Based on their electrical properties, materials are classified into three categories:
A conductor is a material that easily allows the flow of electrical current due to the presence of a large number of free electrons.
Free electrons are those electrons that have escaped from the control of the nucleus and become free, or those electrons that are in the conduction band. These free electrons move randomly within the material, similar to how air molecules move within containers.
The best conductors are copper (Cu), silver (Ag), gold (Au), and aluminum (Al), which are characterized by atoms with only one valence electron very loosely bound to the atom.
From the energy band diagram of a conductor as shown in the figure, it is evident that the valence band and conduction band overlap. This overlap means that no external energy is required for the valence electrons to become free electrons, allowing them to easily move and conduct electricity.
Due to the presence of a large number of free electrons, the resistivity of conductors is very low. This abundance of free electrons allows electric current to flow easily through the material, resulting in minimal resistance.
A semiconductor is a material that lies between conductors and insulators in its ability to conduct electrical current.
From the energy band diagram of a semiconductor, as shown in the figure, it is evident that there is a small gap between the valence band and the conduction band, which is around 1 eV.
In general, the resistivity of intrinsic (pure) semiconductors like silicon and germanium at room temperature ranges from about 10⁻² to 10² ohm-meters (refer to this figure).
In its pure (intrinsic) state, a semiconductor is neither a good conductor nor a good insulator.
Single-element semiconductors include
Compound semiconductors, such as
are also commonly used.
The single-element semiconductors are characterized by atoms with four valence electrons, with silicon being the most commonly used semiconductor.
Semiconductors can be classified based on their purity and doping, as shown in the figure.
An insulator is a material that does not conduct electrical current under normal conditions because it has either no free electrons or a very small number of them.
Insulators do not have free electrons or have only a very small number of them because they require a significant amount of energy (around 6 eV) to free them. This is evident from the energy band diagram shown in the figure.
Most effective insulators are compounds rather than single-element materials and have very high resistivities as shown in the figure.
Examples of insulators include rubber, plastics, glass, mica, and quartz.
Most good insulators are compounds rather than single-element materials.
