We normally think of something as either a conductor of electricity or an insulator.
Transistors are semi-conductors which means that their ability to conduct electricity can be controlled. There’s an older video I made that explains DC analysis of transistors that you can watch if you visit Facebook.com/ElectronicThinking.
Let’s examine conductors and insulators first. Every time you use a cloth to pick up something hot, you’re using an insulator. In this case a heat insulator. And the reason we use metal skillets and pots is because metal conducts heat.
It all comes to this. A conductor allows something to move from one place to another and an insulator stops this movement. Silicon and a few other elements are a bit different though. Silicon is normally an insulator of electricity. But it can be turned into a conductor by adding impurities. This is called doping and it involves hitting the silicon with some fast moving elements such as boron which then embed themselves inside the silicon.
In order for anything to conduct electricity, it has to contain ions which are positive or negative charged atoms. By carefully choosing what elements to inject into the silicon and how many, we can introduce atoms that can either easily supply extra electrons or accept electrons. It’s the electrons that move around. That’s the charge. By injecting boron for example, it’ll pull an electron from a nearby silicon atom which creates a positive charged silicon ion. And by injecting arsenic, it’ll provide an extra electron to a nearby silicon atom creating a negative charged silicon ion.
There are two basic types called NPN and PNP transistors. They consist of three alternating doped sections of silicon which is why there are two types. Each transistor has three connections each leading to one of the N or P sections. The connections are called the emitter, base, and collector. Because of how the sections are made, the transistor will initially not conduct any current from the emitter to the collector. But by raising the voltage at the base above a certain threshold until current starts to flow through the base, it has the effect of causing a larger amount of current to flow through the emitter and collector. This allows a transistor to operate as an amplifier.
Listen to the full episode to learn how transistors can operate as switches and why this is needed by computers.