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 the 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.
Various types of semiconductors have become widespread in industry and energy microelectronics. With their help, one energy can be converted into another; without them, many electronic devices will not work normally. There are a large number of types of these elements, depending on the principle of their operation, purpose, material, and design features. In order to understand the mode of action of semiconductors, it is necessary to know their basic physical properties.
Properties and characteristics of semiconductors
The basic electrical properties of semiconductors allow them to be considered as a cross between standard conductors and materials that do not conduct electricity. The semiconductor group includes significantly more different substances than the total number.
Semiconductors made from silicon, germanium, selenium and other materials are widely used in electronics. Their main characteristic is considered to be a pronounced dependence on the influence of temperature. At very low temperatures, comparable to absolute zero, semiconductors acquire the properties of insulators, and as the temperature rises, their resistance decreases while their conductivity increases. The properties of these materials can also change under the influence of light, when a significant increase in photoconductivity occurs.
Semiconductors convert light energy into electricity, unlike conductors, which do not have this property. In addition, the introduction of atoms of certain elements into the semiconductor contributes to an increase in electrical conductivity. All these specific properties allow the use of semiconductor materials in various fields of electronics and electrical engineering.
Types and applications of semiconductors
Due to their qualities, all types of semiconductors are divided into several main groups.
Diodes. They include two crystals made of semiconductors with different conductivities. An electron-hole transition is formed between them. They are produced in various designs, mainly point and flat types. In planar cells, the germanium crystal is alloyed with indium. Point diodes consist of a silicon crystal and a metal needle.
Transistors. They consist of three crystalline semiconductors. Two crystals have the same conductivity, and in the third, the conductivity has the opposite value. They are called collector, base and emitter. In electronics, amplifies electrical signals.
Thyristors. They are elements that convert electricity. They have three electron-hole junctions with gate properties. Their properties allow thyristors to be widely used in automation, computers, and control devices.
How does a semiconductor differ from insulators and conductors?
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.
We talked about conductors and dielectrics and briefly mentioned that there is an intermediate form of conductivity, which under certain conditions can take on the properties of a conductor or dielectric. This type of substance is called semiconductor.
Let me remind you: in terms of electrical properties, semiconductors occupy a middle place between conductors and non-conductors of current.
Most often, germanium and silicon are used for the production of semiconductors; less often, selenium, cuprous oxide and other substances are used.
The electrical conductivity of semiconductors is highly dependent on the ambient temperature. At temperatures close to absolute zero (-273C), they behave as insulators in relation to electric current. Most conductors, on the contrary, at this temperature become superconducting, that is, they offer almost no resistance to current. As the temperature of conductors increases, their resistance to electric current increases, and the resistance of semiconductors decreases. The electrical conductivity of conductors does not change when exposed to light. The electrical conductivity of semiconductors under the influence of light, the so-called photoconductivity, increases.
Semiconductors can convert light energy into electrical current. This is absolutely not typical for conductors. The electrical conductivity of semiconductors increases sharply when atoms of some other elements are introduced into them. The electrical conductivity of conductors decreases when impurities are introduced into them.
Germanium and silicon, which are the starting materials of many modern semiconductor devices, each have four valence electrons in the outer layers of their shells. In total, there are 32 electrons in a germanium atom, and 14 in a silicon atom. But 28 germanium electrons and 10 silicon electrons, located in the inner layers of their shells, are firmly held by the nuclei and under no circumstances are separated from them. Only four valence electrons of the atoms of these semiconductors can, and even then not always, become free. A semiconductor atom that has lost at least one electron becomes a positive ion. In a semiconductor, the atoms are arranged in a strict order: each of them is surrounded by four similar atoms. They are also located so close to each other that their valence electrons form single orbits passing around all neighboring atoms, binding them into a single substance.
