What is a semiconductor and what is it eaten with?
Semiconductor- a material without which the modern world of technology and electronics is unthinkable. Semiconductors exhibit properties of metals and non-metals under certain conditions. In terms of electrical resistivity, semiconductors occupy an intermediate position between good conductors and dielectrics. Semiconductor differs from conductors in the strong dependence of specific conductivity on the presence of impurity elements (impurity elements) in the crystal lattice and the concentration of these elements, as well as on temperature and exposure to various types of radiation.
Basic property of a semiconductor- increase in electrical conductivity with increasing temperature.
Semiconductors are substances whose band gap is on the order of several electron volts (eV). For example, diamond can be classified as a wide-gap semiconductor, and indium arsenide can be classified as a narrow-gap semiconductor. The band gap is the width of the energy gap between the bottom of the conduction band and the top of the valence band, in which there are no allowed states for the electron.
The magnitude of the band gap is important when generating light in LEDs and semiconductor lasers and determines the energy of the emitted photons.
Semiconductors include many chemical elements: Si silicon, Ge germanium, As arsenic, Se selenium, Te tellurium and others, as well as all kinds of alloys and chemical compounds, for example: silicon iodide, gallium arsenide, mercury tellurite, etc.). In general, almost all inorganic substances in the world around us are semiconductors. The most common semiconductor in nature is silicon, which, according to rough estimates, makes up almost 30% of the earth's crust.
Depending on whether an atom of an impurity element gives up an electron or captures it, impurity atoms are called donor or acceptor atoms. The donor and acceptor properties of an atom of an impurity element also depend on which atom of the crystal lattice it replaces and in which crystallographic plane it is embedded.
As mentioned above, the conductive properties of semiconductors strongly depend on temperature, and when the temperature reaches absolute zero (-273 ° C), semiconductors have the properties of dielectrics.
Based on the type of conductivity, semiconductors are divided into n-type and p-type
n-type semiconductor
Based on the type of conductivity, semiconductors are divided into n-type and p-type.
An n-type semiconductor has an impurity nature and conducts electric current like metals. Impurity elements that are added to semiconductors to produce n-type semiconductors are called donor elements. The term "n-type" comes from the word "negative", which refers to the negative charge carried by a free electron.
The theory of the charge transfer process is described as follows:
An impurity element, pentavalent As arsenic, is added to tetravalent Si silicon. During the interaction, each arsenic atom enters into a covalent bond with silicon atoms. But a fifth free arsenic atom remains, which has no place in saturated valence bonds, and it moves to a distant electron orbit, where less energy is needed to remove an electron from the atom. The electron breaks away and becomes free, capable of carrying charge. Thus, charge transfer is carried out by an electron, not a hole, that is, this type of semiconductor conducts electric current like metals.
Antimony Sb also improves the properties of one of the most important semiconductors - germanium Ge.
p-type semiconductor
A p-type semiconductor, in addition to the impurity base, is characterized by the hole nature of conductivity. The impurities that are added in this case are called acceptor impurities.
“p-type” comes from the word “positive,” which refers to the positive charge of the majority carriers.
For example, a small amount of trivalent indium atoms is added to a semiconductor, tetravalent Si silicon. In our case, indium will be an impurity element, the atoms of which establish a covalent bond with three neighboring silicon atoms. But silicon has one free bond while the indium atom does not have a valence electron, so it captures a valence electron from the covalent bond between neighboring silicon atoms and becomes a negatively charged ion, forming a so-called hole and, accordingly, a hole transition.
According to the same scheme, In ndium imparts hole conductivity to Ge germanium.
Investigating the properties of semiconductor elements and materials, studying the properties of contact between a conductor and a semiconductor, experimenting in the manufacture of semiconductor materials, O.V. Losev created the prototype of the modern LED in the 1920s.
There is nothing extraordinarily important or interesting in this article, just an answer to a simple question for “dummies”: what are the main properties that distinguish semiconductors from metals and dielectrics?
Semiconductors are materials (crystals, polycrystalline and amorphous materials, elements or compounds) with the existence of a band gap (between the conduction band and the valence band).
Electronic semiconductors are crystals and amorphous substances that, in terms of electrical conductivity, occupy an intermediate position between metals (σ = 10 4 ÷10 6 Ohm -1 cm -1) and dielectrics (σ = 10 -10 ÷10 -20 Ohm -1 cm -1). However, the given boundary values of conductivity are very arbitrary.
Band theory makes it possible to formulate a criterion that makes it possible to divide solids into two classes - metals and semiconductors (insulators). Metals are characterized by the presence of free levels in the valence band, to which electrons can move, receiving additional energy, for example, due to acceleration in an electric field. A distinctive feature of metals is that in their ground, unexcited state (at 0 K) they have conduction electrons, i.e. electrons that participate in ordered movement under the influence of an external electric field.
