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Properties of semiconductors

In semiconductors (e.g. silicon, germanium), the band gap is smaller than that of insulators (around $$1\text{ eV}$$). It would take a moderate amount of energy to excite an electron into the conduction band.

The conductivity of a semiconductor increases with temperature as more electrons are able to cross into the conduction band, increasing the number of electron-hole pairs.

There are two main types of semiconductors: intrinsic and extrinsic.

  • An intrinsic (or pure) semiconductor is made up completely of one type of semiconductive material (e.g. pure silicon or gallium arsenide).

    This means that it contains no other additives or impurities. It has a small number of electron-hole pairs which act as mobile charge carriers.

  • An extrinsic semiconductor is created when certain impurities are added into the structure of an intrinsic semiconductor.

Doping is the process of introducing impurities into a semiconductor. The impurities used are known as dopants.

The dopants in an extrinsic semiconductor are normally atoms of a different valence (i.e. a different number of electrons in the outermost shell).

An electron shell is a group of electron orbitals where the maximum distance of the electrons from the nucleus is roughly equal.

The valence shell is the outermost shell of an atom. It is typically only partially filled and determines many of the chemical properties of the atom.

Doping introduces either additional electrons or holes into the semiconductor, depending on the type of dopant.

Doping therefore increases the conductivity of a semiconductor by increasing the number of charge carriers it contains. However, the overall charge of extrinsic semiconductors is still neutral as the total number of electrons and protons is equal.