Semiconductors

These are very important class of material. The components made using them have revolutionized technology and the way we live in recent years.

A semiconductor is basically just a material that neither has a high resistance or a low resistance but is somewhere in between. We can produce two very useful types of material, n type and p type semiconductors, by a process called doping.

Silicon has a valency of 4. This means that a silicon atom is able to form covalent bonds with 4 other atoms by sharing these 4 electrons. In pure silicon this is what happens.

n type
If we add a very small amount of something with a valency of 5, e.g. phosphorous, then these atoms will have an an electron which isn't needed for bonding. If it has enough energy then this electron could become a free electron and be available for conduction. The n in n type stands for negative, the "majority charge carriers" in this type of semiconductor are electrons. If we heat up the material then more charge carriers become available for conduction so the resistance of the material decreases.

p type
If we add a very small amount of something with a valency of 3 to our silicon, e.g. boron, then we create "holes". These are places where an electron is needed to complete a bond. Holes act as positive charge carriers (an electron traveling in one direction is equivalent to a hole traveling in the other) so in a p type semiconductor the majority charge carriers are positive.

It is possible to create a crystal of silicon where one half is p type and the other n type. This is a p n junction. It has the very useful property that current will flow easily from p to n (the direction that the majority charge carriers like to go) but very little current can flow in the opposite direction. This is a diode.
If we make an n p n sandwich we have created a transistor. Current from collector to emitter is controlled by a tiny current from the base, rather like the current in a pipe being controlled by a valve.
An integrated circuit is made by depositing layers of different materials onto a wafer of silicon. Layers of different materials can form tiny diodes and transistors. Many thousands can be put onto a tiny crystal so incredibly complicated circuits can take up very small amounts of space.