The most famous semiconductor is silicon (Si). But besides 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. The family of semiconductors, including those synthesized in laboratories, is one of the most versatile classes of materials known to man.
Semiconductor Characteristics
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 conductivity increases significantly with increasing temperature. At low temperatures, they behave like dielectrics, and at high temperatures they behave like conductors. In this, semiconductors differ from metals: the resistance of a metal increases in proportion to an increase in temperature.
Another difference between a semiconductor and a metal is that the resistance of a semiconductor falls under the influence of light, while the latter does not affect 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. These can be elements such as silicon and selenium, or double compounds such as gallium arsenide. Many organic compounds, such as 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 non-metallic semiconducting phases. For example, La 2 CuO 4 is a semiconductor, but when an alloy with Sr is formed, it becomes the (La 1-x Sr x ) 2 CuO 4 superconductor.
Physics textbooks define a semiconductor as a material with an electrical resistance of 10 -4 to 10 7 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 exceptions. For example, a semiconductor diamond has a band gap of 6 eV wide, a semi-insulating GaAs of 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 zone located below. These levels, depending on the symmetry of the crystal lattice and the composition of the atoms, can intersect or be located at a distance from each other. In the latter case, an energy gap or, in other words, a forbidden zone arises between the zones.
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 varies between 0.01–3 eV, and the dielectric energy gap 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 zone, or conduction band, is fenced off from the valence energy gap - a region of the 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 fall into the conduction band. They are not able to move around the crystal lattice and become carriers of electric current.
In order to excite electrical conductivity, the electron at the valence level must be given the energy that would be enough to bridge the energy gap. Only when the amount of energy is absorbed that is not less than the magnitude of the energy gap does the electron transfer 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 semiconductor’s conductivity by irradiation or heating is practically impossible — the electron excitation energy at the melting temperature is insufficient to jump through the energy gap. When heated, the crystal will melt until electronic conductivity occurs. These substances include quartz (dE = 5.2 eV), diamond (dE = 5.1 eV), many salts.
Impurity and intrinsic conductivity of semiconductors
Pure semiconductor crystals have their own conductivity. Such semiconductors are called proprietary. 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 of the number of generated electron-hole pairs and the number of recombining electrons and holes, which remain constant under these conditions.
The presence of impurities has a significant effect on the electrical conductivity of semiconductors. Adding them can significantly 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 conductivity level. Impurity semiconductors are impurity conductors.
Impurities that easily give electrons are called donor impurities. Donor impurities can be chemical elements with atoms, the valence levels of which contain a greater number of electrons than the atoms of the basic substance. For example, phosphorus and bismuth are donor impurities of silicon.
The energy required for the electron to jump into the conduction region is called the activation energy. Impurity semiconductors need much less of it than the main substance. With a little heating or lighting, mainly the electrons of the atoms of impurity semiconductors are released. A hole takes the place of an electron that has left an atom. But the recombination of electrons into holes practically does not occur. The hole conductivity of the donor is negligible. This is because a small number of impurity atoms does not allow free electrons to often approach and occupy a hole. Electrons are located near the holes, but are not able to fill them due to insufficient energy level.
An insignificant addition of a donor impurity increases the number of conduction electrons by several orders of magnitude compared to the number of free electrons in a semiconductor. Electrons here are the main charge carriers of atomic impurity semiconductors. These substances are classified as n-type semiconductors.
The impurities that bind the electrons of the semiconductor, increasing the number of holes in it, are called acceptor. Acceptor impurities are chemical elements with fewer electrons at the valence level than that of the base semiconductor. Boron, gallium, indium - acceptor impurities for silicon.
The characteristics of a semiconductor are dependent on defects in its crystal structure. This is the reason for the need to grow extremely pure crystals. The semiconductor conductivity parameters are controlled by the addition of dopants. 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 are created in a similar way to move it to the middle of the forbidden zone.
Singleton Semiconductors
The most common semiconductor is, of course, silicon. Together with Germany, he 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 that form a tetrahedron. This coordination is called fourfold. Tetradically coupled crystals have become fundamental to 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 threefold (P), twofold (S, Se, Te) or fourfold coordination. As a result, such elements can exist in several different crystalline 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).
- 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 is a semiconductor used in solar panels, and in an amorphous form - in thin-film solar panels. It is the most used semiconductor in solar cells, it is easy to manufacture, has good electrical and mechanical qualities. dE = 1.12 eV.
