What happens in the contact of two conductors. Contact of semiconductors with different types of conductivity
The most interesting phenomena occur when n- and p-type semiconductors come into contact. These phenomena are used in most semiconductor devices. Recombination of electrons and holes occurs in them. When a contact is formed, electrons partially move from an n-type semiconductor to a p-type semiconductor, and holes move in the opposite direction. As a result, the n-type semiconductor is charged positively, and the p-type - negatively. Diffusion stops after the electric field arising in the transition zone begins to prevent further movement of electrons and holes.
Semiconductor diodes
The basis of a semiconductor diode is the p-n junction, which determines its properties, characteristics and parameters. According to their purpose, semiconductor diodes are divided into rectifier diodes, pulse diodes, high-frequency and ultra-high-frequency diodes, zener diodes, three-layer switching diodes, tunnel diodes, varicaps, photo and LEDs. Depending on the source semiconductor material, diodes are divided into germanium and silicon. Germanium diodes operate at temperatures no higher than +80 °C, and silicon diodes up to +140 °C. Based on their design and technological characteristics, diodes are divided into planar and point diodes. The most common are planar alloy diodes, the use of which is difficult only at higher frequencies. The advantage of point diodes is the low capacitance of the p-n junction, which makes them possible to operate at high ultra-high frequencies. High-frequency diodes are universal devices. They can operate in AC rectifiers over a wide frequency range, as well as in modulators, detectors and other nonlinear electrical signal converters. High-frequency diodes usually contain a point pn junction and are therefore called point diodes. Pulse diodes are a type of high-frequency diodes and are intended for use as key elements in high-speed pulse circuits. Zener diodes are silicon planar diodes designed to stabilize the level DC voltage in the circuit when the current through the diode changes within certain limits. A varicap is a specially designed semiconductor diode, used as a variable capacitor. A photodiode is a semiconductor photoelectric device with an internal photoelectric effect that reflects the process of converting light energy into electrical energy. LEDs (electroluminescent diodes) convert electric field energy into non-thermal optical radiation called electroluminescence. A tunnel diode is a semiconductor diode that uses the phenomenon of tunnel breakdown when turned on in the forward direction.
NUCLEAR CHAIN REACTION.
It is a process in which one reaction carried out causes subsequent reactions of the same type. During the fission of one uranium nucleus, the resulting neutrons can cause the fission of other uranium nuclei, and the number of neutrons increases like an avalanche.
The chain reaction is accompanied by the release of a large amount of energy. To carry out a chain reaction, it is impossible to use any nuclei that fission under the influence of neutrons. The chemical element uranium, used as fuel for nuclear reactors, naturally consists of two isotopes: uranium-235 and uranium-238.
In nature, uranium-235 isotopes make up only 0.7% of the total uranium reserve, but they are the ones that are suitable for carrying out a chain reaction, because fission under the influence of slow neutrons. The first controlled chain reaction - USA in 1942 (E. Fermi)
In the USSR - 1946 (I.V. Kurchatov).
NUCLEAR REACTOR - This is a device in a nuclear power plant for producing atomic energy.
Purpose of a nuclear reactor: converting the internal energy of an atomic nucleus into electrical energy.
In a nuclear reactor, a controlled chain reaction of nuclear fission occurs. All nuclear power plants (nuclear power plants) are equipped with nuclear reactors.
Reactor operation:
The reactor operates on slow neutrons. The reactor core contains nuclear fuel - uranium rods and a moderator - water. The water around the uranium rods is not only a neutron moderator, but also serves to remove heat, because The internal energy of the flying fragments transforms into the internal energy of the environment - water. The core is surrounded by a reflector for returning neutrons and a protective layer of concrete.
Achieving the critical mass of the fuel is achieved by introducing control rods (until the mass of uranium = critical mass is reached).
The active zone is connected into a ring through pipes (1st circuit).
Water is pumped through the pipes of the circuit by a pump and gives off its energy to the coil in the heat exchanger, heating the water in the coil (in the 2nd circuit).
The water in the coil turns into steam, the temperature of which can reach 540 degrees.
The steam rotates the turbine, the energy of the steam is converted into mechanical energy.
The turbine axis rotates the rotor of the electric generator, converting mechanical energy into electrical energy.
The exhaust (cooled) steam enters the condenser, where it turns into water, returning to the 1st circuit. The first nuclear power plant was built in Obninsk (USSR).
Advantages of nuclear power plants: nuclear reactors do not consume oxygen and organic fuel. Do not pollute environment ash and organic fuel products harmful to humans. The biosphere is reliably protected from radioactive effects when normal mode operation of nuclear power plants.
Disadvantages of nuclear power plants: the need to dispose of radioactive waste and dismantle old reactors. Danger of radioactive contamination of the area during emergency releases. The danger of environmental disasters (1986 - Chernobyl nuclear power plant).
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1.TRANSISTOR, a semiconductor device designed to amplify and control electrical current. Transistors are produced in the form of discrete components in individual packages or in the form of active elements, so-called. integrated circuits, where their dimensions do not exceed 0.025 mm. Due to the fact that transistors are very easy to adapt to different conditions applications, they have almost completely replaced vacuum tubes. One of the first industrial applications of the transistor was found in telephone switching stations. The first consumer product to use transistors was hearing aids, which went on sale in 1952. Today, transistors and multi-transistor integrated circuits are used in everything from radios to ground and airborne surveillance systems in missile forces. The list of applications for transistors is almost endless and continues to grow. In 1954, slightly more than 1 million transistors were produced. Now this figure is impossible to even indicate. Initially, transistors were very expensive. Today, transistor signal processing devices can be purchased for a few cents.
A thermistor is a semiconductor resistor whose electrical resistance depends significantly on temperature. The thermistor is characterized by a large temperature coefficient of resistance (TCR) (tens of times higher than this coefficient for metals), simplicity of design, ability to operate in various climatic conditions under significant mechanical loads, and stability of characteristics over time. The thermistor was invented by Samuel Ruben in 1930 and has a patent.
PHOTORESISTOR
A semiconductor resistor that changes its electricity. resistance under external influence el.-magn. radiation. They belong to photoelectricity radiation receivers, their operating principle is based on internal. photoelectric effect in semiconductors. To expand the function and capabilities of F., they are supplemented with filters, lenses, rasters, and presets. amplifiers, thermostats, lighting, cooling systems, etc. Basic parameters of a photoresistor: dark resistance (10 1 -10 14 Ohm); spectral sensitivity range (0.5-120 µm); time constant (10 -2 - 10 -9 s); voltage sensitivity (10 3 -10 6 V/W); detection ability (10 8 -10 16 cm Hz 1/2 W -1); temperature coefficient sensitivity (0.1-5%/K); operating voltage (0.1 -100 V).
Thermonuclear reactions
In 1939, the famous American physicist Bethe gave a quantitative theory of nuclear sources of stellar energy. As you know, stars are mostly made of hydrogen (although there are exceptions), so the probability of a collision between two protons is very high. When a proton collides with another proton, it can be attracted to the nucleus due to nuclear forces. Nuclear forces act over distances of the order of the size of the nucleus itself (i.e. 10 m). In order to approach the nucleus at such a short distance, the proton must overcome a very significant force of electrostatic repulsion. After all, the nucleus is also positively charged.
Ticket 20
ELECTRIC CURRENT IN VACUUM
create email current in a vacuum is possible if you use a source of charged particles.
The action of a source of charged particles can be based on the phenomenon of thermionic emission: this is the emission of electrons by solid or liquid bodies when they are heated to temperatures corresponding to the visible glow of a hot metal.
Vacuum diode
A vacuum diode is a two-electrode (A - anode and K - cathode) electron tube.
A very low pressure is created inside the glass container.
H - filament placed inside the cathode to heat it. The surface of the heated cathode emits electrons. If the anode is connected to + of the current source, and the cathode to -, then a constant thermionic current flows in the circuit. The vacuum diode has one-way conductivity. Those. current in the anode is possible if the anode potential is higher than the cathode potential. In this case, electrons from the electron cloud are attracted to the anode, creating an electric current in a vacuum.
Application of atomic energy.
