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 are 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 affected 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 matter 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. With decreasing temperature, the resistance of metals decreases. In semiconductors, on the contrary, with decreasing temperature, the resistance increases and near absolute zero they practically become insulators.
Resistivity of a pure semiconductor as a function of temperature.
Such a course of dependence ρ (T) shows that the concentration of free charge carriers in semiconductors does not remain constant, but increases with increasing temperature. Mechanism electric current in semiconductors cannot be explained in terms of the free electron gas model. Let us consider this mechanism qualitatively using germanium (Ge) as an example. In a silicon (Si) crystal, the mechanism is similar.
Germanium atoms in the outer shell have four weakly bound electrons. They are called covalent electrons. In a crystal lattice, each atom is surrounded by 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
The valence electrons in a germanium crystal are bound to atoms much more strongly than in metals; therefore, the concentration of conduction electrons at room temperature in semiconductors is many orders of magnitude smaller than in metals. Near absolute zero temperature in a germanium crystal, all electrons are engaged in the formation of bonds. Such a crystal does not conduct electricity.
As the temperature rises, some of the valence electrons can gain enough energy to break covalent bonds. Then free electrons (conduction electrons) will appear in the crystal. At the same time, vacancies that are not occupied by electrons are formed at the sites of bond breaking. These vacancies are called holes. A 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 is going on - when a free electron meets a hole, the electronic bond between germanium atoms is restored. This process is called recombination. Electron-hole pairs can also be produced when a semiconductor is illuminated due to energy electromagnetic radiation. In the absence of 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 mechanism of conduction manifests itself only in pure (i.e., without impurities) semiconductors. It is called With own electrical conductivity semiconductors .
In the presence of impurities, the electrical conductivity of semiconductors changes greatly. For example, adding phosphorus impurities to a silicon crystal in the amount of 0.001 atomic percent 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. Necessary condition A sharp decrease in the resistivity of a semiconductor with the introduction of impurities is the difference in the valency 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 conduction - electronic and hole.
Electronic conductivity occurs when pentavalent atoms (for example, arsenic, As) are introduced into a germanium crystal with tetravalent atoms. 
Semiconductor n - type. Arsenic atom in germanium crystal lattice.
The figure shows a pentavalent arsenic atom in a lattice site of germanium. The four valence electrons of the arsenic atom are involved in the formation of covalent bonds with four neighboring germanium atoms. The fifth valence electron turned out to be redundant; it easily detaches from the arsenic atom and becomes free. An atom that has lost an electron turns into a positive ion located at a site in the crystal lattice. An admixture of atoms with a valency greater than the valency 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. This leads to a sharp decrease in the resistivity of the semiconductor - by thousands and even millions of times. The resistivity of a conductor with a high content of impurities can approach that of a metallic conductor.
In a germanium crystal with an arsenic impurity, there are electrons and holes responsible for the intrinsic conductivity of the crystal. 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, In) are introduced into a germanium crystal. The figure shows an indium atom that, with the help of 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 an indium atom from a 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 in the crystal, many covalent bonds are broken and vacant sites (holes) are formed. Electrons can jump to these places from neighboring covalent bonds, which leads to random wandering of holes around 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 have arisen due to the mechanism of intrinsic electrical conductivity of the semiconductor: n p >> n n . This type of conduction 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-race 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 currents and voltages, provided that the concentrations of free carriers are constant.
With the discovery of semiconductors and the study of their properties, it became possible to create circuits based on diodes and transistors. Soon due to the best performance characteristics and smaller sizes, they replaced vacuum tubes, then it became possible to produce integrated circuits based on semiconductor elements.
To define semiconductors is to characterize them in terms of their ability to conduct electric current. Data crystalline substances electrical conductivity increases with increasing temperature, exposure to light, the presence of various impurities.
Semiconductors are wide-gap and narrow-gap, which determines the properties of semiconductor materials. The band gap, measured in electron volts (eV), determines the electrical conductivity. This parameter can be represented as the energy that an electron needs to penetrate into the zone of electric current. On average, for semiconductors, it is 1 eV, it can be more or less.
If the regularity of the crystal lattice of semiconductors is violated by a foreign atom, then such conductivity will be an impurity. When semiconductor substances are intended to create microcircuit elements, impurities are specially added to them, which form increased accumulations of holes or electrons:
Important! The main factor affecting the electrical conductivity of conductors is temperature.
Examples of semiconductors are silicon, germanium. In the crystals of these substances, the atoms have covalent bonds. As the temperature rises, some electrons can be released. The atom that has lost an electron then becomes a positively charged ion. And the electron, not being able to move to another atom due to the saturation of the bonds, turns out to be free. Under the influence of an electric field, the released electrons can move in a directed stream.
An ion that has lost an electron tends to “take away” another from the nearest atom. If he succeeds, then this atom will already be stopped by an ion, in turn, trying to replace the lost electron. Thus, there is a movement of "holes" (positive charges), which can also become ordered in an electric field.
An increased temperature allows electrons to be released more energetically, which leads to a decrease in the resistance of the semiconductor and an increase in conductivity. Electrons and holes are related approximately in equal proportions in pure crystals, such conductivity is called intrinsic.
Impurity types of conductivity are divided into:
Semiconductor elements mainly function based on the features of the p-n junction. When two materials different type conductivity to bring into contact, at the boundary between them there will be an interpenetration of electrons and holes in opposite zones.
Important! The process of interchange of semiconductor materials by positive and negative charge carriers has time limits - before the formation of the barrier layer.
Carriers of positive and negative charge accumulate in the connected parts, on both sides of the line of contact. The resulting potential difference can reach 0.6 V.
