A semiconductor is a material which has an
electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
value falling between that of a
conductor, such as copper, and an
insulator, such as glass. Its
resistivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
falls as its temperature rises; metals behave in the opposite way. Its conducting properties may be altered in useful ways by introducing impurities ("
doping") into the
crystal structure. When two differently doped regions exist in the same crystal, a
semiconductor junction
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 created. The behavior of
charge carrier
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 used ...
s, which include
electrons,
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...
s, and
electron hole
In physics, chemistry, and electronic engineering, an electron hole (often simply called a hole) is a quasiparticle which is the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or c ...
s, at these junctions is the basis of
diodes,
transistors, and most modern
electronics. Some examples of semiconductors are
silicon,
germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors si ...
,
gallium arsenide, and elements near the so-called "
metalloid staircase
A metalloid is a type of chemical element which has a preponderance of properties in between, or that are a mixture of, those of metals and nonmetals. There is no standard definition of a metalloid and no complete agreement on which elements are ...
" on the
periodic table
The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...
. After silicon, gallium arsenide is the second-most common semiconductor and is used in laser diodes,
solar cells, microwave-frequency
integrated circuits, and others. Silicon is a critical element for fabricating most electronic circuits.
Semiconductor device
A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity ...
s can display a range of useful properties, such as passing current more easily in one direction than the other, showing variable resistance, and having sensitivity to light or heat. Because the electrical properties of a semiconductor material can be modified by doping and by the application of electrical fields or light, devices made from semiconductors can be used for amplification, switching, and
energy conversion
Energy transformation, also known as energy conversion, is the process of changing energy from one form to another. In physics, energy is a quantity that provides the capacity to perform work or moving, (e.g. Lifting an object) or provides heat. ...
.
The conductivity of silicon is increased by adding a small amount (of the order of 1 in 10
8) of pentavalent (
antimony,
phosphorus, or
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, but ...
) or trivalent (
boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
,
gallium
Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (alumini ...
,
indium
Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 parts pe ...
) atoms. This process is known as doping, and the resulting semiconductors are known as doped or extrinsic semiconductors. Apart from doping, the conductivity of a semiconductor can be improved by increasing its temperature. This is contrary to the behavior of a metal, in which conductivity decreases with an increase in temperature.
The modern understanding of the properties of a semiconductor relies on
quantum physics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, q ...
to explain the movement of charge carriers in a
crystal lattice
In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by
: \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n_ ...
.
Doping greatly increases the number of charge carriers within the crystal. When a doped semiconductor contains free holes, it is called "
p-type", and when it contains free
electrons, it is known as "
n-type". The semiconductor materials used in electronic devices are doped under precise conditions to control the concentration and regions of p- and n-type dopants. A single semiconductor device
crystal can have many p- and n-type regions; the
p–n junction
A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contai ...
s between these regions are responsible for the useful electronic behavior. Using a
hot-point probe
A hot point probe is a method of quickly determining whether a semiconductor sample is n-type or p-type. The sample is probed using a voltmeter or ammeter and a heat source, such as a soldering iron, is placed on one of the leads. The heat will ...
, one can determine quickly whether a semiconductor sample is p- or n-type.
Some of the properties of semiconductor materials were observed throughout the mid-19th and first decades of the 20th century. The first practical application of semiconductors in electronics was the 1904 development of the
cat's-whisker detector
A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (demo ...
, a primitive semiconductor diode used in early
radio receivers. Developments in quantum physics led in turn to the invention of the
transistor in 1947 and the integrated circuit in 1958.
Properties
Variable electrical conductivity
Semiconductors in their natural state are poor conductors because a
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
requires the flow of electrons, and semiconductors have their
valence bands filled, preventing the entire flow of new electrons. Several developed techniques allow semiconducting materials to behave like conducting materials, such as
doping or
gating. These modifications have two outcomes: n-type and p-type. These refer to the excess or shortage of electrons, respectively. A balanced number of electrons would cause a current to flow throughout the material.
Heterojunctions
Heterojunctions occur when two differently doped semiconducting materials are joined. For example, a configuration could consist of p-doped and n-doped
germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors si ...
