eli5 How are transistors made so crazy small?
In: 7
Okay, basic electronic construction and physics. Some substances are able to easily gain or lose electrons. These allow electricity to flow well, as electrical current is simply electrons moving around. These are “conductors.” Other substances are highly resistant to gaining or losing electrons, which means they do not allow electricity to flow well. These are called “insulators.”
There is a third kind of substance that falls in between them, that holds on to its electrons harder than conductors but not as hard as insulators. They are called “semiconductors,” of which silicon is the most important one.
Since everything that happens here is on the atomic level, it is very easy to make transistors on the small scale. A mechanical switch with copper contacts would have to be much larger than a transistor. Copper is a conductor, one of the best ones we have, so electrons can jump from one contact to another over a “decent distance.” A gap of a couple millimeters is enough to break the circuit, but compared to transistors, that’s a massive gulf. Plus, you need something to mechanically move the contacts. Usually an electromagnet is used. Put an electromagnet in a formation that it will cause contacts to open or close when the magnet is energized, and you have a “relay.” That’s what we used before transistors, and are often used today, though we no longer use them for “thinking” in electronics.
But with semiconductors, they can change from being a conductor to being an insulator very easily. The trick is to add just the right amount of impurities in just the right structure. This is called “doping,” and in the world of electronics, it’s a good thing. All it takes is a single atom to switch a properly doped piece of silicon from an insulator to conductor and back again. Plus the process is purely electronic. There are no moving parts, so no mechanical components are needed. All you need to do is apply an electrical current to the third leg of a transistor, and the other two legs will go from “open” to “closed.” Once the current on the third leg stops, the transistor “opens” again and electricity can’t pass through.
Since the conductor itself changes to an insulator, you don’t need an air gap for the contacts like you do in a relay. And since you don’t need to physically move the contacts, you don’t need a magnet. So a transistor is able to do the same job at 1/100 the scale of a relay.
Hope this helps.
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