Semiconductors

There are materials that conduct current. They are called conductors. And there are materials that do not conduct current. They are called insulators. And then, there are materials that are somewhere between a conductor and an insulator — they partly conduct.

These materials, semiconductors, are very useful. Semiconductors are needed to build almost every device you have at home. Your mobile, computer, TV and many other things. To understand how semiconductors work, we need to get down to the atomic level.. All materials are made up of atoms.

Electrons spin around the atomic nucleus. Here we have several silicon atoms next to each other, in a crystal. Silicon atoms have four electrons that border other atoms. Each silicon atom can "share" its electrons with four of its neighbours. At the same time, these neighbours can "borrow" electrons from those atoms.

In this way, each silicon atom gets eight electrons — exactly what it wants. Then there aren’t so many free electrons traveling around the crystal. And that's why silicon does not conduct current so well. To make more electrons want to move, we can add another element — we can dope the silicon. Let’s replace one silicon atom with a phosphorus atom and see what happens.

A phosphorus atom has five electrons outermost. When it gets four more from its neighbors, it has nine electrons outermost. Then there is one electron, the ninth, which all atomic nuclei prefer to avoid. That extra electron can move freely in the silicon crystal. More electrons in motion means that the silicon crystal conducts current better.

When we dope silicon with phosphorus, it becomes a better conductor. Because electrons are negatively charged, and we get more electrons when we dope silicon with phosphorus, we say that this piece of silicon is negatively doped, or n-doped. There is another way to dope silicon. The element boron has three electrons outermost, instead of the five that phosphorus has. Let’s insert one boron atom in the middle of the silicon atoms.

The silicon atoms share a total of four electrons with the boron atom. Now the boron atom has seven electrons outermost. But it wants eight. One electron is missing. It's like a hole.

So the boron atom grabs one electron from an adjacent atom. But then, that atom loses one electron and there is a new hole. Now, it is the lack of electrons that gives us more moving, free electrons. More moving electrons in the material means that silicon conducts current better. With fewer negatively charged electrons, the silicon is positively doped, p-doped.

So, pure silicon conducts current quite poorly. But with the help of other elements used to dope the silicon, we can make silicon conduct current better. Silicon and other elements with similar properties are semiconductors. Doped semiconductors are in your mobile, computer, tv and lots of other electronics. Without semiconductors, you wouldn’t be able to watch this video on your phone or laptop!

And what makes semiconductors so good to use... nah, we'll save that for another time.