Application of nuclear energy in modern world turns out to be so important that if we woke up tomorrow and the energy from a nuclear reaction had disappeared, the world as we know it would probably cease to exist. The use of nuclear energy creates many problems. Basically, all these problems are related to the fact that using the binding energy of the atomic nucleus for one’s benefit, a person receives significant evil in the form of highly radioactive waste that cannot simply be thrown away. Waste from nuclear energy sources must be processed, transported, buried, and stored for a long time in safe conditions.
Pros and cons, benefits and harms of using nuclear energy
Let's consider the pros and cons of using atomic-nuclear energy, their benefits, harm and significance in the life of Mankind. It is obvious that nuclear energy today is needed only by industrialized countries. That is, peaceful nuclear energy is mainly used in facilities such as factories, processing plants, etc. It is energy-intensive industries that are remote from sources of cheap electricity (such as hydroelectric power plants) that use nuclear power plants to ensure and develop their internal processes.
Agrarian regions and cities do not have much need for nuclear energy. It is quite possible to replace it with thermal and other stations. It turns out that the mastery, acquisition, development, production and use of nuclear energy is for the most part aimed at meeting our needs for industrial products. Let's see what kind of industries they are: automotive industry, military production, metallurgy, chemical industry, oil and gas complex, etc.
Does a modern person want to drive a new car? Want to dress in fashionable synthetics, eat synthetics and pack everything in synthetics? Want colorful products in different shapes and sizes? Wants all new phones, TVs, computers? Do you want to buy a lot and often change the equipment around you? Do you want to eat delicious chemical food from colored packages? Do you want to live in peace? Want to hear sweet speeches from the TV screen? Does he want there to be a lot of tanks, as well as missiles and cruisers, as well as shells and guns?
Wants?
And he gets it all. It does not matter that in the end the discrepancy between word and deed leads to war. It doesn't matter that recycling it also requires energy. For now the man is calm. He eats, drinks, goes to work, sells and buys.
And all this requires energy. And this also requires a lot of oil, gas, metal, etc. And all these industrial processes require nuclear energy. Therefore, no matter what anyone says, until the first industrial thermonuclear fusion reactor is put into production, nuclear energy will only develop.
We can safely list everything that we are used to as the advantages of nuclear energy. The downside is the sad prospect of imminent death due to the collapse of resource depletion, problems of nuclear waste, population growth and degradation of arable land. In other words, nuclear energy allowed man to begin to take control of nature even more, raping it beyond measure to such an extent that in a few decades he overcame the threshold of reproduction of basic resources, launching the process of collapse of consumption between 2000 and 2010. This process objectively no longer depends on the person. Everyone will have to eat less, live less and enjoy the natural environment less. Here lies another plus or minus of nuclear energy, which is that countries that have mastered the atom will be able to more effectively redistribute the scarce resources of those who have not mastered the atom. Moreover, only the development of the thermonuclear fusion program will allow humanity to simply survive. Now let’s explain in detail what kind of “beast” this is - atomic (nuclear) energy and what it is eaten with.
Ticket 21
1. Law of electrolysis
1833 - Faraday
The law of electrolysis determines the mass of the substance released on the electrode during electrolysis during the passage of electric current. 
k is the electrochemical equivalent of the substance, numerically equal to the mass of the substance released on the electrode when a charge of 1 C passes through the electrolyte.
Knowing the mass of the released substance, you can determine the charge of the electron.
2. Preparation of radioactive isotopes and their application.
Of all the isotopes known to us, only hydrogen isotopes have proper names. Thus, the isotopes 2H and 3H are called deuterium and tritium and are designated D and T, respectively (the 1H isotope is sometimes called protium).
Application isotopes One of the most outstanding studies carried out using “tagged atoms” was the study of metabolism in organisms. It has been proven that in a relatively short time the body undergoes almost complete renewal. The atoms that make it up are replaced by new ones. Only iron, as experiments on isotope studies of blood have shown, is an exception to this rule. Iron is part of the hemoglobin of red blood cells. When radioactive iron atoms were introduced into food, it was found that the free oxygen released during photosynthesis was originally part of water, not carbon dioxide. Radioactive isotopes are used in medicine, both for diagnosis and for therapeutic purposes.
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1.PLASMA– a partially or fully ionized gas formed from neutral atoms and charged particles (ions and electrons). The most important feature of plasma is its quasineutrality, which means that the volume densities of positive and negative charged particles from which it is formed are almost the same. A gas turns into a plasma state if some of its constituent atoms for some reason have lost one or more electrons, i.e. turned into positive ions. In some cases, negative ions can also appear in the plasma as a result of the “attachment” of electrons to neutral atoms. Plasma is the fourth state of matter, it obeys the gas laws and behaves like a gas in many respects. The term “plasma” itself, as applied to a quasi-neutral ionized gas, was introduced by the American physicists Langmuir and Tonks in 1923 when describing phenomena in a gas discharge. Until then, the word “plasma” was used only by physiologists and meant the colorless liquid component of blood, milk or living tissues, but soon the concept of “plasma” firmly entered the international physical dictionary and became widely used.
2. Biological effects of radioactive radiation was not immediately established. Becquerel, who discovered radioactivity in 1896, did not even suspect the biological effect of this type of radiation. In 1898, Maria Skladovskaya-Curie and Pierre Curie discovered radium and Becquerel took a few milligrams into a glass test tube for research, putting it in his breast pocket. After some time, a painful non-healing ulcer formed on the body opposite the pocket. He was forced to see a doctor, the ulcer was healed, but after some time it opened again. All scientists who worked with radioactive elements had hands covered with non-healing ulcers. Before the biological effect of penetrating radiation was established, science suffered irreparable losses. Maria and Pierre Curie, Irene and Frederic Curie and V. Kurchatov die from radiation sickness. To date, science has established enough facts in this area. But the mechanism of the effect of penetrating radiation on a cell has not been fully established. The effect of radiation on living organisms is characterized by the radiation dose. The natural background radiation per year is 2*10 -3 Gy per person (1 Gy=1J/kg). A radiation dose of 3-10 Gy received in a short time is lethal.
Ticket 23
1. Structure of gaseous, liquid and solid bodies
Gases. In gases, the distance between atoms or molecules is on average many times greater than the size of the molecules themselves. For example, at atmospheric pressure the volume of a vessel is tens of thousands of times greater than the volume of the molecules in it. Gases are easily compressed, and the average distance between molecules decreases, but the shape of the molecule does not change. Molecules move at enormous speeds - hundreds of meters per second - in space. When they collide, they bounce off each other different sides like billiard balls. The weak attractive forces of gas molecules are not able to hold them near each other. Therefore, gases can expand without limit. They retain neither shape nor volume. Numerous impacts of molecules on the walls of the vessel create gas pressure. Liquids. Liquid molecules are located almost close to each other, so a liquid molecule behaves differently than a gas molecule. In liquids, there is so-called short-range order, i.e., the ordered arrangement of molecules is maintained over distances equal to several molecular diameters. Solids. Atoms or molecules of solids, unlike atoms and molecules of liquids, vibrate around certain equilibrium positions. For this reason, solids retain not only volume, but also shape. The potential energy of interaction between solid molecules is significantly greater than their kinetic energy.
2. Three stages in the development of particle physics
1
. From electron to positron: 1897-1932. When the Greek philosopher Democritus called the simplest, indivisible particles atoms, then everything seemed to him, in principle, not very complicated. But at the end of the 19th century, the complex structure of atoms was discovered and the electron was isolated as component atom. Then, already in the 20th century, the proton and neutron were discovered - particles that make up the atomic nucleus.
2
. From positron to quarks: 1932-1970 (All elementary particles turn into each other)
Everything turned out to be much more complicated: as it turned out, there are no unchanging particles at all. The word elementary particle itself has a double meaning. On the one hand, the elementary simplest. On the other hand, by elementary we mean something fundamental that lies at the basis of things.
3
. From the quark hypothesis (1964) to the present day. In the 60s, doubts arose that all particles now called elementary fully justify this name. The discovery of an elementary particle has always been and still is an outstanding triumph of science. Triumphs began to follow literally one after another. A group of so-called “strange” particles was discovered: K-mesons and hyperons with masses exceeding the mass of nucleons. In the 70s, a large group of “charmed” particles with even larger masses was added to them. In addition, short-lived particles with a lifetime of the order of 10-22-10-23 s were discovered. These particles were called resonances, and their number exceeded two hundred. It was then, in 1964, that M. Gell-Mann and J. Zweig proposed a model according to which all particles participating in strong interactions are built from more fundamental particles - quarks. At present, almost no one doubts the reality of quarks, although they have not been discovered in a free state.