This relationship of atoms in a semiconductor crystal can be imagined in the form of a flat diagram, as shown in Fig. 1, a. Here, large balls with the “+” sign conventionally represent atomic nuclei with inner layers of electron shell (positive ions), and small balls - valence electrons
. Each atom is surrounded by four exactly the same. Any of them is connected with each neighboring one by two valence electrons, one of which is “its own”, and the second is borrowed from the “neighbor”. This is a two-electron, or valence, bond. The strongest connection!
In turn, the outer layer of the electron shell of each atom contains eight electrons: four of its own and one each from four neighboring atoms. Here it is no longer possible to distinguish which of the valence electrons is “yours” and which is “foreign”, since they have become common. With such a connection of atoms in the entire mass of a germanium or silicon crystal, we can consider that the semiconductor crystal is one large molecule. The diagram of the interconnection of atoms in a semiconductor can be simplified for clarity by depicting it as shown in Fig. 1, 6. Here, the nuclei of atoms with internal electron shells are shown as circles with a plus sign, and interatomic bonds are shown as two lines symbolizing valence electrons.
Electrical conductivity of semiconductors
At temperatures close to absolute zero, a semiconductor behaves like an absolute nonconductor because it has no free electrons. If there is no increase in temperature, the connection of valence electrons with atomic nuclei weakens and some of them may leave their atoms due to thermal movement. An electron escaped from an interatomic bond becomes free (in Fig. 1, b - black dot), and where it was before, an empty space is formed. This empty space in the interatomic bond of a semiconductor is conventionally called hole (in Fig. 1,b there is a broken line). The higher the temperature, the more free electrons and holes appear. Thus, the formation of a hole in the mass of a semiconductor is associated with the departure of a valence electron from the shell of an atom, and the appearance of a hole corresponds to the appearance of a positive electric charge equal to the negative electron.
Figure 1. Diagram of the relationship of atoms in a semiconductor crystal (a) and a simplified diagram of its structure (b).
Now look at the figure. 2. It schematically shows the phenomenon of current generation in a semiconductor. The cause of the current is the voltage applied to the poles (in Fig. 2, the voltage source is symbolized by the signs “+” and “-”). Due to thermal phenomena, a certain number of electrons are released from interatomic bonds throughout the entire mass of the semiconductor (in Fig. 2 they are indicated by dots with arrows). Electrons released near the positive pole of the voltage source are attracted by this pole and leave the semiconductor mass, leaving behind holes. Electrons that leave interatomic bonds at some distance from the positive pole are also attracted by it and move towards it. But, having encountered holes on their way, the electrons seem to “jump” into them (Fig. 2, a), and the interatomic bonds are filled. And the holes closest to the negative pole are filled with other electrons escaped from atoms located even closer to the negative pole (Fig. 2, b). While the electric field is active in the semiconductor, this process continues: some interatomic bonds are broken - valence electrons leave them, holes appear - and other interatomic bonds are filled - electrons released from some other interatomic bonds “jump” into the holes (Fig. 2 , b-c).
Figure 2. Scheme of the movement of electrons and holes.
At temperatures above absolute zero, free electrons and holes continuously appear and disappear in a semiconductor, even when there are no external electric fields. But electrons and holes move chaotically in different directions and do not leave the semiconductor. In a pure semiconductor, the number of electrons released at each moment of time is equal to the number of holes formed in this case. Their total number at room temperature is relatively small. Therefore, the electrical conductivity of such a semiconductor is (called own) , is small, it provides quite a lot of resistance to electric current. But if even an insignificant amount of impurity in the form of atoms of other elements is added to a pure semiconductor, its electrical conductivity will increase sharply. In this case, depending on the structure of atoms of impurity elements, the electrical conductivity of the semiconductor will be electronic or hole .
Electronic conductivity
If any atom in a semiconductor crystal is replaced by an antimony atom, which has five valence electrons in the outer layer of the electron shell, this “alien” atom will bond with four electrons to four neighboring atoms of the semiconductor. The fifth valence electron of the antimony atom will be “extra” and will become free. The more antimony atoms are introduced into the semiconductor, the more free electrons will be in its mass. Consequently, a semiconductor with an admixture of antimony is close in its properties to a metal: in order for an electric current to pass through it, interatomic bonds in it do not necessarily have to be destroyed. They are called electrically conductive or type (n) semiconductors. Here the Latin letter n is the initial letter of the Latin word negativ (negative), which means “negative” . This term in this case should be understood in the sense that in an n-type semiconductor the main current carriers are negative charges, i.e. electrons.
