Transistors, diodes, LEDs, integrated circuits and many other electronic devices use a semiconductor instead of a conductor. A semiconductor is a material such as silicon, that has some of the properties of both conductors and insulators. Silicon is pretty cool stuff. In fact, they’ve named a whole valley in California after it. In its pure state, silicon conducts an electric current poorly.
But if you add contaminates, such as boron or phosphorus, to the silicon, it conducts. When you add phosphorus, silicon becomes an “n”-type semiconductor. When you add boron, silicon becomes a “p”-type semiconductor. An “n”-type semiconductor has more electrons than a pure semiconductor and a “p”-type semiconductor has fewer electrons than a pure semiconductor. When the regions containing boron and phosphorus are next to each other in silicon, you have a “pn” junction. Current flows in only one direction across a “pn” junction.
Diodes, components that can convert AC to DC by limiting the flow of current to one direction, are an example of a component that contains a “pn” junction. A “pn” junction generates an electric current when exposed to light; this property is used when building solar cells. On the other hand, when you run an electric current through a “pn” junction, it emits light, as light-emitting diodes (LEDs) do. Transistors use junctions in which three adjacent areas have contaminants added.
For example, one region with phosphorus, one with boron, and another with phosphorus result in an “npn” junction. In a transistor, you apply a current to the middle of the three regions (the base), allowing a current to flow. Most electronics projects you work on use components such as transistors, diodes and integrated circuits, and these are made with semiconductors. It’s semiconductors that have made possible much tinier electronic gadgets (like handheld computers and palm-sized radios).