Ticket 24
1. Gas laws Isothermal process (Boyle Mariotto’s law). The process of changing the state of a system of macroscopic bodies at a constant temperature. To maintain a constant gas temperature, it is necessary that it can exchange heat with a large system - a thermostat. Otherwise, during compression or expansion, the temperature of the gas will change. For a gas of a given mass at a constant temperature, the product of the gas pressure and its volume is constant. This law was discovered experimentally (1627-1691). The Boyle-Mariotte law is usually valid for any gases, as well as for their mixtures, for example, air.
Only at pressures several hundred times greater than atmospheric pressure do deviations from this law become significant. The dependence of gas pressure on volume at a constant temperature is graphically represented by a curve called an isotherm.
Isobaric process. The process of changing the state of a thermodynamic system at constant pressure is called isobaric.
For a gas of a given mass at constant pressure, the ratio of volume to temperature is constant. This law was established experimentally in 1802 by the French scientist J. Gay-Lussac (1778-1850). This dependence is graphically represented by a straight line, which is called an isobar; different pressures correspond to different isobars. With increasing pressure, the volume of gas at a constant temperature decreases according to the Boyle-Mariotte law. Therefore, the isobar corresponding to the higher pressure p 2 lies below the isobar corresponding to the lower pressure p 1 .
Isochoric process. The process of changing the state of a thermodynamic system at a constant volume is called isochoric. For a gas of a given mass, the ratio of pressure to temperature is constant if the volume does not change. This gas law was established in 1787 by the French physicist J. Charles (1746-1823) and is called Charles's law. This dependence is depicted by a straight line called an isochore. Different isochores correspond to different volumes. As the volume of a gas increases at a constant temperature, its pressure decreases according to the Boyle-Mariotte law. Therefore, the isochore corresponding to the larger volume V 2 lies below the isochore corresponding to the smaller volume V 1 .
DISCOVERY OF THE POSITRON. ANTI-PARTICLES
The existence of the electron's twin - the positron - was theoretically predicted by the English physicist P. Dirac in 1931. At the same time, he predicted that when a positron meets an electron, both particles should disappear, giving rise to photons high energy. The reverse process can also occur - the birth of an electron-positron pair, for example: when a photon of sufficiently high energy collides with a nucleus. Two years later, the positron was discovered using a cloud chamber placed in a magnetic field. The direction of curvature of the particle track was indicated by the sign of its charge. Based on the radius of curvature and energy of the particle, the ratio of its charge to mass was determined. It turned out to be the same in modulus as that of the electron. At one time, the discovery of the birth and annihilation of electron-positron pairs caused a real sensation in science. Until then, no one had imagined that the electron, the oldest of particles, the most important building material of atoms, might not be eternal. Subsequently, twins - antiparticles - were found in all particles. Antiparticles are opposed to particles precisely because when any particle meets the corresponding antiparticle, their annihilation occurs. Both particles disappear, turning into radiation quanta or other particles. Relatively recently discovered: antiproton and antineutron. The electric charge of the antiproton is negative. Atoms whose nuclei consist of antinucleons and the shell of positrons form antimatter. In 1969, antihelium was first obtained in our country.
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1. The mathematical notation of the universal gas law is simple:
pV = nRT. It contains the main characteristics of the behavior of gases: p, V and T are the pressure, volume and absolute temperature of the gas, respectively, R is the universal gas constant common to all gases, and n is a number proportional to the number of molecules or atoms of the gas. This law is what in physics is commonly called the equation of state of matter, since it describes the nature of changes in the properties of matter when external conditions change. Strictly speaking, this law is exactly true only for an ideal gas. This formula was obtained in 1874 by D. I. Mendeleev by combining Avogadro’s law and the general gas law (pV/T = const), formulated in 1834 by B. P. E. Clapeyron. Therefore, this law is usually called the Mendeleev-Clapeyron law. Essentially, this law made it possible to introduce all previously made empirical conclusions about the nature of the behavior of gases into the framework of the new molecular kinetic theory.
« Physics - 10th grade"
Which current carriers in a semiconductor are majority and which are minority?
How does impurity conductivity differ from intrinsic conductivity?
The most interesting phenomena occur when n- and p-type semiconductors come into contact. These phenomena are used in most semiconductor devices.
p-n-transition.
Let's consider what will happen if we bring two identical semiconductors into contact, but with different types of conductivity: on the left is an n-type semiconductor, and on the right is a p-type semiconductor (Fig. 16.10).
The contact of two semiconductors with different types of conductivity is called p-n- or n-p junction.
Electrons in the figure are shown in blue circles, holes - in gray.
On the left side there are many free electrons, and on the right side their concentration is very low. On the right side, on the contrary, there are many holes, i.e., vacant places for electrons. As soon as the semiconductors are brought into contact, electrons begin to diffuse from the region with n-type conductivity to the region with p-type conductivity and, accordingly, the transition of holes in the opposite direction. The electrons that have passed into the p-type semiconductor occupy free spaces, the process of recombination of electrons and holes occurs, and the holes that have entered the n-type semiconductor also disappear due to the electrons occupying the vacant space. Thus, near the interface between semiconductors with different types of conductivity, a layer appears that is depleted of current carriers (it is called the contact layer). This layer is actually a dielectric; its resistance is very high. In this case, the n-type semiconductor is charged positively, and the p-type semiconductor is charged negatively. A stationary electric field of intensity k arises in the contact zone, preventing further diffusion of electrons and holes.
The total resistance of the semiconductors brought into contact is the sum of the resistance of the l-type semiconductor, the p-n junction and the p-type semiconductor: R = R n + R pn + R p. Since the resistances of areas with n- and p-types of conductivity are small (there are many charge carriers there - electrons and holes), the total resistance is determined mainly by the resistance of the p-n junction: R ≈ R pn.
Let's turn on a semiconductor with a p-n junction in electrical circuit so that the potential of the p-type semiconductor is positive, and the n-type is negative (Fig. 16.11). In this case, the tension external field will be directed in the direction opposite to the tension of the contact layer.
Module of total tension E = E to - E ext. Since the field holding the current carriers weakens, the electrons already have enough energy to overcome it.
A current will flow through the junction, and it will be created by the majority carriers - electrons go from the region with n-type conductivity to the region with p-type conductivity, and holes come from the region with p-type to the region with n-type. In this case, the pn junction is called direct.
Note that the electric current flows throughout the circuit: from the positive contact through the p-type region to the p-n junction, then through the n-type region to the negative contact (Fig. 16.12). The conductivity of the entire sample is high and the resistance is low. The greater the voltage applied to the contact, the greater the current.
The dependence of the current on the potential difference - the current-voltage characteristic of the direct junction - is shown in Figure (16.13) with a solid line.
Note that a change in the applied voltage leads to a sharp increase in current. Thus, an increase in voltage by 0.25 V can lead to an increase in current strength by 20,000 times.
In a direct junction, the resistance of the blocking layer is small, and it also depends on the applied voltage, as the voltage increases, the resistance decreases.
Let us now change the polarity of the battery connection. In this case, the strengths of the external and contact fields are directed in the same direction (Fig. 16.14) and the module of the total strength E = E to - E ext. The external field pulls electrons and holes away from the contact layer, causing it to expand. In this regard, electrons no longer have enough energy to overcome this layer. Now the transition through the contact is carried out by minority carriers, the number of which is small.
The resistance of the contact layer is very high. No current flows through the p-n junction. A so-called barrier layer is formed. This transition is called reverse.
The current-voltage characteristic of the reverse transition is shown in Figure 16.13 with a dashed line.
The pn junction turns out to be asymmetrical with respect to the current: in the forward direction the junction resistance is much less than in the reverse direction. Thus, a pn junction can be used to rectify electric current.
A device containing a p-n junction and capable of passing current in one direction and not passing in the opposite direction is called semiconductor diode.
