Most electronic components combine p type and n type semiconductors. Where they join is called the pn junction, an area where some electrons and holes cancel each other out. A thin layer forms, that acts like an insulator. Since it has so few charge carriers, it is called a depletion layer.
A semiconductor diode has a single pn junction. If it is connected to a current source, so that the p region is connected to a negative pole, and the n region to a positive pole, the negative pole attracts the holes, and the positive pole, the electrons. This enlarges the depletion layer, and insulated space. As a result, current cannot flow across it.
If the current source is connected in reverse, holes flow in large numbers across the junction, towards the negative pole, and electrons move in the opposite direction, towards the positive pole. The pn junction floods with charge carriers, the depletion layer disappears, and with it, the insulator effect. In this direction, the diode lets current flow. This principle of the pn junction operates in most semiconductors.
Semiconductors are widely useful because their conductivity can be manipulated and precisely controlled – by such means as introducing impurities, or an electric field, or just light.
Semiconductor devices are replacing many other kinds of switching. They are small, light, and use low operating voltages. They're reliable, need no maintenance, and they're relatively easy to manufacture.
Some semiconductor materials