In semiconductors and insulators at 0 K, the valence band is completely populated, and the conduction band is separated from it by a band gap and does not contain carriers. Therefore, a not too strong electric field is not able to strengthen the electrons located in the valence band and transfer them to the conduction band. In other words, such crystals at 0 K should be ideal insulators. When the temperature increases or such a crystal is irradiated, electrons can absorb quanta of thermal or radiant energy sufficient to move into the conduction band. During this transition, holes appear in the valence band, which can also participate in the transfer of electricity. The probability of an electron transferring from the valence band to the conduction band is proportional to ( -Eg/ kT), Where Eg - width of the forbidden zone. With a large value Eg (2-3 eV) this probability turns out to be very small.
Thus, the division of substances into metals and non-metals has a very definite basis. In contrast, the division of nonmetals into semiconductors and dielectrics does not have such a basis and is purely conditional.
Previously, it was believed that substances with a band gap could be classified as dielectrics Eg≈ 2÷3 eV, but later it turned out that many of them are typical semiconductors. Moreover, it was shown that, depending on the concentration of impurities or excess (above the stoichiometric composition) atoms of one of the components, the same crystal can be both a semiconductor and an insulator. This applies, for example, to crystals of diamond, zinc oxide, gallium nitride, etc. Even such typical dielectrics as barium and strontium titanates, as well as rutile, upon partial reduction, acquire the properties of semiconductors, which is associated with the appearance of excess metal atoms in them.
The division of nonmetals into semiconductors and dielectrics also has a certain meaning, since a number of crystals are known whose electronic conductivity cannot be noticeably increased either by introducing impurities or by illumination or heating. This is due either to the very short lifetime of photoelectrons, or to the existence of deep traps in crystals, or to the very low mobility of electrons, i.e. with an extremely low speed of their drift in an electric field.
Electrical conductivity is proportional to the concentration n, the charge e and the mobility of charge carriers. Therefore, the temperature dependence of the conductivity of various materials is determined by the temperature dependences of the indicated parameters. For all electronic conductors charge e constant and independent of temperature. In most materials, the mobility value usually decreases slightly with increasing temperature due to an increase in the intensity of collisions between moving electrons and phonons, i.e. due to electron scattering by vibrations of the crystal lattice. Therefore, the different behavior of metals, semiconductors and dielectrics is mainly associated with the charge carrier concentration and its temperature dependence:
1) in metals, the concentration of charge carriers n is high and changes slightly with temperature changes. The variable included in the equation for electrical conductivity is mobility. And since mobility slightly decreases with temperature, electrical conductivity also decreases;
2) in semiconductors and dielectrics n usually increases exponentially with temperature. This rapid growth n makes the most significant contribution to changes in conductivity than a decrease in mobility. Therefore, electrical conductivity increases rapidly with increasing temperature. In this sense, dielectrics can be considered as a certain limiting case, since at ordinary temperatures the value n in these substances is extremely small. At high temperatures, the conductivity of individual dielectrics reaches the semiconductor level due to an increase n. The opposite is also observed - at low temperatures, some semiconductors become insulators.
Bibliography
- West A. Solid State Chemistry. Part 2 Per. from English - M.: Mir, 1988. - 336 p.
- Modern crystallography. T.4. Physical properties of crystals. - M.: Nauka, 1981.
Students of group 501 of the Faculty of Chemistry: Bezzubov S.I., Vorobyova N.A., Efimov A.A.
Semiconductor devices, which have a number of properties that make their use preferable to vacuum devices, are increasingly used in electronic technology. In recent years, characterized by progress in semiconductor electronics, devices based on new physical principles have been developed.
Semiconductors include many chemical elements, such as silicon, germanium, indium, phosphorus, etc., most oxides, sulfides, selenides and tellurides, some alloys, and a number of minerals. According to academician A.F. Ioffe, “semiconductors are almost the entire inorganic world around us.”
Semiconductors are crystalline, amorphous and liquid. In semiconductor technology, only crystalline semiconductors are usually used (single crystals with impurities of no more than one impurity atom per 1010 atoms of the main substance). Typically, semiconductors include substances that, in terms of electrical conductivity, occupy an intermediate position between metals and dielectrics (hence the origin of their name). At room temperature, their specific electrical conductivity ranges from 10-8 to 105 S/m (for metals - 106-108 S/m, for dielectrics - 10-8-10-13 S/m). The main feature of semiconductors is the increase in electrical conductivity with increasing temperature (for metals it falls). The electrical conductivity of semiconductors depends significantly on external influences: heating, irradiation, electric and magnetic fields, pressure, acceleration, as well as the content of even small amounts of impurities. The properties of semiconductors are well explained using the band theory of solids.