Hole conductivity
A completely different picture will turn out if atoms with three valence electrons, for example indium, are introduced into the semiconductor. Each indium metal atom with its three electrons will fill bonds with only three neighboring atoms of the semiconductor, and it lacks one electron to fill the bond with the fourth. A hole is formed. It, of course, can be filled with some kind of electron that has escaped from the valence bond with other atoms of the semiconductor. However, no matter where the holes are, there will not be enough electrons in the mass of the indium-doped semiconductor to fill them. And the more indium impurity atoms are introduced into the semiconductor, the more holes are formed in it. In order for electrons to move in such a semiconductor, valence bonds between atoms must be destroyed. The electrons that escape from them or the electrons that enter the semiconductor from the outside move from hole to hole. And in the entire mass of the semiconductor at any time, the number of holes will be greater than the total number of free electrons. They are called semiconductors with hole electrical conductivity or type (p).
Latin letter r - the first letter of the Latin word positiv (positive), which means “positive”.
This term in this case should be understood in the sense that the phenomenon of electric current in the mass of a semiconductor of type (p) is accompanied by the continuous appearance and disappearance of positive charges - holes. Moving through the mass of the semiconductor, the holes act as current carriers. Semiconductors of type p, as well as type n, have many times better electrical conductivity compared to pure ones.
It must be said that there are practically no both completely pure semiconductors and absolutely electrically conductive types n and p. A semiconductor with an admixture of indium necessarily contains a small number of atoms of some other elements that give it electronic conductivity, and with an admixture of antimony there are atoms of elements that create hole electrical conductivity in it. For example, in a semiconductor, which has an overall electrical conductivity of type n, there are holes that can be filled with free electrons from antimony impurity atoms. As a result, the electrical conductivity will deteriorate somewhat, but in general it will retain electronic conductivity. A similar phenomenon will be observed if free electrons enter a semiconductor with a hole character.
Therefore, in n-type semiconductors, the main current carriers are electrons (electronic electrical conductivity predominates), and in p-type semiconductors, the main current carriers are holes (hole electrical conductivity predominates).
Semiconductors got their name because they occupy an intermediate position between conductors (metals, electrolytes, coal), which have high electrical conductivity, and insulators (porcelain, mica, rubber, and others), which conduct almost no electric current.
If we compare the specific volume resistance in Ohm × cm for various substances, it turns out that the conductors have: ρ U= 10 -6 - 10 -3 Ohm × cm; resistivity of semiconductors: ρ U= 10 -3 - 10 8 Ohm × cm; and for dielectrics: ρ U= 10 8 - 10 20 Ohm × cm. Semiconductors include: metal oxides - oxides (Al 2 O 3, Cu 2 O, ZnO, TiO 2, VO 2, WO 2, MoO 3); sulfur compounds - sulfides (Cu 2 S, Ag 2 S, ZnS, CdS, HgS); compounds with selenium - selenides; compounds with tellurium - tellurides; some alloys (MgSb 2, ZnSb, Mg 2 Sb, CdSb, AlSb, ClSb); chemical elements - germanium, silicon, tellurium, selenium, boron, carbon, sulfur, phosphorus, arsenic, as well as a large number of complex compounds (galene, carborundum and others).
Figure 1. Germanium
Figure 2. Silicon
Figure 3. Tellurium
A complete and extensive study of the properties of semiconductors was carried out by the Soviet scientist A.F. Ioffe and his colleagues.
