Examples of semiconductors. Types, properties, practical application

The most famous semiconductor is silicon (Si). But, apart from him, there are many others. An example is natural semiconductor materials such as zinc blende (ZnS), cuprite (Cu ₂ O), galena (PbS) and many others. A family of semiconductors, including semiconductors synthesized in laboratories, is one of the most versatile classes of materials known to man.

Characteristics of semiconductors

Of the 104 elements of the periodic table, 79 are metals, 25 are non-metals, of which 13 chemical elements possess semiconductor properties and 12 are dielectric. The main difference between semiconductors is that their electrical conductivity increases significantly with increasing temperature. At low temperatures they behave like dielectrics, and at high temperatures they behave like conductors. These semiconductors are different from metals: the resistance of the metal increases in proportion to the temperature increase.

Another difference between the semiconductor and the metal is that the resistance of the semiconductor falls under the action of light, while the metal does not influence the metal. The conductivity of semiconductors also changes with the introduction of a small amount of impurity.

Semiconductors are found among chemical compounds with a variety of crystalline structures. It can be elements such as silicon and selenium, or double compounds, like gallium arsenide. Many organic compounds, for example polyacetylene (CH) _n, are semiconductor materials. Some semiconductors exhibit magnetic (Cd _1-x Mn _x Te) or ferroelectric properties (SbSI). Others with sufficient doping become superconductors (GeTe and SrTiO ₃ ). Many of the recently discovered high-temperature superconductors have nonmetallic semiconducting phases. For example, La ₂ CuO ₄ is a semiconductor, but when it forms an alloy with Sr it becomes a superconductor (La _1-x Sr _x ) ₂ CuO ₄ .

Physics textbooks give a semiconductor definition as a material with an electrical resistance of 10 ^-4 to 10 ⁷ Ω · m. An alternative definition is also possible. The width of the forbidden band of a semiconductor is from 0 to 3 eV. Metals and semimetals are materials with zero energy break, and substances in which it exceeds 3 eV are called insulators. There are exceptions. For example, a semiconductor diamond has a forbidden band of width 6 eV, semi-insulating GaAs - 1.5 eV. GaN, a material for optoelectronic devices in the blue region, has a forbidden band of width 3.5 eV.

Energy gap

The valent orbitals of atoms in the crystal lattice are divided into two groups of energy levels-a free zone located at the highest level and determining the electrical conductivity of semiconductors, and a valence band located lower. These levels, depending on the symmetry of the crystal lattice and the composition of the atoms, may intersect or be spaced apart. In the latter case, there is an energy gap between the zones or, in other words, the forbidden zone.

The location and filling of the levels determines the conductive properties of the substance. On this basis, substances are divided into conductors, insulators and semiconductors. The width of the forbidden band of a semiconductor varies within the range of 0.01-3 eV, the dielectric energy gap exceeds 3 eV. Metals are not due to overlapping levels of energy gaps.

Semiconductors and dielectrics, in contrast to metals, have a valence band filled with electrons, and the nearest free zone or conduction band is fenced off from the valence energy gap-the portion of forbidden electron energies.

In dielectrics of thermal energy or a small electric field is not enough to make a jump through this gap, electrons do not enter the conduction band. They are not able to move around the crystal lattice and become carriers of electric current.

To excite electrical conductivity, an electron at the valence level needs to be given energy that would be enough to overcome the energy gap. Only by absorbing an amount of energy not less than the magnitude of the energy gap, the electron will go from the valence level to the conductivity level.

In the event that the width of the energy gap exceeds 4 eV, the excitation of the conductivity of the semiconductor by irradiation or heating is practically impossible - the excitation energy of the electrons at the melting temperature proves insufficient for a jump through the energy-discontinuity zone. When heated, the crystal melts before the appearance of electronic conduction. Such substances include quartz (dE = 5.2 eV), diamond (dE = 5.1 eV), many salts.

