Semiconductors, diodes, and transistors

Materials can be sorted by how easily charge moves through them: conductors like copper have many free electrons, insulators like glass and rubber have almost none, and semiconductors like silicon fall in between. A pure semiconductor conducts only a little, but its conductivity can be controlled precisely by doping, adding tiny amounts of other elements. Adding an element such as phosphorus, which has one more outer electron than silicon, makes n-type silicon with extra free electrons. Adding boron, which has one fewer, makes p-type silicon with holes, places missing an electron that behave like moving positive charges. Where p-type and n-type silicon meet, they form a p-n junction, the heart of a diode, which lets current flow easily in one direction and almost completely blocks it in the other at normal voltages; diodes can be used to turn alternating current into direct current. A light-emitting diode (LED) gives off light when current flows through its junction, and a solar cell does the reverse, using light to produce a current. A transistor uses more layers of doped semiconductor so that a small signal can switch or amplify a larger current. Billions of tiny transistors acting as on-off switches make up the chips inside computers and phones.

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