A dopant (also called a doping agent) is a small amount of a substance added to a material to alter its physical properties, such as
electrical
Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
or
optical
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
properties. The amount of dopant is typically very low compared to the material being doped.
When doped into
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
line substances, the dopant's atoms get incorporated into the crystal lattice of the substance. The crystalline materials are frequently either crystals of a
semiconductor
A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
such as
silicon
Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
and
germanium
Germanium is a chemical element; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...
for use in
solid-state electronics
Solid-state electronics are semiconductor electronics: electronic equipment that use semiconductor devices such as transistors, diodes and integrated circuits (ICs). The term is also used as an adjective for devices in which semiconductor elec ...
, or
transparent crystals for use in the production of various
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
types; however, in some cases of the latter, noncrystalline substances such as
glass
Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
can also be doped with impurities.
In solid-state electronics using the proper types and amounts of dopants in semiconductors is what produces the
p-type semiconductors and
n-type semiconductor
N-type, N type or Type N may refer to:
* N-type semiconductor is a key material in the manufacture of transistors and integrated circuits
* An N-type connector is a threaded RF connector used to join coaxial cables
* The MG N-type Magnette was p ...
s that are essential for making
transistor
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...
s and
diode
A diode is a two-Terminal (electronics), terminal electronic component that conducts electric current primarily in One-way traffic, one direction (asymmetric electrical conductance, conductance). It has low (ideally zero) Electrical resistance ...
s.
Transparent crystals
Lasing media
The procedure of doping tiny amounts of the metals
chromium
Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal.
Chromium ...
(Cr),
neodymium
Neodymium is a chemical element; it has Symbol (chemistry), symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth element, rare-earth metals. It is a hard (physics), hard, sli ...
(Nd),
erbium
Erbium is a chemical element; it has Symbol (chemistry), symbol Er and atomic number 68. A silvery-white solid metal when artificially isolated, natural erbium is always found in chemical combination with other elements. It is a lanthanide, a rare- ...
(Er),
thulium (Tm),
ytterbium (Yb), and a few others, into transparent
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s,
ceramic
A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...
s, or
glass
Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
es is used to produce the
active medium for
solid-state laser
A solid-state laser is a laser that uses a active laser medium, gain medium that is a solid, rather than a liquid as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as ...
s. It is in the electrons of the dopant atoms that a
population inversion
In physics, specifically statistical mechanics, a population inversion occurs when a system (such as a group of atoms or molecules) exists in a state in which more members of the system are in higher, excited states than in lower, unexcited energy ...
can be produced, and this population inversion is essential for the
stimulated emission
Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to ...
of photons in the operation of ''all''
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
s.
In the case of the natural
ruby
Ruby is a pinkish-red-to-blood-red-colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapph ...
, what has occurred is that a tiny amount of chromium dopant has been naturally distributed through a crystal of
aluminium oxide
Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several Aluminium oxide (compounds), aluminium oxides, and specifically identified as alum ...
(
corundum
Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium, and chromium. It is a rock (geology), rock-forming mineral. It is a naturally transparency and translucency, transparent material, but ...
). 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 the
neodymium YAG laser, the crystal is synthetically made and does not occur in nature. The human-made
yttrium aluminium garnet
Yttrium aluminium garnet (YAG, Yttrium, Y3Aluminium, Al5Oxygen, O12) is a synthetic crystalline material of the garnet group. It is a Crystal system, cubic yttrium aluminium oxide phase, with other examples being YAlO3 (YAP) in a Crystal system, ...
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 the
rare-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 many
optically-transparent hosts, such active centers may keep their excitation for a time on the order of milliseconds, and relax with
stimulated emission
Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to ...
, providing the laser action. The amount of dopant is usually measured in
atomic 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 the
refraction index in
optical fiber
An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at ...
s, especially in the
double-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.
Examples
The medical field has some use for
erbium
Erbium is a chemical element; it has Symbol (chemistry), symbol Er and atomic number 68. A silvery-white solid metal when artificially isolated, natural erbium is always found in chemical combination with other elements. It is a lanthanide, a rare- ...
-doped laser crystals for the
laser scalpels that are used in
laser surgery
Laser surgery is a type of surgery that cuts tissue using a laser in contrast to using a scalpel.
Soft-tissue laser surgery is used in a variety of applications in humans ( general surgery, neurosurgery, ENT, dentistry, orthodontics, and ...
.
Europium
Europium is a chemical element; it has symbol Eu and atomic number 63. It is a silvery-white metal of the lanthanide series that reacts readily with air to form a dark oxide coating. Europium is the most chemically reactive, least dense, and soft ...
,
neodymium
Neodymium is a chemical element; it has Symbol (chemistry), symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth element, rare-earth metals. It is a hard (physics), hard, sli ...
, and other rare-earth elements are used to dope
glass
Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
es for lasers.
Holmium
Holmium is a chemical element; it has symbol Ho and atomic number 67. It is a rare-earth element and the eleventh member of the lanthanide series. It is a relatively soft, silvery, fairly corrosion-resistant and malleable metal. Like many other ...
-doped and
neodymium
Neodymium is a chemical element; it has Symbol (chemistry), symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth element, rare-earth metals. It is a hard (physics), hard, sli ...
yttrium aluminium garnets (YAGs) are used as the
active laser medium
The active laser medium (also called a gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from ...
in some laser scalpels.
Phosphors and scintillators
In context of
phosphor
A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...
s and
scintillators, dopants are better known as
activators, and are used to enhance the luminescence process.
Semiconductors
The addition of a dopant to a
semiconductor
A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
, known as
doping, has the effect of shifting the
Fermi level
The Fermi level of a solid-state body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by ''μ'' or ''E''F
for brevity. The Fermi level does not include the work required to re ...
s within the material. This results in a material with predominantly negative (
n-type) or positive (
p-type)
charge carrier
In solid state physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. ...
s depending on the dopant variety. Pure semiconductors that have been altered by the presence of dopants are known as
extrinsic semiconductors (see
intrinsic semiconductor
An intrinsic semiconductor, also called a pure semiconductor, undoped semiconductor or i-type semiconductor, is a semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties ...
). Dopants are introduced into semiconductors in a variety of techniques: solid sources, gases, spin on liquid, and ion implanting. See
ion implantation
Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the target's physical, chemical, or electrical properties. Ion implantation is used in semiconductor device fabrica ...
, surface
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
, and solid sources footnote.
Others
The color of some
gemstone
A gemstone (also called a fine gem, jewel, precious stone, semiprecious stone, or simply gem) is a piece of mineral crystal which, when cut or polished, is used to make jewellery, jewelry or other adornments. Certain Rock (geology), rocks (such ...
s is caused by dopants. For example, ruby and
sapphire
Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name ''sapphire ...
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.
See also
*
List of semiconductor materials
Semiconductor materials are nominally small band gap Insulator (electricity), insulators. The defining property of a semiconductor material is that it can be compromised by Doping (semiconductor), doping it with impurities that alter its electronic ...
References
{{reflist
Semiconductor properties
Materials science
Physical properties
Chemical properties
Crystals