. This results in an exchange of electrons and holes between the differently doped semiconducting materials. The n-doped germanium would have an excess of electrons, and the p-doped germanium would have an excess of holes. The transfer occurs until an equilibrium is reached by a process called
recombination, which causes the migrating electrons from the n-type to come in contact with the migrating holes from the p-type. The result of this process is a narrow strip of immobile
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...
s, which causes an
electric field across the junction.
Excited electrons
A difference in electric potential on a semiconducting material would cause it to leave thermal equilibrium and create a non-equilibrium situation. This introduces electrons and holes to the system, which interact via a process called
ambipolar diffusion Ambipolar diffusion (ambipolar: relating to or consisting of both electrons and positive ions moving in opposite directions) is diffusion of positive and negative species with opposite electrical charge due to their interaction via an electric field ...
. Whenever thermal equilibrium is disturbed in a semiconducting material, the number of holes and electrons changes. Such disruptions can occur as a result of a temperature difference or
photons, which can enter the system and create electrons and holes. The process that creates and annihilates electrons and holes are called
generation and recombination, respectively.
Light emission
In certain semiconductors, excited electrons can relax by emitting light instead of producing heat. These semiconductors are used in the construction of
light-emitting diodes and fluorescent
quantum dot
Quantum dots (QDs) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanotechnology. When the ...
s.
High thermal conductivity
Semiconductors with high thermal conductivity can be used for heat dissipation and improving thermal management of electronics.
Thermal energy conversion
Semiconductors have large
thermoelectric power factor
Thermoelectric materials show the thermoelectric effect in a strong or convenient form.
The ''thermoelectric effect'' refers to phenomena by which either a temperature difference creates an electric potential or an electric current creates a te ...
s making them useful in
thermoelectric generators, as well as high
thermoelectric figures of merit making them useful in
thermoelectric cooler
Thermoelectric cooling uses the Peltier effect to create a heat flux at the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side o ...
s.
Materials
A large number of elements and compounds have semiconducting properties, including:
[B. G. Yacobi, ''Semiconductor Materials: An Introduction to Basic Principles'', Springer 2003 , pp. 1–3.]
* Certain pure elements are found in
group 14 of the
periodic table
The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...
; the most commercially important of these elements are
silicon and
germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors si ...
. Silicon and germanium are used here effectively because they have 4 valence electrons in their outermost shell, which gives them the ability to gain or lose electrons equally at the same time.
*
Binary compound
In materials chemistry, a binary phase or binary compound is a chemical compound containing two different elements. Some binary phase compounds are molecular, e.g. carbon tetrachloride (CCl4). More typically binary phase refers to extended soli ...
s, particularly between elements in groups 13 and 15, such as
gallium arsenide, groups 12 and 16, groups 14 and 16, and between different group-14 elements, e.g.
silicon carbide
Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal si ...
.
* Certain ternary compounds, oxides, and alloys.
*
Organic semiconductors, made of
organic compounds.
* Semiconducting
metal–organic framework
Metal–organic frameworks (MOFs) are a class of compounds consisting of metal ions or clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. The organic ligands included are sometimes referred to as "stru ...
s.
The most common semiconducting materials are crystalline solids, but
amorphous
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal.
Etymology
The term comes from the Greek ''a'' ("wit ...
and liquid semiconductors are also known. These include
hydrogenated amorphous silicon
Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs.
Used as semiconductor material for a-Si solar cells, or thin-film silicon solar cells, it is deposited in thin films onto ...
and mixtures of
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, but ...
,
selenium, and
tellurium
Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...
in a variety of proportions. These compounds share with better-known semiconductors the properties of intermediate conductivity and a rapid variation of conductivity with temperature, as well as occasional
negative resistance. Such disordered materials lack the rigid crystalline structure of conventional semiconductors such as silicon. They are generally used in
thin film
A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many a ...
structures, which do not require material of higher electronic quality, being relatively insensitive to impurities and radiation damage.
Preparation of semiconductor materials
Almost all of today's electronic technology involves the use of semiconductors, with the most important aspect being the
integrated circuit (IC), which are found in
desktops,
laptops
A laptop, laptop computer, or notebook computer is a small, portable personal computer (PC) with a screen and alphanumeric keyboard. Laptops typically have a clam shell form factor with the screen mounted on the inside of the upper li ...
