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 and Extrinsic 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.
The 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.
An extrinsic semiconductor, or doped semiconductor, is a semiconductor, that was intentionally doped for the purpose of modulating its electrical, optical and structural properties. In case of semiconductor detectors of ionizing radiation, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of changes in their electrical properties. Therefore, intrinsic semiconductors are also known as pure semiconductors or i-type semiconductors.
The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, since these foreign atoms incorporated into the crystal structure of the semiconductor provide free charge carriers (electrons or electron holes) in the semiconductor. In an extrinsic semiconductor it is these foreign dopant atoms in the crystal lattice that mainly provide the charge carriers which carry electric current through the crystal. In general, there are two types of dopant atoms resulting in two types of extrinsic semiconductors. These dopants that produce the desired controlled changes are classified as either electron acceptors or donors and the corresponding doped semiconductors are known as:
Extrinsic semiconductors are components of many common electrical devices, as well of many detectors of ionizing radiation. For these purpose, a semiconductor diode (devices that allow current in only one direction) usually consists of p-type and n-type semiconductors placed in junction with one another.