Metals, dielectrics and semiconductors

Solids are divided into metals, dielectrics, and semiconductors, primarily in terms of electrical conductivity. For typical metals, this value is 10 8 ... 10 6 (Ohm m) -1. In dielectrics, the electrical conductivity is negligible:< 10 -8 (Ом м) -1 . Для хороших диэлектриков удельная электропроводность достигает величины 10 -11 (Ом м) -1 . Твердые тела с промежуточной электропроводностью относят к полупроводникам. Оказывается, что столь большие различия в электрических свойствах твердых тел связаны со структурой и степенью заполнения электронами энергетических зон в этих телах.

Despite the fact that the energy bands are quasi-continuous, they consist of a very large but finite number of energy levels. The number of these levels is determined by the number of atoms N combined into a crystal and by the orbital quantum number l:

In each energy zone, in accordance with the Pauli principle, no more than 2 (2 l+ 1) electrons - two with opposite spins at each level. The number of electrons in a crystal is also finite and depends both on the number of atoms N, and the number of electrons in the atom. Since the electrons tend to occupy the energy levels with the lowest energy, in the crystal the lower energy bands are completely filled, and the uppermost ones are either partially filled or completely free.

A partially filled zone is formed, for example, near a sodium crystal. This element has completely filled 1s, 2s and 2p levels, which contain a total of 10 electrons. In the Na crystal, the corresponding 1s, 2s, and 2p bands will also be completely filled. The eleventh valence electron in the Na atom is located at the 3s level, which can contain 2 electrons. Consequently, the 3s band of crystalline sodium will be only half filled. The band structure of Na is shown in fig. 2.8a. The bands filled with electrons and part of the 3s band are shaded. E g - band gap.

Often a partially filled zone is formed as a result of the overlap of a completely filled zone with the next completely free one. An example of such a band structure is shown in Fig. . 2.8b for beryllium, in which the filled 2s and free 2p bands overlap.

A large group consists of crystals in which completely empty bands are located above the completely filled bands, and the band gap varies in them from several tens of eV to units of eV. Typical examples of this group of crystals are shown in fig. 2.8, c, d. This is carbon in the modification of diamond and silicon.

The structure of the energy bands of a crystal has a decisive influence on the magnitude of its electrical conductivity. Since the electric current is a directed movement of charges (in metals - electrons), the occurrence of an electric current is associated with an increase in the momentum of the electrons along the direction of the force acting on it. Together with the momentum of the electron, its wave vector changes. Since the energy and wave vector of an electron are two interrelated quantities, the relationship between which is determined by the dispersion relation, an increase in the wave number must necessarily be accompanied by an increase in the energy of the electron. It is easy to estimate what is the increase in the energy of an electron due to its acceleration in an electric field that causes an electric current in the conductors. If the electric field strength is 10 4 V/m, then at a distance equal to the mean free path of an electron in a crystal, which is usually ~10 -8 m, the electron acquires an energy of approximately 10 -4 eV. It is clear that these energy values ​​allow the electron to move from level to level only within one energy band. The transition between the bands requires an energy greater than the band gap E g , which, as mentioned above, is 0.1 ... 10 eV.

Fig.2.8. Filling of energy bands with electrons

These considerations lead to the conclusion that in order for bodies to have high conductivity, it is necessary that their energy spectrum contain zones that are partially filled. Electrons that have increased their energy under the action of an external electric field can pass to the free levels of these zones (Fig. 2.9). Therefore, bodies with partially filled energy bands are conductors. Partially filled zones have all metals.

Rice. 2.9. Scheme of distribution of electrons in the valence band of an alkali metal: a - in the absence of an electric field; b - in the presence of an electric field.

Now consider crystals, the upper energy band of which is completely filled with electrons (Fig. 2.8, c, d). An external electric field is not able to change the nature of the motion of electrons, since it is not able to raise electrons to the overlying free zone. Inside the completely filled band itself, which does not contain a single free level, it can only cause a rearrangement of electrons in places, which does not violate the symmetry of their velocity distribution. This does not lead to the appearance of an electric current in such crystals.

Thus, solids with energy bands completely filled with electrons are non-conductors. According to the band gap, non-conductors are divided into dielectrics and semiconductors.

Dielectrics are bodies that have a relatively wide bandgap. For typical dielectrics, E g > 3 eV. Thus, for diamond E g = 5.2 eV; for boron nitride E g = 4.6 eV; Al 2 O 3 E g = 7 eV.

