastic. But there's a whole group of materials that fall in between. Their conductivity is in between metals and insulators. And their conductivity can be modified transiently, by shining a light on them or injecting charges. They're known as semiconductors, and they first became interesting to physicists in the late 1920s. 

At first no one could figure out how they worked. Scientists once thought that certain atoms simply held onto their electrons more strongly than others. But as physicists got a better understanding of what an atom looked like, they understood what was really going on. 

Different kinds of atoms have different numbers of electrons swarming around them. These electrons can only sit in specific places around the atom. It's sort of like rows of seats in a theater-in-the-round: a few electrons get to sit in the first row around the stage, and when that's filled the next electrons sit in the next row and so on. Electrons in a filled row stay put -- just as in a theater it's harder to get out when you've got people sitting on each side of you. In an insulator, every row is completely filled. Consequently the electrons rarely move. No moving electrons means no electricity can pass through. 

But if you're sitting in the back row of a movie theater and the seats aren't full, you could easily get up, switch seats, maybe even decide to check out a different movie in the next theater. In a metal, the last row isn't filled with electrons. The outer electrons have little loyalty to the atom they're with and readily wander off in search of other atoms. This translates to many moving electrons, which means metals can easily conduct electricity. 

So what happens with semiconductors? They reside somewhere in the middle. They are mostly made of atoms that don't conduct electricity, but they have a handful of atoms with loose electrons. Under certain circumstances -- by changing things like temperature or how much energy is injected -- these loose electrons will start a flowing current. 

That means that depending on what you do, semiconductors can transiently conduct more or less electricity. It's just that property that transistors exploit. 

-- Crystal Fire by Michael Riordan and Lillian Hoddeson 
-- Feynman, "Lectures on Physics"

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