When an element with a p-n junction enters an electric field, its conductivity will depend on the connection of the power supply (PS). With "plus" on the part with p-conductivity and "minus" on the part with n-conductivity, the blocking layer will be destroyed, and current will flow through the junction. If the power supply is connected in the opposite way, the blocking layer will increase even more and let through an electric current of negligible magnitude.
Important! P-n-junction has one-sided conductivity.
Based on the properties of semiconductors, various devices have been created that are used in radio engineering, electronics and other fields.
The one-way conductance of semiconductor diodes has determined the scope of their application - mainly in the rectification of alternating current. Other types of diodes:
This diode does not consist of two semiconductor materials, instead the semiconductor is in contact with the metal. Since the metal does not have a crystalline structure, there cannot be holes in it. This means that at the point of contact with the semiconductor material, only electrons from both sides are capable of penetrating, performing the work function. This becomes possible when:
At the point of contact, the semiconductor will lose charge carriers, its conductivity will decrease. A barrier is created, which is overcome by a direct voltage of the required value. Reverse voltage practically blocks the diode, which works as a rectifier. Due to their high speed, Schottky diodes are used in pulse circuits, in computing devices, they also serve as power diodes for rectifying a significant current.
Almost no microcircuit can do without transistors, semiconductor elements with two p-n junctions. The transistor element has three output contacts:
If a low power control signal is applied to the base, much more current is passed between the collector and emitter. When no signal is applied to the base, no current is conducted. Thus, the current strength can be adjusted. A device is used to amplify the signal and contactless switching of the circuit.
Types of semiconductor transistors:
Doping is the introduction of impurity elements, donor and acceptor, into semiconductor crystals to control their conductivity. This occurs during the crystal growth period or by local introduction in certain zones.
Applied methods:
There are other ways of doping: chemical etching, the creation of thin films by sputtering.
The main thing in obtaining semiconductors is their purification from unnecessary impurities. Among the many ways to obtain them, two of the most commonly used can be distinguished:
Differences in composition determine the scope of semiconductors:
Now it is difficult to name a field of technology where there would be no semiconductor materials used, including in the absence of a p-n junction, for example, thermal resistance in temperature sensors, photoresistors in remote controls and others.
Our article will consider examples of semiconductors, their properties and applications. These materials have their place in radio engineering and electronics. They are something between a dielectric and a conductor. By the way, plain glass can also be considered a semiconductor - in its normal state it does not conduct current. But with strong heating (almost to a liquid state), a change in properties occurs and the glass becomes a conductor. But this is an exceptional example; other materials are a little different.
The conductivity index is about 1000 Ohm * m (at a temperature of 180 degrees). Compared to metals, semiconductors have a decrease in conductivity with increasing temperature. Dielectrics have the same property. Semiconductor materials have a fairly strong dependence of the conductivity index on the amount and type of impurities.
For example, if only a thousandth of arsenic is introduced into pure germanium, the conductivity will increase by about 10 times. Without exception, all semiconductors are sensitive to external influences - nuclear radiation, light, electromagnetic fields, pressure, etc. Examples of semiconductor materials can be given - these are antimony, silicon, germanium, tellurium, phosphorus, carbon, arsenic, iodine, boron, as well as various compounds of these substances.
Due to the fact that semiconductor materials have such specific properties, they are quite widespread. Diodes, transistors, triacs, lasers, thyristors, sensors for pressure, magnetic field, temperature, etc. are made on their basis. After the development of semiconductors, a radical transformation took place in automation, radio engineering, cybernetics and electrical engineering. It was through the use of semiconductors that it was possible to achieve such small dimensions of equipment - there is no need to use massive power supplies and radio tubes the size of a one and a half liter jar.
In conductors, the current is determined by where the free electrons move. There are a lot of free electrons in semiconductor materials, and there are reasons for this. All valence electrons that are present in a semiconductor are not free, as they bond with their atoms.
In semiconductors, current can appear and change over a fairly wide range, but only if there is an external influence. The current changes with heating, irradiation, the introduction of impurities. All influences can significantly increase the energy of valence electrons, which contributes to their detachment from atoms. And the applied voltage causes these electrons to move in a certain direction. In other words, these electrons become current carriers.
With an increase in temperature or intensity of external irradiation, an increase in the number of free electrons occurs. Therefore, the current increases. Those atoms in a substance that have lost electrons become positive ions, they do not move. A hole remains on the outside of the atom from which the electron has left. Another electron can get into it, which has left its place in the atom nearby. As a result, a hole is formed on the outer part of the neighboring atom - it turns into an ion (positive).
If a voltage is applied to the semiconductor, then the electrons will begin to move from some atoms to neighboring ones in a certain direction. The holes will begin to move in the opposite direction. A hole is a positively charged particle. Moreover, its charge modulo is the same as that of an electron. With the help of such a definition, it is possible to significantly simplify the analysis of all processes that occur in a semiconductor crystal. The current of holes (denoted by I D) is the movement of particles in the direction opposite to the movement of electrons.
A semiconductor has two types of electrical conductivity - electronic and hole. In pure semiconductors (without impurities), the concentration of holes and electrons (N D and N E, respectively) is the same. For this reason, such electrical conductivity is called intrinsic. The total value of the current will be equal to:
But if we take into account the fact that electrons have a greater mobility value than holes, we can come to the following inequality:
Charge mobility is denoted by the letter M, this is one of the main properties of semiconductors. Mobility is the ratio of two parameters. The first is the speed of movement of the charge carrier (indicated by the letter V with the index "E" or "D", depending on the type of carrier), the second is the electric field strength (indicated by the letter E). It can be expressed in the form of formulas:
M E \u003d (V E / E).