, scanners,
cell-phone
A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whil ...
s, and other electronic devices. Semiconductors for ICs are mass-produced. To create an ideal semiconducting material, chemical purity is paramount. Any small imperfection can have a drastic effect on how the semiconducting material behaves due to the scale at which the materials are used.
A high degree of crystalline perfection is also required, since faults in the crystal structure (such as
dislocations,
twins, and
stacking faults) interfere with the semiconducting properties of the material. Crystalline faults are a major cause of defective semiconductor devices. The larger the crystal, the more difficult it is to achieve the necessary perfection. Current mass production processes use crystal
ingots between in diameter, grown as cylinders and sliced into
wafers
A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...
.
There is a combination of processes that are used to prepare semiconducting materials for ICs. One process is called
thermal oxidation
In microfabrication, thermal oxidation is a way to produce a thin layer of oxide (usually silicon dioxide) on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The ra ...
, which forms
silicon dioxide on the surface of the
silicon. This is used as a
gate insulator and
field oxide. Other processes are called
photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography and the production of integrated circuits (ICs or "chips") in particular. Masks are used ...
s and
photolithography. This process is what creates the patterns on the circuit in the integrated circuit.
Ultraviolet light is used along with a
photoresist layer to create a chemical change that generates the patterns for the circuit.
The etching is the next process that is required. The part of the silicon that was not covered by the
photoresist layer from the previous step can now be etched. The main process typically used today is called
plasma etching
Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge ( plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as etch spec ...
. Plasma etching usually involves an
etch gas pumped in a low-pressure chamber to create
plasma. A common etch gas is
chlorofluorocarbon
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and prop ...
, or more commonly known
Freon. A high
radio-frequency voltage between the
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
and
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...
is what creates the plasma in the chamber. The
silicon wafer
In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serv ...
is located on the cathode, which causes it to be hit by the positively charged ions that are released from the plasma. The result is silicon that is etched
anisotropically.
The last process is called
diffusion. This is the process that gives the semiconducting material its desired semiconducting properties. It is also known as
doping. The process introduces an impure atom to the system, which creates the
p–n junction
A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contai ...
. To get the impure atoms embedded in the silicon wafer, the wafer is first put in a 1,100 degree Celsius chamber. The atoms are injected in and eventually diffuse with the silicon. After the process is completed and the silicon has reached room temperature, the doping process is done and the semiconducting material is ready to be used in an integrated circuit.
Physics of semiconductors
Energy bands and electrical conduction
Semiconductors are defined by their unique electric conductive behavior, somewhere between that of a conductor and an insulator. The differences between these materials can be understood in terms of the
quantum states for electrons, each of which may contain zero or one electron (by the
Pauli exclusion principle
In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously. This principle was formula ...
). These states are associated with the
electronic band structure of the material.
Electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
arises due to the presence of electrons in states that are
delocalized
In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref>
The term delocalization is general and can have slightly diff ...
(extending through the material), however in order to transport electrons a state must be ''partially filled'', containing an electron only part of the time. If the state is always occupied with an electron, then it is inert, blocking the passage of other electrons via that state. The energies of these quantum states are critical since a state is partially filled only if its energy is near the
Fermi level (see
Fermi–Dirac statistics
Fermi–Dirac statistics (F–D statistics) is a type of quantum statistics that applies to the physics of a system consisting of many non-interacting, identical particles that obey the Pauli exclusion principle. A result is the Fermi–Dirac d ...
).
High conductivity in material comes from it having many partially filled states and much state delocalization.
Metals are good
electrical conductors and have many partially filled states with energies near their Fermi level.
Insulators
Insulator may refer to:
* Insulator (electricity), a substance that resists electricity
** Pin insulator, a device that isolates a wire from a physical support such as a pin on a utility pole
** Strain insulator, a device that is designed to work ...
, by contrast, have few partially filled states, their Fermi levels sit within
band gap
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (i ...
s with few energy states to occupy. Importantly, an insulator can be made to conduct by increasing its temperature: heating provides energy to promote some electrons across the bandgap, inducing partially filled states in both the band of states beneath the band gap (
valence band) and the band of states above the bandgap (
conduction 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 wh ...