Typical semiconductors have a band gap less than 3 eV. For example, germanium has E g = 0.66 eV; silicon has E g = 1.12 eV; for indium antimonide E g = 0.17 eV.

The upper filled band of semiconductors and dielectrics is called valence band, the free zone following it is called conduction band. In metals, a partially filled band is called both the valence band and the conduction band.

All substances consist of molecules, molecules of atoms, atoms of positively charged nuclei around which are located negative electrons. Under certain conditions, electrons are able to leave their nucleus and move to neighboring ones. In this case, the atom itself becomes positively charged, and the neighboring one receives a negative charge. The movement of negative and positive charges under the influence of an electric field is called an electric current.

Depending on the property of materials to conduct electric current, they are divided into:

1. Semiconductors.

Conductor Properties

Conductors are different good electrical conductivity. This is due to the presence of a large number of free electrons that do not specifically belong to any of the atoms, which under the action of an electric field can move freely.

Most conductors have low resistivity and conduct electricity with very little loss. Due to the fact that elements that are ideally pure in chemical composition do not exist in nature, any material contains impurities in its composition. Impurities in conductors occupy places in the crystal lattice and, as a rule, prevent the passage of free electrons under the action of an applied voltage.

Impurities degrade the properties of the conductor. The more impurities, the more they affect the conductivity parameters.

Good conductors with low resistivity are the following materials:

• Gold.
• Silver.
• Copper.
• Aluminum.
• Iron.

Gold and silver are good conductors, but due to their high cost, they are used where it is necessary to obtain good quality conductors with a small volume. These are mainly electronic circuits, microcircuits, conductors of high-frequency devices in which the conductor itself is made of cheap material (copper), which is covered with a thin layer of silver or gold on top. This makes it possible, with a minimum consumption of precious metal, to have good frequency characteristics of the conductor.

Copper and aluminum are cheaper metals. With a slight decrease in the characteristics of these materials, their price is orders of magnitude lower, which makes it possible for their mass application. Used in electronics and electrical engineering. In electronics, these are tracks of printed circuit boards, legs of radioelements, radiators, etc. In electrical engineering, it is very widely used in motor windings, for laying high and low voltage electrical networks, wiring electricity in apartments, houses, and transport.

The conductivity parameter is very dependent on the temperature of the material itself. As the temperature of the crystal increases, the vibrations of electrons in the crystal lattice increase, preventing the free passage of free electrons. With a decrease, on the contrary, the resistance decreases and at a certain value close to absolute zero, the resistance becomes zero and the effect of superconductivity occurs.

Properties of dielectrics

Dielectrics in their crystal lattice contain very few free electrons capable of carrying a charge under the action of an electric field. In this regard, when creating a potential difference on a dielectric, the current passing through it is so small that it is considered equal to zero - the dielectric does not conduct electric current. Along with this, impurities contained in any dielectric, as a rule, worsen its dielectric properties. The current passing through a dielectric under the action of an applied voltage is mainly determined by the amount of impurities.

Dielectrics are most widely used in electrical engineering where it is necessary to protect maintenance personnel from the harmful effects of electric current. These are insulating handles of various devices, measuring devices. In electronics - capacitor gaskets, wire insulation, dielectric gaskets necessary for heat removal of active elements, instrument cases.

Semiconductors are materials that conduct electricity under certain conditions, otherwise behave as dielectrics.

Table: what is the difference between conductors and dielectrics?

		Dielectric
The presence of free electrons	Present in large numbers	Absent, or present, but very few
The ability of materials to conduct electricity	Conducts well	Does not conduct or the current is slightly low
What happens when the applied voltage is increased	The current passing through the conductor increases according to Ohm's law	The current passing through the dielectric changes slightly and, when a certain value is reached, an electrical breakdown occurs
materials	Gold, silver, copper and its alloys, aluminum and alloys, iron and others	Ebonite, fluoroplastic, rubber, mica, various plastics, polyethylene and other materials
Resistance	from 10 -5 to 10 -8 degrees Ohm/m	10 10 – 10 16 Ohm/m
Influence of foreign impurities on the resistance of the material	Impurities degrade the conductivity property of the material, which degrades its properties	Impurities improve the conductivity of the material, which degrades its properties.
Change in properties when the ambient temperature changes	As the temperature increases, the resistance increases; as the temperature decreases, it decreases. At very low temperatures - superconductivity.	As the temperature increases, the resistance decreases.