Atoms of all substances consist of a nucleus and electrons moving in a closed orbit around the nucleus. Electrons in an atom are grouped into shells. The main semiconductors used to create semiconductor devices - silicon and germanium - have a tetrahedral crystal lattice (has the shape of a regular triangular pyramid) (Fig. 16.1). The projection of the Ge structure onto a plane is shown in Fig. 16.2. Each valence electron, i.e., an electron located on the outer, unfilled, shell of an atom, in a crystal belongs not only to its own, but also to the nucleus of a neighboring atom. All atoms in the crystal lattice are located at the same distance from each other and are connected by covalent bonds (a bond between a pair of valence electrons of two atoms is called covalent; it is shown in Fig. 16.2 by two lines). These connections are strong; To break them, you need to apply energy from outside.
The electron energy W is discrete, or quantized, so the electron can only move in the orbit that corresponds to its energy. Possible values of electron energy can be represented on a diagram by energy levels (Fig. 16.3). The further away the orbit is from the nucleus, the greater the energy of the electron and the higher its energy level. Energy levels are separated by zones II, corresponding to the forbidden energy for electrons (forbidden zones). Since neighboring atoms in a solid are very close to each other, this causes a shift and splitting of energy levels, resulting in the formation of energy bands called allowed bands (I, III, IV in Fig. 16.3). The width of the allowed bands is usually several electron volts. In the energy band, the number of allowed levels is equal to the number of atoms in the crystal. Each allowed zone occupies a certain energy region and is characterized by minimum and maximum energy levels, which are called the bottom and ceiling of the zone, respectively.
Allowed zones in which there are no electrons are called free (I). The free zone, in which there are no electrons at a temperature of 0 K, but at a higher temperature they can be present, is called the conduction band.
It is located above the valence band (III) - the upper of the filled bands in which all energy levels are occupied by electrons at a temperature of 0 K.
In band theory, the division of solids into metals, semiconductors, and insulators is based on the band gap between the valence and conduction bands and the degree of filling of the allowed energy bands (Fig. 16.4). The band gap ΔWa is called the activation energy of intrinsic electrical conductivity. For metal ΔWa = 0 (Fig. 16.4, a); conventionally, at ΔWa ≤ 2 eV the crystal is a semiconductor (Fig. 16.4,6), at ΔWa ≥ 2 eV it is a dielectric (Fig. 16.4, c). Since the value of ΔWa in semiconductors is relatively small, it is enough to impart an energy comparable to the energy of thermal motion to the electron so that it moves from the valence band to the conduction band. This explains the peculiarity of semiconductors - an increase in electrical conductivity with increasing temperature.
Electrical conductivity of semiconductors. Intrinsic electrical conductivity. In order for a substance to have electrical conductivity, it must contain free charge carriers. Such charge carriers in metals are electrons. Semiconductors contain electrons and holes.
Let us consider the electrical conductivity of intrinsic semiconductors (i-type), i.e., substances that do not contain impurities and have no structural defects in the crystal lattice (empty sites, lattice shifts, etc.) At a temperature of 0 K, there are no free charge carriers in such a semiconductor. However, with increasing temperature (or other energetic influences, such as lighting), some of the covalent bonds can be broken and the valence electrons, becoming free, can move away from their atom (Fig. 16.5). The loss of an electron turns the atom into a positive ion. In bonds, in the place where the electron used to be, a free ("vacant") space appears - a hole. The charge of a hole is positive and in absolute value is equal to the charge of an electron.
The free space - a hole - can be filled by a valence electron of a neighboring atom, in whose place a new hole is formed in a covalent bond, etc. Thus, simultaneously with the movement of valence electrons, holes will also move. It should be borne in mind that in a crystal lattice the atoms are “rigidly” fixed at the nodes. The departure of an electron from an atom leads to ionization, and the subsequent movement of a hole means the alternate ionization of “stationary” atoms. If there is no electric field, conduction electrons undergo chaotic thermal motion. If a semiconductor is placed in an external electric field, then electrons and holes, continuing to participate in chaotic thermal motion, will begin to move (drift) under the influence of the field, which will create an electric current. In this case, electrons move against the direction of the electric field, and holes, like positive charges, move in the direction of the field. The electrical conductivity of a semiconductor resulting from the disruption of covalent bonds is called intrinsic electrical conductivity.
