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 called crystals and amorphous substances, which 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. Distinctive feature 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 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. So not too strong electric field unable to enhance 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 conductivity various materials is determined by the temperature dependences of these 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 the increase n. The opposite is also observed - when low temperatures some semiconductors become insulators.
Bibliography
Students of group 501 of the Faculty of Chemistry: Bezzubov S.I., Vorobyova N.A., Efimov A.A.
You, young friend, are a contemporary of the technical revolution in all areas of radio electronics. Its essence lies in the fact that electron tubes have been replaced by semiconductor devices, and now they are increasingly crowded out by microcircuits.
The ancestor of one of the most characteristic representatives of the “army” of semiconductor devices - the transistor - was the so-called generating detector, invented back in 1922 by the Soviet radiophysicist O. V. Losev. This device, which is a semiconductor crystal with two wires adjacent to it - conductors, under certain conditions could generate and amplify electrical oscillations. But then, due to imperfections, it could not compete with an electron tube. A worthy semiconductor contender vacuum tube, called a transistor, was created in 1948 by American scientists Brattain, Bardeen and Shockley. In our country, a great contribution to the development of semiconductor devices was made by A.F. Ioffe, L.D. Landau, B.I. Davydova, V.E. Loshkarev and a number of other scientists and engineers, many scientific teams.
To understand the essence of the phenomena occurring in modern semiconductor devices, we will have to “look” into the structure of the semiconductor and understand the reasons for the formation of electric current. But before that, it would be good for you to remember that part of the first conversation where I talked about the structure of atoms.
Let me remind you: by electrical properties Semiconductors occupy a middle place between conductors and non-conductors of current. To what has been said, I will add that the group of semiconductors includes many more substances than to groups of conductors and non-conductors taken together. To semiconductors who have found practical use in technology, include germanium, silicon, selenium, cuprous oxide and some other substances. But for semiconductor devices, only germanium and silicon are mainly used.
What are the most characteristic properties semiconductors, distinguishing them from conductors and non-conductors of current? The electrical conductivity of semiconductors is highly dependent on the ambient temperature. At very low temperatures, close to absolute zero (-273°C), they behave as insulators in relation to electric current. Most conductors, on the contrary, at this temperature become superconducting, i.e. 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. These and some other properties of semiconductors have been known for a relatively long time, but they began to be widely used relatively recently.
Germanium and silicon, which are starting materials Many modern semiconductor devices 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 electrons of a germanium atom and 10 electrons of a silicon atom, 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. Remember: four! 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 atom 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 as flat diagram, as shown in Fig. 72, a. Here, large balls with a “+” sign conventionally represent the nuclei of atoms with inner layers electron shell (positive ions), and the small balls are valence electrons. Each atom, as you can see, is surrounded by four exactly the same atoms. Any of the atoms is connected to each neighboring one with 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!
Rice. 72. Diagram of the relationship of atoms in a semiconductor crystal (a) and a simplified diagram of its structure (b)
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 in the atom 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. 72, b. Here, the nuclei of atoms with inner electron shells are shown as circles with a plus sign, and interatomic bonds are shown as two lines symbolizing valence electrons.
According to the value of electrical resistivity semiconductors occupy an intermediate position between good conductors (σ = 10 6 -10 4 Ohm -1 cm -1) and dielectrics (σ = -12 - 10 -10 Ohm -1 cm -1). Semiconductors include many chemical elements(germanium, silicon, selenium, indium, tellurium, arsenic, etc.), a huge number of alloys and chemical compounds. Almost all inorganic substances the world around us - semiconductors. The most common semiconductor in nature is silicon, which makes up about 30% of the earth's crust.
In addition to temperature, the electrical conductivity of semiconductors is influenced by a strong electric field, pressure, exposure to optical and ionizing radiation, the presence of impurities and other factors that can change the structure of the substance and the state of electrons. This circumstance plays a decisive role in the numerous and varied uses semiconductors.
The qualitative difference between semiconductors and metals is manifested primarily in the dependence of resistivity on temperature. As the temperature decreases, the resistance of metals decreases. In semiconductors, on the contrary, the resistance increases with decreasing temperature and near absolute zero they practically become insulators.
Dependence of resistivity of a pure semiconductor on temperature.
This behavior of the ρ(T) dependence shows that in semiconductors the concentration of free charge carriers does not remain constant, but increases with increasing temperature. The mechanism of electric current in semiconductors cannot be explained within the framework of the free electron gas model. Let us qualitatively consider this mechanism using the example of germanium (Ge). In a silicon (Si) crystal the mechanism is similar.