It is known that in a substance placed in an electric field, under the influence of the forces of this field, the movement of free electrons or ions is formed in the direction of the field forces. In other words, an electric current occurs in the substance.

The property that determines the ability of a substance to conduct an electric current is called "electrical conductivity". The electrical conductivity is directly dependent on the concentration of charged particles: the higher the concentration, the higher the electrical conductivity.

According to this property, all substances are divided into 3 types:

1. Conductors.
2. Semiconductors.

Description of conductors

Conductors have highest electrical conductivity from all types of substances. All conductors are divided into two large subgroups:

• Metals(copper, aluminum, silver) and their alloys.
• electrolytes(aqueous solution of salt, acid).

In substances of the first subgroup, only electrons are able to move, since their connection with the nuclei of atoms is weak, and therefore, they are quite simply detached from them. Since the occurrence of current in metals is associated with the movement of free electrons, the type of electrical conductivity in them is called electronic.

Of the conductors of the first subgroup, they are used in the windings of electric machines, power lines, wires. It is important to note that the electrical conductivity of metals is affected by its purity and the absence of impurities.

In substances of the second subgroup, when exposed to a solution, the molecule breaks up into a positive and negative ion. Ions move due to the action of an electric field. Then, when the current passes through the electrolyte, ions are deposited on the electrode, which is lowered into this electrolyte. The process when a substance is released from an electrolyte under the influence of an electric current is called electrolysis. The electrolysis process is usually used, for example, when a non-ferrous metal is extracted from a solution of its compound, or when the metal is coated with a protective layer of other metals.

Description of dielectrics

Dielectrics are also commonly referred to as electrical insulators.

All electrical insulating substances have the following classification:

• Depending on the state of aggregation, dielectrics can be liquid, solid and gaseous.
• Depending on the methods of obtaining - natural and synthetic.
• Depending on the chemical composition - organic and inorganic.
• Depending on the structure of the molecules - neutral and polar.

These include gas (air, nitrogen, SF6 gas), mineral oil, any rubber and ceramic substance. These substances are characterized by the ability to polarization in an electric field. Polarization is the formation of charges with different signs on the surface of a substance.

Dielectrics contain a small number of free electrons, while the electrons have a strong bond with the nuclei of atoms and only in rare cases are detached from them. This means that these substances do not have the ability to conduct current.

This property is very useful in the production of products used in protection against electric current: dielectric gloves, rugs, boots, insulators for electrical equipment, etc.

About semiconductors

The semiconductor acts as intermediate substance between conductor and dielectric. The brightest representatives of this type of substances are silicon, germanium, selenium. In addition, it is customary to refer to these substances the elements of the fourth group of the periodic table of Dmitry Ivanovich Mendeleev.

Semiconductors have additional "hole" conduction in addition to electronic conduction. This type of conductivity is dependent on a number of environmental factors, including light, temperature, electric and magnetic fields.

These substances have weak covalent bonds. Under the influence of one of the external factors, the bond is destroyed, after which free electrons are formed. At the same time, when an electron is detached, a free "hole" remains in the composition of the covalent bond. Free "holes" attract neighboring electrons, and so this action can be performed indefinitely.

It is possible to increase the conductivity of semiconductor substances by introducing various impurities into them. This technique is widely used in industrial electronics: in diodes, transistors, thyristors. Let us consider in more detail the main differences between conductors and semiconductors.

What is the difference between a conductor and a semiconductor?

The main difference between a conductor and a semiconductor is the ability to conduct electric current. At the conductor it is an order of magnitude higher.

When the temperature value rises, the conductivity of semiconductors also increases; the conductivity of the conductors decreases with increasing.

In pure conductors, under normal conditions, the passage of current releases a much larger number of electrons than in semiconductors. At the same time, the addition of impurities reduces the conductivity of conductors, but increases the conductivity of semiconductors.

Electrical materials: semiconductors, dielectrics, conductors, superconductors.

According to their electrical properties, materials are divided into dielectrics, semiconductors, conductors and superconductors. They differ from each other in electrical conductivity and its mechanism, the nature of the dependence of electrical resistance on temperature.