The electrical conductivity of semiconductors can also be explained using band theory. In accordance with it, all energy levels of the valence band at a temperature of 0 K are occupied by electrons. If the electrons are given an energy from the outside that exceeds the activation energy ΔWa, then some of the valence electrons will move to the conduction band, where they will become free, or conduction electrons. Due to the departure of electrons from the valence band, holes are formed in it, the number of which, naturally, is equal to the number of electrons that have left. The holes can be occupied by electrons whose energy corresponds to the energy of the valence band levels. Consequently, in the valence band, the movement of electrons causes holes to move in the opposite direction. Although electrons move around in the valence band, it is usually more convenient to consider the movement of holes.
The process of formation of a conduction electron–conduction hole pair is called the generation of a pair of charge carriers (1 in Fig. 16.6). We can say that the intrinsic electrical conductivity of a semiconductor is the electrical conductivity caused by the generation of conduction electron–conduction hole pairs. The resulting electron-hole pairs can disappear if the hole is filled with an electron: the electron will become unfree and lose the ability to move, and the excess positive charge of the atomic ion will be neutralized. In this case, both the hole and the electron disappear simultaneously. The process of reuniting an electron and a hole is called recombination (2 in Fig. 16.6). Recombination, in accordance with band theory, can be considered as the transition of electrons from the conduction band to free places in the valence band. Note that the transition of electrons from a higher energy level to a lower one is accompanied by the release of energy, which is either emitted in the form of light quanta (photons) or transferred to the crystal lattice in the form of thermal vibrations (phonons). The average lifetime of a pair of charge carriers is called the carrier lifetime. The average distance that a charge carrier travels during its lifetime is called the diffusion length of the charge carrier (Lр, - for holes, Ln - for electrons).
At a constant temperature (and in the absence of other external influences), the crystal is in a state of equilibrium: the number of generated pairs of charge carriers is equal to the number of recombined pairs. The number of charge carriers per unit volume, i.e. their concentration, determines the value of specific electrical conductivity. For an intrinsic semiconductor, the electron concentration ni is equal to the hole concentration pi (ni = pi).
Impurity electrical conductivity. If an impurity is introduced into a semiconductor, it will also have an impurity in addition to its own electrical conductivity. Impurity electrical conductivity can be electronic or hole. As an example, consider the case when an impurity of a pentavalent element, for example arsenic, is introduced into pure germanium (a tetravalent element) (Fig. 16.7, a). The arsenic atom is bonded in the germanium crystal lattice by covalent bonds. But only four valence electrons of arsenic can participate in the bond, and the fifth electron turns out to be “extra”, less strongly bound to the arsenic atom. In order to tear this electron away from an atom, much less energy is needed, so already at room temperature it can become a conduction electron without leaving a hole in the covalent bond. Thus, a positively charged impurity ion appears at a site of the crystal lattice, and a free electron appears in the crystal. Impurities whose atoms donate free electrons are called donors.
In Fig. Figure 16.7b shows the energy band diagram of a semiconductor with a donor impurity. In the band gap near the bottom of the conduction band, an allowed energy level (impurity, donor) is created, on which “extra” electrons are located at a temperature close to 0 K. To transfer an electron from an impurity level to the conduction band requires less energy than to transfer an electron from the valence band. The distance from the donor level to the bottom of the conduction band is called the ionization (activation) energy of donors ΔWand.
The introduction of a donor impurity into a semiconductor significantly increases the concentration of free electrons, while the hole concentration remains the same as it was in the native semiconductor. In such an impurity semiconductor, electrical conductivity is mainly due to electrons, it is called electronic, and semiconductors are called n-type semiconductors. Electrons in n-type semiconductors are the majority charge carriers (their concentration is high), and holes are minority carriers.
If an impurity of a trivalent element (for example, indium) is introduced into germanium, then one electron is not enough for indium to form an eight-electron covalent bond with germanium. One connection will remain empty. With a slight increase in temperature, an electron from a neighboring germanium atom can move into an unfilled valence bond, leaving a hole in its place (Fig. 16.8, a), which can also be filled with an electron, etc. Thus, the hole seems to move in the semiconductor. The impurity atom turns into a negative ion. Impurities whose atoms, when excited, are capable of accepting valence electrons from neighboring atoms, creating a hole in them, are called acceptors or acceptors.
In Fig. Figure 16.8b shows a diagram of the energy bands of a semiconductor with an acceptor impurity. An impurity energy level (acceptor) is created in the band gap near the top of the valence band. At temperatures close to 0 K, this level is free; with increasing temperature, it can be occupied by an electron in the valence band, in which a hole is formed after the electron leaves. The distance from the top of the valence band to the acceptor level is called the ionization (activation) energy of acceptors ΔWа. The introduction of an acceptor impurity into a semiconductor significantly increases the hole concentration, while the electron concentration remains the same as it was in the native semiconductor. In this impurity semiconductor, electrical conductivity is mainly due to holes, it is called hole conductivity, and semiconductors are called p-type semiconductors. For a p-type semiconductor, holes are the majority charge carriers, and electrons are minority charge carriers.
