An extrinsic

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

is one that has been '' doped''; during manufacture of the semiconductor crystal a trace element or chemical called a doping agent has been incorporated chemically into the crystal, for the purpose of giving it different electrical properties than the pure semiconductor crystal, which is called an ''

intrinsic semiconductor An intrinsic (pure) semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the ...

''. In an extrinsic semiconductor it is these foreign dopant atoms in the crystal lattice that mainly provide the

charge carriers In 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. The term is ...

which carry electric current through the crystal. The doping agents used are of two types, resulting in two types of extrinsic semiconductor. An ''

electron donor In chemistry, an electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process. Typical reducing agents undergo permanent chemi ...

'' dopant is an atom which, when incorporated in the crystal, releases a mobile conduction

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

into the crystal lattice. An extrinsic semiconductor which has been doped with electron donor atoms is called an n-type semiconductor, because the majority of charge carriers in the crystal are negative electrons. An '' electron acceptor'' dopant is an atom which accepts an electron from the lattice, creating a vacancy where an electron should be called a '' hole'' which can move through the crystal like a positively charged particle. An extrinsic semiconductor which has been doped with electron acceptor atoms is called a p-type semiconductor, because the majority of charge carriers in the crystal are positive holes. Doping is the key to the extraordinarily wide range of electrical behavior that semiconductors can exhibit, and extrinsic semiconductors are used to make semiconductor electronic devices such as diodes,

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

s, integrated circuits, semiconductor lasers, LEDs, and

photovoltaic cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

s. Sophisticated semiconductor fabrication processes like photolithography can implant different dopant elements in different regions of the same semiconductor crystal wafer, creating semiconductor devices on the wafer's surface. For example a common type of transistor, the n-p-n

bipolar transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar t ...

, consists of an extrinsic semiconductor crystal with two regions of n-type semiconductor, separated by a region of p-type semiconductor, with metal contacts attached to each part.

Conduction in semiconductors

A solid substance can conduct electric current only if it contains charged particles,

s, which are free to move about and not attached to atoms. In a

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

conductor, it is the metal atoms that provide the electrons; typically each metal atom releases one of its outer orbital electrons to become a conduction electron which can move about throughout the crystal, and carry electric current. Therefore the number of conduction electrons in a metal is equal to the number of atoms, a very large number, making metals good conductors. Unlike in metals, the atoms that make up the bulk semiconductor crystal do not provide the electrons which are responsible for conduction. In semiconductors, electrical conduction is due to the mobile charge carriers, electrons or holes which are provided by impurities or dopant atoms in the crystal. In an extrinsic semiconductor, the concentration of doping atoms in the crystal largely determines the density of charge carriers, which determines its electrical conductivity, as well as a great many other electrical properties. This is the key to semiconductors' versatility; their conductivity can be manipulated over many orders of magnitude by doping.

Semiconductor doping

Semiconductor doping In semiconductor production, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as an extrinsic se ...

is the process that changes an intrinsic semiconductor to an extrinsic semiconductor. During doping, impurity atoms are introduced to an intrinsic semiconductor. Impurity atoms are atoms of a different element than the atoms of the intrinsic semiconductor. Impurity atoms act as either donors or acceptors to the intrinsic semiconductor, changing the electron and hole concentrations of the semiconductor. Impurity atoms are classified as either donor or acceptor atoms based on the effect they have on the intrinsic semiconductor. Donor impurity atoms have more

valence electron In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair form ...

