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A
semiconductor material has an
electrical conductivity value between a
conductor, such as copper, and an
insulator, such as glass. Semiconductors are the foundation of modern
electronics. The modern understanding of the properties of a semiconductor relies on
quantum physics to explain the movement of
electrons and
holes in a
crystal lattice.
An increased knowledge of semiconductor materials and fabrication
processes has made possible continuing increases in the complexity and
speed of
microprocessors and memory devices.
The
electrical conductivity of a semiconductor material increases with increasing temperature, which is behaviour opposite to that of a metal.
Semiconductor devices can display a range of useful properties such as passing current more easily in one direction than the other, showing variable
resistance,
and sensitivity to light or heat. Because the electrical properties of a
semiconductor material can be modified by controlled addition of
impurities, or by the application of electrical fields or light, devices
made from semiconductors can be used for amplification, switching, and
energy conversion.
Current
conduction in a semiconductor occurs through the movement of free
electrons and "holes", collectively known as charge carriers. Adding
impurity atoms to a semiconducting material, known as "
doping",
greatly increases the number of charge carriers within it. When a doped
semiconductor contains mostly free holes it is called "p-type", and
when it contains mostly free electrons it is known as "n-type". The
semiconductor materials used in electronic devices are doped under
precise conditions to control the location and concentration of p- and
n-type dopants. A single semiconductor crystal can have many p- and
n-type regions; the
p–n junctions between these regions are responsible for the useful electronic behaviour.
Some of the properties of semiconductor materials were observed
throughout the mid 19th and first decades of the 20th century.
Development of quantum physics in turn allowed the development of the
transistor in 1948. Although some pure elements and many compounds display semiconductor properties,
silicon,
germanium, and compounds of
gallium are the most widely used in electronic devices.
Properties
- Variable conductivity
- A pure semiconductor is a poor electrical conductor as a consequence
of having just the right number of electrons to completely fill its valence bonds. Through various techniques (e.g., doping or gating), the semiconductor can be modified to have an excess of electrons (becoming an n-type semiconductor) or a deficiency of electrons (becoming a p-type semiconductor).
In both cases, the semiconductor becomes much more conductive (the
conductivity can be increased by one million fold or more).
Semiconductor devices exploit this effect to shape electrical current.
- Depletion
- When doped semiconductors are joined to metals, to different semiconductors, and to the same semiconductor with different doping, the resulting junction often strips the electron excess or deficiency out from the semiconductor near the junction. This depletion region is rectifying (only allowing current to flow in one direction), and used to further shape electrical currents in semiconductor devices.
- Energetic electrons travel far
- Electrons can be excited across the energy band gap (see Physics below) of a semiconductor by various means. These electrons can carry their excess energy over distance scales of micrometers
before dissipating their energy into heat – a significantly longer
distance than is possible in metals. This property is essential to the
operation of, e. g., bipolar junction transistors and solar cells.
- 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 dots.
- Thermal energy conversion
- Semiconductors have large thermoelectric power factors making them useful in thermoelectric generators, as well as high thermoelectric figures of merit making them useful in thermoelectric coolers.
Materials
A large number of elements and compounds have semiconducting properties, including:
[1]
- Certain pure elements are found in Group XIV of the periodic table; the most commercially important of these elements are silicon and germanium.
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 compounds, particularly between elements in Groups III and V, such as gallium arsenide, Groups II and VI, groups IV and VI, and between different group IV elements, e.g. silicon carbide.
- Certain ternary compounds, oxides and alloys.
- Organic semiconductors, made of organic compounds.
Most common semiconducting materials are crystalline solids, but
amorphous and liquid semiconductors are also known. These include
hydrogenated amorphous silicon and mixtures of
arsenic,
selenium and
tellurium
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
structures, which do not require material of higher electronic quality,
being relatively insensitive to impurities and radiation damage.
Preparation of semiconductor materials
Semiconductors with predictable, reliable electronic properties are necessary for
mass production.
The level of chemical purity needed is extremely high because the
presence of impurities even in very small proportions can have large
effects on the properties of the material. A high degree of crystalline
perfection is also required, since faults in 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 100 and 300 mm (4 and 12 in) in diameter which are grown as cylinders and sliced into
wafers.
Because of the required level of chemical purity and the perfection
of the crystal structure which are needed to make semiconductor devices,
special methods have been developed to produce the initial
semiconductor material. A technique for achieving high purity includes
growing the crystal using the
Czochralski process. An additional step that can be used to further increase purity is known as
zone refining.
In zone refining, part of a solid crystal is melted. The impurities
tend to concentrate in the melted region, while the desired material
recrystalizes leaving the solid material more pure and with fewer
crystalline faults.
In manufacturing semiconductor devices involving
heterojunctions between different semiconductor materials, it is often important to align the crystal lattices of the two materials by using
epitaxial techniques. The
lattice constant,
which is the length of the repeating element of the crystal structure,
is important for determining the compatibility of materials.
