Semiconductors are materials which have a conductivity between conductors and nonconductors or insulators. It can be pure elements, such as silicon or germanium.
Types of Semiconductor:
Semiconductors are of following two types:
- Intrinsic Semiconductor
- Extrinsic Semiconductor
The chemically pure semiconductors like Ge, Si, etc are called intrinsic semiconductors. As Ge and Si have four valence electrons, each Ge or Si atom forms covalent bonds with the neighboring four Ge or Si atoms giving a diamond like structure. At low temperature, all the valence electrons are bound to the bonds and hence no free electrons are available. In other words, the conduction band is completely empty giving rise to zero electrical conductivity.
If the temperature be increased, due to thermal energy even at room temperature a few bonds are broken. A few electrons are made free and lifted from the valence band to the conduction band leaving an equal number of vacancies, that is called holes. A hole behaves like an apparent free particle having positive charge of magnitude equal to that of electrons.
Thus, in intrinsic semiconductor, the number of free electrons per unit volume (ne) is equal to the number of holes per unit volume (nh).
∴ ne = nh = ni (Where ni is called intrinsic carrier concentration).
The electrical conductivity of intrinsic semiconductor at room temperature is too low to be used for practical purposes. The conductivity of intrinsic semiconductor can be increased by adding a very small amount of suitable impurity to it. The impurity is added to pure semiconductor by solid state diffusion method is called doping. Such impure semiconductor is called extrinsic semiconductor.