Semiconductor devices

If part of one crystal is n-type and the other p-type, at the boundary electrons fill holes and a thin barrier (depletion) layer almost free of carriers forms. With forward bias (the p-region to the positive terminal, the n-region to the negative) the layer narrows, carriers cross it and the current is large; with reverse bias (p negative, n positive) the layer widens and the current is very small. So a device with one p–n junction – a semiconductor diode – conducts mainly in one direction (in a silicon diode the current rises sharply above a forward voltage of about 0.6–0.7 V). A diode is used to rectify alternating current: a half-wave rectifier conducts during the positive half-cycle. A device with two p–n junctions is a transistor (p–n–p or n–p–n): its terminals are the emitter, base and collector. The emitter–base junction is forward-biased and the base–collector junction reverse-biased; because the base is very thin most carriers from the emitter reach the collector. A small change of base current causes a large change of collector current – the current and signal are amplified (β = Ic/Ib). Transistors and diodes are packed in integrated circuits (microchips) in thousands to billions (computers, phones).