). An (intrinsic) semiconductor has a bandgap that is smaller than that of an insulator and at room temperature, significant numbers of electrons can be excited to cross the band gap.
Charles Kittel
Charles Kittel (July 18, 1916 – May 15, 2019) was an American physicist. He was a professor at University of California, Berkeley from 1951 and was professor emeritus from 1978 until his death.
Life and work
Charles Kittel was born in New Yo ...
(1995) '' Introduction to Solid State Physics'', 7th ed. Wiley, .
A pure semiconductor, however, is not very useful, as it is neither a very good insulator nor a very good conductor.
However, one important feature of semiconductors (and some insulators, known as ''semi-insulators'') is that their conductivity can be increased and controlled by
doping with impurities and
gating with electric fields. Doping and gating move either the conduction or valence band much closer to the Fermi level and greatly increase the number of partially filled states.
Some
wider-bandgap semiconductor materials are sometimes referred to as semi-insulators. When undoped, these have electrical conductivity nearer to that of electrical insulators, however they can be doped (making them as useful as semiconductors). Semi-insulators find niche applications in micro-electronics, such as substrates for
HEMT
A high-electron-mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field-effect transistor incorporating a junction between two materials with different band gaps (i.e. a heterojunction ...
. An example of a common semi-insulator is
gallium arsenide. Some materials, such as
titanium dioxide, can even be used as insulating materials for some applications, while being treated as wide-gap semiconductors for other applications.
Charge carriers (electrons and holes)
The partial filling of the states at the bottom of the conduction band can be understood as adding electrons to that band. The electrons do not stay indefinitely (due to the natural thermal
recombination) but they can move around for some time. The actual concentration of electrons is typically very dilute, and so (unlike in metals) it is possible to think of the electrons in the conduction band of a semiconductor as a sort of classical
ideal gas, where the electrons fly around freely without being subject to the
Pauli exclusion principle
In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously. This principle was formula ...
. In most semiconductors, the conduction bands have a parabolic
dispersion relation, and so these electrons respond to forces (electric field, magnetic field, etc.) much as they would in a vacuum, though with a different
effective mass.
Because the electrons behave like an ideal gas, one may also think about conduction in very simplistic terms such as the
Drude model, and introduce concepts such as
electron mobility.
For partial filling at the top of the valence band, it is helpful to introduce the concept of an
electron hole
In physics, chemistry, and electronic engineering, an electron hole (often simply called a hole) is a quasiparticle which is the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or c ...
. Although the electrons in the valence band are always moving around, a completely full valence band is inert, not conducting any current. If an electron is taken out of the valence band, then the trajectory that the electron would normally have taken is now missing its charge. For the purposes of electric current, this combination of the full valence band, minus the electron, can be converted into a picture of a completely empty band containing a positively charged particle that moves in the same way as the electron. Combined with the ''negative'' effective mass of the electrons at the top of the valence band, we arrive at a picture of a positively charged particle that responds to electric and magnetic fields just as a normal positively charged particle would do in a vacuum, again with some positive effective mass.
This particle is called a hole, and the collection of holes in the valence band can again be understood in simple classical terms (as with the electrons in the conduction band).
Carrier generation and recombination
When
ionizing radiation strikes a semiconductor, it may excite an electron out of its energy level and consequently leave a hole. This process is known as
''electron-hole pair generation''. Electron-hole pairs are constantly generated from
thermal energy as well, in the absence of any external energy source.
Electron-hole pairs are also apt to recombine.
Conservation of energy
In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be ''conserved'' over time. This law, first proposed and tested by Émilie du Châtelet, means that ...
demands that these recombination events, in which an electron loses an amount of
energy larger than the
band gap
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (i ...
, be accompanied by the emission of thermal energy (in the form of
phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phonon is an excited state in the quantum mechanical ...
s) or radiation (in the form of
photons).
In some states, the generation and recombination of electron-hole pairs are in equipoise. The number of electron-hole pairs in the
steady state at a given temperature is determined by
quantum statistical mechanics
Quantum statistical mechanics is statistical mechanics applied to quantum mechanical systems. In quantum mechanics a statistical ensemble (probability distribution over possible quantum states) is described by a density operator ''S'', which is ...