In impurity semiconductors, along with impurity electrical conductivity, there is also intrinsic conductivity, due to the presence of minority carriers. The concentration of minority carriers in an impurity semiconductor decreases as many times as the concentration of majority carriers increases, therefore for n-type semiconductors the relation nnpn = nipi = ni2 = pi2 is valid, and for p-type semiconductors the relation is ppnp = ni2 = pi2, where nn and pn is the concentration of the majority carriers, and pp and np are the concentration of minority charge carriers, respectively, in the n- and p-type semiconductor.
The specific electrical conductivity of an impurity semiconductor is determined by the concentration of the majority carriers and the higher the higher their concentration. In practice, there is often a case when a semiconductor contains both donor and acceptor impurities. Then the type of electrical conductivity will be determined by the impurity, the concentration of which is higher. A semiconductor in which the concentrations of Nd donors and Na acceptors are equal (Nd = Na)) is called compensated.
Along with conductors of electricity, there are many substances in nature that have significantly lower electrical conductivity than metal conductors. Substances of this kind are called semiconductors.
Semiconductors include: some chemical elements, such as selenium, silicon and germanium, sulfur compounds, such as thallium sulfide, cadmium sulfide, silver sulfide, carbides, such as carborundum,carbon (diamond),boron, gray tin, phosphorus, antimony, arsenic, tellurium, iodine and a number of compounds that include at least one of the elements of the 4th - 7th groups of the periodic system. There are also organic semiconductors.
The nature of the electrical conductivity of a semiconductor depends on the type of impurities present in the base material of the semiconductor and on the manufacturing technology of its components.
A semiconductor is a substance with 10 -10 - 10 4 (ohm x cm) -1, which, according to these properties, is between a conductor and an insulator. The difference between conductors, semiconductors and insulators according to band theory is as follows: in pure semiconductors and electronic insulators, there is an energy gap between the filled band (valence) and the conduction band.
Why do semiconductors conduct current?
A semiconductor has electronic conductivity if the outer electrons in its impurity atoms are relatively weakly bound to the nuclei of these atoms. If an electric field is created in a semiconductor of this kind, then, under the influence of the forces of this field, the outer electrons of the impurity atoms of the semiconductor will leave the confines of their atoms and turn into free electrons.
Free electrons will create an electric conduction current in the semiconductor under the influence of electric field forces. Consequently, the nature of the electric current in semiconductors with electronic conductivity is the same as in metal conductors. But since there are many times fewer free electrons in a unit volume of a semiconductor than in a unit volume of a metal conductor, it is natural that, under all other identical conditions, the current in a semiconductor will be many times less than in a metal conductor.
A semiconductor has “hole” conductivity if its impurity atoms not only do not give up their outer electrons, but, on the contrary, tend to capture electrons from the atoms of the main substance of the semiconductor. If an impurity atom takes an electron from an atom of the main substance, then in the latter something like a free space for an electron is formed - a “hole”.
A semiconductor atom that has lost an electron is called an “electron hole,” or simply a “hole.” If the “hole” is filled with an electron transferred from a neighboring atom, then it is eliminated and the atom becomes electrically neutral, and the “hole” is displaced to the neighboring atom that has lost the electron. Consequently, if a semiconductor with “hole” conductivity is exposed to an electric field, then the “electron holes” will shift in the direction of this field.
Bias "electron holes" in the direction of the electric field is similar to the movement of positive electric charges in the field and therefore represents the phenomenon of electric current in a semiconductor.
Semiconductors cannot be strictly distinguished by the mechanism of their electrical conductivity, since, along withWith “hole” conductivity, a given semiconductor may, to one degree or another, also have electronic conductivity.
Semiconductors are characterized by:
type of conductivity (electronic - n-type, hole - p-type);
resistivity;
lifetime of charge carriers (minority) or diffusion length, surface recombination rate;
dislocation density.
Silicon is the most common semiconductor material
Temperature has a significant influence on the characteristics of semiconductors. An increase in it predominantly leads to a decrease in resistivity and vice versa, i.e. semiconductors are characterized by the presence of a negative . Near absolute zero, a semiconductor becomes an insulator.
Semiconductors are the basis of many devices. In most cases they must be obtained in the form of single crystals. To impart specified properties, semiconductors are doped with various impurities. Increased demands are placed on the purity of source semiconductor materials.
Semiconductors have found the widest application in modern technology; they have had a very strong influence on technical progress. Thanks to them, it is possible to significantly reduce the weight and dimensions of electronic devices. The development of all areas of electronics leads to the creation and improvement of a large number of various equipment based on semiconductor devices. Semiconductor devices serve as the basis for microcells, micromodules, solid-state circuits, etc.
Electronic devices based on semiconductor devices are practically inertia-free. A carefully constructed and well-sealed semiconductor device can last tens of thousands of hours. However, some semiconductor materials have a low temperature limit (for example, germanium), but not very complex temperature compensation or replacing the main material of the device with another (for example, silicon, silicon carbide) largely eliminates this disadvantage. Improving the technology of manufacturing semiconductor devices leads to a reduction in the existing scatter and instability of parameters.
The semiconductor-metal contact and electron-hole junction (n-p junction) created in semiconductors are used in the manufacture of semiconductor diodes. Double junctions (p-n-p or n-p-n) - transistors and thyristors. These devices are mainly used for rectifying, generating and amplifying electrical signals.
Based on the photoelectric properties of semiconductors, photoresistors, photodiodes and phototransistors are created. The semiconductor serves as the active part of oscillation generators (amplifiers). When electric current is passed through a pn junction in the forward direction, charge carriers - electrons and holes - recombine with the emission of photons, which is used to create LEDs.
The thermoelectric properties of semiconductors made it possible to create semiconductor thermal resistances, semiconductor thermoelements, thermopiles and thermoelectric generators, and thermoelectric cooling of semiconductors, based on the Peltier effect, - thermoelectric refrigerators and thermostabilizers.
Semiconductors are used in machineless converters of thermal and solar energy into electricity - thermoelectric generators, and photoelectric converters (solar batteries).
Mechanical stress applied to a semiconductor changes its electrical resistance (the effect is stronger than in metals), which was the basis of the semiconductor strain gauge.
Semiconductor devices have become widespread in world practice, revolutionizing electronics; they serve as the basis for the development and production of:
measuring equipment, computers,
equipment for all types of communications and transport,
for process automation in industry,
devices for scientific research,
rocket technology,
medical equipment
other electronic devices and instruments.
The use of semiconductor devices makes it possible to create new equipment and improve old ones, which means a reduction in its dimensions, weight, power consumption, and therefore a decrease in heat generation in the circuit, an increase in strength, immediate readiness for action, and can increase the service life and reliability of electronic devices. devices.
Properties of semiconductors - the property of amber, after rubbing with wool, to attract small objects to itself, was noticed a very long time ago. But electrical phenomena, fickle and transient, were long in the shadow of magnetic phenomena, more stable over time.
In the 17th and 18th centuries, electrical experiments became widely available, and a number of new discoveries were made. In 1729, the Englishman Stephen Gray discovered that all substances are divided into 2 classes: insulators unable to carry an electric charge (called “electric bodies” because they could be electrified by friction), and conductors capable of carrying a charge (called “non-electric bodies”).
Modern ideas about the electrical properties of substances
With the development of further ideas, the properties of substances to conduct electric current began to be characterized quantitatively - by the value of specific electrical conductivity, measured in siemens per meter (S/m). At room temperature, the conductivity of conductors lies in the range from 10 6 to 10 8 S/m, and for dielectrics (insulators) it is less than 10 -8 S/m.
Substances that occupy an intermediate position in conductivity are logically called semiconductors or semi-insulators. The first name has been historically fixed. The conductivity of semiconductors lies in the range from 10 -8 to 10 6 S/m. There are no sharp boundaries between these 3 types of substances; qualitative differences are determined by the difference in quantitative properties.
It is known from physics that an electron in a solid cannot have arbitrary energy; this energy can only take on certain values, called energy levels. The closer the electron in an atom is to the nucleus, the lower its energy. The distant electron has the highest energy. Only the electrons of the outer shell of the atom (electrons of the so-called valence band) participate in electrical and chemical processes.
Electrons with higher energy than the valence band electrons are classified as conduction band electrons. These electrons are not associated with individual atoms, and they move randomly within the body to enable conduction.
The atoms of a substance that has donated an electron to the conduction band are considered as positively charged ions; they are immobile and form a crystal lattice of the substance within which conduction electrons move. In conductors (metals), the conduction band is adjacent to the valence band, and each metal atom without interference gives up one or more electrons to the conduction band, which provides metals with the property of electrical conductivity.
