Anextrinsicsemiconductor is one that has beendoped; during manufacture of the semiconductor crystal a trace element or chemical called adoping 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 anintrinsic semiconductor. In an extrinsic semiconductor it is these foreign dopant atoms in the crystal lattice that mainly provide thecharge carriers which carryelectric current through the crystal. The doping agents used are of two types, resulting in two types of extrinsic semiconductor. Anelectron donor dopant is an atom which, when incorporated in the crystal, releases a mobile conductionelectron into the crystal lattice. An extrinsic semiconductor that has been doped with electron donor atoms is called ann-type semiconductor, because the majority of charge carriers in the crystal are negative electrons. Anelectron acceptor dopant is an atom which accepts an electron from the lattice, creating a vacancy where an electron should be called ahole which can move through the crystal like a positively charged particle. An extrinsic semiconductor which has been doped with electron acceptor atoms is called ap-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 makesemiconductor electronic devices such asdiodes,transistors,integrated circuits,semiconductor lasers,LEDs, andphotovoltaic cells. Sophisticatedsemiconductor fabrication processes likephotolithography 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-nbipolar transistor, 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.

A solid substance can conduct electric current only if it contains charged particles,electrons, which are free to move about and not attached to atoms. In ametal conductor, it is the metal atoms that provide the electrons; typically each metal atom releases one of its outer orbital electrons to become aconduction 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 mobilecharge carriers, electrons orholes 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 itselectrical 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 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 eitherdonors oracceptors 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 morevalence electrons 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 theperiodic 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.

GroupIV semiconductors usegroup V atoms as donors andgroup III atoms as acceptors.

GroupIII–V semiconductors, thecompound semiconductors, usegroup VI atoms as donors andgroup II atoms as acceptors. Group III–V semiconductors can also usegroup IV 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 asamphoteric impurities.

N-type semiconductors are created bydoping an intrinsic semiconductor with an electrondonor element during manufacture. The termn-type comes from the negative charge of the electron. Inn-type semiconductors, electrons are themajority carriers and holes are theminority carriers. A common dopant forn-type silicon isphosphorus orarsenic. In ann-type semiconductor, theFermi level is greater than that of the intrinsic semiconductor and lies closer to theconduction band than thevalence band.

Examples:phosphorus,arsenic,antimony, etc.

P-type semiconductors are created bydoping an intrinsic semiconductor with an electronacceptor element during manufacture. The termp-type refers to the positive charge of a hole. As opposed ton-type semiconductors,p-type semiconductors have a larger hole concentration than electron concentration. Inp-type semiconductors, holes are the majority carriers and electrons are the minority carriers. A commonp-type dopant for silicon isboron orgallium. Forp-type semiconductors the Fermi level is below the intrinsic semiconductor and lies closer to the valence band than the conduction band.

Examples:boron,aluminium,gallium, etc.

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

Transistors (devices that enable current switching) also make use of extrinsic semiconductors.Bipolar junction transistors (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 calledunipolar because they involve single carrier type operation – either 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), which are four-terminal semiconductors.

Other devices implementing the extrinsic semiconductor:

• Lasers
• Solar cells
• Photodetectors
• Light-emitting diodes
• Thyristors

• Intrinsic semiconductor
• Doping (semiconductor)
• List of semiconductor materials

• Neamen, Donald A. (2003).Semiconductor Physics and Devices: Basic Principles (3rd ed.). McGraw-Hill Higher Education.ISBN 0-07-232107-5.

• Howstuffworks: How Semiconductors Work