. The precise
quantum mechanical mechanisms of generation and recombination are governed by the
conservation of energy
In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be ''conserved'' over time. This law, first proposed and tested by Émilie du Châtelet, means that ...
and
conservation of momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass and ...
.
As the probability that electrons and holes meet together is proportional to the product of their numbers, the product is in the steady-state nearly constant at a given temperature, providing that there is no significant electric field (which might "flush" carriers of both types, or move them from neighbor regions containing more of them to meet together) or externally driven pair generation. The product is a function of the temperature, as the probability of getting enough thermal energy to produce a pair increases with temperature, being approximately exp(−''E''
G/''kT''), where ''k'' is
Boltzmann's constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...
, ''T'' is the absolute temperature and ''E''
G is bandgap.
The probability of meeting is increased by carrier traps – impurities or dislocations which can trap an electron or hole and hold it until a pair is completed. Such carrier traps are sometimes purposely added to reduce the time needed to reach the steady-state.
Doping
The conductivity of semiconductors may easily be modified by introducing impurities into their
crystal lattice
In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by
: \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n_ ...
. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an ''
intrinsic
In science and engineering, an intrinsic property is a property of a specified subject that exists itself or within the subject. An extrinsic property is not essential or inherent to the subject that is being characterized. For example, mass ...
'' (pure) semiconductor varies its level of conductivity. Doped semiconductors are referred to as
''extrinsic''. By adding impurity to the pure semiconductors, the electrical conductivity may be varied by factors of thousands or millions.
A 1 cm
3 specimen of a metal or semiconductor has the order of 10
22 atoms. In a metal, every atom donates at least one free electron for conduction, thus 1 cm
3 of metal contains on the order of 10
22 free electrons, whereas a 1 cm
3 sample of pure germanium at 20°C contains about atoms, but only free electrons and holes. The addition of 0.001% of
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, but ...
(an impurity) donates an extra ''10
17'' free electrons in the same volume and the electrical conductivity is increased by a factor of ''10,000''.
The materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. In general, dopants that produce the desired controlled changes are classified as either electron
acceptors or
donors
A donor in general is a person, organization or government which donates something voluntarily. The term is usually used to represent a form of pure altruism, but is sometimes used when the payment for a service is recognized by all parties as rep ...
. Semiconductors doped with ''donor'' impurities are called ''n-type'', while those doped with ''acceptor'' impurities are known as ''p-type''. The n and p type designations indicate which charge carrier acts as the material's
majority carrier
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 used ...
. The opposite carrier is called the
minority carrier
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 exists due to thermal excitation at a much lower concentration compared to the majority carrier.
For example, the pure semiconductor
silicon has four valence electrons that bond each silicon atom to its neighbors. In silicon, the most common dopants are ''group III'' and ''group V'' elements. Group III elements all contain three valence electrons, causing them to function as acceptors when used to dope silicon. When an acceptor atom replaces a silicon atom in the crystal, a vacant state (an electron "hole") is created, which can move around the lattice and function as a charge carrier. Group V elements have five valence electrons, which allows them to act as a donor; substitution of these atoms for silicon creates an extra free electron. Therefore, a silicon crystal doped with
boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
creates a p-type semiconductor whereas one doped with
phosphorus results in an n-type material.
During
manufacture, dopants can be diffused into the semiconductor body by contact with gaseous compounds of the desired element, or
ion implantation can be used to accurately position the doped regions.
Amorphous semiconductors
Some materials, when rapidly cooled to a glassy amorphous state, have semiconducting properties. These include B,
Si, Ge, Se, and Te, and there are multiple theories to explain them.
Early history of semiconductors
The history of the understanding of semiconductors begins with experiments on the electrical properties of materials. The properties of the time-temperature coefficient of resistance, rectification, and light-sensitivity were observed starting in the early 19th century.
