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Adopant (also called adoping agent) is a small amount of a substance added to a material to alter its physical properties, such aselectrical oroptical properties. The amount of dopant is typically very low compared to the material being doped.
When doped intocrystalline substances, the dopant's atoms get incorporated into the crystal lattice of the substance. The crystalline materials are frequently either crystals of asemiconductor such assilicon andgermanium for use insolid-state electronics, ortransparent crystals for use in the production of variouslaser types; however, in some cases of the latter, noncrystalline substances such asglass can also be doped with impurities.
In solid-state electronics using the proper types and amounts of dopants in semiconductors is what produces thep-type semiconductors andn-type semiconductors that are essential for makingtransistors anddiodes.
The procedure of doping tiny amounts of the metalschromium (Cr),neodymium (Nd),erbium (Er),thulium (Tm),ytterbium (Yb), and a few others, into transparentcrystals,ceramics, orglasses is used to produce theactive medium forsolid-state lasers. It is in the electrons of the dopant atoms that apopulation inversion can be produced, and this population inversion is essential for thestimulated emission of photons in the operation ofalllasers.
In the case of the naturalruby, what has occurred is that a tiny amount of chromium dopant has been naturally distributed through a crystal ofaluminium oxide (corundum). This chromium both gives a ruby its red color, and also enables a ruby to undergo a population inversion and act as a laser. The aluminium and oxygen atoms in the transparent crystal of aluminium oxide served simply to support the chromium atoms in a good spatial distribution, and otherwise, they do not have anything to do with the laser action.
In other cases, such as in theneodymium YAG laser, the crystal is synthetically made and does not occur in nature. The human-madeyttrium aluminium garnet crystal contains millions of yttrium atoms in it, and due to its physical size, chemical valence, etc., it works well to take the place of a small minority of yttrium atoms in its lattice, and to replace them with atoms from therare-earth series of elements, such as neodymium. Then, these dopant atoms actually carry out the lasing process in the crystal. The rest of the atoms in the crystal consist of yttrium, aluminium, and oxygen atoms, but just as above, these other three elements function to simply support the neodymium atoms. In addition, the rare-earth element erbium can readily be used as the dopant rather than neodymium, giving a different wavelength of its output.
In manyoptically-transparent hosts, such active centers may keep their excitation for a time on the order of milliseconds, and relax withstimulated emission, providing the laser action. The amount of dopant is usually measured inatomic percent. Usually the relative atomic percent is assumed in the calculations, taking into account that the dopant ion can substitute in only part of a site in a crystalline lattice. The doping can be also used to change therefraction index inoptical fibers, especially in thedouble-clad fibers. The optical dopants are characterized with lifetime of excitation and the effective absorption and emission cross-sections, which are main parameters of an active dopant. Usually, the concentration of optical dopant is of order of few percent or even lower. At large density of excitation, the cooperative quenching (cross-relaxation) reduces the efficiency of the laser action.
The medical field has some use forerbium-doped laser crystals for thelaser scalpels that are used inlaser surgery.Europium,neodymium, and other rare-earth elements are used to dopeglasses for lasers.Holmium-doped andneodymium yttrium aluminium garnets (YAGs) are used as theactive laser medium in some laser scalpels.[1]
In context ofphosphors andscintillators, dopants are better known asactivators, and are used to enhance the luminescence process.[2]
The addition of a dopant to asemiconductor, known asdoping, has the effect of shifting theFermi levels within the material.[citation needed] This results in a material with predominantly negative (n-type) or positive (p-type)charge carriers depending on the dopant variety. Pure semiconductors that have been altered by the presence of dopants are known asextrinsic semiconductors (seeintrinsic semiconductor). Dopants are introduced into semiconductors in a variety of techniques: solid sources, gases, spin on liquid, and ion implanting. Seeion implantation, surfacediffusion, and solid sources footnote.
The color of somegemstones is caused by dopants. For example, ruby andsapphire are both aluminium oxide, the former getting its red color from chromium atoms, and the latter doped with any of several elements, giving a variety of colors.