Impurity and intrinsic conductivity of semiconductors

Pure semiconductor crystals have intrinsic conductivity. Such semiconductors are called proprietary. The intrinsic semiconductor contains an equal number of holes and free electrons. When heated, the intrinsic conductivity of semiconductors increases. At a constant temperature, a state of dynamic equilibrium arises between the number of electron-hole pairs formed and the number of recombining electrons and holes that remain constant under the given conditions.

The presence of impurities has a significant effect on the electrical conductivity of semiconductors. Adding them makes it possible to greatly increase the number of free electrons with a small number of holes and increase the number of holes with a small number of electrons at the level of conductivity. Impurity semiconductors are conductors that have impurity conductivity.

Impurities, which easily give up electrons, are called donor ones. Donor impurities can be chemical elements with atoms, the valence levels of which contain more electrons than the atoms of the base material. For example, phosphorus and bismuth are donor impurities of silicon.

The energy necessary for the electron to jump into the conduction region is called activation energy. The impurity semiconductors need much less of it than the main substance. With slight heating or illumination, electrons of impurity semiconductor atoms are predominantly released. The place of the electron that left the atom occupies a hole. But there is practically no recombination of electrons into holes. Hole conductivity of the donor is negligible. This is because a small number of impurity atoms does not allow free electrons to often approach the hole and occupy it. The electrons are near the holes, but they can not fill them because of the lack of energy.

The insignificant addition of the donor impurity by several orders increases the number of conduction electrons in comparison with the number of free electrons in the intrinsic semiconductor. The electrons here are the main carriers of the charges of atoms of impurity semiconductors. These substances are classified as n-type semiconductors.

The impurities that bind electrons of a semiconductor, increasing the number of holes in it, are called acceptor ones. Acceptor impurities are chemical elements with a smaller number of electrons at the valence level than in the base semiconductor. Boron, gallium, indium are acceptor impurities for silicon.

The characteristics of the semiconductor are dependent on the defects of its crystal structure. This is the reason for the need to grow extremely pure crystals. The conductivity parameters of the semiconductor are controlled by the addition of alloying additives. The silicon crystals are doped with phosphorus (element V of the subgroup), which is a donor to create an n-type silicon crystal. To obtain a crystal with hole conductivity, a boron acceptor is introduced into silicon. Semiconductors with a compensated Fermi level to move it to the middle of the forbidden band are created in a similar way.

Single-element semiconductors

The most common semiconductor is, of course, silicon. Together with germanium, it became the prototype of a wide class of semiconductors with similar crystal structures.

The structure of Si and Ge crystals is the same as that of diamond and α-tin. In it, each atom is surrounded by 4 nearest atoms, which form a tetrahedron. Such coordination is called fourfold. Crystals with a tetradic bond have become basic for the electronics industry and play a key role in modern technology. Some elements of the V and VI groups of the periodic table are also semiconductors. Examples of semiconductors of this type are phosphorus (P), sulfur (S), selenium (Se), and tellurium (Te). In these semiconductors, atoms can have triple (P), double (S, Se, Te) or quadruple coordination. As a result, such elements can exist in several different crystal structures, and can also be obtained in the form of glass. For example, Se was grown in monoclinic and trigonal crystal structures or in the form of glass (which can also be considered a polymer).

- The diamond has excellent thermal conductivity, excellent mechanical and optical characteristics, high mechanical strength. The width of the energy gap is dE = 5.47 eV.

- Silicon - a semiconductor used in solar batteries, and in amorphous form - in thin-film solar cells. It is the most used semiconductor in photocells, easy to manufacture, has good electrical and mechanical properties. DE = 1.12 eV.

- Germanium - a semiconductor used in gamma spectroscopy, high-efficiency photocells. Used in the first diodes and transistors. Requires less cleaning than silicon. DE = 0.67 eV.

- Selenium - a semiconductor, which is used in selenium rectifiers, which have high radiation resistance and the ability to self-repair.