Thomas Johann Seebeck was the first to notice an
effect
Effect may refer to:
* A result or change of something
** List of effects
** Cause and effect, an idiom describing causality
Pharmacy and pharmacology
* Drug effect, a change resulting from the administration of a drug
** Therapeutic effect, a ...
due to semiconductors, in 1821. In 1833,
Michael Faraday reported that the resistance of specimens of
silver sulfide decreases when they are heated. This is contrary to the behavior of metallic substances such as copper. In 1839,
Alexandre Edmond Becquerel
Alexandre-Edmond Becquerel (24 March 1820 – 11 May 1891), known as Edmond Becquerel, was a French physicist who studied the solar spectrum, magnetism, electricity and optics. He is credited with the discovery of the photovoltaic effect, the ...
reported observation of a voltage between a solid and a liquid electrolyte, when struck by light, the
photovoltaic effect. In 1873,
Willoughby Smith
Willoughby Smith (6 April 1828, in Great Yarmouth, Norfolk – 17 July 1891, in Eastbourne, Sussex) was an English electrical engineer who discovered the photoconductivity of the element selenium. This discovery led to the invention of photoelec ...
observed that
selenium resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active ...
s exhibit decreasing resistance when light falls on them. In 1874,
Karl Ferdinand Braun observed conduction and
rectification in metallic
sulfide
Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
s, although this effect had been discovered much earlier by Peter Munck af Rosenschold (
sv) writing for the Annalen der Physik und Chemie in 1835,
and
Arthur Schuster
Sir Franz Arthur Friedrich Schuster (12 September 1851 – 14 October 1934) was a German-born British physicist known for his work in spectroscopy, electrochemistry, optics, X-radiography and the application of harmonic analysis to physics. ...
found that a copper oxide layer on wires has rectification properties that ceases, when the wires are cleaned.
William Grylls Adams and Richard Evans Day observed the photovoltaic effect in selenium in 1876.
A unified explanation of these phenomena required a theory of
solid-state physics, which developed greatly in the first half of the 20th century. In 1878
Edwin Herbert Hall
Edwin Herbert Hall (November 7, 1855 – November 20, 1938) was an American physicist, who discovered the eponymous Hall effect. Hall conducted thermoelectric research and also wrote numerous physics textbooks and laboratory manuals.
Biograp ...
demonstrated the deflection of flowing charge carriers by an applied magnetic field, the
Hall effect. The discovery of the
electron by
J.J. Thomson
Sir Joseph John Thomson (18 December 1856 – 30 August 1940) was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be discovered.
In 1897, Thomson showed that ...
in 1897 prompted theories of electron-based conduction in solids.
Karl Baedeker
Karl Ludwig Johannes Baedeker ( , ; 3 November 1801 – 4 October 1859) was a German publisher whose company, Baedeker, set the standard for authoritative guidebooks for tourists.
Karl Baedeker was descended from a long line of printers, book ...
, by observing a Hall effect with the reverse sign to that in metals, theorized that copper iodide had positive charge carriers. Johan Koenigsberger classified solid materials like metals, insulators, and "variable conductors" in 1914 although his student Josef Weiss already introduced the term ''Halbleiter'' (a semiconductor in modern meaning) in his Ph.D. thesis in 1910.
Felix Bloch
Felix Bloch (23 October 1905 – 10 September 1983) was a Swiss- American physicist and Nobel physics laureate who worked mainly in the U.S. He and Edward Mills Purcell were awarded the 1952 Nobel Prize for Physics for "their development of new ...
published a theory of the movement of electrons through atomic lattices in 1928. In 1930, B. Gudden stated that conductivity in semiconductors was due to minor concentrations of impurities. By 1931, the band theory of conduction had been established by
Alan Herries Wilson
Sir Alan Herries Wilson (2 July 1906 – 30 September 1995), was a British mathematician and industrialist. He was educated at Wallasey Grammar School and at Emmanuel College, Cambridge, obtaining a BA degree in mathematics in 1926. His graduate ...
and the concept of band gaps had been developed.