M D \u003d (V D / E).
Mobility allows you to determine the path that a hole or electron travels in one second at a tension value of 1 V/cm. one can now calculate the intrinsic current of the semiconductor material:
I \u003d N * e * (M E + M D) * E.
But it should be noted that we have equalities:
N \u003d N E \u003d N D.
The letter e in the formula denotes the charge of an electron (this is a constant value).
You can immediately give examples semiconductor devices- these are transistors, thyristors, diodes, and even microcircuits. Of course, this is far from full list. To make a semiconductor device, you need to use materials that have hole or electronic conductivity. To obtain such a material, it is necessary to introduce an additive into an ideally pure semiconductor with an impurity concentration of less than 10 -11% (it is called a dopant).
Those impurities, in which the valency is greater than that of the semiconductor, give away free electrons. These impurities are called donors. But those whose valency is less than that of a semiconductor tend to grab and hold electrons. They are called acceptors. In order to get a semiconductor that will have only electronic type conductivity, in raw material it is enough to introduce a substance whose valency will be only one more. For an example of semiconductors in the physics of a school course, germanium is considered - its valence is 4. A donor is added to it - phosphorus or antimony, their valency is five. There are few semiconductor metals, they are practically not used in technology.
In this case, 4 electrons in each atom carry out the installation of four pair (covalent) bonds with germanium. The fifth electron does not have such a bond, which means that it is in a free state. And if you apply voltage to it, it will form an electronic current.
When the electron current is greater than the holes, the semiconductor is called n-type (negative). Consider an example - a little acceptor impurity (say, boron) is introduced into ideally pure germanium. In this case, each acceptor atom will begin to establish covalent bonds with germanium. But the fourth atom of germanium has no connection with boron. Therefore, a certain number of germanium atoms will have only one electron without a covalent bond.
But a slight influence from the outside is enough for the electrons to start leaving their places. In this case, holes are formed in germanium.
The figure shows that on the 2nd, 4th, and 6th atoms, free electrons begin to attach to boron. For this reason, no current is generated in the semiconductor. Holes with numbers 1, 3 and 5 are formed on the surface of germanium atoms - with their help, electrons from adjacent atoms pass to them. On the latter, holes begin to appear, as electrons fly away from them.
Each hole that arises will begin to move between the germanium atoms. When a voltage is applied, the holes begin to move in an orderly manner. In other words, a current of holes appears in the substance. This type of semiconductor is called hole or p-type. When a voltage is applied, not only electrons move, but also holes - they meet various obstacles in their path. In this case, there is a loss of energy, a deviation from the original trajectory. In other words, the carrier charge is dissipated. All this is due to the fact that the semiconductor contains contaminants.
A little higher, examples of semiconductor substances that are used in modern technology were considered. All materials have their own characteristics. In particular, one of the key properties is the non-linearity of the current-voltage characteristic.
In other words, when there is an increase in voltage that is applied to the semiconductor, there is a rapid increase in current. In this case, the resistance decreases sharply. This property has found application in a variety of valve arresters. Examples of disordered semiconductors can be considered in more detail in the specialized literature, their use is strictly limited.
A good example: at the operating voltage value, the arrester has a high resistance, so the current does not go to the ground from the power line. But as soon as lightning strikes a wire or a support, the resistance very quickly decreases to almost zero, all the current goes into the ground. And the voltage drops back to normal.
When the voltage polarity is reversed, the current in the semiconductor begins to flow in the opposite direction. And it changes according to the same law. This suggests that the semiconductor element has a symmetrical current-voltage characteristic. In the event that one part of the element is of the hole type, and the second is of the electronic type, then a p-n junction (electron-hole) appears at the boundary of their contact. It is these transitions that are found in all elements - transistors, diodes, microcircuits. But only in microcircuits on one crystal several transistors are assembled at once - sometimes their number is more than a dozen.
Now let's look at how the p-n junction is formed. If the contact between the hole and electron semiconductors is not of very high quality, then a system consisting of two regions is formed. One will have hole conductivity, and the second - electronic.
And the electrons that are in the n-region will begin to diffuse to where their concentration is less - that is, to the p-region. Holes move simultaneously with electrons, but their direction is reversed. With mutual diffusion, there is a decrease in the concentration in the n-region of electrons and in the p-region of holes.
Having considered examples of conductors, semiconductors and dielectrics, one can understand that their properties are different. For example, the main quality of semiconductors is the ability to pass current in only one direction. For this reason, devices made using semiconductors have become widespread in rectifiers. In practice, using several measuring instruments, you can see the operation of semiconductors and evaluate a lot of parameters - both at rest and under the influence of external "stimuli".
Until now, speaking about the ability of substances to conduct electric current, we divided them into conductors and dielectrics. The specific resistance of ordinary conductors is in the range of Ohm m; the resistivity of dielectrics exceeds these values on average by orders of magnitude: Ohm m.
But there are also substances that, in their electrical conductivity, occupy an intermediate position between conductors and dielectrics. it semiconductors: their resistivity at room temperature can take on values in a very wide range of ohm m. Semiconductors include silicon, germanium, selenium, and some other chemical elements and compounds (Semiconductors are extremely common in nature. For example, about 80% of the mass of the earth's crust falls on substances that are semiconductors). Silicon and germanium are the most widely used.
The main feature of semiconductors is that their electrical conductivity increases sharply with increasing temperature. The resistivity of a semiconductor decreases with increasing temperature approximately as shown in Fig. 1 .
Rice. 1. Dependence for a semiconductor
In other words, at low temperatures, semiconductors behave like dielectrics, and at high temperatures, they behave like fairly good conductors. This is the difference between semiconductors and metals: the resistivity of the metal, as you remember, increases linearly with increasing temperature.
