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 better performance and smaller size, they replaced vacuum tubes, then it became possible to produce integrated circuits based on semiconductor elements.

What are semiconductors

To define semiconductors is to characterize them in terms of their ability to conduct electric current. For these crystalline substances, the electrical conductivity increases with increasing temperature, exposure to light, and 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:

• donor - with a higher valency, donate electrons;
• acceptor - with a lower valence, take away electrons, forming holes.

Important! The main factor affecting the electrical conductivity of conductors is temperature.

How is conductivity provided?

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 flow.

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.

p-type and n-type conductivity

Impurity types of conductivity are divided into:

1. R-type. Formed upon addition of an acceptor impurity. The lower impurity valency causes the formation of an increased number of holes. For tetravalent silicon, trivalent boron can serve as such an impurity;
2. N-type. If pentavalent antimony is added to silicon, then the number of released negative charge carrier electrons in the semiconductor will increase.

Semiconductor elements mainly function based on the features of the p-n junction. When two materials with different types of conductivity are brought into contact, at the boundary between them, electrons and holes will interpenetrate into 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.

Use of semiconductors

Based on the properties of semiconductors, various devices have been created that are used in radio engineering, electronics and other fields.

Diode

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:

1. Tunnel. It uses semiconductor materials with such an impurity content that the width of the p-n junction decreases sharply, and the tunnel breakdown effect becomes possible with direct connection. Used in RF devices, generators, measurement equipment;
2. Converted. A slightly modified tunnel diode. With a direct connection, the voltage that opens it is much lower compared to classic diodes. This predetermines the use of a tunnel diode for converting low voltage currents;
3. Varicap. When the p-n junction is closed, its capacitance is quite high. The varicap is used as a capacitor, the capacitance of which can be varied by changing the voltage. The capacitance will decrease if the reverse voltage rises;

1. Zener diode. Connected in parallel, stabilizes the voltage in a given area;
2. Pulse. Due to short transients, they are used for pulsed RF circuits;
3. Avalanche-flying. Used to generate ultra-high frequency oscillations. It is based on the avalanche-like multiplication of charge carriers.

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:

• there is an n-type semiconductor, and the work function of its electrons is less than that of a metal;
• there is a p-type semiconductor with a work function of its electrons greater than that of a metal.

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 current of considerable magnitude.

Almost no microcircuit can do without transistors, semiconductor elements with two p-n junctions. The transistor element has three output contacts:

• collector;
• base;
• emitter.

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:

1. Bipolar. They have positive and negative charge carriers. The flowing current is able to pass in the forward and reverse direction. Used as amplifiers;
2. Field. Their outputs are called drain, source, gate. Control is carried out by means of an electric field of a certain polarity. The signal applied to the gate can change the conductance of the transistor. Charge carriers in field devices can have only one sign: positive or negative. Powerful field effect transistors are used in audio amplifiers. Their main application is integrated circuits. Compact dimensions and low power consumption make it possible to install them in devices with low power voltage sources (hours);
3. Combined. They can be located together with other transistor elements, resistors in one monolithic structure.

Doping of semiconductors

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:

1. High temperature diffusion. The semiconductor crystal is heated, and the impurity atoms that are in contact with its surface, fall into the depths. At some sites of the crystal lattice, impurity atoms replace the atoms of the main substance;
2. Ionic implantation. Ionization and acceleration of impurity atoms occur, which bombard the single crystal, creating local inhomogeneities and forming p-n junctions;
3. laser irradiation. The advantage of the method is that, using directed radiation, individual sections can be heated to any temperature values, which facilitates the introduction of impurities;
4. neutron doping. Used relatively recently. It consists in irradiating a single crystal with thermal neutrons in a reactor, as a result of which a mutation of atomic nuclei occurs. Silicon atoms are converted to phosphorus.

There are other ways of doping: chemical etching, the creation of thin films by sputtering.

How are semiconductors made?

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:

1. Zone melting. The process is carried out in a sealed quartz container, where an inert gas is supplied. A narrow zone of the ingot is melted, which gradually moves. In the process of melting, impurities are redistributed and recrystallized, releasing a pure part;
2. Czochralski method. It consists in growing a crystal from a seed by gradually pulling it out of the molten composition.

Varieties of semiconductor materials

Differences in composition determine the scope of semiconductors:

1. Simple - include homogeneous substances that are used independently, as well as as impurities and constituent parts of complex materials. Silicon, selenium and germanium are used independently. Boron, antimony, tellurium, arsenic, sulfur, iodine serve as additives;
2. Complex materials are chemical compounds of two or more elements: sulfides, tellurides, carbides;
3. Oxides of cobalt, copper, europium are used in rectifiers and photocells;
4. Organic semiconductors: indole, acridone, flavantron, pentacene. One area of ​​their use is optical electronics;
5. Magnetic semiconductors. These are ferromagnetic materials, for example, europium sulfide and oxide, as well as antiferromagnetic materials - nickel oxide, europium telluride. They are used in radio engineering, optical devices controlled by a magnetic field.

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, photoresistance in remote controls, and others.

Video

Semiconductors are a wide class of substances characterized by electrical conductivity values ​​that lie in the range between the electrical conductivity of metals and good dielectrics, that is, these substances cannot be classified as either dielectrics (since they are not good insulators) or metals (they are not good conductors of electricity). Semiconductors, for example, include substances such as germanium, silicon, selenium, tellurium, as well as some oxides, sulfides and metal alloys.

Properties:

1) With increasing temperature, the resistivity of semiconductors decreases, in contrast to metals, in which the resistivity increases with increasing temperature. Moreover, as a rule, in a wide temperature range, this increase occurs exponentially. The resistivity of semiconductor crystals can also decrease when exposed to light or strong electronic fields.

2) The property of one-sided conduction of the contact of two semiconductors. It is this property that is used to create a variety of semiconductor devices: diodes, transistors, thyristors, etc.

3) Contacts of various semiconductors under certain conditions, when illuminated or heated, are sources of photo-e. d.s. or, respectively, thermo-e. d.s.

Semiconductors differ from other classes of solids in many specific features, the most important of which are:

1) positive temperature coefficient of electrical conductivity, that is, with increasing temperature, the electrical conductivity of semiconductors increases;

2) the specific conductivity of semiconductors is less than that of metals, but more than that of insulators;

3) large values ​​of thermoelectromotive force in comparison with metals;

4) high sensitivity of semiconductor properties to ionizing radiation;

5) the ability of a sharp change in physical properties under the influence of negligible concentrations of impurities;

6) the effect of current rectification or non-ohmic behavior on the contacts.

3. Physical processes in p-n - transition.

The main element of most semiconductor devices is the electron-hole junction ( district junction), which is a transition layer between two regions of a semiconductor, one of which has electronic electrical conductivity, and the other has hole conductivity.

Education pn transition. Pn equilibrium transition

Let's take a closer look at the education process pn transition. The equilibrium state is called such a transition state when there is no external voltage. Recall that in R- region there are two types of main charge carriers: immobile negatively charged ions of acceptor impurity atoms and free positively charged holes; and in n-region there are also two types of main charge carriers: immobile positively charged ions of acceptor impurity atoms and free negatively charged electrons.

Before touch p and n regions, electrons, holes, and impurity ions are uniformly distributed. On contact at the border p and n regions, a concentration gradient of free charge carriers and diffusion arise. Under the action of diffusion, electrons from n-area goes into p and recombines there with holes. holes from R-areas go to n region and recombine with electrons there. As a result of such a movement of free charge carriers in the boundary region, their concentration decreases almost to zero and, at the same time, R region, a negative space charge of acceptor impurity ions is formed, and in n-region positive space charge of donor impurity ions. Between these charges there is a contact potential difference φ to and electric field E to, which prevents the diffusion of free charge carriers from the depth R- and n- areas through p-n- transition. Thus, the region united by free charge carriers with its electric field is called p-n- transition.

Pn The transition is characterized by two main parameters:

1. Potential barrier height. It is equal to the contact potential difference φ to. This is the potential difference in the transition due to the concentration gradient of charge carriers. This is the energy that a free charge must have in order to overcome the potential barrier:

where k is the Boltzmann constant; e is the electron charge; T- temperature; N a and N D are the concentrations of acceptors and donors in the hole and electron regions, respectively; p p and p n are the concentrations of holes in R- and n- areas respectively; n i - own concentration of charge carriers in an undoped semiconductor,  t \u003d kT / e- temperature potential. At a temperature T\u003d 27 0 С  t=0.025V, for germanium transition  to=0.6V, for silicon junction  to\u003d 0.8V.

2. p-n junction width(Fig. 1) is a border region depleted in charge carriers, which is located in p and n areas: l p-n = l p + l n:

From here,

where ε is the relative permittivity of the semiconductor material; ε 0 is the dielectric constant of free space.

The thickness of electron-hole transitions is of the order of (0.1-10) µm. If , then and pn-transition is called symmetric, if , then and pn- transition is called asymmetric, and it is mainly located in the region of the semiconductor with a lower impurity concentration.

In the equilibrium state (without external voltage) through district transition, two counter currents of charges move (two currents flow). These are the drift current of minority charge carriers and the diffusion current, which is associated with the majority charge carriers. Since there is no external voltage, and there is no current in the external circuit, the drift current and diffusion current are mutually balanced and the resulting current is zero

I dr + I diff = 0.

This relation is called the condition of dynamic equilibrium of diffusion and drift processes in an isolated (equilibrium) pn-transition.

The surface on which they are in contact p and n area is called the metallurgical boundary. In reality, it has a finite thickness - δ m. If a δ m<< l p-n , then pn The transition is called a sharp one. If δ m >> lp-n, then pn The transition is called smooth.

Р-n transition at an external voltage applied to it

External voltage disturbs the dynamic balance of currents in pn-transition. Pn- the transition goes into a non-equilibrium state. Depending on the polarity of the voltage applied to the areas in pn-transition possible two modes of operation.

1) Forward biaspn transition. R-n- the junction is considered to be forward biased if the positive pole of the power supply is connected to R-region, and negative to n- areas (Fig. 1.2)

With forward bias, the voltages  to and U are directed oppositely, the resulting voltage on pn-transition decreases to the value  to - U. This leads to the fact that the electric field strength decreases and the process of diffusion of the main charge carriers resumes. In addition, forward offset reduces the width pn transition, because lp-n ≈( to - U) 1/2. The diffusion current, the current of the main charge carriers, becomes much larger than the drift current. Through pn-transition direct current flows

I p-n \u003d I pr \u003d I diff + I dr I differential .

When a direct current flows, the majority charge carriers in the p-region pass into the n-region, where they become minor. The diffusion process of introducing majority charge carriers into a region where they become minority is called injection, and direct current - diffusion current or injection current. To compensate for the minority charge carriers accumulated in the p and n regions, an electron current is generated in the external circuit from a voltage source, i.e. the principle of electroneutrality is preserved.

With an increase U the current increases sharply, - the temperature potential, and can reach large values. associated with the main carriers, the concentration of which is high.

2) reverse bias, occurs when R-area is applied a minus, and to n-area plus, an external voltage source (Fig. 1.3).

This external tension U included according to  to. It: increases the height of the potential barrier to a value  to + U; the electric field strength increases; width pn transition increases, because l p-n ≈( to + U) 1/2; the diffusion process stops completely and after pn transition flows drift current, minority carrier current. Such a current pn-transition is called reverse, and since it is associated with minor charge carriers that arise due to thermal generation, it is called thermal current and denoted - I 0, i.e.

I p-n \u003d I arr \u003d I diff + I dr I dr \u003d I 0.

This current is small in magnitude. associated with minority charge carriers, the concentration of which is low. Thus, pn transition has one-sided conductivity.

With a reverse bias, the concentration of minority charge carriers at the transition boundary somewhat decreases compared to the equilibrium one. This leads to the diffusion of minority charge carriers from the depth p and n-areas to the border pn transition. Having reached it, minority carriers fall into a strong electric field and are transferred through pn transition, where they become the majority charge carriers. Diffusion of minor charge carriers to the boundary pn transition and drift through it to the region where they become the main charge carriers is called extraction. Extraction and creates a reverse current pn transition is the current of minor charge carriers.

The magnitude of the reverse current is highly dependent on: ambient temperature, semiconductor material and area pn transition.

The temperature dependence of the reverse current is determined by the expression , where is the nominal temperature, is the actual temperature, is the doubling temperature of the thermal current.

The thermal current of the silicon junction is much less than the thermal current of the germanium-based junction (by 3–4 orders of magnitude). It's connected with  to material.

With an increase in the transition area, its volume increases, and, consequently, the number of minority carriers appearing as a result of thermal generation and the thermal current increase.

So the main property pn-transition is its one-way conduction.

4. Current-voltage characteristic p-n - transition.

We get the current-voltage characteristic of the p-n junction. To do this, we write the continuity equation in general form:

We will consider the stationary case dp/dt = 0.

Consider the current in the quasi-neutral volume of an n-type semiconductor to the right of the depleted region of the p-n junction (x > 0). The generation rate G in a quasi-neutral volume is zero: G = 0. The electric field E is also zero: E = 0. The drift component of the current is also zero: I E = 0, therefore, the current is diffusion. The recombination rate R at a low injection level is described by the relation:

Let us use the following relationship relating the diffusion coefficient, diffusion length, and minority carrier lifetime: Dτ = L p 2 .

Taking into account the above assumptions, the continuity equation has the form:

The boundary conditions for the diffusion equation in the p-n junction are:

The solution of differential equation (2.58) with boundary conditions (*) has the form:

Relation (2.59) describes the law of distribution of injected holes in the quasi-neutral volume of an n-type semiconductor for an electron-hole transition (Fig. 2.15). All carriers that have crossed the SCR boundary with a quasi-neutral volume of the p-n junction take part in the p-n junction current. Since the entire current is diffusion, substituting (2.59) into the expression for the current, we obtain (Fig. 2.16):

Relation (2.60) describes the diffusion component of the p-n junction hole current, which arises during the injection of minority carriers under forward bias. For the electronic component of the p-n junction current, we similarly obtain:

At V G = 0, the drift and diffusion components balance each other. Hence, .

