The most famous semiconductor is silicon (Si). But besides him, there are many others. An example is such natural semiconductor materials as zinc blende (ZnS), cuprite (Cu2O), galena (PbS) and many others. The semiconductor family, including laboratory-synthesized semiconductors, is one of the most versatile classes of materials known to man.
Characterization of semiconductors
Of the 104 elements of the periodic table, 79 are metals, 25 are non-metals, of which 13 chemical elements have 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. This is how semiconductors differ from metals: the resistance of the metal increases in proportion to the increase in temperature.
Another difference between a semiconductor and a metal is that the resistance of a semiconductorfalls under the action of light, while the latter does not affect the metal. The conductivity of semiconductors also changes when a small amount of impurity is introduced.
Semiconductors are found among chemical compounds with a variety of crystal structures. These can be elements such as silicon and selenium, or binary compounds such as gallium arsenide. Many organic compounds, such as polyacetylene (CH)n, are semiconductor materials. Some semiconductors exhibit magnetic (Cd1-xMnxTe) or ferroelectric properties (SbSI). Others with sufficient doping become superconductors (GeTe and SrTiO3). Many of the recently discovered high temperature superconductors have non-metallic semiconducting phases. For example, La2CuO4 is a semiconductor, but when alloyed with Sr it becomes a superconductor (La1-x Srx)2CuO4.
Physics textbooks define a semiconductor as a material with electrical resistance from 10-4 to 107 Ohm·m. An alternative definition is also possible. The band gap of a semiconductor is from 0 to 3 eV. Metals and semimetals are materials with a zero energy gap, and substances in which it exceeds 3 eV are called insulators. There are also exceptions. For example, semiconductor diamond has a band gap of 6 eV, semi-insulating GaAs - 1.5 eV. GaN, a material for optoelectronic devices in the blue region, has a band gap of 3.5 eV.
Energy gap
The valence orbitals of atoms in the crystal lattice are divided into two groups of energy levels - the free zone located at the highest level and determining the electrical conductivity of semiconductors, and the valence band located below. These levels, depending on the symmetry of the crystal lattice and the composition of atoms, can intersect or be located at a distance from each other. In the latter case, an energy gap appears between the zones, or, in other words, a 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 semiconductor band gap width varies within 0.01–3 eV, the energy gap of the dielectric exceeds 3 eV. Metals do not have energy gaps due to overlapping levels.
Semiconductors and dielectrics, in contrast to metals, have a valence band filled with electrons, and the nearest free band, or conduction band, is fenced off from the valence band by an energy gap - a region of forbidden electron energies.
In dielectrics, thermal energy or an insignificant 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 along the crystal lattice and become carriers of electric current.
To excite electrical conductivity, an electron at the valence level must be given energy that would be enough to overcome the energygap. Only when absorbing an amount of energy not less than the value of the energy gap, the electron will move from the valence level to the conduction level.
In the event that the width of the energy gap exceeds 4 eV, excitation of semiconductor conductivity by irradiation or heating is practically impossible - the excitation energy of electrons at the melting temperature is insufficient to jump through the energy gap zone. When heated, the crystal will melt until electronic conduction occurs. These substances include quartz (dE=5.2 eV), diamond (dE=5.1 eV), many s alts.
Impurity and intrinsic conductivity of semiconductors
Pure semiconductor crystals have their own conductivity. Such semiconductors are called intrinsic. An 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 in the number of electron-hole pairs formed and the number of recombining electrons and holes, which remain constant under given conditions.
The presence of impurities has a significant impact 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 to increase the number of holes with a small number of electrons at the conduction level. Impurity semiconductors are conductors with impurity conductivity.
Impurities that easily donate electrons are called donor impurities. Donor impurities can be chemical elements with atoms whose valence levels contain more electrons than the atoms of the base substance. For example, phosphorus and bismuth are silicon donor impurities.
The energy needed to jump an electron into the conduction region is called the activation energy. Impurity semiconductors need much less of it than the base material. With a slight heating or illumination, it is predominantly the electrons of the atoms of the impurity semiconductors that are released. The place of the electron leaving the atom is occupied by a hole. But the recombination of electrons into holes practically does not occur. The hole conductivity of the donor is negligible. This is because the small number of impurity atoms does not allow free electrons to often approach the hole and occupy it. Electrons are near holes, but are not able to fill them due to an insufficient energy level.
Insignificant addition of a donor impurity by several orders of magnitude increases the number of conduction electrons compared to the number of free electrons in the intrinsic semiconductor. Electrons here are the main charge carriers of atoms of impurity semiconductors. These substances are classified as n-type semiconductors.