There are other differences between semiconductors and metals. Thus, illumination of a semiconductor causes a decrease in its resistance (and light has almost no effect on the resistance of a metal). In addition, the electrical conductivity of semiconductors can change very strongly with the introduction of even a negligible amount of impurities.
Experience shows that, as in the case of metals, when current flows through a semiconductor, there is no transfer of matter. Therefore, the electric current in semiconductors is due to the movement of electrons.
A decrease in the resistance of a semiconductor when it is heated indicates that an increase in temperature leads to an increase in the number of free charges in the semiconductor. Nothing like this happens in metals; therefore, semiconductors have a different mechanism of electrical conductivity than metals. And the reason for this is different nature chemical bond between metal and semiconductor atoms.
The metallic bond, remember, is provided by a gas of free electrons, which, like glue, holds the positive ions at the lattice sites. Semiconductors are arranged differently - their atoms are held together covalent bond. Let's remember what it is.
Electrons located in the outer electronic level and called valence, are weaker bound to the atom than the rest of the electrons, which are located closer to the nucleus. In the process of forming a covalent bond, two atoms contribute "to the common cause" one of their valence electrons. These two electrons are socialized, that is, they now belong to both atoms, and therefore are called common electron pair(Fig. 2).
Rice. 2. Covalent bond
The socialized pair of electrons just holds the atoms near each other (with the help of electrical attraction forces). A covalent bond is a bond that exists between atoms due to common electron pairs.. For this reason, a covalent bond is also called pair-electron.
We are now ready to take a closer look internal organization semiconductors. As an example, consider the most common semiconductor in nature - silicon. The second most important semiconductor, germanium, has a similar structure.
The spatial structure of silicon is shown in fig. 3 (image by Ben Mills). Silicon atoms are depicted as balls, and the tubes connecting them are channels of covalent bonding between atoms.
Rice. 3. Crystal structure of silicon
Note that each silicon atom is bonded to four neighboring atoms. Why is it so?
The fact is that silicon is tetravalent - on the outer electron shell of the silicon atom there are four valence electrons. Each of these four electrons is ready to form a common electron pair with the valence electron of another atom. And so it happens! As a result, the silicon atom is surrounded by four docked atoms, each of which contributes one valence electron. Accordingly, there are eight electrons around each atom (four own and four alien).
We see this in more detail in flat pattern silicon crystal lattice (Fig. 4).
Rice. 4. Crystal lattice of silicon
Covalent bonds are shown as pairs of lines connecting atoms; these lines share electron pairs. Each valence electron located on such a line spends most of its time in the space between two neighboring atoms.
However, valence electrons are by no means "tightly tied" to the corresponding pairs of atoms. Electron shells overlap all neighboring atoms, so that any valence electron is the common property of all neighboring atoms. From some atom 1, such an electron can go to its neighboring atom 2, then to its neighboring atom 3, and so on. Valence electrons can move throughout the space of the crystal - they are said to belong to the whole crystal(rather than any single atomic pair).
However, silicon's valence electrons are not free (as is the case in metal). In a semiconductor, the bond between valence electrons and atoms is much stronger than in a metal; silicon covalent bonds do not break at low temperatures. The energy of the electrons is not enough to start an orderly movement from a lower potential to a higher one under the action of an external electric field. Therefore, at sufficiently low temperatures, semiconductors are close to dielectrics - they do not conduct electric current.
If included in electrical circuit semiconductor element and begin to heat it, then the current in the circuit increases. Therefore, the semiconductor resistance decreases with an increase in temperature. Why is this happening?
As the temperature rises, the thermal vibrations of silicon atoms become more intense, and the energy of valence electrons increases. For some electrons, the energy reaches values sufficient to break covalent bonds. Such electrons leave their atoms and become free(or conduction electrons) is exactly the same as in metal. In an external electric field, free electrons begin an ordered movement, forming an electric current.
The higher the silicon temperature, the greater the energy of the electrons, and the large quantity covalent bonds does not withstand and breaks. The number of free electrons in a silicon crystal increases, which leads to a decrease in its resistance.
The breaking of covalent bonds and the appearance of free electrons is shown in fig. five . At the site of a broken covalent bond, a hole is a vacancy for an electron. The hole has positive charge, since with the departure of a negatively charged electron, an uncompensated positive charge of the nucleus of the silicon atom remains.
Rice. 5. Formation of free electrons and holes
Holes do not stay in place - they can wander around the crystal. The fact is that one of the neighboring valence electrons, "traveling" between atoms, can jump to the formed vacancy, filling the hole; then the hole in this place will disappear, but will appear in the place where the electron came from.
In the absence of an external electric field, the movement of holes is random, because valence electrons wander between atoms randomly. However, in an electric field directed hole movement. Why? It's easy to understand.
On fig. 6 shows a semiconductor placed in an electric field. On the left side of the figure is the initial position of the hole.
Rice. 6. Motion of a hole in an electric field
Where will the hole go? It is clear that the most probable are hops "electron > hole" in the direction vs field lines (that is, to the "pluses" that create the field). One of these jumps is shown in the middle part of the figure: the electron jumped to the left, filling the vacancy, and the hole, accordingly, shifted to the right. The next possible jump of an electron caused by an electric field is shown on the right side of the figure; as a result of this jump, the hole took a new place, located even more to the right.
We see that the hole as a whole moves towards field lines - that is, where positive charges are supposed to move. We emphasize once again that the directed motion of a hole along the field is caused by hops of valence electrons from atom to atom, occurring predominantly in the direction against the field.