The electrical properties of semiconductors differ sharply from the properties of conductors and insulators. The electrical conductivity of conductors strongly depends on temperature, illumination, the presence and intensity of the electric field, and the amount of impurities. At ordinary temperatures, semiconductors contain a certain number of free electrons resulting from the breaking of electronic bonds. Semiconductors have two types of conductivity: electron and hole. Charge carriers in semiconductors with electronic conduction are free electrons, and with hole conduction they are bonds devoid of electrons.
Consider the following experiment. Let's take a metal conductor and heat one end of it, then the heated end of the conductor will receive a positive charge. This is due to the movement of electrons from the hot end to the cold end, resulting in a shortage of electrons at the hot end of the conductor (positive charge) and an excess of electrons at the cold end (negative charge). The short-term flow of current through a conductor was caused by the movement of electrons from one end of the conductor to the other. Thus, here we are talking about a conductor with electronic conductivity. However, there are substances that behave differently during such an experiment: the heated edge of such a substance receives a negative charge, and the cold edge receives a positive charge. This is possible if we assume that current transfer is carried out by positive charges.
Figure 4. Bonding between atoms of a substance
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| Figure 5. Intrinsic conductivity of semiconductors |
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| Figure 6. Electronic conductivity of a semiconductor |
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| Figure 7. Hole conductivity of a semiconductor |
Let's get acquainted with another type of conductivity in semiconductors - hole conductivity. In pure semiconductors, all electrons weakly bound to the nuclei participate in electronic bonds. In Figure 4, A the filled bond between the atoms of the substance is conventionally shown. A “hole” is an element of the crystal lattice of a substance that has lost an electron, which corresponds to the appearance of a positive charge (Figure 4, b).
A released bond may be filled again if the “hole” captures an electron from a neighboring bond (Figure 4, V). This will cause the "hole" to move to a new location. In a semiconductor substance under normal conditions, the direction of electron emission and the location of the “hole” formation are chaotic. If a constant voltage is applied to a pure semiconductor, then electrons and “holes” will move (the first against the direction of the field forces, the second in the opposite direction). If the number of “holes” formed is equal to the number of released electrons, then, as is the case with pure semiconductors, the conductivity of semiconductors is low (intrinsic conductivity). The presence of even a small amount of foreign impurities can change the mechanism of electrical conductivity: make it electronic or hole. Let's look at a specific example. Let's take germanium (Ge) as a semiconductor. In a germanium crystal, each atom is bonded to four other atoms. When the temperature increases or as a result of irradiation, the pair bonds of the crystal can be broken. In this case, an equal number of electrons and “holes” are formed (Figure 5).
Let's add arsenic to germanium as an impurity. Such an impurity has a large number of weakly bound electrons. Impurity atoms have their own energy level, located between the energy levels of the free and filled bands, closer to the latter (Figure 6). Such impurities give up their electrons to the free zone and are called donor impurities. The semiconductor will have free electrons, while all bonds will be filled. The semiconductor will have electronic conductivity in the free band.
If now indium, rather than arsenic, is added as an impurity to germanium, the following will happen. Such an impurity has a small number of weakly bound electrons, and the energy level of the impurity is located between the energy levels of the free and filled zones, closer to the free zone (Figure 7). Impurities of this kind accept electrons from an adjacent filled zone into their zone and are called acceptor impurities. In the semiconductor there will be unfilled bonds - “holes” in the absence of free electrons. The semiconductor will have hole conductivity in the filled band.
Now the experience of heating a semiconductor will become clear, when the heated end received a negative charge, and the cold end received a positive charge. Under the influence of heat, bonds at the hot end will begin to break down, creating “holes” and free electrons. If the semiconductor contains impurities, then the “holes” will begin to move to the cold end, charging it positively, and the heated end of the semiconductor will become negatively charged.
Concluding our consideration of semiconductors, we draw the following conclusion.
By adding impurities to a semiconductor, one can give it predominant electronic or hole conductivity. Based on this, the following types of semiconductors are obtained. Semiconductors with electronic conductivity are called semiconductors n-type (negative), and with hole conductivity - p-type (positive).
We also invite you to watch educational videos about semiconductors:
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