s than the atoms they replace in the intrinsic semiconductor lattice. Donor impurities "donate" their extra valence electrons to a semiconductor's conduction band, providing excess electrons to the intrinsic semiconductor. Excess electrons increase the electron carrier concentration (n₀) of the semiconductor, making it n-type. Acceptor impurity atoms have fewer valence electrons than the atoms they replace in the intrinsic semiconductor lattice. They "accept" electrons from the semiconductor's valence band. This provides excess holes to the intrinsic semiconductor. Excess holes increase the hole carrier concentration (p₀) of the semiconductor, creating a p-type semiconductor. Semiconductors and dopant atoms are defined by the column of the periodic table in which they fall. The column definition of the semiconductor determines how many valence electrons its atoms have and whether dopant atoms act as the semiconductor's donors or acceptors. Group IV semiconductors use

group V A pnictogen ( or ; from grc, πνῑ́γω "to choke" and -gen, "generator") is any of the chemical elements in group 15 of the periodic table. Group 15 is also known as the nitrogen group or nitrogen family. Group 15 consists of the el ...

atoms as donors and group III atoms as acceptors. Group III–V semiconductors, the

compound semiconductor Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way. Because of ...

s, use group VI atoms as donors and group II atoms as acceptors. Group III–V semiconductors can also use

group IV The carbon group is a periodic table group consisting of carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and flerovium (Fl). It lies within the p-block. In modern IUPAC notation, it is called group 14. In the field of semicon ...

atoms as either donors or acceptors. When a group IV atom replaces the group III element in the semiconductor lattice, the group IV atom acts as a donor. Conversely, when a group IV atom replaces the group V element, the group IV atom acts as an acceptor. Group IV atoms can act as both donors and acceptors; therefore, they are known as

amphoteric In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphipro ...

impurities.

The two types of semiconductor

N-type semiconductors

''N-type'' semiconductors are created by doping an intrinsic semiconductor with an electron donor element during manufacture. The term ''n-type'' comes from the negative charge of the electron. In ''n-type'' semiconductors, electrons are the majority carriers and holes are the minority carriers. A common dopant for ''n-type'' silicon is

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...

. In an ''n-type'' semiconductor, the Fermi level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the

valence band In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...

. Examples:

antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient t ...

, etc.

P-type semiconductors

''P-type'' semiconductors are created by doping an intrinsic semiconductor with an electron acceptor element during manufacture. The term ''p-type'' refers to the positive charge of a hole. As opposed to ''n-type'' semiconductors, ''p-type'' semiconductors have a larger hole concentration than electron concentration. In ''p-type'' semiconductors, holes are the majority carriers and electrons are the minority carriers. A common ''p-type'' dopant for silicon is boron or gallium. For ''p-type'' semiconductors the Fermi level is below the intrinsic semiconductor and lies closer to the valence band than the conduction band. Examples: boron,

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

, gallium, etc.

Use of extrinsic semiconductors

Extrinsic semiconductors are components of many common electrical devices. A semiconductor diode (devices that allow current in only one direction) consists of p-type and n-type semiconductors placed in junction with one another. Currently, most semiconductor diodes use doped silicon or germanium.

Transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

s (devices that enable current switching) also make use of extrinsic semiconductors.

Bipolar junction transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...

s (BJT), which amplify current, are one type of transistor. The most common BJTs are NPN and PNP type. NPN transistors have two layers of n-type semiconductors sandwiching a p-type semiconductor. PNP transistors have two layers of p-type semiconductors sandwiching an n-type semiconductor. Field-effect transistors (FET) are another type of transistor which amplify current implementing extrinsic semiconductors. As opposed to BJTs, they are called unipolar because they involve single carrier type operationeither N-channel or P-channel. FETs are broken into two families, junction gate FET (JFET), which are three terminal semiconductors, and insulated gate FET (

IGFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the thermal oxidation, controlled oxidation of silicon. It has an insulated gate, the ...

), which are four terminal semiconductors. Other devices implementing the extrinsic semiconductor: * Lasers * Solar cells *

Photodetector Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or ...

s *

Light-emitting diode A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (co ...

s *

Thyristor A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...

References

* {{cite book, author=Neamen, Donald A., title=Semiconductor Physics and Devices: Basic Principles (3rd ed.), publisher=McGraw-Hill Higher Education, year=2003, isbn=0-07-232107-5, url-access=registration, url=https://archive.org/details/semiconductorphy00neam

External links

Howstuffworks: How Semiconductors Work
Semiconductor material types