Thus, there are two types of charge carriers in a silicon crystal: free electrons and holes. When an external electric field is applied, an electric current appears, caused by their ordered counter motion: free electrons move opposite to the field strength vector, and holes move in the direction of the vector.
The occurrence of current due to the movement of free electrons is called electronic conductivity, or n-type conductivity. The process of orderly movement of holes is called hole conductivity,or p-type conductivity(from the first letters of the Latin words negativus (negative) and positivus (positive)). Both conductivities - electron and hole - together are called own conductivity semiconductor.
Each departure of an electron from a broken covalent bond generates a “free electron-hole” pair. Therefore, the concentration of free electrons in a pure silicon crystal is equal to the concentration of holes. Accordingly, when the crystal is heated, the concentration of not only free electrons, but also holes increases, which leads to an increase in the intrinsic conductivity of the semiconductor due to an increase in both electronic and hole conductivity.
Along with the formation of “free electron-hole” pairs, the reverse process also takes place: recombination free electrons and holes. Namely, a free electron, meeting with a hole, fills this vacancy, restoring the broken covalent bond and turning into a valence electron. Thus, in a semiconductor, dynamic balance: the average number of breaks of covalent bonds and the resulting electron-hole pairs per unit time is equal to the average number of recombining electrons and holes. This state of dynamic equilibrium determines the equilibrium concentration of free electrons and holes in a semiconductor under given conditions.
Change external conditions shifts the state of dynamic equilibrium in one direction or another. The equilibrium value of the concentration of charge carriers naturally changes in this case. For example, the number of free electrons and holes increases when a semiconductor is heated or illuminated.
At room temperature, the concentration of free electrons and holes in silicon is approximately equal to cm. The concentration of silicon atoms is about cm. In other words, there is only one free electron per silicon atom! This is very little. In metals, for example, the concentration of free electrons is approximately equal to the concentration of atoms. Respectively, intrinsic conductivity of silicon and other semiconductors under normal conditions is small compared to the conductivity of metals.
The most important feature of semiconductors is that their resistivity can be reduced by several orders of magnitude by introducing even a very small amount of impurities. In addition to its own conductivity, a semiconductor has a dominant impurity conductivity. It is thanks to this fact that semiconductor devices have found such wide application in science and technology.
Suppose, for example, that a little pentavalent arsenic is added to the silicon melt. After crystallization of the melt, it turns out that arsenic atoms occupy places in some sites of the formed silicon crystal lattice.
The outer electronic level of an arsenic atom has five electrons. Four of them form covalent bonds with the nearest neighbors - silicon atoms (Fig. 7). What is the fate of the fifth electron not occupied in these bonds?
Rice. 7. N-type semiconductor
And the fifth electron becomes free! The fact is that the binding energy of this "extra" electron with an arsenic atom located in a silicon crystal is much less than the binding energy of valence electrons with silicon atoms. Therefore, already at room temperature, almost all arsenic atoms as a result thermal motion remain without the fifth electron, turning into positive ions. And the silicon crystal, respectively, is filled with free electrons, which are unhooked from the arsenic atoms.
The filling of a crystal with free electrons is not new to us: we have seen it above when it was heated clean silicon (without any impurities). But now the situation is fundamentally different: the appearance of a free electron leaving the arsenic atom is not accompanied by the appearance of a mobile hole. Why? The reason is the same - the bond of valence electrons with silicon atoms is much stronger than with the arsenic atom on the fifth vacancy, so the electrons of neighboring silicon atoms do not tend to fill this vacancy. Thus, the vacancy remains in place; it is, as it were, "frozen" to the arsenic atom and does not participate in the creation of the current.
In this way, the introduction of pentavalent arsenic atoms into the silicon crystal lattice creates electronic conductivity, but does not lead to the symmetrical appearance of hole conductivity. The main role in creating the current now belongs to free electrons, which in this case are called main carriers charge.
The intrinsic conduction mechanism, of course, continues to operate even in the presence of an impurity: covalent bonds are still broken due to thermal motion, generating free electrons and holes. But now there are much fewer holes than free electrons, which are provided in large quantities by arsenic atoms. Therefore, the holes in this case will be minority carriers charge.
Impurities whose atoms donate free electrons without the appearance of an equal number of mobile holes are called donor. For example, pentavalent arsenic is a donor impurity. In the presence of a donor impurity in the semiconductor, free electrons are the main charge carriers, and holes are the minor ones; in other words, the concentration of free electrons is much higher than the concentration of holes. Therefore, semiconductors with donor impurities are called electronic semiconductors, or n-type semiconductors(or simply n-semiconductors).
And how much, interestingly, can the concentration of free electrons exceed the concentration of holes in an n-semiconductor? Let's do a simple calculation.
Suppose that the impurity is , that is, there is one arsenic atom per thousand silicon atoms. The concentration of silicon atoms, as we remember, is on the order of cm.
The concentration of arsenic atoms, respectively, will be a thousand times less: cm. The concentration of free electrons donated by the impurity will also turn out to be the same - after all, each arsenic atom gives off an electron. And now let's remember that the concentration of electron-hole pairs that appear when silicon covalent bonds are broken at room temperature is approximately equal to cm. Do you feel the difference? The concentration of free electrons in this case is greater than the concentration of holes by orders of magnitude, that is, a billion times! Accordingly, the resistivity of a silicon semiconductor decreases by a factor of a billion when such a small amount of impurity is introduced.
The above calculation shows that in n-type semiconductors, the main role is indeed played by electronic conductivity. Against the background of such a colossal superiority in the number of free electrons, the contribution of the motion of holes to the total conductivity is negligibly small.
It is possible, on the contrary, to create a semiconductor with a predominance of hole conductivity. This will happen if a trivalent impurity is introduced into a silicon crystal - for example, indium. The result of such implementation is shown in Fig. 8 .
