Diode

A diode lets electric current flow in one direction and blocks it in the other, like a one-way valve. It has two terminals, the anode and the cathode, and the cathode end is usually marked with a stripe.

The heart of a diode is a p–n junction in doped silicon. One side is doped with boron to make it p-type, the other with phosphorus to make it n-type. Where they meet, electrons and holes cancel out, leaving a thin layer with no free charges. Push current forward, and once the voltage passes about 0.6 to 0.7 volts, that layer collapses and current flows. Push backward, and it blocks. To package a diode, the small silicon chip is soldered to a lead frame, joined to the other leg, and encased in mold compound.

Mains electricity is AC, reversing direction 50 or 60 times a second, but electronics need DC, which flows one way. On the power supply board, four diodes arranged as a bridge rectifier flip the backward halves of each wave forward, and more diodes rectify the outputs of the transformer. Light-emitting diodes, like the blue LED chip and infrared LED, are diodes too, made from other semiconductors that give off light at the junction.