Germanium atoms have four weakly bound electrons in their outer shell. These are called covalent electrons. In a crystal lattice, each atom is surrounded by its four nearest neighbors. The bond between atoms in a germanium crystal is covalent, that is, it is carried out by pairs of valence electrons. Each valence electron belongs to two atoms.
Pair-electron bonds in a germanium crystal and the formation of an electron-hole pair
Valence electrons in a germanium crystal are much more strongly bound to atoms than in metals; therefore, the concentration of conduction electrons at room temperature in semiconductors is many orders of magnitude less than in metals. Near absolute zero temperature in a germanium crystal, all electrons are occupied in the formation of bonds. Such a crystal does not conduct electric current.
As the temperature increases, some of the valence electrons may gain enough energy to break covalent bonds. Then free electrons (conduction electrons) will appear in the crystal. At the same time, vacancies are formed in places where bonds are broken, which are not occupied by electrons. These vacancies are called holes. The vacant place can be occupied by a valence electron from a neighboring pair, then the hole will move to a new place in the crystal. At a given semiconductor temperature, a certain number of electron-hole pairs are formed per unit time. At the same time, the reverse process occurs - when a free electron meets a hole, the electronic bond between the germanium atoms is restored. This process is called recombination. Electron-hole pairs can also be created when illuminating a semiconductor due to energy electromagnetic radiation. In the absence of an electric field, conduction electrons and holes participate in chaotic thermal motion.
The concentration of conduction electrons in a semiconductor is equal to the concentration of holes: n n = n p. The electron-hole conductivity mechanism manifests itself only in pure (i.e., without impurities) semiconductors. It is called With private electrical conductivity semiconductors .
In the presence of impurities, the electrical conductivity of semiconductors changes greatly. For example, adding phosphorus impurities in an amount of 0.001 atomic percent to a silicon crystal reduces the resistivity by more than five orders of magnitude. Such a strong influence of impurities can be explained on the basis of the above ideas about the structure of semiconductors. A necessary condition A sharp decrease in the resistivity of a semiconductor with the introduction of impurities is the difference in the valence of the impurity atoms from the valence of the main atoms of the crystal.
The conductivity of semiconductors in the presence of impurities is called impurity conductivity . There are two types of impurity conductivity - electron and hole.
Electronic conductivity occurs when pentavalent atoms (for example, arsenic atoms, As) are introduced into a germanium crystal with tetravalent atoms. 
Semiconductor n - type. An arsenic atom in a germanium crystal lattice.
The figure shows a pentavalent arsenic atom found in a site of the germanium crystal lattice. The four valence electrons of the arsenic atom are included in the formation of covalent bonds with four neighboring germanium atoms. The fifth valence electron turned out to be redundant; it easily breaks away from the arsenic atom and becomes free. An atom that has lost an electron becomes a positive ion located at a site in the crystal lattice. An impurity of atoms with a valency exceeding the valence of the main atoms of a semiconductor crystal is called donor impurity . As a result of its introduction, a significant number of free electrons appear in the crystal. It leads to sharp decrease resistivity of a semiconductor - thousands and even millions of times. The resistivity of a conductor with a high content of impurities may approach that of a metal conductor.
In a germanium crystal with an admixture of arsenic, there are electrons and holes responsible for the crystal’s own conductivity. But the main type of free charge carriers are electrons detached from arsenic atoms. In such a crystal n n >> n p . Such conductivity is called electronic, and a semiconductor with electronic conductivity is called n-type semiconductor.
Hole conduction occurs when trivalent atoms (for example, indium atoms, In) are introduced into a germanium crystal. The figure shows an indium atom that, using its valence electrons, has created covalent bonds with only three neighboring germanium atoms.
P-type semiconductor. India atom in germanium crystal lattice
The indium atom does not have an electron to form a bond with the fourth germanium atom. This missing electron can be captured by the indium atom from the covalent bond of neighboring germanium atoms. In this case, the indium atom turns into a negative ion located at a site of the crystal lattice, and a vacancy is formed in the covalent bond of neighboring atoms. An admixture of atoms capable of capturing electrons is called acceptor impurity. As a result of the introduction of an acceptor impurity, many covalent bonds are broken in the crystal and vacancies (holes) are formed. Electrons from neighboring covalent bonds can jump to these places, which leads to chaotic wandering of holes throughout the crystal.
The presence of an acceptor impurity sharply reduces the resistivity of the semiconductor due to the appearance of a large number of free holes. The concentration of holes in a semiconductor with an acceptor impurity significantly exceeds the concentration of electrons that arose due to the mechanism of the semiconductor’s own electrical conductivity: n p >> n n . This type of conductivity is called hole conductivity. An impurity semiconductor with hole conductivity is called p-type semiconductor. The main free charge carriers in p-type semiconductors are holes.