Two-element connections

The properties of semiconductors formed by elements of groups 3 and 4 of the Mendeleyev table recall the properties of substances in group 4. The transition from 4 groups of elements to 3-4 gr compounds. Makes the bonds partially ionic due to the transfer of the electron charge from the atom of group 3 to the atom of group 4. Ionicity changes the properties of semiconductors. It is the cause of an increase in the Coulomb interionic interaction and energy of the energy discontinuity of the electron band structure. An example of a binary compound of this type is indium antimonide InSb, gallium arsenide GaAs, gallium antimonide GaSb, indium phosphide InP, aluminum antimonide AlSb, gallium phosphide GaP.

Ionicity increases, and its value grows even more in compounds of substances of 2-6 groups, such as cadmium selenide, zinc sulphide, cadmium sulfide, cadmium telluride, zinc selenide. As a result, in most compounds of groups 2-6, the forbidden zone is wider than 1 eV, except for mercury compounds. Mercury telluride is a semiconductor without an energy gap, a semimetal, like α-tin.

Semiconductors of 2-6 groups with a large energy gap find application in the production of lasers and displays. Binary compounds of 2-6 groups with a narrowed energy gap are suitable for infrared receivers. Binary compounds of the elements of groups 1-7 (copper bromide CuBr, silver iodide AgI, copper chloride CuCl) have a forbidden zone wider than 3 eV due to high ionicity. They are actually not semiconductors, but insulators. The increase in the binding energy of the crystal due to Coulomb interionic interaction promotes the structuring of rock salt atoms with sixfold rather than quadratic coordination. Compounds of the 4-6 groups - lead sulfide and lead telluride, tin sulfide - are also semiconductors. The degree of ionicity of these substances also contributes to the formation of sixfold coordination. Significant ionicity does not prevent them from having very narrow forbidden bands, which allows them to be used to receive infrared radiation. Gallium nitride is a compound of 3-5 groups with a wide energy gap, found application in semiconductor lasers and light-emitting diodes working in the blue part of the spectrum.

- GaAs, gallium arsenide is the second semiconductor that is in demand after silicon, usually used as a substrate for other conductors, for example, GaInNAs and InGaAs, in IR-LEDs, high-frequency microcircuits and transistors, high-efficiency photocells, laser diodes, nuclear-radiation detectors. DE = 1.43 eV, which allows to increase the power of devices in comparison with silicon. Brittle, contains more impurities, is complicated in manufacturing.

- ZnS, zinc sulphide - zinc salt of hydrogen sulfide with a band gap of 3.54 and 3.91 eV, is used in lasers and as a phosphor.

- SnS, tin sulfide is a semiconductor used in photoresistors and photodiodes, dE = 1.3 and 10 eV.

Oxides

Metal oxides are predominantly excellent insulators, but there are exceptions. Examples of semiconductors of this type are nickel oxide, copper oxide, cobalt oxide, copper dioxide, iron oxide, europium oxide, zinc oxide. Since copper dioxide exists in the form of a cuprite mineral, its properties have been intensively studied. The procedure for growing semiconductors of this type is not yet fully understood, therefore their application is still limited. The exception is zinc oxide (ZnO), compound 2-6 groups, used as a converter and in the production of adhesive tapes and patches.

The situation changed drastically after superconductivity was discovered in many compounds of copper and oxygen. The first high-temperature superconductor, discovered by Mueller and Bednorz, was a compound based on a La ₂ CuO ₄ semiconductor with an energy gap of 2 eV. By replacing the trivalent lanthanum with bivalent barium or strontium, the carriers of the hole charge are introduced into the semiconductor. Achieving the necessary hole concentration converts La ₂ CuO ₄ into a superconductor. At the present time, the highest temperature of the transition to the superconducting state belongs to the compound HgBaCa ₂ Cu ₃ O ₈ . At high pressures, its value is 134 K.

ZnO, zinc oxide, used in varistors, blue LEDs, gas sensors, biological sensors, window coatings to reflect infrared light, as a conductor in LCD displays and solar panels. DE = 3.37 eV.