If an alternating voltage is applied to the contacts of a semiconductor diode, then the current through the circuit will flow only in one direction.
Semiconductor diodes are made from germanium, silicon, selenium and other substances.
Let's consider how a pn junction is created using germanium, which has n-type conductivity, with a small addition of a donor impurity. This transition cannot be obtained by mechanically connecting two semiconductors with different types of conductivity, since this results in too large a gap between the semiconductors. The thickness of the p-n junction should be no greater than the interatomic distances, so indium is melted into one of the surfaces of the sample. To create a semiconductor diode, a p-doped semiconductor containing indium atoms is heated to a high temperature. Vapors of n-type impurities (for example, arsenic) are deposited on the surface of the crystal. Due to diffusion, they are introduced into the crystal, and on the surface of the crystal with p-type conductivity, a region with electronic type conductivity is formed (Fig. 16.15).
To prevent harmful effects of air and light, the germanium crystal is placed in a sealed metal case.
Semiconductor diodes are used in receiver detectors to isolate low-frequency signals and to protect against improper connection of the source to the circuit.
Traffic lights use special semiconductor diodes. At direct connection In such a diode, active recombination of electrons and holes occurs. In this case, energy is released in the form of light radiation.
A schematic representation of the diode is shown in Figure 16.16. Semiconductor rectifiers are highly reliable and have a long service life. However, they can only operate in a limited temperature range (from -70 to 125 °C)
Transistors.
Another application of semiconductors with impurity type conductivity - transistors - devices used to amplify electrical signals.
Let's consider one of the types of transistors made of germanium or silicon with donor and acceptor impurities introduced into them. The distribution of impurities is such that a very thin (on the order of several micrometers thick) layer of n-type semiconductor is created between two layers of p-type semiconductor (Fig. 16.17). This thin layer is called basis or base.
Two p-n junctions are formed in the crystal, the direct directions of which are opposite. Three terminals from areas with different types of conductivity allow you to include a transistor in the circuit shown in Figure 16.17. In this circuit, when battery B1 is connected, the left p-n junction is direct. A left-handed semiconductor with p-type conductivity is called emitter. If there were no right p-n junction, there would be a current in the emitter-base circuit, depending on the voltage of the sources (battery B1 and source AC voltage) and circuit resistance, including the low resistance of the direct emitter-base junction.
Battery B2 is connected so that the right n-p junction in the circuit (see Fig. 16.17) is reverse. The right region with p-type conductivity is called collector. If there were no left pn junction, the current in the collector circuit would be close to zero, since the resistance of the reverse junction is very high. When a current exists in the left p-n junction, a current appears in the collector circuit, and the current strength in the collector is only slightly less than the current strength in the emitter. (If a negative voltage is applied to the emitter, then the left p-n junction will be reversed, and there will be practically no current in the emitter circuit or in the collector circuit.)
This is explained as follows. When a voltage is created between the emitter and the base, the majority carriers of the p-type semiconductor (holes) penetrate into the base, where they are already non-major media. Since the thickness of the base is very small and the number of main carriers (electrons) in it is small, the holes that get into it almost do not combine (do not recombine) with the electrons of the base and penetrate into the collector due to diffusion. The right pn junction is closed to the main charge carriers of the base - electrons, but not to holes. In the collector, holes are carried away by the electric field and complete the circuit. The strength of the current branching into the emitter circuit from the base is very small, since the cross-sectional area of the base in the horizontal (see Fig. 16.17) plane is much smaller than the cross-section in the vertical plane.
The current in the collector is almost equal to strength current in the emitter varies with the current through the emitter. The resistance of the resistor R has little effect on the current in the collector, and this resistance can be made quite large. By controlling the emitter current using an alternating voltage source connected to its circuit, we obtain a synchronous change in the voltage across resistor R.
With a large resistor resistance, the change in voltage across it can be tens of thousands of times greater than the change in signal voltage in the emitter circuit. This means increased tension. Therefore, at load R it is possible to obtain electrical signals whose power is many times greater than the power entering the emitter circuit.
Application of transistors.
Modern electronics are based on microcircuits and microprocessors, which include a colossal number of transistors.
The first integrated circuit went on sale in 1964. It contained six elements - four transistors and two resistors. Modern microcircuits contain millions of transistors.
Computers, made up of chips and microprocessors, have actually changed the world around us. Currently, there is not a single area of human activity where computers do not serve as active human assistants. For example, in space research or high-tech production, microprocessors operate, the level of organization of which corresponds to artificial intelligence.
Transistors (Fig. 16.18, 16 19) have become extremely widespread in modern technology. They replaced vacuum tubes in electrical circuits of scientific, industrial and household equipment. Portable radios that use such devices are commonly called transistors. The advantage of transistors (as well as semiconductor diodes) compared to electron tubes is, first of all, the absence of a heated cathode, which consumes significant power and takes time to warm up. In addition, these devices are tens and hundreds of times smaller in size and weight than vacuum tubes.
Pn junction or electron-hole junction is the contact of two semiconductors with electron and hole conductivity. These junctions play an important role in modern electronics. Possessing one-way conductivity, p-n junctions are used to rectify alternating current as independent products (diodes), allow the creation of devices for controlling electric current (transistors), and are also used in integrated circuits to isolate its elements.
In Fig. 4.3. The diagram of the p-n junction is shown.
Fig.4.3. p-n scheme transition: distribution of space charge (a) and impurities (b), band diagram in n and p-type semiconductors (c) and in p-n junction (d).
Volume charges formed in p-n areas transition, create a potential barrier to the passage of mobile carriers. By controlling the value of the barrier, you can change the amount of current in the electrical circuit.
The resulting contact potential difference (the magnitude of the potential barrier) is determined from the expression
where p p , n n – equilibrium concentration of the main carriers;
n i is the concentration of intrinsic carriers.
Electric field in the space charge region (d=d p +d n) is determined by the distribution law of these charges and can be found from the solution of the Poisson equation.
For d p≤x≤0 (4.9)
For 0 ≤x≤d n (4.10)
Considering that E=dφ/dx you can get an expression for φ(x)
at 0≤x≤dn; (4.11)
at d p ≤x≤0 (4.12)
Space charge region thickness (d=d p +d n)
where U is the applied external voltage.
For an asymmetric transition, when N D >>N A the equation is simplified
When turning the transition back on (- to p- areas, + to n- region) the space charge layer can be considered as a kind of capacitor with a capacitance (C b), called a barrier.
Volt-ampere characteristics. Let's consider the current-voltage characteristic of the p-n junction. As in the case of a metal-semiconductor contact, the type of current-voltage characteristic significantly depends on the structure of the pn junction, more precisely, on its thickness. Thus, if the thickness of a pn junction is less than the mean free path of carriers (thin junction), then electrons or holes fly through the junction without colliding with the lattice. In the case of a thick junction, when its width significantly exceeds the mean free path, the transport of charge carriers has a diffusion character. However, since, unlike a metal-semiconductor contact in a p-n junction, current transfer is carried out by minority charge carriers, the main thing is not the nature of the transfer, but the intensity of generation and recombination of carriers in the region of the p-n junction. In the case of a thin transition, recombination in the space charge layer is insignificant. On the contrary, in a thick pn junction, a significant part of the minority carriers recombine, which should noticeably affect the shape of the current-voltage characteristic.
Let us first consider the current-voltage characteristic of a thin junction. Charge carriers pass through a thin transition without having time to recombine, so both hole currents and electron currents on both sides of the p-n transition are equal. The hole current at the boundary of the space charge layer on the side of the electronic semiconductor at x = -L n is completely determined by the diffusion component, since at this point the electric field strength is zero.
The hole current density in this case
A similar expression can be obtained for the electron current density :
The total current flowing through the pn junction can be calculated in any section (S) of the sample. The easiest way to calculate it is at the boundary of the pn junction,
I =S(J p + J p)(4.18)
From the resulting formula it is clear that in the forward direction the current increases exponentially with increasing voltage, and in the shut-off direction it tends to the saturation current I S, caused by thermal generation of minority carriers at the boundary of the p-n junction And independent of external voltage:
If the p-n transition is sharply asymmetric, then one of the terms in formula (4.20) will be vanishingly small. Indeed, let, for example, the n-region be doped much more heavily than the p-region. Then, in accordance with the law of mass action, we have pno<
In general, the degree of asymmetry of the p-n junction is characterized by a parameter called injection ratio . The injection coefficient is equal to the ratio of the larger current component to the total current. In case n n >>p p the injection coefficient is equal to
Let us next consider the current-voltage characteristic of a thick junction using the example of a so-called p-i-n diode. The structure of such a diode consists of two n- and p-type layers, separated by a high-resistivity layer of intrinsic conductivity with a thickness of d. In such a diode, it is no longer possible to neglect the processes of generation and recombination inside the p-n junction. In the case when the external potential difference is switched on in the shut-off direction, charge carriers are generated in the intermediate i-layer at a rate n i /τ i . When the voltage is switched on in the forward direction, recombination of injected carriers occurs in this layer and the current density associated with the generation and recombination of carriers in the intermediate layer of thickness d is equal to
where τ i is the lifetime of intrinsic carriers;
n i is the intrinsic concentration of carriers.
Total current flowing through p-i-n junction, can be considered as the sum of the current calculated without taking into account generation and recombination inside the junction and the generation-recombination component:
The resulting formula is valid not only in the case of a clearly defined i-layer, but also with a smooth change in the concentration of impurities in the region of a conventional p-n junction. In this case, the role of the parameter d overall width plays a role p-p- transition. From formula (4.24) follows a condition for determining whether a given p–n transition belongs to the category of thin or thick: if the third term in parentheses is significantly less than the sum of the first two, then the transition can be considered thin. Otherwise, the pn junction must be considered thick.
Breakdown p-n transition. With an increase in the reverse voltage at the p-n junction, when a certain voltage value U of the samples is reached, a sharp increase in the current through the diode begins, leading to breakdown. The average electric field strength in the space charge region of the p-n junction can be written as
E=V/d= (q/2εε 0) 1/2 (UN D) 1/2 (4.25)
Since the breakdown begins when a certain (for each specific conditions) value of the electric field strength E is reached, the more d(less N D), the higher the sample voltage U, the breakdown begins. Obviously, the largest U samples has p-i-n junction, because N D in its base is smallest, and the width of the space charge region d greatest.
Heterojunctions. In contrast to a p-n junction formed by a change in the concentration of impurities in one semiconductor material (homojunction), A heterojunction is a junction formed by semiconductors of different physicochemical natures. Examples of heterojunctions can be germanium - silicon, germanium - gallium arsenide, gallium arsenide - gallium phosphorus transitions, etc. To obtain heterojunctions with a minimum number of defects at the interface of the crystal lattice one semiconductor should pass into the crystal lattice of another with minimal disruption. In this regard, the semiconductors used to create a heterojunction must have similar lattice constants and identical crystal structures. Of greatest practical interest at present are heterojunctions formed by semiconductors with different band gaps, and not only heterojunctions between p- and n-type semiconductors, but also heterojunctions between semiconductors with one type of conductivity: n-n or p have interesting properties for semiconductor devices -R.
Let's consider the energy diagram of a heterojunction between an n-type semiconductor with a wide bandgap and a p-type semiconductor with a narrow bandgap (Fig. 4.4). The energy of an electron located in a vacuum is taken as the reference point (0). Magnitude χ in this case, the true work function of the electron. from semiconductor to vacuum. The thermodynamic work function is designated A.
When a contact is made between two semiconductors, the Fermi levels become equal. Differences between heterojunction and energy p-n diagrams transitions consist in the presence of discontinuities in the conduction band (Δ E C) and in the valence band (Δ E V). In the zone. conductivity, the magnitude of the gap is determined by the difference in the true work functions of electrons from p and n semiconductors:
ΔE C = χ 2 – χ 1 (4.26)
and in the valence band, in addition, there is also inequality in energy values E V .
Therefore, the potential barriers for electrons and holes will be different: the potential barrier for electrons in the conduction band is less than for holes in the valence band. When a voltage is applied in the forward direction, the potential barrier for electrons will decrease and electrons from n-semiconductor is injected into R-semiconductor. Potential barrier for holes in R-area will also decrease, but will still remain large enough for injection of holes from R-regions in n-the area was practically non-existent. In this case, the injection coefficient (γ) can be equal to unity.
Rice. 4.4. Energy diagram of two semiconductors R- and n-type with different band gaps (a) and р–n heterojunction (b)
To achieve the best device parameters, this value should be maximum. In a homojunction, this is achieved by stronger doping of the n-region with impurities relative to the p-region. However, this path cannot be followed endlessly, since, on the one hand, there is a limit to the solubility of the impurity in the semiconductor and, on the other hand, when the semiconductor is heavily doped, many different defects are introduced into it simultaneously with the impurity, which worsen the parameters of the p-n junction. In this direction, the use of a heterojunction is promising.
If a heterojunction is formed by semiconductors with an equal amount of impurities (p p =p p ) and for simplicity we assume that the effective masses and other parameters of charge carriers are equal, then we can write
I p /I n =exp[-(E gn –E g p )/kT](4.27)
When using, for example, n-silicon and p-germanium E gn –E gp =0.4 eV. Because kT/q=0.025 B, then 1 r /1 p = e - 16 , which is practically equal to zero, i.e. the current through the heterojunction consists only of electrons injected from n- area in R-region. In a homojunction under the same conditions I r /I n=:1, i.e. the currents of electrons and holes are equal.
Thus, the heterojunction allows for almost one-sided injection of charge carriers. It is important to note that one-sided injection is preserved even with an increase in the current through the heterojunction, whereas in the homojunction it is disrupted.
26.01.2015
Lesson No. 37 (9th grade)
Topic: Thermistors
Electricity through contact of p- and n-type semiconductors
( p-n junction)
Of particular importance in technology is the bringing into contact of semiconductors of different conductivities. What will happen with such contact? Due to charge diffusion, electrons will begin to penetrate into the p-semiconductor, and holes into the n-semiconductor. As a result, a so-called blocking layer is formed at the boundary, which, with its electric field, prevents further charge exchange (Fig. 6).
Rice. 1. Blocking layer at p-n junction
To construct the current-voltage characteristic of the n-p junction, the following circuit was assembled (see Fig. 2), thanks to which it is possible to both change the polarity and the magnitude of the voltage supplied to the p-n junction.
Fig.2. The circuit for obtaining the characteristics and the volt-ampere itself p-n characteristic transition accordingly.
Figure 16.10 shows a diagram of a semiconductor, the right side of which contains donor impurities and is therefore a semiconductor n-type, and the left one is acceptor impurities and represents a semiconductor R-type; between them - transition zone- zone depleted of charges. Recombination of electrons and holes occurs in it. Electrons are depicted as blue circles, holes as gray circles. The contact between two semiconductors is called р-n- or n-r-transition.
When a contact is formed, electrons partially transfer from the semiconductor n-type semiconductor R-type, and the holes are in the opposite direction. As a result, the semiconductor n-type is charged positively, and R-type - negative. Diffusion stops after the electric field arising in the transition zone begins to prevent further movement of electrons and holes.
Let's turn on the semiconductor with р-n-transition to an electrical circuit ( Fig.16.11). Let's first connect the battery so that the potential of the semiconductor R-was kind of positive, but n-type - negative. In this case, the current through р-n- the transition is created by the main carriers: from the area n to the region R- electrons, and from the region R to the region n- holes ( Fig.16.12).
As a result, the conductivity of the entire sample is high and the resistance is low.
The transition considered here is called direct. The dependence of the current on the potential difference - the current-voltage characteristic of the direct junction - is shown in Figure 16.13 with a solid line.
Let us now change the polarity of the battery connection. Then, at the same potential difference, the current strength in the circuit will be significantly less than with a direct transition. This is due to the following. Electrons now go through the contact from the region R to the region n, and the holes are from the region n to the region R. But in a semiconductor R-type there are few free electrons, and in a semiconductor n-like few holes. Now the transition through the contact is carried out by minority carriers, the number of which is small ( Fig.16.14). As a result, the conductivity of the sample turns out to be insignificant, and the resistance is large. A so-called barrier layer is formed. This transition is called reverse. The current-voltage characteristic of the reverse transition is shown in Figure 16.13 with a dashed line.