It should be emphasized that hole conductivity is actually due to the relay movement of electrons through vacancies from one germanium atom to another, which carry out a covalent bond.
For n- and p-type semiconductors, Ohm's law is satisfied in certain ranges of current and voltage, provided that the concentrations of free carriers are constant.
What are its features? What is the physics of semiconductors? How are they built? What is conductivity of semiconductors? What physical characteristics do they have?
This refers to crystalline materials that do not conduct electricity as well as metals do. But still this indicator is better than that of insulators. Such characteristics are due to the number of moving carriers. Generally speaking, there is a strong attachment to cores. But when several atoms, say antimony, which has an excess of electrons, are introduced into the conductor, this situation will be corrected. When indium is used, elements with a positive charge are obtained. All these properties are widely used in transistors - special devices that can amplify, block or pass current in only one direction. If we consider an NPN-type element, we can note a significant amplifying role, which is especially important when transmitting weak signals.
Conductors have many free electrons. Insulators practically do not have them at all. Semiconductors contain both a certain number of free electrons and gaps with a positive charge that are ready to accept released particles. And most importantly, they all conduct. The type of NPN transistor discussed earlier is not the only possible semiconductor element. So, there are also PNP transistors, as well as diodes.
If we talk about the latter briefly, then this is an element that can transmit signals only in one direction. Also the diode can turn alternating current to permanent. What is the mechanism of this transformation? And why does it only move in one direction? Depending on where the current comes from, electrons and gaps can either diverge or go towards each other. In the first case, due to an increase in distance, the supply supply is interrupted, and therefore negative voltage carriers are transmitted in only one direction, that is, the conductivity of semiconductors is one-way. After all, current can be transmitted only if the constituent particles are nearby. And this is only possible when current is supplied from one side. These are the types of semiconductors that exist and are currently in use.
The electrical and optical properties of conductors are due to the fact that when energy levels are filled with electrons, they are separated from possible states by a band gap. What are its features? The fact is that there are no energy levels in the band gap. This can be changed with the help of impurities and structural defects. The highest completely filled band is called the valence band. This is followed by one that is allowed, but empty. It is called the conduction band. Semiconductor physics - quite interesting topic, and it will be well covered within the article.
For this, concepts such as the number of the allowed zone and quasi-pulse are used. The structure of the first is determined by the dispersion law. He says that it is influenced by the dependence of energy on quasi-momentum. Thus, if the valence band is completely filled with electrons (which carry charge in semiconductors), then they say that there are no elementary excitations in it. If for some reason there is no particle, then this means that a positively charged quasiparticle has appeared here - a gap or a hole. They are charge carriers in semiconductors in the valence band.
The valence band in a typical conductor is sixfold degenerate. This is without taking into account the spin-orbit interaction and only when the quasi-momentum is zero. Under the same condition, it can split into doubly and quadruple degenerate zones. The energy distance between them is called the spin-orbit splitting energy.
They can be electrically inactive or active. The use of the former makes it possible to obtain a positive or negative charge in semiconductors, which can be compensated by the appearance of a hole in the valence band or an electron in the conductive band. Inactive impurities are neutral, and they have a relatively weak effect on the electronic properties. Moreover, what can often matter is the valence of the atoms that take part in the charge transfer process and the structure
Depending on the type and amount of impurities, the ratio between the number of holes and electrons may also change. Therefore, semiconductor materials must always be carefully selected to obtain the desired result. This is preceded by a significant number of calculations, and subsequently experiments. The particles that most people call majority charge carriers are minority ones.
Dosed introduction of impurities into semiconductors makes it possible to obtain devices with the required properties. Defects in semiconductors can also be in an inactive or active electrical state. The important ones here are dislocation, interstitial atom and vacancy. Liquid and non-crystalline conductors react to impurities differently than crystalline ones. The lack of a rigid structure ultimately results in the displaced atom receiving a different valency. It will be different from the one with which he initially saturates his connections. It becomes unprofitable for the atom to give or gain an electron. In this case, it becomes inactive, and therefore impurity semiconductors have a high chance of failure. This leads to the fact that it is impossible to change the type of conductivity by doping and create, for example, a pn junction.
Some amorphous semiconductors can change their electronic properties when exposed to doping. But this applies to them to a much lesser extent than to crystalline ones. The sensitivity of amorphous elements to doping can be increased by using technological processing. Ultimately, I would like to note that thanks to long and hard work, impurity semiconductors are still represented by a number of results with good characteristics.
When the number of holes and electrons exists is determined solely by temperature, band structure parameters and the concentration of electrically active impurities. When the ratio is calculated, it is assumed that some of the particles will be in the conduction band (at the acceptor or donor level). Also taken into account is the fact that part may leave the valence territory, and gaps are formed there.