Dielectrics. These are substances that do not have good electronic conductivity and are therefore insulators. Dielectrics have electrical resistivity in the range from 10 8 to 10 16 Ohm∙m. Some of them, like metals, have a crystalline structure. The type of chemical bond in dielectrics is mainly ionic or covalent. There are no free charge carriers. There is a wide band gap between the valence band and the conduction band. Dielectrics include polymeric materials: salts, oxides, polyethylene, rubber, textile materials.

Dielectrics such as ceramics, glass, and plastics have high dielectric constants ranging from 2 to 20. But individual dielectrics have relative dielectric constants of about a thousand or more. Such dielectrics are called ferroelectrics.

Rice. 1. Diagram of the arrangement of energy bands in a metal (a), a semiconductor (b),

insulator (c).

Semiconductors. Semiconductors are intermediate between insulators and conductors, they differ from both metals and insulators. At low temperatures, the electrical resistance of semiconductors is high and in this respect they are similar to dielectrics, although the dependence of electrical resistivity on temperature differs from that of insulators. When heated, the electrical conductivity of semiconductors increases, reaching values ​​characteristic of metals.

Semiconductors have electrical resistivity from 10 -5 to 10 8 Ohm∙m. Semiconductors include B, C, Si, Ge, Sn, P, As, Sb, S, Se, Te, I. Semiconductors are such binary compounds ZnO, FeO, ZnS, CdS, GaAs, ZnSb, SiC, as well as more complex connections.

The band gap in semiconductors varies from 0.08 eV (for the metal Sn) to 5.31 eV (for the non-metal diamond). The dependence of the electrical properties of semiconductors on temperature and illumination is explained by the electronic structure of their crystals. In them, as in insulators, the valence band is separated from the conduction band by a band gap (Fig. 1). However, the band gap in the case of semiconductors is much smaller than that of dielectrics. Due to this, under the action of irradiation or heating, the electrons occupying the upper levels of the valence band can go into the conduction band and participate in the transfer of electric current. With an increase in temperature and an increase in illumination, the number of electrons passing into the conduction band increases, which leads to an increase in the electrical conductivity of the semiconductor.

In semiconductors with a covalent bond, the appearance of an electron in the conduction band simultaneously creates its vacancy in the valence band. These vacancies are called holes. They can participate in the movement under the influence of an electric field. Therefore, the electric current in semiconductors is determined by the movement of electrons in the conduction band and the movement of holes in the valence band. In the first case, electrons move to unoccupied molecular orbitals, in the second case, to partially occupied molecular orbitals.

Of the simple semiconductors, silicon and germanium are the most common. Semiconductors are used in electronic devices.

Conductors. These are substances that conduct electricity. Conductors are metals. The specific electrical resistance of the conductors varies from 10 -8 to 10 -5 Ohm∙m. As the temperature rises, the electrical resistance increases, and this is what distinguishes them from semiconductors. The charge carriers in conductors are electrons. The valence band and the conduction band of the electronic structure of metals intersect (Fig. 1a). This allows electrons from the valence band to move, with a slight excitation, to the molecular orbitals of the conduction band.

Conductors are used to transmit electrical energy over long distances, as resistors, heating elements, lighting fixtures.

Superconductors. Materials whose electrical resistance drops sharply to zero at some critical temperature are called superconductors. In ordinary substances, a drop in electrical resistance to almost zero is possible only at low temperatures. For example, for mercury it is 4.2 K. Therefore, the wide practical use of superconductivity is inappropriate, since it is associated with high energy costs for cooling to very low temperatures.

In 1988, the phenomenon of high-temperature superconductivity was discovered. Substances have been found that exhibit superconducting properties at sufficiently high temperatures of the order of 90–135 K. Such temperatures can be achieved in a liquid nitrogen medium. This opens up possibilities for the practical use of the phenomenon of superconductivity.

High-temperature properties were found in the following substances: Y-Ba-Cu-O (T c = 90 K), Bi - Ca - Cu - O (T c = 110 K), Hg - Ba - Ca - Cu - O (T c = 135K).

Currently, searches are underway for new systems that could be in the superconducting state at the boiling point of carbon dioxide, which is 194.7 K.

conductor resistance. Conductivity. Dielectrics. The use of conductors and insulators. Semiconductors.