The total p-n junction current is the sum of all four p-n junction current components:

The expression in brackets has the physical meaning of the reverse current of the p-n junction. Indeed, at negative voltages V G< 0 ток дрейфовый и обусловлен неосновными носителями. Все эти носители уходят из цилиндра длиной L n со скоростью L n /τ p . Тогда для дрейфовой компоненты тока получаем:

Rice. 2.15. Distribution of non-equilibrium carriers injected from the emitter over the quasi-neutral volume of the p-n junction base

It is easy to see that this relation is equivalent to that obtained earlier in the analysis of the continuity equation.

If it is required to implement the condition of one-sided injection (for example, only injection of holes), then it follows from relation (2.61) that a small value of the concentration of minority carriers n p0 in the p-region should be chosen. It follows that the p-type semiconductor must be heavily doped compared to the n-type semiconductor: N A >> N D . In this case, the hole component will dominate in the p-n junction current (Fig. 2.16).

Rice. 2.16. Currents in a single-ended p-n junction with forward bias

Thus, the I–V characteristic of the p-n junction has the form:

The saturation current density J s is:

CVC p-n transition, described by relation (2.62), is shown in Figure 2.17.

Rice. 2.17. Current-voltage characteristic of an ideal p-n junction

As follows from relation (2.16) and Figure 2.17, the current-voltage characteristic of an ideal p-n junction has a pronounced asymmetric form. In the region of direct voltages, the current of the p-n junction is diffusion and exponentially increases with increasing applied voltage. In the region of negative voltages, the p-n junction current is drift and does not depend on the applied voltage.

5. Capacitance p-n - junction.

Any system in which the electric charge Q changes when the potential φ changes has a capacitance. The capacitance value C is determined by the ratio: .

For the p-n junction, two types of charges can be distinguished: the charge in the region of the space charge of ionized donors and acceptors Q B and the charge of injected carriers into the base from the emitter Q p . With different biases on the p-n junction, one or another charge will dominate when calculating the capacitance. In this regard, for the capacitance of the p-n junction, barrier capacitance C B and diffusion capacitance C D are distinguished.

Barrier capacitance C B is the capacitance of the p-n junction at reverse bias V G< 0, обусловленная изменением заряда ионизованных доноров в области пространственного заряда.

The charge value of ionized donors and acceptors Q B per unit area for an asymmetric p-n junction is:

Differentiating expression (2.65), we obtain:

It follows from equation (2.66) that the barrier capacitance C B is the capacitance of a flat capacitor, the distance between the plates of which is equal to the width of the space charge region W. Since the width of the SCR depends on the applied voltage V G, the barrier capacitance also depends on the applied voltage. Numerical estimates of the barrier capacitance show that its value is tens or hundreds of picofarads.

Diffusion capacitance C D is the capacitance of a p-n junction at a forward bias V G > 0, due to a change in the charge Q p of the injected carriers into the base from the emitter Q p .

The dependence of the barrier capacitance C B on the applied reverse voltage V G is used for instrumental implementation. A semiconductor diode that implements this dependence is called a varicap. The maximum capacitance value of the varicap is at zero voltage V G . As the reverse bias increases, the capacitance of the varicap decreases. The functional dependence of the varicap capacitance on voltage is determined by the doping profile of the varicap base. In the case of uniform doping, the capacitance is inversely proportional to the root of the applied voltage V G . By setting the doping profile in the base of the varicap N D (x), one can obtain various dependences of the varicap capacitance on the voltage C(V G) - linearly decreasing, exponentially decreasing.

6. Semiconductor diodes: classification, design features, symbols and marking.

semiconductor diode- a semiconductor device with one electrical junction and two leads (electrodes). Unlike other types of diodes, the principle of operation of a semiconductor diode is based on the phenomenon pn-transition.

Hello dear readers of the site. The site has a section dedicated to beginner radio amateurs, but so far I haven’t really written anything for beginners taking their first steps into the world of electronics. I fill this gap, and from this article we begin to get acquainted with the device and operation of radio components (radio components).

Let's start with semiconductor devices. But in order to understand how a diode, thyristor or transistor works, one must understand what semiconductor. Therefore, we will first study the structure and properties of semiconductors at the molecular level, and then we will deal with the operation and design of semiconductor radio components.

General concepts.

Why exactly semiconductor diode, transistor or thyristor? Because the basis of these radio components is semiconductors Substances capable of both conducting electrical current and preventing its passage.

This is a large group of substances used in radio engineering (germanium, silicon, selenium, copper oxide), but for the manufacture of semiconductor devices, they mainly use only Silicon(Si) and Germanium(Ge).

According to their electrical properties, semiconductors occupy a middle place between conductors and non-conductors of electric current.

Properties of semiconductors.

The electrical conductivity of conductors is highly dependent on the ambient temperature.
At very low temperatures close to absolute zero (-273°C), semiconductors do not carry out electric current, and promotion temperature, their resistance to current decreases.

If you point at the semiconductor light, then its electrical conductivity begins to increase. Using this property of semiconductors, were created photovoltaic appliances. Semiconductors are also capable of converting light energy into electrical current, for example, solar panels. And when introduced into semiconductors impurities certain substances, their electrical conductivity increases dramatically.

The structure of semiconductor atoms.

Germanium and silicon are the main materials of many semiconductor devices and have four valence electron.

Atom Germany is made up of 32 electrons, and an atom silicon out of 14. But only 28 electrons of the germanium atom and 10 electrons of the silicon atom, located in the inner layers of their shells, are firmly held by the nuclei and never come off from them. Just four valence electrons of the atoms of these conductors can become free, and even then not always. And if a semiconductor atom loses at least one electron, then it becomes positive ion.

In a semiconductor, the atoms are arranged in a strict order: each atom is surrounded by four the same atoms. Moreover, they are located so close to each other that their valence electrons form single orbits passing around neighboring atoms, thereby binding the atoms into a single whole substance.

Let us represent the interconnection of atoms in a semiconductor crystal in the form of a flat diagram.
In the diagram, red balls with a plus, conventionally, denote nuclei of atoms(positive ions), and the blue balls are valence electrons.

Here you can see that around each atom are located four exactly the same atoms, and each of these four has a connection with four other atoms, and so on. Each of the atoms is connected to each neighboring two valence electrons, and one electron is its own, and the other is borrowed from a neighboring atom. Such a bond is called a two-electron bond. covalent.

In turn, the outer layer of the electron shell of each atom contains eight electrons: four their own, and alone, borrowed from four neighboring atoms. Here it is no longer possible to distinguish which of the valence electrons in the atom is "one's own" and which one is "foreign", since they have become common. With such a bond of atoms in the entire mass of a germanium or silicon crystal, we can assume that a semiconductor crystal is one large molecule. In the figure, pink and yellow circles show the connection between the outer layers of the shells of two neighboring atoms.

Semiconductor electrical conductivity.

Consider a simplified drawing of a semiconductor crystal, where atoms are denoted by a red ball with a plus, and interatomic bonds are shown by two lines symbolizing valence electrons.

At a temperature close to absolute zero, a semiconductor does not conduct current, since it does not have free electrons. But with an increase in temperature, the bond of valence electrons with the nuclei of atoms weakens and some of the electrons, due to thermal motion, may leave their atoms. The electron escaping from the interatomic bond becomes " free", and where he was before, an empty place is formed, which is conventionally called hole.

How higher semiconductor temperature, the more it becomes free electrons and holes. As a result, it turns out that the formation of a "hole" is associated with the departure of a valence electron from the shell of an atom, and the hole itself becomes positive electric charge equal to negative charge of an electron.

Now let's look at the figure, which schematically shows the phenomenon of the occurrence of current in a semiconductor.

If you apply some voltage to the semiconductor, the "+" and "-" contacts, then a current will appear in it.
Due to thermal phenomena, in a semiconductor crystal from interatomic bonds will begin be released some number of electrons (blue balls with arrows). Electrons are attracted positive pole of the voltage source will be move towards him, leaving behind holes, which will be filled in by others released electrons. That is, under the action of an external electric field, charge carriers acquire a certain speed of directional movement and thereby create electricity.

For example: the freed electron closest to the positive pole of the voltage source attracted this pole. Breaking the interatomic bond and leaving it, the electron leaves after myself hole. Another freed electron, which is located on some removal from the positive pole, also attracted pole and moving towards him, but having met a hole in its path, is attracted to it core atom, restoring the interatomic bond.

The resulting new hole after the second electron, fills the third released electron, located next to this hole (Figure No. 1). In its turn holes, which are closest to negative pole, filled with other released electrons(Figure No. 2). Thus, an electric current arises in the semiconductor.

As long as the semiconductor operates electric field, this process continuous: interatomic bonds are broken - free electrons appear - holes are formed. The holes are filled with released electrons - interatomic bonds are restored, while other interatomic bonds are broken, from which electrons leave and fill the following holes (Figure No. 2-4).

From this we conclude: electrons move from the negative pole of the voltage source to the positive, and holes move from the positive pole to the negative.

Electron-hole conductivity.

In a "pure" semiconductor crystal, the number released electrons at the moment is equal to the number emerging in this case, there are holes, so the electrical conductivity of such a semiconductor small, since it provides an electric current big resistance, and this electrical conductivity is called own.

But if we add to the semiconductor in the form impurities a certain number of atoms of other elements, then its electrical conductivity will increase significantly, and depending on structures atoms of impurity elements, the electrical conductivity of the semiconductor will be electronic or perforated.

electronic conductivity.

Suppose, in a semiconductor crystal, in which atoms have four valence electrons, we have replaced one atom with an atom in which five valence electrons. This atom four electrons will bond with four neighboring atoms of the semiconductor, and fifth the valence electron will remain superfluous' means free. And than more more will be free electrons, which means that such a semiconductor will approach a metal in its properties, and in order for an electric current to pass through it, it interatomic bonds do not have to be destroyed.

Semiconductors with such properties are called semiconductors with conductivity of the type " n", or semiconductors n-type. Here the Latin letter n comes from the word "negative" (negative) - that is, "negative". It follows that in a semiconductor n-type main charge carriers are - electrons, and not the main ones - holes.

hole conduction.

Let us take the same crystal, but now we will replace its atom with an atom in which only three free electron. With its three electrons, it will only bond with three neighboring atoms, and to bond with the fourth atom, he will not have enough one electron. As a result, it forms hole. Naturally, it will be filled with any other free electron nearby, but, in any case, there will be no such semiconductor in the crystal. grab electrons to fill holes. And than more there will be such atoms in the crystal, so more there will be holes.

In order for free electrons to be released and move in such a semiconductor, valence bonds between atoms must be destroyed. But the electrons will still not be enough, since the number of holes will always be more number of electrons at any given time.

Such semiconductors are called semiconductors with perforated conductivity or conductors p-type, which in Latin "positive" means "positive". Thus, the phenomenon of electric current in a p-type semiconductor crystal is accompanied by a continuous emergence and disappearance positive charges are holes. And this means that in a semiconductor p-type main charge carriers are holes, and not basic - electrons.

Now that you have some understanding of the phenomena occurring in semiconductors, it will not be difficult for you to understand the principle of operation of semiconductor radio components.

Let's stop at this, and in we will consider the device, the principle of operation of the diode, we will analyze its current-voltage characteristic and switching circuits.
Good luck!

Source:

1 . Borisov V.G. - A young radio amateur. 1985
2 . Website academic.ru: http://dic.academic.ru/dic.nsf/es/45172.

What are its features? What is the physics of semiconductors? How are they built? What is semiconductor conductivity? What physical properties do they have?

What is a semiconductor?

This refers to crystalline materials that do not conduct electricity as well as metals do. But still, this indicator is better than insulators. Such characteristics are due to the number of mobile carriers. Generally speaking, there is a strong attachment to the cores. But when several atoms are introduced into the conductor, for example, antimony, which has an excess of electrons, this situation will be corrected. When using indium, 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, then we can note a significant amplifying role, which is especially important when transmitting weak signals.

Design features possessed by electrical semiconductors

Conductors have many free electrons. Insulators practically do not possess them at all. Semiconductors, on the other hand, contain both a certain amount of free electrons and gaps with a positive charge, which are ready to receive the 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 such an element that can transmit signals in only one direction. A diode can also turn alternating current into direct current. What is the mechanism of such a 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 the distance, the supply is interrupted, and therefore the transfer of negative voltage carriers is carried out only in one direction, that is, the conductivity of semiconductors is one-sided. After all, the current can be transmitted only if the constituent particles are nearby. And this is possible only when current is applied from one side. These types of semiconductors exist and are currently used.

Band structure

The electrical and optical properties of conductors are related to the fact that, when energy levels are filled with electrons, they are separated from possible states by a band gap. What are her features? The fact is that there are no energy levels in the band gap. With the help of impurities and structural defects, this can be changed. The highest completely filled band is called the valence band. Then follows the allowed, but empty. It is called the conduction band. Semiconductor physics is a rather interesting topic, and within the framework of the article it will be well covered.

Electron state

For this, concepts such as the number of the allowed zone and the quasi-momentum are used. The structure of the first is determined by the dispersion law. He says that it is affected by the dependence of the energy on the quasi-momentum. So, 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.

Degenerate zones

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. It can be split under the same condition into doubly and quadruple degenerate bands. The energy distance between them is called the spin-orbit splitting energy.

Impurities and defects in semiconductors

They may 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 relatively little effect on the electronic properties. Moreover, it can often matter what valency the atoms that take part in the charge transfer process have, and the structure

Depending on the type and amount of impurities, the ratio between the number of holes and electrons can 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 refer to as majority charge carriers are non-primary.

The 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. Dislocation, interstitial atom, and vacancy are important here. Liquid and non-crystalline conductors react differently to impurities than crystalline ones. The absence of a rigid structure ultimately results in the fact that the displaced atom receives a different valence. It will be different from the one with which he initially saturates his ties. It becomes unprofitable for an atom to give or add an electron. In this case, it becomes inactive, and therefore doped semiconductors have a high chance of failure. This leads to the fact that it is impossible to change the type of conductivity with the help of doping and create, for example, a p-n junction.

Some amorphous semiconductors can change their electronic properties under the influence of doping. But this applies to them to a much lesser extent than to crystalline ones. The sensitivity of amorphous elements to doping can be improved by processing. In the end, I would like to note that, thanks to long and hard work, doped semiconductors are still represented by a number of results with good characteristics.

Electron statistics in a semiconductor

When it exists, the number of holes and electrons is determined solely by the temperature, the parameters of the band structure, 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). It also takes into account the fact that a part can leave the valence territory, and gaps are formed there.

Electrical conductivity

In semiconductors, in addition to electrons, ions can also act as charge carriers. But their electrical conductivity in most cases is negligible. As an exception, only ionic superconductors can be cited. There are three main mechanisms of electron transfer in semiconductors:

1. Main zone. In this case, the electron comes into motion due to a change in its energy within the same allowed territory.
2. Hopping transfer over localized states.
3. Polaron.

exciton

A hole and an electron can form a bound state. It is called the Wannier-Mott exciton. In this case, which corresponds to the absorption edge, decreases by the size of the bond. With sufficient energy, a significant amount of excitons can form in semiconductors. As their concentration increases, condensation occurs, and an electron-hole liquid is formed.

Semiconductor surface

These words denote several atomic layers that are located near the edge of the device. Surface properties are different from bulk properties. 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, one should also inform 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 the 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 products.

Semiconductor. Temperature affects resistance

When the temperature of metals increases, their resistance also increases. With semiconductors, the opposite is true - under the same conditions, this parameter will decrease for them. The point here is that the electrical conductivity of any material (and this characteristic is inversely proportional to the resistance) depends on what current charge the carriers have, on the speed of their movement in an electric field and on their number in one unit volume of the material.

In semiconductor elements, with increasing temperature, the concentration of particles increases, due to this, thermal conductivity increases, and resistance decreases. You can check this if you have a simple set of a young physicist and the necessary material - silicon or germanium, you can also take a semiconductor made from them. An increase in temperature will reduce their resistance. To make sure of this, you need to stock up on measuring instruments that will allow you to see all the changes. This is in the general case. Let's look at a couple of private options.

Resistance and electrostatic ionization

This is due to the tunneling of electrons passing through a very narrow barrier that supplies about one hundredth of a micrometer. It is located between the edges of the energy zones. Its appearance is possible only when the energy bands are tilted, which occurs only under the influence of a strong electric field. When tunneling occurs (which is a quantum mechanical effect), then the electrons pass through a narrow potential barrier, and their energy does not change. This entails an increase in the concentration of charge carriers, and in both bands: both conduction and valence. If the process of electrostatic ionization is developed, then a tunneling 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.

Resistance and impact ionization

In this case, holes and electrons are accelerated while they pass the mean free path under the influence of a strong electric field to values ​​that contribute to the ionization of atoms and the breaking of one of the covalent bonds (the 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 value of the current in the final result is multiplied by the impact ionization coefficient, which is equal to the number of electron-hole pairs that are formed by the charge carrier in one segment 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 use of semiconductors in practice

The special importance of these elements should be noted in computer technologies. 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 work of modern refrigerators, televisions, computer monitors without semiconductors. Do not do without them and advanced automotive development. They are also used in aviation and space technology. Do you understand what semiconductors are, how important they are? Of course, it cannot be said that these are the only irreplaceable elements for our civilization, but they should not be underestimated either.

The use of semiconductors in practice is also due to a number of factors, including the widespread use of the materials from which they are made, and the ease of processing and obtaining the desired result, and other technical features due to which the choice of scientists who developed electronic equipment settled on them.

Conclusion

We examined in detail what semiconductors are, 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 studied the features of their work to the end. But we know their main properties and characteristics, which allow us to apply them in practice. Therefore, you can look for semiconductor materials and experiment with them yourself, being careful. Who knows, perhaps a great explorer is dozing in you?!

The physical properties of solids, and primarily their electrical properties, are determined not by how the zones were formed, but by how they are filled. From this point of view, all crystalline bodies can be divided into two different groups. All bodies included in the first group are conductors. The second group of solids combines semiconductors and dielectrics. The second group includes bodies in which completely empty zones are located above the completely filled zones. This group also includes crystals having a diamond structure: silicon, germanium, gray tin, diamond itself; and many chemical compounds - metal oxides, carbides, metal nitrides, corundum.

Semiconductors are divided into intrinsic (pure) and extrinsic (doped). Semiconductors of a high degree of purity are called intrinsic. In this case, the properties of the entire crystal are determined only by the properties of the intrinsic atoms of the semiconductor element. The appearance of conducting properties in a semiconductor can be due to an increase in temperature, other external influences (light irradiation, bombardment of fast electrons). It is only important that the external action cause the transition of electrons from the valence band to the conduction band or that conditions be created for the generation of free charge carriers in the bulk of the semiconductor. Intrinsic conductivity with strict equality of carrier concentrations of different signs can be realized only in superpure ideal semiconductor crystals. In real conditions, we always deal with crystals contaminated to one degree or another by various impurities. Moreover, it is the impurity semiconductors that are of the greatest interest in semiconductor technology. Impurity semiconductors, depending on the type of introduced impurity, are divided into donor (electronic) and acceptor (hole). The formation of holes in the valence band means the appearance of hole conduction in the crystal. Due to this type of conductivity, the semiconductors themselves are called hole semiconductors or p-type semiconductors. Impurities introduced into a semiconductor to capture electrons from the valence band are called acceptors, which is why the energy levels of these impurities are called acceptor levels, and the semiconductors themselves with such impurities are called acceptor semiconductors.