The properties of semiconductors are determined by the band gap
In semiconductors and dielectrics, there is a so-called between the valence band and the conduction band. prohibited area. Electrons cannot have energy corresponding to the energy of the levels of this zone. The division of substances into dielectrics and semiconductors is carried out depending on the width of the band gap. With a band gap of a few electron volts (eV), valence band electrons have little chance of entering the conduction band, which makes these substances non-conducting. Thus, diamond has a band gap of 5.6 eV. However, with increasing temperature, the electrons in the valence band increase their energy, and some of them enter the conduction band, which worsens the insulating properties of dielectrics.
If the band gap is on the order of one electron volt, the substance acquires noticeable conductivity already at room temperature, becoming even more conductive with increasing temperature. We classify such substances as semiconductors, and the properties of semiconductors are determined by the band gap.
At room temperature, the band gap of semiconductors is less than 2.5-3 eV. As an example, the band gap of germanium is 0.72 eV, and silicon is 1.12 eV. Wide-bandgap semiconductors include semiconductors with a bandgap greater than 2 eV. Typically, the higher the bandgap of a semiconductor, the higher its melting point. Thus, germanium has a melting point of 936 °C, and silicon has a melting point of 1414 °C.
Two types of semiconductor conductivity - electron and hole
At absolute zero temperature (-273 °C), in a pure semiconductor (intrinsic semiconductor, or semiconductor i-type) all electrons are found in atoms, and the semiconductor is an insulator. As the temperature increases, some of the electrons in the valence band enter the conduction band, and electronic conduction occurs. But when an atom loses an electron, it becomes positively charged.
An atom occupying a place in a crystal lattice cannot move under the influence of an electric field, but it is capable of attracting an electron from a neighboring atom, filling a “hole” in its valence band. The atom that has lost an electron, in turn, will also look for an opportunity to fill the “hole” formed in the outer shell. A hole has all the properties of a positive charge, and we can assume that in a semiconductor there are 2 types of carriers - negatively charged electrons and positively charged holes.
Conduction electrons can occupy free places in the valence band, i.e. merge with holes. This process is called recombination, and since the generation and recombination of carriers occurs simultaneously, at a given temperature the number of carrier pairs is in a state of dynamic equilibrium - the number of resulting pairs is compared with the number of recombining ones.
Intrinsic conductivity of a semiconductor i-type consists of electronic and hole conductivity, with electronic conductivity predominating, since electrons are more mobile than holes. The specific electrical conductivity of metals or semiconductors depends on the number of charge carriers per cubic meter. cm, or on the concentration of electrons and holes.
If the number of atoms in 1 cubic cm of a substance of the order of 10 22, then at room temperature in metals the number of conduction electrons is not less than the number of atoms, i.e. also of the order of 10 22, while in pure germanium the concentration of charge carriers is about 10 13 cm -3, and in silicon 10 10 cm -3, which is significantly less than that of the metal, which is why the conductivity of semiconductors is millions and billions of times worse than that of metals .
It's all about the impurities
When a voltage is applied to a semiconductor, the electric field that arises in it accelerates electrons and holes, their movement becomes ordered, and an electric current arises - conduction current. In addition to intrinsic conductivity, in semiconductors there is also impurity conductivity, which, as you might guess from the name, is due to the presence of impurities in the semiconductor.
If an insignificant amount of 5-valent antimony, arsenic or phosphorus is added to 4-valent germanium, the impurity atoms will use 4 electrons to bond with the germanium atoms, and the fifth will be in the conduction band, which dramatically improves the conductivity of the semiconductor. Such impurities, the atoms of which donate electrons, are called donors. Since electronic conductivity predominates in such semiconductors, they are called semiconductors n-type (from the English word negative- negative). In order for all donor atoms to donate an electron to the conduction band, the energy band of the donor atoms must be located as close as possible to the conduction band of the semiconductor, slightly below it.
When an impurity of 3-valent boron, indium or aluminum is added to 4-valent germanium, the impurity atoms take away electrons from the germanium atoms, and germanium acquires hole conductivity and becomes a semiconductor p-type (from the English word positive– positive). Impurities that create hole conductivity are called acceptors.
In order for acceptors to easily capture electrons, the energy levels of the acceptor atoms must be adjacent to the levels of the valence band of the semiconductor, located just above it.
Impurity conductivity usually significantly exceeds intrinsic conductivity, since the concentration of donor or acceptor atoms significantly exceeds the concentration of intrinsic carriers. It is very difficult to obtain a semiconductor with a strictly dosed amount of impurity, and the initial semiconductor must also be very pure. Thus, for germanium, no more than one atom of foreign impurity (i.e., neither donor nor acceptor) is allowed per 10 billion germanium atoms, and for silicon, the purity requirements are even 1000 times higher.