Rice. 8. p-type semiconductor
What happens in this case? The outer electronic level of the indium atom has three electrons that form covalent bonds with the three surrounding silicon atoms. For the fourth neighboring silicon atom, the indium atom no longer has enough electron, and a hole appears in this place.
And this hole is not simple, but special - with a very high binding energy. When an electron from a neighboring silicon atom enters it, it will “stuck forever” in it, because the attraction of an electron to an indium atom is very large - more than to silicon atoms. The indium atom will turn into a negative ion, and in the place where the electron came from, a hole will appear - but now an ordinary mobile hole in the form of a broken covalent bond in the silicon crystal lattice. This hole in the usual way will begin to wander around the crystal due to the "relay" transfer of valence electrons from one silicon atom to another.
And so, each impurity atom of indium generates a hole, but does not lead to the symmetrical appearance of a free electron. Such impurities, the atoms of which "tightly" capture electrons and thereby create a mobile hole in the crystal, are called acceptor.
Trivalent indium is an example of an acceptor impurity.
If an acceptor impurity is introduced into a pure silicon crystal, then the number of holes generated by the impurity will be much more number free electrons that have arisen due to the breaking of covalent bonds between silicon atoms. A semiconductor with an acceptor dopant is hole semiconductor, or p-type semiconductor(or simply p-semiconductor).
holes play leading role when creating a current in a p-semiconductor; holes - major charge carriers. Free electrons - minor carriers charge in a p-semiconductor. The motion of free electrons in this case does not make a significant contribution: the electric current is provided primarily by hole conduction.
The contact point of two semiconductors with various types conductivity (electronic and hole) is called electron-hole transition, or p–n junction. In the region of the p–n junction, an interesting and very important phenomenon arises - one-way conduction.
On fig. 9 shows the contact of p- and n-type regions; colored circles are holes and free electrons, which are the majority (or minor) charge carriers in the respective regions.
Rice. 9. Blocking layer p–n junction
By performing thermal motion, charge carriers penetrate through the interface between the regions.
Free electrons pass from the n-region to the p-region and recombine there with holes; holes diffuse from the p-region to the n-region and recombine there with electrons.
As a result of these processes, an uncompensated charge of the positive ions of the donor impurity remains in the electronic semiconductor near the contact boundary, while in the hole semiconductor (also near the boundary), an uncompensated negative charge of the acceptor impurity ions arises. These uncompensated space charges form the so-called barrier layer, whose internal electric field prevents further diffusion of free electrons and holes through the contact boundary.
Let us now connect a current source to our semiconductor element by applying the “plus” of the source to the n-semiconductor, and the “minus” to the p-semiconductor (Fig. 10).
Rice. 10. Turn on in reverse: no current
We see that the external electric field takes the majority charge carriers farther from the contact boundary. The width of the barrier layer increases, and its electric field increases. The resistance of the barrier layer is high, and the main carriers are not able to overcome the p–n junction. The electric field allows only minority carriers to cross the boundary, however, due to the very low concentration of minority carriers, the current they create is negligible.
The considered scheme is called turning on the p–n junction in the opposite direction. There is no electric current of the main carriers; there is only a negligible minority carrier current. In this case, the p–n junction is closed.
Now let's change the polarity of the connection and apply "plus" to the p-semiconductor, and "minus" to the n-semiconductor (Fig. 11). This scheme is called switching in forward direction.
Rice. 11. Forward switching: current flows
In this case, the external electric field is directed against the blocking field and opens the way for the main carriers through the p–n junction. The barrier layer becomes thinner, its resistance decreases.
There is a mass movement of free electrons from the n-region to the p-region, and holes, in turn, rush together from the p-region to the n-region.
A current arises in the circuit, caused by the movement of the main charge carriers (Now, however, the electric field prevents the current of minority carriers, but this negligible factor does not have a noticeable effect on the overall conductivity).
One-sided conduction of the p–n junction is used in semiconductor diodes. A diode is a device that conducts current in only one direction; in opposite direction no current flows through the diode (the diode is said to be closed). A schematic representation of the diode is shown in fig. 12 .
Rice. 12. Diode
In this case, the diode is open in the direction from left to right: the charges seem to flow along the arrow (see it in the figure?). In the direction from right to left, the charges seem to rest against the wall - the diode is closed.
Semiconductors are characterized by both the properties of conductors and dielectrics. In semiconductor crystals, atoms establish covalent bonds (that is, one electron in a silicon crystal, like diamond, is bonded by two atoms), electrons need a level of internal energy to be released from an atom (1.76 10 −19 J versus 11.2 10 −19 J, which characterizes the difference between semiconductors and dielectrics). This energy appears in them when the temperature rises (for example, at room temperature, the energy level of the thermal motion of atoms is 0.4 10 −19 J), and individual atoms receive energy to detach an electron from an atom. With increasing temperature, the number of free electrons and holes increases, therefore, in a semiconductor that does not contain impurities, the resistivity decreases. It is conventionally accepted to consider as semiconductors elements with an electron binding energy of less than 1.5-2 eV. The electron-hole mechanism of conduction manifests itself in intrinsic (that is, without impurities) semiconductors. It is called intrinsic electrical conductivity of semiconductors.
When the bond between the electron and the nucleus is broken, a free space appears in the electron shell of the atom. This causes the transfer of an electron from another atom to an atom with free space. The atom from which the electron passed, another electron enters from another atom, etc. This is due to the covalent bonds of atoms. Thus, there is a movement of a positive charge without moving the atom itself. This conditional positive charge is called a hole.
