# Semiconductor Physics

 Cross-section Conducting Insulating

## Preamble ...

This is where it all starts: carefully controlling the composition of tiny pieces of silicon so they conduct electricity how and where and when we want it. Advances in semiconductor physics allow us to cram ever more transistors (electrically-controlled switches, essentially) onto a chip of silicon.

So here's an idea of how a Field Effect Transistor works. This is heavily simplified, and omits many details and options, but it's intended to give a basic idea of how it works.

## Basic operation

So in the diagram at right we've got silicon in yellow, and that's the underlying physical stratum for all this to happen. In various patterns we then infuse a small and carefully controlled amount of various different contaminants. These are atoms of different elements introduced into the silicon crystalline array.

Some of these contaminants have an extra electron, and some have one less electron.

## The diagrams

In green we've marked a channel where there's a lot of extra electrons. As a result, the green channels conduct electricity without too much resistance. The blue section is where there is very little contaminant.

Then in red we have a completely insulating layer. And finally, we have metal electrical connections, one to each green channel, and one connected to a large metal plate hovering above the blue section, separated from it by the insulating layer.

Got that?

## How does it work?

Now let's remove electrons from the metal plate. That makes it positively charged, attracting electrons from the surrounding silicon into the blue area. That, in turn, provides enough extra electrons to convert it from being semi-conducting to being properly conducting. Thus we can now have current flowing between the green channels. It becomes conducting.

On the other hand, let's put more charge onto the metal plate. Thus it drives excess electrons from the blue section and makes it insulating.

So by putting a positive voltage on the plate the outer connections are connected, as if by a switch, and by putting a negative charge on the metal plate we effectively turn the switch off, isolating the contacts.

And thus we have a switch, controlled by a voltage.

 More accurate diagram
 Schematic representation

## Variations

By using different contaminants we can change the characteristics, having it turn on with a negative charge instead of a positive one, and off with a positive charge instead of a negative one. The elements boron, arsenic, phosphorus, and occasionally gallium are used to dope (contaminate) the silicon. Combining the different types allows us to create all sorts of gates, and hence combinatorial circuits. That leads us to our next level:

 Lower level: Root node: Higher level: