Intrinsic Semiconductor – Pure Semiconductor

silicon - semiconducting material
Purified silicon. Source: wikipedia.org License: Public Domain

In general, semiconductors are materials, inorganic or organic, which have the ability to control their conduction depending on chemical structure, temperature, illumination, and presence of dopants. The name semiconductor comes from the fact that these materials  have an electrical conductivity between that of a metal, like copper, gold, etc. and an insulator, such as glass. They have an energy gap less than 4eV (about 1eV). In solid-state physics, this energy gap or band gap is an energy range between valence band and conduction band where electron states are forbidden. In contrast to conductors, electrons in a semiconductor must obtain energy (e.g. from ionizing radiation) to cross the band gap and to reach the conduction band. Properties of semiconductors are determined by the energy gap between valence and conduction bands.

Intrinsic Semiconductor – Pure Semiconductor

An intrinsic semiconductor is completely pure semiconductor without any significant dopant species present. Therefore, intrinsic semiconductors are also known as pure semiconductors or i-type semiconductors.

intrinsic semiconductorsThe number of charge carriers at certain temperature is therefore determined by the properties of the material itself instead of the amount of impurities. Note that, a 1 cm3 sample of pure germanium at 20 °C contains about 4.2×1022 atoms, but also contains about 2.5 x 1013 free electrons and 2.5 x 1013 holes. These charge carriers are produced by thermal excitation. In intrinsic semiconductors, the number of excited electrons and the number of holes are equal: n = p. Electrons and holes are created by excitation of electron from valence band to the conduction band. An electron hole (often simply called a hole) is the lack of an electron at a position where one could exist in an atom or atomic lattice. This equality may even be the case after doping the semiconductor, though only if it is doped with both donors and acceptors equally. In this case, n = p still holds, and the semiconductor remains intrinsic, though doped.

Semiconductors have an energy gap less than 4eV (about 1eV). Band gaps are naturally different for different materials. For example, diamond is a wide-band gap semiconductor (Egap= 5.47 eV) with high potential as an electronic device material in many devices. On the other side, germanium has a small band gap energy (Egap = 0.67 eV), which requires to operate the detector at cryogenic temperatures. In solid-state physics, this energy gap or band gap is an energy range between valence band and conduction band where electron states are forbidden. In contrast to conductors, electrons in a semiconductor must obtain energy (e.g. from ionizing radiation) to cross the band gap and to reach the conduction band.

Intrinsic semiconductors, however, are not very useful, as they are neither very good insulators nor very good conductors. However, one important feature of semiconductors is that their conductivity can be increased and controlled by doping with impurities and gating with electric fields. Recall, a 1 cm3 sample of pure germanium at 20 °C contains about 4.2×1022 atoms, but also contains about 2.5 x 1013 free electrons and 2.5 x 1013 holes constantly generated from thermal energy. Total absorption of a 1 MeV photon produces around 3 x 105 electron-hole pairs. This value is minor in comparison the total number of free carriers in a 1 cm3 intrinsic semiconductor. As can be seen, the signal to noise ratio (S/N) would be minimal. The addition of 0.001% of arsenic (an impurity) donates an extra 1017 free electrons in the same volume and the electrical conductivity is increased by a factor of 10,000. In doped material the signal to noise ratio (S/N) would be even smaller. Because germanium has relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers to an acceptable level. Otherwise, leakage current induced noise destroys the energy resolution of the detector. Doping and gating move either the conduction or valence band much closer to the Fermi level, and greatly increase the number of partially filled states.

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See above:

Types of Semiconductors