Thus, р-n- the junction can be used to rectify electric current. This device is called a semiconductor diode.
Semiconductor diodes are made from germanium, silicon, selenium and other substances.
Let's look at how they create р-n-transition using conductive germanium n-type, with a small addition of donor impurity. This transition cannot be obtained by mechanically connecting two semiconductors with different types of conductivity, since this results in too large a gap between the semiconductors. Thickness р-n-transition should be no greater than the interatomic distances, so indium is melted into one of the surfaces of the sample. To create a semiconductor diode, a doped semiconductor R-type, containing indium atoms, is heated to a high temperature. Impurity pairs n-type (for example, arsenic) is deposited on the surface of the crystal. Due to diffusion, they are introduced into the crystal, and on the surface of the crystal with conductivity R-type, a region with an electronic type of conductivity is formed ( Fig. 16.15).
To prevent harmful effects of air and light, the germanium crystal is placed in a sealed metal case.
A schematic representation of the diode is shown in Figure 16.16. Semiconductor rectifiers are highly reliable and have a long service life. However, they can only operate in a limited temperature range (from -70 to 125°C).
p-n- the transition turns out to be asymmetrical with respect to the current: in the forward direction the resistance of the transition is much less than in the reverse direction.
Properties р-n-transitions are used to rectify alternating current. During half the period of change of current through the junction, when the potential of the semiconductor R- type positive, current flows freely through р-n-transition. In the next half of the period, the current is practically zero.
2. Semiconductor devices
Small size and very high quality of transmitted signals have made semiconductor devices very common in modern electronic technology. The composition of such devices may include not only the aforementioned silicon with impurities, but also, for example, germanium. One such device is a diode - a device that can pass current in one direction and prevent it from passing in another. It is obtained by implanting a semiconductor of another type into a p- or n-type semiconductor crystal (Fig. 11).
Rice. 3. Designation of the diode on the diagram and the diagram of its device, respectively
Another device, now with two p-n junctions, is called a transistor. It serves not only to select the direction of current transmission, but also to transform it.
It should be noted that modern microcircuits use many combinations of diodes, transistors and other electrical devices.
Rice. 12. Diagram of the structure of the transistor and its designation on the electrical diagram, respectively
A transistor is an ingenious device. It is not easy to understand the principles of operation of a transistor, but they managed to invent it! We hope you can understand how it works even from a short description.
Let's consider one of the types of transistors made of germanium or silicon with donor and acceptor impurities introduced into them. The distribution of impurities is such that a very thin (about several micrometers thick) semiconductor layer is created n-type between two layers of semiconductor R-type ( Fig.16.17). This thin layer is called basis, or base.
Two are formed in the crystal р-n-transitions whose direct directions are opposite. Three terminals from areas with different types of conductivity allow you to include a transistor in the circuit shown in Figure 16.17. In this diagram, the left р-n-transition is direct and separates the base from the region with conductivity R-type called emitter. If there was no right р-n-transition, in the emitter-base circuit there would be a current depending on the voltage of the sources (battery B1 and the alternating voltage source) and the resistance of the circuit, including the low resistance of the direct emitter-base transition.
Battery B2 is turned on so that the right р-n-transition in the circuit (see Fig. 16.17) is reverse. It separates the base from the right region with conductivity R-type called collector. If there were no left pn junction, the current in the collector circuit would be close to zero, since the resistance of the reverse junction is very high. If there is a current in the left р-n-transition, a current also appears in the collector circuit, and the current strength in the collector is only slightly less than the current strength in the emitter.
This is explained as follows. When a voltage is created between the emitter and base, the majority carriers of the semiconductor R-type (holes) penetrate into the base where they are already non-major media. Since the thickness of the base is very small and the number of majority carriers (electrons) in it is small, the holes that get into it almost do not combine (do not recombine) with the electrons of the base and penetrate into the collector due to diffusion. The right pn junction is closed to the main charge carriers of the base - electrons, but not to holes. In the collector, holes are carried away by the electric field and complete the circuit. The strength of the current branching into the emitter circuit from the base is very small, since the cross-sectional area of the base in the horizontal (see Fig. 16.17) plane is much smaller than the cross-section in the vertical plane.
The collector current, which is almost equal to the emitter current, varies with the current through the emitter. Resistor resistance R has little effect on the collector current, and this resistance can be made quite large. By controlling the emitter current using an alternating voltage source connected to its circuit, we obtain a synchronous change in the voltage across the resistor R.
With a large resistor resistance, the change in voltage across it can be tens of thousands of times greater than the change in signal voltage in the emitter circuit. This means increased tension. Therefore, under load R It is possible to obtain electrical signals whose power is many times greater than the power entering the emitter circuit.
Application of transistors. Modern electronics are based on microcircuits and microprocessors, which include a colossal number of transistors. Computers, made up of chips and microprocessors, have actually changed the world around us. Currently, there is not a single area of human activity where computers do not serve as active human assistants. For example, in space research or high-tech production, microprocessors operate, the level of organization of which corresponds to artificial intelligence.
Transistors (Fig. 16.18, 16.19) have become extremely widespread in modern technology. They replace vacuum tubes in electrical circuits of scientific, industrial and household equipment. Portable radios using such devices are commonly called transistors. The advantage of transistors (as well as semiconductor diodes) compared to electron tubes is, first of all, the absence of a heated cathode, which consumes significant power and takes time to warm up. In addition, these devices are tens and hundreds of times smaller in size and weight than vacuum tubes. They operate at lower voltages.
1. Receipt p-n transition. In the second half of the twentieth century, solid-state electronics developed intensively. Bulky vacuum tubes have been replaced by small-sized semiconductor devices. The main element of semiconductor devices is p-n – transition with unique properties. It is a thin layer at the interface between two impurity semiconductors.
Get p-n– a transition by direct contact of two semiconductors is almost impossible. No matter how thoroughly their surfaces are cleaned, they always contain many impurities and contaminants that worsen the properties of semiconductors. Therefore, the problem is solved by introducing an opposite impurity into the same crystal with a certain type of conductivity.
For example, into a single crystal of tetravalent germanium with a donor impurity, which creates Ge conductivity in the germanium crystal n- like, a piece of trivalent indium In is melted in a vacuum at a temperature of about 1000°C. Indium atoms diffuse into germanium to a certain depth. In the region of the crystal where indium atoms penetrate, the conductivity becomes hole ( p– sort of). At the border of this area there arises p-n– transition. As you move deeper into the crystal, the indium concentration gradually decreases. The layer where the concentration of the indium acceptor impurity is equal to the concentration of the donor impurity in the single crystal is actually p-n– transition. Such transitions are called smooth. Sharp p-n junctions obtained by depositing a semiconductor onto a crystal, for example n– type, semiconductor p– type from the gas phase. To do this, pass over the crystal at a temperature of 1200 o WITH such a gas mixture that a semiconductor with the right type conductivity.
2. Equilibrium states p-n– transition. Let us mentally assume that immediately after formation p- And n- areas, we separated them, preventing charges from flowing from one area to another. The situation shown in Fig. 117 arises. Both areas are electrically neutral, their zero levels coincide. Fermi level p–regions above impurity levels, and in n–areas – below. In general, the Fermi levels do not coincide, in n–regions have a higher Fermi level.
However, in reality, after education p-n– layer, the diffusion of the main carriers from one area to another begins. For n–regions the main carriers are electrons, for p– areas – holes. Majority carriers arise almost entirely due to the ionization of donor and acceptor impurities. At temperatures T³ 250 K these impurities are almost completely ionized. Therefore, the electron concentration in n–region is equal to the concentration of donor atoms, and the concentration of holes in p–regions – concentration of acceptor atoms.
The concentration of minority carriers is approximately 10 6 times less than the concentration of major carriers in both regions (). As a result, diffusion flows of conduction electrons from P-regions in R-area and holes from p–regions in n-region. Electrons moving in p–region, recombine near the interface with holes, and holes in n–regions recombine with conduction electrons. Therefore, in the contact layer n– there are practically no free electrons left in the region and a stationary positive charge of ionized donors.