- Germanium - a semiconductor used in gamma spectroscopy, high-efficiency photocells. Used in the first diodes and transistors. It requires less cleaning than silicon. dE = 0.67 eV.
- Selenium - a semiconductor that is used in selenium rectifiers with high radiation resistance and the ability to self-healing.
Two-element compounds
The properties of semiconductors formed by elements of groups 3 and 4 of the periodic table resemble the properties of substances of group 4. Transition from 4 groups of elements to compounds 3-4 gr. makes the bonds partially ionic due to electron charge transfer from an atom of group 3 to an atom of group 4. Ionicity changes the properties of semiconductors. It is the reason for the increase in the Coulomb interionic interaction and the energy of the energy gap of the electron band structure. An example of a binary compound of this type is InSb indium antimonide, GaAs gallium arsenide, GaSb gallium antimonide, InP indium phosphide, AlSb aluminum antimonide, GaP gallium phosphide.
The 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, in most compounds of groups 2–6 the band gap 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 groups 2-6 with a large energy gap are used 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 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 cohesive energy of the crystal due to the Coulomb interionic interaction promotes the structuring of rock salt atoms with six-fold rather than quadratic coordination. Compounds of groups 4–6 — lead sulfide and telluride, and 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 forbidden zones, 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 semiconductor lasers and LEDs operating in the blue part of the spectrum.
- GaAs, gallium arsenide - the second most popular semiconductor after silicon, usually used as a substrate for other conductors, for example, GaInNAs and InGaAs, in IR network diodes, high-frequency circuits and transistors, high-efficiency photocells, laser diodes, nuclear cure detectors. dE = 1.43 eV, which makes it possible to increase the power of devices in comparison with silicon. Fragile, contains more impurities, difficult to manufacture.
- ZnS, zinc sulfide - zinc salt of hydrogen sulfide 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 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 as a cuprite mineral, its properties have been extensively studied. The procedure for growing semiconductors of this type is not yet fully understood, so their use is still limited. The exception is zinc oxide (ZnO), a compound of 2-6 groups, used as a converter and in the production of adhesive tapes and adhesives.
The situation changed dramatically after superconductivity was discovered in many compounds of copper and oxygen. The first high-temperature superconductor discovered by Müller and Bednorz was a compound based on the La 2 CuO 4 semiconductor with an energy gap of 2 eV. Substituting trivalent lanthanum with divalent barium or strontium, hole charge carriers are introduced into the semiconductor. Achieving the required hole concentration turns La 2 CuO 4 into a superconductor. At present, the highest transition temperature to the superconducting state belongs to the HgBaCa 2 Cu 3 O 8 compound. 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, like a conductor in LCD displays and solar panels. dE = 3.37 eV.
Layered crystals
Binary compounds like lead diiodide, gallium selenide and molybdenum disulfide 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 the electrons behave quasi-two-dimensionally in the layers. The interaction of the layers is changed by the introduction of external atoms - intercalation.
MoS 2, 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 are naphthalene, polyacetylene (CH 2 ) n , anthracene, polydiacetylene, phthalocyanides, polyvinylcarbazole. Organic semiconductors have an advantage over inorganic semiconductors: they are easily given the necessary qualities. Substances with conjugated bonds of the form – = – = have significant optical nonlinearity and, due to this, are used in optoelectronics. In addition, the energy gap zones of organic semiconductors are changed by changing the compound formula, which is much easier than with conventional semiconductors. Crystalline carbon allotropes 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 the doping of fullerene C 60 with an alkali metal turns it into a superconductor.
- Graphene is formed by a monatomic layer of carbon connected to 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 several nanometers in diameter. These forms of carbon have a great promise in nanoelectronics. Depending on the clutch, they may exhibit metallic or semiconductor properties.
Magnetic Semiconductors
Compounds with magnetic ions of europium and manganese have interesting magnetic and semiconductor properties. Examples of semiconductors of this type are europium sulfide, europium selenide and solid solutions like Cd 1-x Mn x Te. The content of magnetic ions affects how such magnetic properties as antiferromagnetism and ferromagnetism manifest in substances. – , . . , , .
. , Mn 0,7 Ca 0,3 O 3, -, -. , , , -.
. , PbTiO 3 , BaTiO 3 , GeTe, SnTe, . -, .
In addition to the semiconductor substances mentioned above, there are many others that do not fall under any of the listed types. Compounds of elements according to the 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, similarly to the 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 As 2 Se 3 ) are semiconductors 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.