Metal-semiconductor transition
In semiconductor devices, there is a need to use semiconductor-metal contacts. A substance (metal or semiconductor) is characterized by the energy required for an electron to leave the substance - the work function. Let us denote the work function from the metal as A m, and from the semiconductor as A p.
Ohmic contacts
If it is necessary to create an ohmic contact (i.e., non-rectifying, when the contact resistance is low at any polarity of the applied voltage), it is sufficient to ensure contact of the metal with the semiconductor under the following conditions:
- In contact with n-semiconductor: A m< A п;
- In contact with a p-semiconductor: A m > A p .
Such properties of semiconductors are explained by the fact that majority carriers accumulate in the boundary layer of the semiconductor, which ensures its low resistance. The accumulation of majority carriers is ensured by the fact that electrons always move from a substance with a lower work function to a substance with a higher work function.
Rectifier contacts
But if with a semiconductor n-type there is a metal in contact with A m > A p, then electrons will move from the semiconductor to the metal, and a region depleted of majority carriers and having low conductivity is formed in the boundary layer. In order to overcome the created barrier, a voltage of a certain polarity and sufficient magnitude must be applied to the contact. When reverse polarity is applied, the conductivity of the contact will deteriorate even more - such a contact has rectifying properties. It is easy to see that metal-semiconductor contact has similar properties to semiconductors. p-type at A m< A п.
History of the Semiconductor Detector
Similar properties of metal-semiconductor semiconductors were discovered by the German physicist Ferdinand Braun in 1874. The earliest metal-semiconductor diodes appeared around 1900, when radio receivers began using detectors consisting of a tungsten wire pressed against the surface of a galena (lead sulfide) crystal. Radio amateurs made detectors themselves by fusing lead with sulfur.
In 1906, the French scientist G. Picard designed a detector from a silicon crystal and a spiral contact spring with a tip, and received a patent for it. Electronic devices based on a metal-semiconductor contact are called Schottky diodes after the German physicist Walter Schottky who studied such contacts.
In 1926, powerful cuprox rectifier elements appeared, consisting of a copper plate coated with a layer of cuprous oxide, which were widely used in power units.
Electron-hole transition
Electron-hole transition, or n-p-junction is an area at the boundary of two semiconductors of different conductivity types, and the operation of semiconductor devices is based on the use of the properties of such transitions. In the absence of voltage applied to the junction, charge carriers move from areas of higher concentration to areas of lower concentration - out of the semiconductor n-type semiconductor p-type electrons move, and holes move in the opposite direction.
As a result of these movements, regions with a space charge appear on both sides of the interface, and a contact potential difference arises between these regions. This potential difference forms a potential barrier, which prevents further carriers from passing through the barrier. The barrier height (contact potential difference) depends on the concentration of impurities, and for germanium it is usually 0.3-0.4 V, reaching 0.7 V. In steady state, there is no current through the junction, since p-n- the junction has a high resistance compared to other areas of semiconductors, and the resulting layer is called a blocking layer.
If to n-p-apply an external voltage to the junction, then depending on its polarity, the junction will behave differently.
Direct current flow through junction
If to a semiconductor p-type, apply the “plus” of a voltage source, then the field created by the source acts opposite to the field of the contact potential difference, the total field decreases, the height of the potential barrier decreases, and a larger number of carriers overcome it. A current called direct current begins to flow through the junction. At the same time, the thickness of the protective layer and its electrical resistance decrease.
To generate a significant forward current, it is sufficient to apply a voltage to the junction comparable to the barrier height in the absence of an applied voltage, i.e. tenths of a volt, and at an even higher voltage, the resistance of the barrier layer will become close to zero.
Reverse current flow through the junction
If the external voltage is “reversed”, i.e. attach to p-semiconductor “minus” voltage source, the external voltage field will add up to the field of the contact potential difference. The height of the potential barrier increases, which will impede the diffusion of majority carriers through the junction, and the current through the junction, called "reverse", will be small. The barrier layer becomes thicker and its electrical resistance increases.
The rectifying properties of electron-hole junctions are used in diodes of various powers and purposes - for rectifying alternating current in power supplies and weak signals in devices for various purposes.
Other applications of semiconductor properties
An electron-hole junction under reverse voltage behaves similarly to a charged electrical capacitor with a capacity of a few to hundreds of picofarads. This capacitance depends on the voltage applied to the junction, which allows some types of semiconductor devices to be used as variable capacitors controlled by the applied voltage.
Properties n-p-transitions also significantly depend on the temperature of the medium, which makes it possible to use certain types of semiconductor devices as temperature sensors. Devices with three regions of different conductivity, such as n-p-n, allow you to create devices that have the properties of amplifying electrical signals, as well as generating them.