In the contact layer p– there are practically no holes left in the area, and a negative charge of ionized acceptors. These stationary electric charges create p-n– transition contact electric field with potential difference j to and practically not going beyond its limits.
This situation is illustrated in Fig. 118, where space charges in contact areas are shown at the top, and energy zones at the bottom. Because the p– the area became charged with a negative charge, the energy of the electrons in it increased. As a result, the energy diagram in p– the area rises, in n-areas are omitted. IN p-n layer it is tilted so that the Fermi levels in different regions coincide.
If a minority carrier (an electron from p-area or hole from n-area), then the contact field picks it up and throws it through this layer. As a result, every non-primary carrier that strikes p-n- transition, passes through it.
On the contrary, the majority current carriers (electron from n-area or hole from p-regions) can fly through a layer of space charges only if the kinetic energy of their movement along the axis X sufficient to overcome the contact potential difference, that is, if it is greater than | ej to |. Therefore, as soon as space charges are formed at the boundary of the regions, the flow of majority carriers crossing this boundary decreases. When the flow of majority carriers equals the flow of minority carriers, dynamic equilibrium is established.
3. Direct connection p-n– transition to an electrical circuit. Let's connect to p-n– transition current source, connecting to p– “plus” areas, and n–areas – “minus” (Fig. 119 above). We believe that the current source is capable of creating voltage on ohmic buses j 0 . The additional electric field created by the current source causes an influx of majority carriers into the space charge region p-n– transition. IN p–areas in the direction from the ohmic bus to p-n– holes are moving through the transition. They recombine with the electrons of the negative ions of the acceptor impurity. IN n–areas towards p-n– conduction electrons move through the transition and recombine with positive ions of the donor impurity.
As a result, the volume charge on p-n– transition decreases compared to the equilibrium state. The height of the potential barrier also becomes smaller. This process continues until the contact potential difference is p-n– the transition will not decrease to the value j To - j 0 .
In Fig. 119 below this situation is shown in the energy band diagram. Dashed lines correspond to the equilibrium state p-n– transition.
Electrons in the conduction band of a semiconductor behave like heavy objects sliding along the bottom of the conduction band. Reducing the height of the potential barrier sharply increases the fraction of those electrons in n–areas whose kinetic energy in the direction of movement towards the potential barrier is sufficient to overcome this barrier.
Holes in the valence band behave like air bubbles under ice. How less height barrier, the greater the proportion of holes that can “dive” under it (in Fig. 119 below from left to right). As a result, with a decrease in the height of the potential barrier, the diffusion flux through p-n– transition of conduction electrons from n–areas and holes from p-regions Main carrier current i dp jumps compared to the equilibrium value by several orders of magnitude.
4. Reverse switching p-n – transition shown in Fig. 120 above. The “plus” of the current source is connected to the ohmic bus n-area, and the “minus” is connected to the ohmic bus p-regions The drift of the main carriers arising in the electric field of the current source is directed from p-n– transition to ohmic buses. In this case, new layers of ionized donors and acceptors are exposed, thereby increasing the region of volumetric bound charge.
The flow of electrons and holes to the ohmic contacts occurs until they almost completely compensate for the charges created external source EMF. After this, all applied voltage j 0 falls on p-n– a junction whose resistance is many orders of magnitude greater than the resistance p- And n- regions. Potential barrier p-n– transition increases almost to the value e(j k + j 0). This dramatically reduces the main carrier current i basic, making it less than the equilibrium value. Minority carrier current i neosn depends only on their concentration and therefore changes insignificantly 
clearly (Fig. 120 below).
So, p-n– the transition can be considered as nonlinear conductor, the resistance of which depends only on the sign of the applied voltage. One way conduction p-n– transitions are used not only in semiconductor diodes. Properties p-n– the transitions turned out to be so fruitful that they made it possible to create a series based on them electronic semiconductor devices, which in addition to diodes include transistors, thyristors etc. In the second half of the 20th century there is Fast passage from tube to solid-state electronics.
5.
Semiconductor diodes– nonlinear conductors. Their two electrodes are called anode (+) and cathode ( -
). Diodes have a sharply asymmetrical current-voltage characteristic (Fig. 121). This allows them to be used for rectifying alternating currents.
If an alternating sinusoidal voltage is applied to the primary winding of the transformer, then in the secondary winding, closed to an ohmic resistor R, sinusoidal flows alternating current the same frequency 
, Where j 0 – phase shift (Fig. 122-a). If a semiconductor diode is connected to the break in the secondary circuit, then through a resistor R During one half of the period, a pulsating unidirectional current will flow. The result is a scheme half wave rectifier(Fig. 122-b).
For full wave rectification you need at least two diodes and the output of the middle point of the secondary winding of the transformer (Fig. 122-c). By connecting four diodes according to the circuit rectifier bridge, you can do without the midpoint (Fig. 122-d).
6. Transistors. By using p-n– transitions can not only be straightened, but also electric currents can be amplified. For this purpose they serve transistors– semiconductor devices having three electrodes ( emitter, collector, base). Let's consider the principle of operation of a transistor using the example of its connection according to a circuit with a common base (Fig. 123).
Left in the picture p-n– transition 1 works in the forward direction. Right p-n– transition 2 operates in the locking direction. The distance in the transistor between transitions 1 and 2 (base width) does not exceed several tens of microns. The current in the base-collector circuit is determined by minority carriers and is highly dependent on the concentration of these carriers. IN n– areas of minority carriers are holes.
If current flows in the emitter-base circuit, then the holes from p– areas where they are the main carriers, in large quantities move through transition 1 to the base area. As a result, the concentration of holes in n– the base area increases sharply. They say it's happening injection holes. Because Since the base width is very small, then a large number of holes diffusing through junction 1 reach junction 2. The concentration of minority carriers in n– the area near transition 2 increases significantly, which is why the current in the collector circuit increases.
Voltage U 2 in the collector circuit there is much more voltage U 1 in emitter circuit U 2 >>U 1 . Therefore, standing out at the resistance R the power turns out to be greater than the power consumed in the emitter circuit. The power gain in modern transistors ranges from several tens to tens of thousands of times.
7. Thermoelectric Seebeck and Peltier effects expressed in semiconductors much more strongly than in metals (see §14). Especially if the contacts form semiconductors with different types of conductivity. The differential thermal emf in semiconductors is approximately 1000 times greater than in metals. This makes it possible to create semiconductor thermoelectric generators and refrigerators.
The theory of thermoelectric generators was developed in the early 40s of the 20th century by Abram Ioffe. The first thermal generators in the USSR were built at the beginning of the Great Patriotic War and were used to power radio stations in partisan detachments. In the mid-70s, thermal generators with a power of 150–200 W appeared to power the equipment of meteorological stations and spacecraft. The source of energy in them was the radioactive isotope of cerium 144 Ce.
The maximum efficiency of thermoelectric generators achieved to date is 15% and is unlikely to exceed 20%. Semiconductor thermoelectric generators are expensive, so industrial production based on them electrical energy in the near future is unlikely unless cheap materials are created that combine high electrical conductivity with low thermal conductivity.
Semiconductor refrigerators, built on the basis of the Peltier effect, are most often used to cool elements of radio-electronic circuits.
8. Photovoltaic effect. When illuminated p-n– the transition and adjacent areas with light capable of causing the generation of electron-hole pairs, through p-n– transition occurs when a charge current changes its state compared to the equilibrium state.
Let's say on p– the area where the light falls, as shown in Fig. 124 above. For photons to be absorbed near p-n junction, thickness R– the area should be small and not exceed 1–2 µm. If the photon energy hn greater than the band gap, hn³ E g, then when a photon is absorbed by an electron in the valence band of any region, the electron goes into the conduction band. (We assume that the acceptor and donor impurity levels in p- And n- areas are already completely ionized). A pair of carriers appears - an electron in the conduction band and a hole in the valence band.
Increasing the number of primary carriers (holes in p–regions and electrons in n-regions) essentially does not change anything, since their relative increase is small. And the increase in the number of minority carriers (holes in n–region electrons in p-areas) is very significant. Since the current through p-n– the transition of minority carriers depends only on their concentration, then upon illumination p-n– light transition occurs photovoltaic effect– the appearance of a minority carrier current approximately proportional to the luminous flux F.