Impurities that bind the electrons of a semiconductor, increasing the number of holes in it, are called acceptor. Acceptor impurities are chemical elements with fewer electrons at the valence level than the base semiconductor. Boron, gallium, indium - acceptorimpurities for silicon.
The characteristics of a semiconductor depend on the defects in its crystal structure. This is the reason for the need to grow extremely pure crystals. The semiconductor conductivity parameters are controlled by adding dopants. Silicon crystals are doped with phosphorus (subgroup V element), 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 band gap are created in a similar way.
Single cell semiconductors
The most common semiconductor is, of course, silicon. Together with germanium, it became the prototype for 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. This coordination is called quadruple. Tetra-bonded crystals have become the basis of the electronics industry and play a key role in modern technology. Some elements of groups V and VI 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 three-fold (P), two-fold (S, Se, Te) or four-fold coordination. As a result, similar elements can exist in several differentcrystal structures, and also be obtained in the form of glass. For example, Se has been grown in monoclinic and trigonal crystal structures or as glass (which can also be considered a polymer).
- Diamond has excellent thermal conductivity, excellent mechanical and optical characteristics, high mechanical strength. Energy gap width - dE=5.47 eV.
- Silicon is a semiconductor used in solar cells and in amorphous form in thin-film solar cells. It is the most used semiconductor in solar cells, easy to manufacture, and has good electrical and mechanical properties. dE=1.12 eV.
- Germanium is a semiconductor used in gamma spectroscopy, high-performance photovoltaic cells. Used in the first diodes and transistors. Requires less cleaning than silicon. dE=0.67 eV.
- Selenium is a semiconductor that is used in selenium rectifiers, which have high radiation resistance and self-healing ability.
Two-element compounds
The properties of semiconductors formed by elements of the 3rd and 4th groups of the periodic table resemble the properties of substances of the 4th group. Transition from group 4 elements to compounds 3–4 gr. makes the bonds partially ionic due to the transfer of electron charge from the atom of group 3 to the atom of group 4. Ionicity changes the properties of semiconductors. It is the reason for the increase in the Coulomb interion interaction and the energy of the energy band gapelectron structures. 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 groups 2-6, such as cadmium selenide, zinc sulfide, cadmium sulfide, cadmium telluride, zinc selenide. As a result, most compounds of groups 2-6 have a band gap wider than 1 eV, except for mercury compounds. Mercury telluride is a semiconductor without an energy gap, a semimetal, like α-tin.
Group 2-6 semiconductors with a large energy gap are used in the production of lasers and displays. Binary connections of 2-6 groups with a narrowed energy gap are suitable for infrared receivers. Binary compounds of elements of groups 1–7 (copper bromide CuBr, silver iodide AgI, copper chloride CuCl) due to their high ionicity have a band gap wider than 3 eV. They are actually not semiconductors, but insulators. The increase in the anchoring energy of the crystal due to the Coulomb interionic interaction contributes to the structuring of rock s alt atoms with sixfold rather than quadratic coordination. Compounds of groups 4–6 - lead sulfide and telluride, tin sulfide - are also semiconductors. The degree of ionicity of these substances also contributes to the formation of six-fold coordination. Significant ionicity does not prevent them from having very narrow band gaps, which allows them to be used to receive infrared radiation. Gallium nitride - a compound of 3-5 groups with a wide energy gap, has found application in semiconductorlasers and LEDs operating in the blue part of the spectrum.
- GaAs, gallium arsenide, is the second most used semiconductor after silicon, commonly used as a substrate for other conductors such as GaInNAs and InGaAs, in IR diodes, high-frequency microcircuits and transistors, high-efficiency solar cells, laser diodes, detectors nuclear cure. dE=1.43 eV, which makes it possible to increase the power of devices compared to silicon. Fragile, contains more impurities, difficult to manufacture.
- ZnS, zinc sulfide - zinc s alt of hydrosulfide acid with a band gap of 3.54 and 3.91 eV, used in lasers and as a phosphor.
- SnS, tin sulfide - a semiconductor used in photoresistors and photodiodes, dE=1, 3 and 10 eV.
Oxides
Metal oxides are mostly excellent insulators, but there are exceptions. Examples of semiconductors of this type are nickel oxide, copper oxide, cob alt oxide, copper dioxide, iron oxide, europium oxide, zinc oxide. Since copper dioxide exists as the mineral cuprite, its properties have been extensively researched. The procedure for growing semiconductors of this type is not yet fully understood, so their application is still limited. The exception is zinc oxide (ZnO), a group 2-6 compound used as a converter and in the production of adhesive tapes and plasters.