Physics of semiconductors
Energy bands and electrical conduction
Filling of the electronic
Density of states in various types of materials at
equilibrium.
Here the vertical axis is energy while the horizontal axis is the
Density of states for a particular band in the material listed. In
metals and
semimetals the
Fermi level EF lies inside at least one band. In
insulators and
semiconductors the Fermi level is inside a
band gap; however, in semiconductors the bands are near enough to the Fermi level to be
thermally populated with electrons or
holes.
Semiconductors are defined by their unique electric conductive
behavior, somewhere between that of a metal 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). These states are associated with the
electronic band structure of the material.
Electrical conductivity arises due to the presence of electrons in states that are
delocalized (extending through the material), however in order to transport electrons a state must be
partially filled, containing an electron only part of the time.
[2]
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).
High conductivity in a 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, by contrast, have few partially filled states, their Fermi levels sit within
band gaps
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 band gap, inducing partially filled
states in both the band of states beneath the band gap (
valence band) and the band of states above the band gap (
conduction band).
An (intrinsic) semiconductor has a band gap that is smaller than that
of an insulator and at room temperature significant numbers of electrons
can be excited to cross the band gap.
[3]
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-
band gap 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. An example of a common semi-insulator is
gallium arsenide.
[4] 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 most semiconductors the conduction bands have a parabolic
dispersion relation,
and so these electrons respond to forces (electric field, magnetic
field, etc.) much like they would in a vacuum, though with a different
effective mass.
[3] 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.
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
vacuum, again with some positive effective mass.
[3]
This particle is called a hole, and the collection of holes in the
valence 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 demands that these recombination events, in which an electron loses an amount of
energy larger than the
band gap, be accompanied by the emission of thermal energy (in the form of
phonons) 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. The precise
quantum mechanical mechanisms of generation and recombination are governed by
conservation of energy and
conservation of momentum.
As the probability that electrons and holes meet together is
proportional to the product of their amounts, the product is in 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 neighbour 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(−
EG/
kT), where
k is
Boltzmann's constant,
T is absolute temperature and
EG is band gap.
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. The process of adding controlled impurities to a semiconductor is known as
doping.
The amount of impurity, or dopant, added to an
intrinsic (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 of 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
4.2×1022 atoms, but only
2.5×1013 free electrons and
2.5×1013 holes. The addition of 0.001% of arsenic (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. 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. The opposite carrier is called the
minority carrier, 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 which 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
functions 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 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.
Early history of semiconductors
The history of the understanding of semiconductors begins with
experiments on the electrical properties of materials. The properties of
negative temperature coefficient of resistance, rectification, and
light-sensitivity were observed starting in the early 19th century.
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,
A. E. Becquerel reported observation of a voltage between a solid and a liquid electrolyte when struck by light, the
photovoltaic effect. In 1873
Willoughby Smith observed that
selenium resistors exhibit decreasing resistance when light falls on them. In 1874
Karl Ferdinand Braun observed conduction and
rectification in metallic sulphides, and
Arthur Schuster
found that a copper oxide layer on wires has rectification properties
that ceases when the wires are cleaned. Adams and Day observed the
photovoltaic effect in selenium in 1876.
[5]
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 demonstrated the deflection of flowing charge carriers by an applied magnetic field, the
Hall effect. The discovery of the
electron by
J.J. Thomson in 1897 prompted theories of electron-based conduction in solids.
Karl Baedeker,
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 as metals, insulators and "variable conductors" in 1914.
Felix Bloch
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 and the concept of band gaps had been developed.
Walter H. Schottky and
Nevill Francis Mott developed models of the potential barrier and of the characteristics of a
metal-semiconductor junction. By 1938,
Boris Davydov had developed a theory of the copper-oxide rectifer, identifying the effect of the
p–n junction and the importance of minority carriers and surface states.
[6]
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.
[6]
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
construction of more capable and reliable devices.
Alexander Graham Bell 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
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.
[6] Point-contact microwave detector rectifiers made of lead sulfide were used by
Jagadish Chandra Bose in 1904; the
cat's-whisker detector 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 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.
[5][6]
In the years preceding World War II, infra-red 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.
[6]
Detector and power rectifiers could not amplify a signal. Many
efforts were made to develop a solid-state amplifier, but these were
unsuccessful because of limited theoretical understanding of
semiconductor materials.
[6] In 1922
Oleg Losev developed two-terminal,
negative resistance amplifiers for radio; however, he perished in the
Siege of Leningrad. In 1926
Julius Edgar Lilienfeld
patented a device resembling a modern 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 of one cycle per second, too low for any practical applications, but an effective application of the available theory.
[6] At
Bell Labs,
William Shockley and A. Holden started investigating solid-state amplifiers in 1938. The first p–n junction in silicon was observed by
Russell Ohl
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.
In France, during the war,
Herbert Mataré had observed amplification between adjacent point contacts on a germanium base. After the war, Mataré's group announced their "
Transistron" amplifier only shortly after Bell Labs announced the "
transistor".
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