Minority carriers are captured by the contact field and escape from p- And n- regions. The main carriers remain. As a result, on opposite sides p-n– transition gradually accumulates charges of free carriers – holes in p– area and electrons in n– areas, p– the area is charged positively, n– area – negative.
The field of these free charges is opposite to the contact field and weakens it. Related in the area p-n– transition, the charge of the ionized impurity decreases, the height of the potential barrier becomes smaller (Fig. 124 below). As a result, the diffusion of the main carriers increases. Gradually, such a dynamic equilibrium is established when, for a given luminous flux Ф, the minority carrier current is proportional to it i neosn will become equal to the opposite current of the main carriers, i neosn = i basic The height of the potential barrier takes the value e(j k + j f), where j F – photo-EMF p-n junction.
The photovoltaic effect can be used in photocell or in mode photodiode.
A . Photocell.
For use p-n– transition in mode photocell (in valve mode) just connect p- And n–area of ohmic jumper with load resistance R. When illuminating a photocell by resistance R photocurrent of free people will flow electric charges. Therefore, in photocell mode p-n– the transition allows the direct conversion of light energy into electrical energy. The diagram of the photocell device is shown in Fig. 125. To thin p– area (»1 µm) an even thinner metal layer of silver or gold is deposited, playing the role of an ohmic bus. In order for this metal film to transmit light well enough, its thickness must be much less than the wavelength of light l. Usually this is several tens of atomic layers.
The second ohmic bus is a metal plate, which simultaneously plays the role of a mechanical supporting basis for the entire structure of the photocell. Collected from individual photocells solar panels, used to power space equipment and in ground-based power plants.
Currently, solar cells are made mainly from silicon Si and gallium arsenide GaAs. Achieved efficiency h» 20% is close to theoretically possible.
b . Photodiode
. To use p-n– transition in photodiode mode, voltage is applied to it j 0 from the current source in the shut-off direction (Fig. 126 on the left). If the photodiode is not illuminated, a very small dark minority carrier current flows through it. Voltage U on a resistor R practically equal to zero. When a luminous flux F is directed to a photodiode, the concentration of minority carriers and their current increases in proportion to the flux F. At the resistor R tension arises U(Fig. 126 on the right), which can be used as a signal in communication or control circuits.
9. Light-emitting diode. When passing a direct current, the concentration of minority carriers in the region p-n– transition increases. Major carriers are drawn to the injected minority carriers. As a result, in the area p-n– transition process develops recombination excess carriers over the equilibrium state.
If part of the recombination events occurs with the emission of light and if this light can go out, then a light-emitting diode is obtained - Light-emitting diode.
These two conditions are decisive when designing LEDs. The first task - increasing the role of light-emitting recombination events - is solved by reducing the proportion of non-radiative transitions. To do this, the semiconductor must be up to high degree cleared of non-radiative impurity centers, which is a rather difficult task. The second condition - the release of radiation to the outside - is also a difficult task. The fact is that the refractive index of light in semiconductors is high; gallium arsenide, for example, n=3.45. Therefore, the angle of total internal reflection in semiconductors is very small, 
. Only »2% of the emitted radiation strikes the flat surface of the semiconductor at angles less than b before, having experienced only partial reflection from the conductor-air interface.
The average emission power of LEDs in continuous mode is 3¸5 mW. It is not possible to increase it by increasing the forward current due to heating p-n– a transition that sharply reduces internal efficiency.
LEDs are widely used in modern electronics. In combination with photodetectors they form optocoupler pairs, used for decoupling and amplifying signals in optocouplers logical elements. The performance of LEDs reaches »10 -9 s. LEDs are also used as small-sized light indicators. By choosing semiconductors with different band gaps, it is possible to make LEDs with different emission colors.
10.
Semiconductor lasers. The most widely used now are semiconductor injection lasers based on gallium arsenide, GaAs. Inversion of the population of levels in them is achieved by injection of majority carriers through p-n– transition.
Figure 127-a shows the equilibrium p-n– transition between two degenerate regions of a semiconductor. Regions with coinciding energy levels are called degenerate. As a result, two or more electrons can correspond to one energy value. Fermi level E F in p–region is below the top of the valence band E in, and in n–regions – above the bottom of the conduction band E n. As a result, the ceiling of the valence band is filled to capacity with holes in p–region, and the bottom of the conduction band in n– areas – electrons (Fig. 127-c).
If to this p-n– attach the transition forward voltage j(To p– “plus” areas, to n– “minus” area), sharply reducing the potential barrier, then an area appears in it A with inverse filling of zones (Fig. 127-b). Above the hole-saturated ceiling of the valence band is the bottom of the conduction band, filled to capacity with electrons. Spontaneous radiative recombination of electron-hole pairs causes in these conditions stimulated emission.
A diagram of a semiconductor laser device is shown in Fig. 128 on the left. Monocrystal with p-n– the transition has the shape of a pyramid. Its two opposite faces are made strictly parallel to each other and perpendicular to the plane p-n– transition. These faces act as an optical resonator, causing stimulated emission to appear in the plane p-n– transition, go through it many times. The other two faces remain roughly processed and opaque to light.
The coefficient of light reflection from the crystal faces at n= 3.45 is from 30 to 35% at angles of incidence close to normal. In addition, the light wave, propagating along p-n– transition, absorbed by the passive regions of the diode. Therefore, for generation to occur, it is necessary to create an inversion of the zone population that would cover all light losses.
Current I The time at which this condition is satisfied and generation occurs is called threshold. Up to the threshold current, the laser operates like a regular LED. It emits spontaneous radiation with uniform density in all directions. Therefore, about 2% of the light coming out of the LED is due to radiative recombination.
Upon transition to the lasing mode, almost all radiation is concentrated in the plane p-n– transition, propagating perpendicular to the optical windows of the crystal. The ratio of the probability of radiative recombination to the probability of non-radiative recombination increases. As a result, when I > I there is a sharp increase in luminous flux F(Fig. 128 on the right).
An important disadvantage of semiconductor lasers is the strong dependence of their parameters on temperature. Due to the significant forward current, the LED heats up, the bandgap width, as a rule, decreases, so the maximum radiation shifts towards long waves. This worsens the optical resonance conditions.
Moreover, with increasing temperature the threshold current increases rapidly I pores, since at a constant injection current the energy distribution of current carriers becomes more diffuse with increasing temperature. The filling of energy states with electrons and holes becomes looser. As a result, the radiation power decreases with increasing laser temperature. Therefore, the problem of heat removal from p-n– transitions for semiconductor lasers are of paramount importance.
11. Microelectronics. The development of the technology of semiconductor devices - diodes, transistors, etc. - went not only in the direction of improving their functional characteristics, but also in the direction of reducing their size. It was possible to place first tens, and then hundreds and thousands of semiconductor devices on one chip. At the same time, the technology of forming classical elements in such blocks - capacitors, resistors, inductors - was developed. As a result, in the late 60s of the twentieth century. appears microelectronics.
The main practical products of microelectronics are integrated circuits(IS), which serve as elements of computers, automation control and communication tools. All devices and communication lines between them are formed in a single technological process on a common substrate. For a generalized characteristic of integrated circuits, three quantities are used. Integration degree N equal to the number of elements in the microcircuit. At N < 10 схема называется малой интегральной схемой (МИС), при 10 ≤ N < 100 – средней (СИС), при 100 ≤ N < 1000 – большой (БИС) и при N> 1000 – extra-large (VLSI). Degree of integration N is constantly growing and is currently approaching 10 8 . The second value is s Medium linear dimensions microcircuit elements - currently amounts to about 0.1 microns and tends to further decrease. The third value is p operating frequencies pulse circuits. They amount to several billion hertz.
Integrated electronic circuits are developed and manufactured using a computer. In general, the production technology of modern integrated circuits is quite complex and expensive, requiring a high production culture. In the manufacture of ICs, 3 technologies are used. IN semiconductor make active elements ( p-n- transitions) in the volume of a single crystal. IN film They make passive elements - resistors, capacitors, by depositing layers of metal (Cr) and dielectric (SiO 2) onto a substrate in a vacuum. IN hybrid combines semiconductor and film technologies.
Chapter 3. Physics of the atomic nucleus