The situation changed dramatically after superconductivity was discovered in many compounds of copper with oxygen. FirstThe high-temperature superconductor discovered by Müller and Bednorz was a compound based on the semiconductor La2CuO4 with an energy gap of 2 eV. By replacing trivalent lanthanum with divalent barium or strontium, hole charge carriers are introduced into the semiconductor. Reaching the required concentration of holes turns La2CuO4 into a superconductor. At present, the highest transition temperature to the superconducting state belongs to the compound HgBaCa2Cu3O8. At high pressure, its value is 134 K.
ZnO, zinc oxide, is used in varistors, blue LEDs, gas sensors, biological sensors, window coatings to reflect infrared light, as a conductor in LCDs and solar panels. dE=3.37 eV.
Layer crystals
Double compounds like lead diiodide, gallium selenide and molybdenum disulfide are characterized by a layered crystal structure. Covalent bonds of significant 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 quasi-two-dimensionally in layers. The interaction of the layers is changed by the introduction of foreign atoms - intercalation.
MoS2, molybdenum disulfide 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 - naphthalene, polyacetylene(CH2) , anthracene, polydiacetylene, phthalocyanides, polyvinylcarbazole. Organic semiconductors have an advantage over inorganic ones: it is easy to impart the desired qualities to them. Substances with conjugated bonds of the –С=С–С=type have significant optical nonlinearity and, due to this, are used in optoelectronics. In addition, the energy discontinuity zones of organic semiconductors are changed by changing the compound formula, which is much easier than that of conventional semiconductors. Crystalline allotropes of carbon fullerene, graphene, nanotubes are also semiconductors.
- Fullerene has a structure in the form of a convex closed polyhedron of an even number of carbon atoms. And doping fullerene C60 with an alkali metal turns it into a superconductor.
- Graphene is formed by a monatomic layer of carbon connected into a two-dimensional hexagonal lattice. It has a record thermal conductivity and electron mobility, high rigidity
- Nanotubes are graphite plates rolled into a tube, having a few nanometers in diameter. These forms of carbon hold great promise in nanoelectronics. May exhibit metallic or semi-conductive qualities depending on coupling.
Magnetic semiconductors
Compounds with magnetic europium and manganese ions have curious magnetic and semiconductor properties. Examples of semiconductors of this type are europium sulfide, europium selenide, and solid solutions likeCd1-xMnxTe. The content of magnetic ions influences how magnetic properties such as antiferromagnetism and ferromagnetism appear in substances. Semimagnetic semiconductors are solid magnetic solutions of semiconductors that contain magnetic ions in a small concentration. Such solid solutions attract attention due to their promise and great potential for possible applications. For example, unlike non-magnetic semiconductors, they can achieve a million times greater Faraday rotation.
The strong magneto-optical effects of magnetic semiconductors allow them to be used for optical modulation. Perovskites like Mn0, 7Ca0, 3O3, outperform the metal- a semiconductor, the direct dependence of which on the magnetic field results in the phenomenon of giant magnetoresistance. They are used in radio engineering, optical devices that are controlled by a magnetic field, in waveguides of microwave devices.
Semiconductor ferroelectrics
This type of crystals is distinguished by the presence of electric moments in them and the occurrence of spontaneous polarization. For example, semiconductors such as lead titanate PbTiO3, barium titanate BaTiO3, germanium telluride GeTe, tin telluride SnTe, which at low temperatures have properties ferroelectric. These materials are used in non-linear optical, memory and piezo sensors.
Variety of semiconductor materials
In addition to the abovesemiconductor substances, there are many others that do not fall under any of the listed types. Connections of elements according to the formula 1-3-52 (AgGaS2) and 2-4-52(ZnSiP2) form crystals in the chalcopyrite structure. The bonds of the compounds are tetrahedral, similar to semiconductors of groups 3–5 and 2–6 with the crystal structure of zinc blende. The compounds that form the elements of semiconductors of groups 5 and 6 (like As2Se3) are semiconductor in the form of a crystal or glass. Bismuth and antimony chalcogenides are used in semiconductor thermoelectric generators. The properties of semiconductors of this type are extremely interesting, but they have not gained popularity due to their limited application. However, the fact that they exist confirms the existence of areas of semiconductor physics that have not yet been fully explored.