In semiconductors, in addition to electrons, ions can also act as charge carriers. But their electrical conductivity is in most cases negligible. As an exception, only ionic superconductors can be cited. There are three main electron transfer mechanisms in semiconductors:
A hole and an electron can form a bound state. It is called a Wannier-Mott exciton. In this case, which corresponds to the absorption edge, decreases by the size of the coupling value. If sufficient, a significant number of excitons can be formed in semiconductors. As their concentration increases, condensation occurs and an electron-hole liquid is formed.
These words denote several atomic layers that are located near the border of the device. Surface properties differ from volumetric ones. The presence of these layers breaks the translational symmetry of the crystal. This leads to so-called surface states and polaritons. Developing the theme of the latter, we should also talk about spin and vibrational waves. Due to its chemical activity, the surface is covered with a microscopic layer of foreign molecules or atoms that have been adsorbed from environment. They determine the properties of those several atomic layers. Fortunately, the creation of ultra-high vacuum technology, in which semiconductor elements are created, makes it possible to obtain and maintain a clean surface for several hours, which has a positive effect on the quality of the resulting product.
When the temperature of metals increases, their resistance also increases. With semiconductors, the opposite is true - under the same conditions, this parameter will decrease. The point here is that the electrical conductivity of any material (and this characteristic inversely proportional to resistance) depends on what current charge the carriers have, on the speed of their movement in the electric field and on their number in one unit volume of the material.
In semiconductor elements, as the temperature increases, the concentration of particles increases, due to which thermal conductivity increases and resistance decreases. You can check this with a simple set of young physicists and required material- silicon or germanium, you can also take a semiconductor made from them. Increasing the temperature will reduce their resistance. To make sure of this, you need to stock up measuring instruments, which will allow you to see all the changes. This is in the general case. Let's look at a couple of private options.
This is due to the tunneling of electrons passing through a very narrow barrier that delivers approximately one-hundredth of a micrometer. It is located between the edges of energy zones. Its appearance is possible only when the energy zones are tilted, which occurs only under the influence of a strong electric field. When tunneling occurs (which is a quantum mechanical effect), electrons pass through a narrow potential barrier without changing their energy. This entails an increase in the concentration of charge carriers, in both bands: conductivity and valence. If the process of electrostatic ionization is developed, a tunnel breakdown of the semiconductor may occur. During this process, the resistance of the semiconductors will change. It is reversible, and as soon as the electric field is turned off, all processes will be restored.
IN in this case holes and electrons are accelerated as they travel through the mean free path under the influence of a strong electric field to values that promote ionization of atoms and breaking of one of the covalent bonds (main atom or impurity). Impact ionization occurs like an avalanche, and charge carriers multiply in it like an avalanche. In this case, the newly created holes and electrons are accelerated by an electric current. The current value in the final result is multiplied by the impact ionization coefficient, which equal to the number electron-hole pairs that are formed by a charge carrier on one section of the path. The development of this process ultimately leads to an avalanche breakdown of the semiconductor. The resistance of semiconductors also changes, but, as in the case of tunnel breakdown, it is reversible.
The particular importance of these elements should be noted in computer technology. We have almost no doubt that you would not be interested in the question of what semiconductors are if it were not for the desire to independently assemble an object using them. It is impossible to imagine the operation of modern refrigerators, televisions, and computer monitors without semiconductors. Advanced automotive developments cannot do without them. They are also used in aviation and space technology. Do you understand what semiconductors are and how important they are? Of course, we cannot say that these are the only irreplaceable elements for our civilization, but we should not underestimate them either.
The use of semiconductors in practice is also due to a number of factors, including the widespread availability of the materials from which they are made, the ease of processing and obtaining the desired result, and others technical features, thanks to which the choice of scientists who developed electronic equipment settled on them.
We looked in detail at what semiconductors are and how they work. Their resistance is based on complex physical and chemical processes. And we can notify you that the facts described in the article will not fully understand what semiconductors are, for the simple reason that even science has not fully studied the features of their work. But we know their basic properties and characteristics, which allow us to use them in practice. Therefore, you can look for semiconductor materials and experiment with them yourself, being careful. Who knows, maybe there is a great explorer within you?!
We talked about conductors and dielectrics and briefly mentioned that there are intermediate form 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, silicon are used for the production of semiconductors, and less often - selenium, cuprous oxide and other substances.
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.
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 due to thermal movement can leave their atoms. 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 have left 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 sides and do not go beyond 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 .
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.
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 moment in 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 a 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, 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 doped with indium must contain a small amount 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).