Photoconductivity is a non-equilibrium process in semiconductors, which consists in the appearance or change in the conductive properties of a semiconductor under the influence of any radiation (infrared, visible or ultraviolet). As a rule, irradiation of a semiconductor with light is accompanied by an increase in its electrical conductivity. The increase in conductivity is explained by an increase in the concentration of free carriers (the mobility of nonequilibrium carriers practically does not differ from the mobility of equilibrium ones). The formation of excess mobile carriers when exposed to light is possible for the following three main reasons:

• light quanta, interacting with electrons located at impurity donor levels, and giving them their energy, transfer them to the conduction band, thereby increasing the concentration of conduction electrons;
• light quanta excite electrons located in the valence band and transfer them to acceptor levels, thereby creating free holes in the valence band and increasing the hole conductivity of the semiconductor;
• light quanta transfer electrons from the valence band directly to the conduction band, thereby creating both mobile holes and free electrons at the same time.

Currently, semiconductor devices are used in almost all areas of electronics and radio engineering. However, despite the extreme variety of these devices, they are usually based on the operation of a conventional p-n junction or a system of several p-n junctions. A semiconductor diode contains only one p-n junction, to each of the regions of which metal inputs are connected using ohmic contacts. Semiconductor diodes are mainly used to rectify alternating current.

Unlike semiconductor diodes, transistors are semiconductor systems consisting of three regions separated by two p-n junctions. Each area has its own output. Therefore, by analogy with vacuum triodes, transistors are often called semiconductor triodes. And by appointment, transistors are similar to vacuum triodes: the main area of ​​\u200b\u200btheir use is the amplification of electrical signals in voltage and power. To obtain transistors in a semiconductor single-crystal plate with a certain type of conductivity, on its two opposite faces, an impurity is fused or diffusely penetrated, imparting conductivity of the opposite type to near-surface regions. You can create a transistor as a p-n-p-type and n-p-n-type. There is no fundamental difference between them. It's just that holes play the main role in p-n-p-type transistors, and electrons in n-p-n-type transistors.

Semiconductors rapidly burst into science and technology. Enormous savings in power consumption, amazing compactness of equipment due to the unusually high packing density of elements in circuits, high reliability allowed semiconductors to win a leading position in electronics, radio engineering and science. Research in space, where the requirements for size, weight and energy consumption are so critical, is currently unthinkable without semiconductor devices, which, by the way, receive energy in the autonomous flight of the device from solar batteries operating on semiconductor elements. Surprising prospects in the development of semiconductor technology were opened by microelectronics. However, the possibilities of semiconductors are far from being exhausted, and they are waiting for their new researchers.

Semiconductor applications

Currently, semiconductor devices are used in almost all areas of electronics and radio engineering. However, despite the extreme variety of these devices, they are usually based on the operation of a conventional p-n junction or a system of several p-n junctions.

A semiconductor diode contains only one p-n junction, to each of the regions of which metal inputs are connected using ohmic contacts.

rectifier diodes. Semiconductor diodes are mainly used to rectify alternating current. The simplest scheme for using a semiconductor diode as a rectifying element is shown in Figure 1. An alternating voltage source i-, diode D and a load resistor Rn are connected in series. The flow direction of the diode is indicated by an arrow (from the anode to the cathode).

Let the voltage at the source terminals change according to a sinusoidal law (Fig. 2, a). During the positive half-cycle, when “+” is applied to the anode of the diode, and “-” to the cathode, the diode turns on in the forward direction and current flows through it. In this case, the instantaneous value of the current I is determined by the instantaneous value of the voltage and at the source terminals and the load resistance (the resistance of the diode in the forward direction is small and can be neglected). During the negative half cycle, no current flows through the diode. Thus, a pulsating current flows in the circuit, the graph of which is shown in Figure 2, b. The same pulsating will be the voltage un on the load resistor. Since u=iR, the change in voltage u repeats the course of change in current i. The polarity of the voltage created on the load resistance is always the same, and it is determined in accordance with the direction of the transmitted current: at the end of the resistance facing the cathode, there will be “+”, and at the opposite end “-”.

The considered scheme of rectification is one-half-wave. To reduce the ripple of the rectified voltage, smoothing filters are used. The simplest smoothing method is to connect a capacitor C in parallel with the load resistor (shown in dotted line in Figure 1). During the positive half-cycle, part of the current passed by the diode goes to charge the capacitor. During the negative half-cycle, when the diode is locked, the capacitor is discharged through Rp, creating a current in it in the same direction. Due to this, the voltage ripple across the load resistor is largely smoothed out.