Walter H. Schottky
Walter Hans Schottky (23 July 1886 – 4 March 1976) was a German physicist who played a major early role in developing the theory of electron and ion emission phenomena, invented the screen-grid vacuum tube in 1915 while working at Siemen ...
and
Nevill Francis Mott
Sir Nevill Francis Mott (30 September 1905 – 8 August 1996) was a British physicist who won the Nobel Prize for Physics in 1977 for his work on the electronic structure of magnetic and disordered systems, especially amorphous semiconductor ...
developed models of the potential barrier and of the characteristics of a
metal–semiconductor junction
In solid-state physics, a metal–semiconductor (M–S) junction is a type of electrical junction in which a metal comes in close contact with a semiconductor material. It is the oldest practical semiconductor device. M–S junctions can either ...
. By 1938, Boris Davydov had developed a theory of the copper-oxide rectifier, identifying the effect of the
p–n junction
A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contai ...
and the importance of minority carriers and surface states.
Agreement between theoretical predictions (based on developing quantum mechanics) and experimental results was sometimes poor. This was later explained by
John Bardeen as due to the extreme "structure sensitive" behavior of semiconductors, whose properties change dramatically based on tiny amounts of impurities.
Commercially pure materials of the 1920s containing varying proportions of trace contaminants produced differing experimental results. This spurred the development of improved material refining techniques, culminating in modern semiconductor refineries producing materials with parts-per-trillion purity.
Devices using semiconductors were at first constructed based on empirical knowledge before semiconductor theory provided a guide to the construction of more capable and reliable devices.
Alexander Graham Bell
Alexander Graham Bell (, born Alexander Bell; March 3, 1847 – August 2, 1922) was a Scottish-born inventor, scientist and engineer who is credited with patenting the first practical telephone. He also co-founded the American Telephone and T ...
used the light-sensitive property of selenium to
transmit sound over a beam of light in 1880. A working solar cell, of low efficiency, was constructed by
Charles Fritts
Charles Fritts (1850 – 1903) was the American inventor credited with creating the first working selenium cell in 1883.
The world's first rooftop solar array, using Fritts' selenium cells, was installed in 1884 on a New York City rooftop.
Frit ...
in 1883, using a metal plate coated with selenium and a thin layer of gold; the device became commercially useful in photographic light meters in the 1930s.
Point-contact microwave detector rectifiers made of lead sulfide were used by
Jagadish Chandra Bose in 1904; the
cat's-whisker detector
A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (demo ...
using natural galena or other materials became a common device in the
development of radio. However, it was somewhat unpredictable in operation and required manual adjustment for best performance. In 1906,
H.J. Round observed light emission when electric current passed through
silicon carbide
Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal si ...
crystals, the principle behind the
light-emitting diode.
Oleg Losev observed similar light emission in 1922, but at the time the effect had no practical use. Power rectifiers, using copper oxide and selenium, were developed in the 1920s and became commercially important as an alternative to
vacuum tube rectifiers.
The first semiconductor device
A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity ...
s used galena
Galena, also called lead glance, is the natural mineral form of lead(II) sulfide (PbS). It is the most important ore of lead and an important source of silver.
Galena is one of the most abundant and widely distributed sulfide minerals. It cryst ...
, including German physicist
A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe.
Physicists generally are interested in the root or ultimate caus ...
Ferdinand Braun's crystal detector
A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (dem ...
in 1874 and Bengali physicist Jagadish Chandra Bose's radio crystal detector in 1901.
In the years preceding World War II, infrared detection and communications devices prompted research into lead-sulfide and lead-selenide materials. These devices were used for detecting ships and aircraft, for infrared rangefinders, and for voice communication systems. The point-contact crystal detector became vital for microwave radio systems since available vacuum tube devices could not serve as detectors above about 4000 MHz; advanced radar systems relied on the fast response of crystal detectors. Considerable research and development of silicon materials occurred during the war to develop detectors of consistent quality.
