### 1.08 Number Density

In the previous sections we explained that the only charged particles which can move in solid materials to produce electricity are electrons. We also saw that the atomic structure of metals produces lots of free electrons. The way we express the amount of free charge carriers in a material is by quoting the number density. The number density is the number of free charge carriers per cubic metre of material. For metals this value is around 1029
ie 100,000,000,000,000,000,000,000,000,000 ! charge carriers (free electrons) per cubic metre

Conductors, Semiconductors and Insulators
Materials that have a high number density like metals are called conductors. Electricity flows easily in a conductor. Insulators are materials that have a very low number density. Insulators strongly resist the flow of electricity. Semiconductors are materials that have electrical properties and a number density midway between those of conductors and insulators.

Energy band diagrams The previous diagrams of atomic structure illustrate why metals have a higher number density and hence are better conductors than insulators, however it does not illustrate the difference in the electrical properties of semiconductors and insulators. Energy band diagrams are a better way to illustrate this.

Electron energy levels In the previous section we discussed electron shells and orbits, an alternative and more accurate way of imagining the state of electrons in an atom is to consider their energy levels. In an atom the electrons have different energy levels that they can occupy. To move from one energy level to another the electron must gain or loose a certain amount of energy.

Energy bands
Materials contain many atoms and the energy levels of the individual atoms merge to form energy bands. The outermost of these energy bands is called the valence band. The valence band is the highest energy level for electrons which are still bound to their parent atoms. In addition to the energy bands of electrons orbiting their parent atoms there is another higher energy band called the conduction band. The conduction band is occupied by the free electrons within the material. Free electrons are not bound to the atoms in the material and are therefore able to move within the material. Any electrons in the conduction band can contribute to an electrical current through the material.