Layered crystals

Double compounds like lead diiodide, gallium selenide and molybdenum disulphide are distinguished by the layered structure of the crystal. Covalent bonds of considerable strength act in the layers, much stronger than the van der Waals bonds between the layers themselves. Semiconductors of this type are interesting in that electrons behave in layers quasi-two-dimensional. The interaction of the layers is changed by introducing third-party atoms - by intercalation.

MoS _2, molybdenum disulphide is used in high-frequency detectors, rectifiers, memristors, transistors. DE = 1.23 and 1.8 eV.

Organic Semiconductors

Examples of semiconductors based on organic compounds are naphthalene, polyacetylene (CH ₂ ) _n , anthracene, polydiacetylene, phthalocyanides, polyvinylcarbazole. Organic semiconductors have an advantage over inorganic: they are easy to impart the necessary qualities. Substances with conjugated bonds of the type -C = С-С =, have a significant optical nonlinearity and, owing to this, are used in optoelectronics. In addition, the energy-break zones of organic semiconductors are altered by a change in the compound formula, which is much easier than for conventional semiconductors. Crystalline allotropes of fullerene carbon, graphene, and nanotubes are also semiconductors.

- Fullerene has a structure in the form of a convex closed polytope from an even number of carbon atoms. A doping of fullerene C _{60 with an} alkali metal turns it into a superconductor.

- Graphene is formed by a monatomic carbon layer, connected to a two-dimensional hexagonal lattice. Has a record thermal conductivity and electron mobility, high rigidity

- Nanotubes are rolled into a tube of graphite plates, having several nanometers in diameter. These forms of carbon have a great prospect in nanoelectronics. Depending on the adhesion, metallic or semiconductor qualities can be exhibited.

Magnetic semiconductors

Compounds with magnetic ions of europium and manganese have interesting magnetic and semiconductor properties. Examples of semiconductors of this type are europium sulphide, europium selenide and solid solutions like Cd _1-x Mn _x Te. The content of magnetic ions influences the way in which magnetic properties such as antiferromagnetism and ferromagnetism manifest themselves in substances. Semi-magnetic semiconductors are solid magnetic solutions of semiconductors that contain magnetic ions in a small concentration. Such solid solutions attract attention by their prospects and great potential for possible applications. For example, unlike non-magnetic semiconductors, they can achieve a million times greater Faraday rotation.

Strong magneto-optical effects of magnetic semiconductors make it possible to use them for optical modulation. Perovskites, like Mn _0.7 Ca _0.3 O _{3, have} their properties superior to the metal-semiconductor transition, the direct dependence of which on the magnetic field has a consequence of the phenomenon of giant magneto-resistivity. They are used in radio engineering, optical devices, which are controlled by a magnetic field, in waveguides of microwave devices.

Semiconductor ferroelectrics

This type of crystal is characterized by the presence of electrical moments in them and the appearance of spontaneous polarization. For example, PbTiO ₃ lead titanate semiconductors, BaTiO ₃ barium titanate, GeTe telluride, SnTe telluride, which have ferroelectric properties at low temperatures, have such properties. These materials are used in nonlinear optical, memory devices and piezoelectric sensors.

Variety of semiconductor materials

In addition to the semiconductor substances mentioned above, there are many others that do not fall under any of the listed types. The compounds of the elements of formula 1-3-5 ₂ (AgGaS ₂ ) and 2-4-5 ₂ (ZnSiP ₂ ) form crystals in the structure of chalcopyrite. The bonds of the compounds are tetrahedral, analogous to 3-5 and 2-6 semiconductor semiconductors with a crystal structure of zinc blende. The compounds that form elements of Group 5 and 6 semiconductors (like As ₂ Se ₃ ) are semiconducting in the form of a crystal or glass. Chalcogenides of bismuth and antimony are used in semiconductor thermoelectric generators. The properties of semiconductors of this type are extremely interesting, but they have not gained popularity due to limited application. However, the fact that they exist confirms the presence of even before the end of the unexplored areas of semiconductor physics.