Early transistors
Detector and power rectifiers could not amplify a signal. Many efforts were made to develop a solid-state amplifier and were successful in developing a device called the point contact transistor
The point-contact transistor was the first type of transistor to be successfully demonstrated. It was developed by research scientists John Bardeen and Walter Brattain at Bell Laboratories in December 1947. They worked in a group led by physicis ...
which could amplify 20dB or more.[Peter Robin Morris (1990) ''A History of the World Semiconductor Industry'', IET, , pp. 11–25] In 1922, Oleg Losev developed two-terminal, negative resistance amplifiers for radio, but he perished in the Siege of Leningrad
The siege of Leningrad (russian: links=no, translit=Blokada Leningrada, Блокада Ленинграда; german: links=no, Leningrader Blockade; ) was a prolonged military blockade undertaken by the Axis powers against the Soviet Union, So ...
after successful completion. In 1926, Julius Edgar Lilienfeld patented a device resembling a field-effect transistor, but it was not practical. R. Hilsch and R. W. Pohl in 1938 demonstrated a solid-state amplifier using a structure resembling the control grid of a vacuum tube; although the device displayed power gain, it had a cut-off frequency
In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced ( attenuated or reflected) rather than ...
of one cycle per second, too low for any practical applications, but an effective application of the available theory. At Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
, William Shockley and A. Holden started investigating solid-state amplifiers in 1938. The first p–n junction in silicon was observed by Russell Ohl
Russell Shoemaker Ohl (January 30, 1898 – March 20, 1987) was an American scientist who is generally recognized for patenting the modern solar cell (, "Light sensitive device").
Ohl was a notable semiconductor researcher prior to the invention ...
about 1941 when a specimen was found to be light-sensitive, with a sharp boundary between p-type impurity at one end and n-type at the other. A slice cut from the specimen at the p–n boundary developed a voltage when exposed to light.
The first working transistor was a point-contact transistor invented by John Bardeen, Walter Houser Brattain, and William Shockley at Bell Labs in 1947. Shockley had earlier theorized a field-effect amplifier made from germanium and silicon, but he failed to build such a working device, before eventually using germanium to invent the point-contact transistor. In France, during the war, Herbert Mataré
Herbert Franz Mataré (22 September 1912 – 2 September 2011) was a German physicist. The focus of his research was the field of semiconductor research. His best-known work is the first functional European transistor, which he developed and paten ...
had observed amplification between adjacent point contacts on a germanium base. After the war, Mataré's group announced their "Transistron
A transistor is a semiconductor device with at least three terminals for connection to an electric circuit. In the common case, the third terminal controls the flow of current between the other two terminals. This can be used for amplification, a ...
" amplifier only shortly after Bell Labs announced the " transistor".
In 1954, physical chemist
Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical mecha ...
Morris Tanenbaum
Morris Tanenbaum (November 10, 1928 - February 26, 2023) was an American physical chemist and executive who worked at Bell Laboratories and AT&T Corporation.
Tanenbaum made significant contributions in the fields of transistor development and ...
fabricated the first silicon junction transistor at Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
. However, early junction transistors were relatively bulky devices that were difficult to manufacture on a mass-production basis, which limited them to a number of specialised applications.
See also
* Deathnium
Deathnium is a name given by early electronic engineers to a trap in semiconductors that reduces the lifetime of both electron and hole charge carriers. It is considered the fifth of the imperfections that must be considered in semiconductor crysta ...
* Semiconductor device fabrication
* Semiconductor industry
* Semiconductor characterization techniques
{{Use mdy dates, date = March 2019
Semiconductor characterization techniques are used to characterize a semiconductor material or device (PN junction, Schottky diode, etc.). Some examples of semiconductor properties that could be characterized ...
* Transistor count
References
Further reading
*
*
*
*
*
* G. B. Abdullayev, T. D. Dzhafarov, S. Torstveit (Translator), ''Atomic Diffusion in Semiconductor Structures,'' Gordon & Breach Science Pub., 1987
External links
Feynman's lecture on Semiconductors
HowStuffWorks
* Calculator for th
intrinsic carrier concentration
in silicon
Physical Properties of Semiconductors]
Semiconductor Manufacturer List
ABACUS
Introduction to Semiconductor Devices – by Gerhard Klimeck and Dragica Vasileska, online learning resource with simulation tools on nanoHUB
nanoHUB.org is a science and engineering gateway comprising community-contributed resources and geared toward education, professional networking, and interactive simulation tools for nanotechnology. Funded by the United States National Science F ...
Organic Semiconductors page
DoITPoMS Teaching and Learning Package- "Introduction to Semiconductors"
The Virtual Museum of Semiconductors Organizations
{{Authority control
Semiconductors