At thermodynamic equilibrium, the electron density of a semiconductor is related to temperature by the following relation:
- Planck's constant - electron mass - temperature; - the level of the conducted band - the Fermi level;Also, the hole density of a semiconductor is related to temperature by the following relation:
- Planck's constant; is the mass of the hole; - temperature ; - Fermi level; is the level of the valence band.The intrinsic density is related to and by the following relation:
Semiconductors in which free electrons and "holes" appear in the process of ionization of the atoms from which the entire crystal is built are called semiconductors with intrinsic conductivity. In semiconductors with intrinsic conductivity, the concentration of free electrons is equal to the concentration of "holes".
Conductivity is related to particle mobility by the following relationship:
where is the resistivity, is the mobility of electrons, is the mobility of holes, is their concentration, q is the elementary electric charge (1.602 10 −19 C).
For an intrinsic semiconductor, the carrier concentrations are the same and the formula takes the form:
To create semiconductor devices, crystals with impurity conductivity are often used. Such crystals are made by introducing impurities with atoms of a trivalent or pentavalent chemical element.
n-type semiconductor
Term "n-type" comes from the word "negative", denoting the negative charge of the majority carriers. This type of semiconductor has an impurity nature. An impurity of a pentavalent semiconductor (for example, arsenic) is added to a tetravalent semiconductor (for example, silicon). In the process of interaction, each impurity atom enters into a covalent bond with silicon atoms. However, there is no place for the fifth electron of the arsenic atom in saturated valence bonds, and it passes to the far electron shell. There, a smaller amount of energy is needed to detach an electron from an atom. The electron breaks off and becomes free. In this case, the charge transfer is carried out by an electron, and not by a hole, that is, this type Semiconductors conduct electricity like metals. Impurities that are added to semiconductors, as a result of which they turn into n-type semiconductors, are called donor impurities.
The conductivity of N-semiconductors is approximately equal to:
p-type semiconductor
Term "p-type" comes from the word "positive", denoting the positive charge of the majority carriers. This type of semiconductors, in addition to the impurity base, is characterized by the hole nature of conductivity. In a tetravalent semiconductor (for example, silicon) is added a small amount of atoms of a trivalent element (for example, indium). Each impurity atom establishes a covalent bond with three neighboring silicon atoms. To establish a bond with the fourth silicon atom, the indium atom does not have a valence electron, so it captures a valence electron from a covalent bond between neighboring silicon atoms and becomes a negatively charged ion, as a result of which a hole is formed. The impurities that are added in this case are called acceptor impurities.
The conductivity of p-semiconductors is approximately equal to:
A semiconductor diode consists of two types of semiconductors - hole and electronic. During the contact between these regions, electrons pass from the region with the n-type semiconductor to the region with the p-type semiconductor, which then recombine with holes. As a result, an electric field arises between the two regions, which sets the limit for the division of semiconductors - the so-called p-n junction. As a result, an uncompensated charge from negative ions arises in the region with a p-type semiconductor, and an uncompensated charge from positive ions arises in the region with an n-type semiconductor. The difference between the potentials reaches 0.3-0.6 V.
The relationship between the potential difference and the impurity concentration is expressed by the following formula:
where is the thermodynamic stress, is the concentration of electrons, is the concentration of holes, is the intrinsic concentration .
In the process of applying voltage with a plus to the p-semiconductor and a minus to the n-semiconductor, the external electric field will be directed against the internal electric field of the p-n junction and, with sufficient voltage, the electrons will overcome the p-n junction, and an electric current will appear in the diode circuit (forward conduction). When a voltage is applied minus to the region with a p-type semiconductor and plus to the region with an n-type semiconductor, a region arises between the two regions that does not have free electric current carriers (reverse conductivity). The reverse current of a semiconductor diode is not zero, since there are always minor charge carriers in both regions. For those carriers p-n the transition will be open.
Thus, the p-n junction exhibits the properties of one-way conduction, which is caused by applying voltage with different polarity. This property is used to rectify alternating current.
A transistor is a semiconductor device that consists of two regions with p- or n-type semiconductors, between which there is an area with an n- or p-type semiconductor. Thus, in a transistor there are two area p-n transition. The region of the crystal between the two junctions is called the base, and the outer regions are called the emitter and collector. The most commonly used transistor switching circuit is a common emitter switching circuit, in which current flows through the base and emitter to the collector.
A bipolar transistor is used to amplify electric current.
The following table provides information on a large number of semiconductor elements and their connections, divided into several types:
All types of semiconductors have an interesting dependence of the band gap on the period, namely, as the period increases, the band gap decreases.
| Group | IIB | IIIA | IVA | VA | VIA |
| Period | |||||
| 2 | 5 | 6 | 7 | ||
| 3 | 13 | 14 | 15 | 16 | |
| 4 | 30 | 31 | 32 | 33 | 34 |
| 5 | 48 | 49 | 50 | 51 | 52 |
| 6 | 80 |
First of all, it should be said that the physical properties of semiconductors are the most studied in comparison with metals and dielectrics. To a large extent, this is facilitated by a huge number of effects that cannot be observed in either substance, primarily related to the arrangement of the band structure of semiconductors and the presence of a fairly narrow band gap. Of course, the main stimulus for the study of semiconductors is the production of semiconductor devices and integrated circuits - this primarily applies to silicon, but also affects other compounds (, GaAs, InP, InSb).
Due to the fact that technologists can obtain very pure substances, the question arises of a new standard for the Avogadro number.
