1) Insulators - These materials have high electrical resistance, meaning that electrons within the material do not move (they have strong covalent bonds). An example of this is Silicon Dioxide (SiO2).

2) Conductors - These materials have low electrical resistance which allows electrons to easily move about. These materials have valence electrons, meaning that its atoms have incomplete outer shells, so that when they get close to each other, they share electrons. Examples of these materials are gold, aluminum, and (most recently) copper. Polysilicon is also a conductor.

3) Semiconductors - These materials resist the flow of electricity, and are between conductors and insulators. They can carry electrical current via holes or free electrons. Silicon has 4 valence electrons per atom, Boron has 3, and Phosphorus has 5. An atom of Silicon bonds with 4 other silicon atoms, but these bonds randomly break. As a result, electrons are freed (hence the term "free electrons"), and fill in holes. Holes are formed when an electron is freed. By adding Boron or Phosphorus, the number of holes (Boron) or free electrons (Phosphorus) are increased. This process is called "doping". Silicon doped with Boron is called a p-type semiconductor, and Silicon doped with Phosphorus is called an n-type semiconductor.

When a wafer of silicon is created, it is doped with Boron. A layer of Silicon Dioxide then covers the surface, but can be removed as needed. Phosphorus can form a n-type region in the silicon substrate when enough of it is used to nullify the Boron in that area.