Physical substances are diverse in their electrical properties. The most extensive classes of matter are conductors and dielectrics.

conductors

The main feature of conductors- the presence of free charge carriers that participate in thermal motion and can move throughout the volume of matter.
As a rule, such substances include salt solutions, melts, water (except distilled water), moist soil, the human body and, of course, metals.

Metals considered to be the best conductors of electric charge.
There are also very good conductors that are not metals.
Among such conductors, carbon is the best example.
All conductors have properties such as resistance and conductivity . Due to the fact that electric charges, colliding with atoms or ions of a substance, overcome some resistance to their movement in an electric field, it is customary to say that conductors have electrical resistance ( R).
The reciprocal of resistance is called conductivity ( G).

G = 1/R

That is, the conductivity – is the property or ability of a conductor to conduct an electric current.
You need to understand that good conductors represent a very small resistance to the flow of electric charges and, accordingly, have high conductivity. The better the conductor, the greater its conductivity. For example, a copper conductor has b about greater conductivity than an aluminum conductor, and the conductivity of a silver conductor is higher than that of a copper conductor.

Dielectrics

Unlike conductors., in dielectrics at low temperatures there are no free electric charges. They are composed of neutral atoms or molecules. Charged particles in a neutral atom are bound to each other and cannot move under the action of an electric field throughout the entire volume of the dielectric.

The dielectrics are, in the first place, gases that conduct electrical charges very poorly. As well as glass, porcelain, ceramics, rubber, cardboard, dry wood, various plastics and resins.

Items made of dielectrics are called insulators. It should be noted that the dielectric properties of insulators largely depend on the state of the environment. So, in conditions of high humidity (water is a good conductor), some dielectrics may partially lose their dielectric properties.

On the use of conductors and insulators

Both conductors and insulators are widely used in engineering to solve various technical problems.

For example, all electrical wires in the house are made of metal (most often copper or aluminum). And the sheath of these wires or the plug that is plugged into the outlet must be made of various polymers, which are good insulators and do not allow electrical charges to pass through.

It should be noted that the terms "conductor" or "insulator" do not reflect qualitative characteristics: the characteristics of these materials in fact are in a wide range - from very good to very bad.
Silver, gold, platinum are very good conductors, but these are expensive metals, so they are used only where the price is less important compared to the function of the product (space, defense industry).
Copper and aluminum are also good conductors and at the same time inexpensive, which predetermined their widespread use.
Tungsten and molybdenum, on the contrary, are poor conductors and for this reason cannot be used in electrical circuits (they will disrupt the operation of the circuit), but the high resistance of these metals, combined with infusibility, predetermined their use in incandescent lamps and high-temperature heating elements.

insulators there are also very good ones, just good ones and bad ones. This is due to the fact that in real dielectrics there are also free electrons, although there are very few of them. The appearance of free charges even in insulators is due to thermal vibrations of electrons: under the influence of high temperature, some electrons still manage to break away from the nucleus and the insulating properties of the dielectric deteriorate. In some dielectrics, there are more free electrons and the quality of their insulation is, accordingly, worse. It is enough to compare, for example, ceramics and cardboard.

The best insulator is an ideal vacuum, but it is practically unattainable on Earth. Absolutely pure water would also be a great insulator, but has anyone seen it in real life? And water with the presence of any impurities is already a fairly good conductor.
The quality criterion of an insulator is its compliance with the functions that it must perform in a given circuit. If the dielectric properties of a material are such that any leakage through it is negligible (does not affect the operation of the circuit), then such a material is considered a good insulator.

Semiconductors

There are substances, which in their conductivity occupy an intermediate position between conductors and dielectrics.
Such substances are called semiconductors. They differ from conductors in the strong dependence of the conductivity of electric charges on temperature, as well as on the concentration of impurities, and can have the properties of both conductors and dielectrics.

Unlike metallic conductors, in which the conductivity decreases with increasing temperature, for semiconductors, the conductivity increases with increasing temperature, and the resistance, as the reciprocal of conductivity, decreases.

At low temperatures semiconductor resistance, as seen from rice. one, tends to infinity.
This means that at a temperature of absolute zero, a semiconductor has no free carriers in the conduction band and, unlike conductors, behaves like a dielectric.
With an increase in temperature, as well as with the addition of impurities (doping), the conductivity of the semiconductor increases and it acquires the properties of a conductor.

Rice. one. The dependence of the resistance of conductors and semiconductors on temperature