The bulk properties of a semiconductor can be highly dependent on the presence of defects in the crystal structure. And so they strive to grow very pure substances, mainly for the electronics industry. Dopants are introduced to control the magnitude and type of semiconductor conductivity. For example, widespread silicon can be doped with an element of the V subgroup of the periodic system of elements - phosphorus, which is a donor, and create n-Si. To obtain silicon with a hole type of conductivity (p-Si), boron (acceptor) is used. Compensated semiconductors are also created in order to fix the Fermi level in the middle of the band gap.
To obtain single crystals of semiconductors use various methods physical and chemical deposition. The most precise and expensive tool in the hands of technologists for the growth of single-crystal films is the molecular-beam epitaxy installation, which allows growing a crystal with an accuracy of a monolayer.
The absorption of light by semiconductors is due to transitions between the energy states of the band structure. Given the Pauli exclusion principle, electrons can only move from a filled energy level to an unfilled one. In an intrinsic semiconductor, all states of the valence band are filled, and all states of the conduction band are unfilled, so transitions are possible only from the valence band to the conduction band. To carry out such a transition, the electron must receive energy from light that exceeds the band gap. Photons with lower energy do not cause transitions between the electronic states of a semiconductor, so such semiconductors are transparent in the frequency range , where is the band gap, is Planck's constant. This frequency defines the fundamental absorption edge for a semiconductor. For semiconductors, which are often used in electronics (silicon, germanium, gallium arsenide), it lies in the infrared region of the spectrum.
Additional restrictions on the absorption of light by semiconductors are imposed by selection rules, in particular the law of conservation of momentum. The law of conservation of momentum requires that the quasi-momentum of the final state differ from the quasi-momentum of the initial state by the magnitude of the momentum of the absorbed photon. The photon wave number , where is the wavelength, is very small compared to the wave vector of the reciprocal lattice of the semiconductor, or, equivalently, the wavelength of the photon in the visible region is much larger than the characteristic interatomic distance in the semiconductor, which leads to the requirement that the quasi-momentum of a finite state during the electronic transition was practically equal to the quasi-momentum of the initial state. At frequencies close to the fundamental absorption edge, this is only possible for direct-gap semiconductors. Optical transitions in semiconductors, in which the electron momentum almost does not change, are called direct or vertical. The momentum of the final state can differ significantly from the momentum of the initial state if another, third particle, for example, a phonon, participates in the process of absorption of a photon. Such transitions are also possible, although less likely. They're called indirect transitions.
Thus, direct-gap semiconductors such as gallium arsenide begin to absorb light strongly when the quantum energy exceeds the band gap. Such semiconductors are very suitable for use in optoelectronics.
Indirect-gap semiconductors, for example, silicon, absorb much weaker in the frequency range of light with a quantum energy slightly more than the band gap, only due to indirect transitions, the intensity of which depends on the presence of phonons, and therefore on temperature. The limiting frequency of direct transitions of silicon is more than 3 eV, that is, it lies in the ultraviolet region of the spectrum.
When an electron passes from the valence band to the conduction band, free charge carriers appear in the semiconductor, and hence photoconductivity.
At frequencies below the fundamental absorption edge, light absorption is also possible, which is associated with the excitation of excitons, electronic transitions between impurity levels and allowed bands, as well as with the absorption of light on lattice vibrations and free carriers. Exciton bands are located in the semiconductor somewhat below the bottom of the conduction band due to the binding energy of the exciton. Exciton absorption spectra have a hydrogen-like structure energy levels. Similarly, impurities, acceptors or donors, create acceptor or donor levels that lie in the bandgap. They significantly modify the absorption spectrum of the doped semiconductor. If a phonon is absorbed simultaneously with a light quantum during an indirect-gap transition, then the energy of the absorbed light quantum can be lower by the phonon energy, which leads to absorption at frequencies somewhat lower in energy from the fundamental absorption edge.
Semiconductor compounds are divided into several types:
The following compounds are widely used:
A III B V
as well as some oxides of lead, tin, germanium, silicon, as well as ferrite, amorphous glasses and many other compounds (A I B III C 2 VI, A I B V C 2 VI, A II B IV C 2 V, A II B 2 II C 4 VI, A II B IV C 3 VI).
Based on most of the above binary compounds, it is possible to obtain their solid solutions: (CdTe) x (HgTe) 1-x, (HgTe) x (HgSe) 1-x, (PbTe) x (SnTe) 1-x, (PbSe) x (SnSe) 1-x and others.
Connections A III B V are mainly used for electronic products operating at microwave frequencies
A II B V compounds are used as visible region phosphors, LEDs, Hall sensors, modulators.
Compounds A III B V , A II B VI and A IV B VI are used in the manufacture of light sources and receivers, indicators and radiation modulators.
Oxide semiconductor compounds are used for the manufacture of photocells, rectifiers and high-frequency inductor cores.
| Options | AlSb | GaSb | InSb | AlAs | GaAs | InAs |
|---|---|---|---|---|---|---|
| Melting point, K | 1333 | 998 | 798 | 1873 | 1553 | 1218 |
| lattice constant, | 6,14 | 6,09 | 6,47 | 5,66 | 5,69 | 6,06 |
| Band gap Δ E, eV | 0,52 | 0,7 | 0,18 | 2,2 | 1,32 | 0,35 |
| Dielectric constant ε | 8,4 | 14,0 | 15,9 | - | - | - |
| Mobility, cm²/(V s): | ||||||
| electrons | 50 | 5000 | 60 000 | - | 4000 | 3400 |
| holes | 150 | 1000 | 4000 | - | 400 | 460 |
| Refractive index, n | 3,0 | 3,7 | 4,1 | - | 3,2 | 3,2 |
| Linear thermal coefficient extensions, K -1 |
- | 6.9 10 -6 | 5.5 10 -6 | 5.7 10 -6 | 5.3 10 -6 | - |
