What happens in the contact of two conductors. Physics lesson on the topic "Semiconductors
« 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 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 small. 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 transferred to the p-type semiconductor occupy free places, the process of recombination of electrons and holes occurs, and the holes that enter the n-type semiconductor also disappear due to the electrons occupying the vacant place. 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 connect a semiconductor with a p-n junction to an 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 achieved by mechanically connecting two semiconductors with various types 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 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 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 an impurity type of conductivity is 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-hand pn junction, there would be a current in the emitter-base circuit, depending on the voltage of the sources (battery B1 and the AC voltage source) and the resistance of the circuit, 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 have replaced vacuum tubes in electrical circuits in scientific, industrial and household equipment. Portable radios, in which such devices are used, 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 transitions play an important role in modern electronics. Possessing one-sided conductivity p-n transitions 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).
Space charges formed in the region of the pn junction 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
At reverse switching transition (- 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 the thermal generation of minority carriers on p-n boundary transition 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 р-n junction 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 p-i-n junction has the largest U samples, since 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 V in this case is 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 deteriorate p-n parameters transition. 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.
What is a semiconductor and what is it eaten with?
Semiconductor- a material without which the modern world of technology and electronics is unthinkable. Semiconductors exhibit properties of metals and non-metals under certain conditions. In terms of electrical resistivity, semiconductors occupy an intermediate position between good guides and dielectrics. Semiconductor differs from conductors in the strong dependence of specific conductivity on the presence of impurity elements (impurity elements) in the crystal lattice and the concentration of these elements, as well as on temperature and exposure to various types of radiation.
Basic property of a semiconductor- increase in electrical conductivity with increasing temperature.
Semiconductors are substances whose band gap is on the order of several electron volts (eV). For example, diamond can be classified as a wide-gap semiconductor, and indium arsenide can be classified as a narrow-gap semiconductor. The band gap is the width of the energy gap between the bottom of the conduction band and the top of the valence band, in which there are no allowed states for the electron.
The magnitude of the band gap is important when generating light in LEDs and semiconductor lasers and determines the energy of the emitted photons.
Semiconductors include many chemical elements: Si silicon, Ge germanium, As arsenic, Se selenium, Te tellurium and others, as well as all kinds of alloys and chemical compounds, for example: silicon iodide, gallium arsenide, mercury tellurite, etc.). In general, almost all inorganic substances in the world around us are semiconductors. The most common semiconductor in nature is silicon, which, according to rough estimates, makes up almost 30% of the earth's crust.
Depending on whether an atom of an impurity element gives up an electron or captures it, impurity atoms are called donor or acceptor atoms. The donor and acceptor properties of an atom of an impurity element also depend on which atom of the crystal lattice it replaces and in which crystallographic plane it is embedded.
As mentioned above, the conductive properties of semiconductors strongly depend on temperature, and when the temperature reaches absolute zero (-273 ° C), semiconductors have the properties of dielectrics.
Based on the type of conductivity, semiconductors are divided into n-type and p-type
n-type semiconductor
Based on the type of conductivity, semiconductors are divided into n-type and p-type.
An n-type semiconductor has an impurity nature and conducts electric current like metals. Impurity elements that are added to semiconductors to produce n-type semiconductors are called donor elements. The term "n-type" comes from the word "negative", which refers to the negative charge carried by a free electron.
The theory of the charge transfer process is described as follows:
An impurity element, pentavalent As arsenic, is added to tetravalent Si silicon. During the interaction, each arsenic atom enters into a covalent bond with silicon atoms. But a fifth free arsenic atom remains, which has no place in saturated valence bonds, and it moves to a distant electron orbit, where less energy is needed to remove an electron from the atom. The electron breaks away and becomes free, capable of carrying charge. Thus, charge transfer is carried out by an electron and not a hole, that is this type Semiconductors conduct electric current like metals.
Antimony Sb also improves the properties of one of the most important semiconductors - germanium Ge.
p-type semiconductor
A p-type semiconductor, in addition to the impurity base, is characterized by the hole nature of conductivity. The impurities that are added in this case are called acceptor impurities.
“p-type” comes from the word “positive,” which refers to the positive charge of the majority carriers.
For example, a small amount of trivalent indium atoms is added to a semiconductor, tetravalent Si silicon. In our case, indium will be an impurity element, the atoms of which establish a covalent bond with three neighboring silicon atoms. But silicon has one free bond while the indium atom does not have a valence electron, so it captures a valence electron from the covalent bond between neighboring silicon atoms and becomes a negatively charged ion, forming a so-called hole and, accordingly, a hole transition.
According to the same scheme, In ndium imparts hole conductivity to Ge germanium.
Investigating the properties of semiconductor elements and materials, studying the properties of contact between a conductor and a semiconductor, experimenting in the manufacture of semiconductor materials, O.V. Losev created the prototype of the modern LED in the 1920s.
Plan - outline
physics lesson
Lesson topic: Electricity via semiconductor contact RAnd ntype.
Lesson topic . Electric current through contact
semiconductorspAndntypes.
Semiconductor diode.
The purpose of the lesson : explain the mechanism of passage of electric current through the contact of semiconductors p andntypes, consider direct and reverse transitions, study the structure and principle of operation of a semiconductor diode, repeat previously studied material using reference notes and TSO.
Lesson objectives:
Educational - create conditions for mastering new educational material using problem-based learning;
Introduce the concepts of direct and reverse junction, semiconductor diode;
Developmental – to develop the creative and mental activity of students in the classroom by solving research problems, the intellectual qualities of the student’s personality such as independence, the ability to make evaluative actions, generalize, and quickly switch; promote skills development independent work; develop the ability to clearly and clearly express your thoughts.
Educational - to instill a culture of mental work, to instill in students an interest in the subject through the use of information technology (using a computer); develop the ability to accurately and competently perform mathematical notations.
Equipment : basic notes, a set of semiconductor
diodes, computers with software
"Open Physics".
Lesson steps
Time,
min
Techniques and methods
1.Repetition of previously studied material
2. Studying a new material: electric current through a semiconductor contact
r andntype. Semiconductor diode.
3. Formation of skills and abilities.
4. Primary test of knowledge acquisition. Reflection.
5. Repetition of material.
5. Summing up.
6.Homework.
Conversation. Survey on supporting notes.
Teacher's story. Conversation. Supporting notes. Show step-by-step animation.
Answers to student questions.
Survey on supporting notes.
Open Physics Program
Teacher's message.
Writing on the board.
Lesson Plan
Course and content of the lesson.
Introductory word from the teacher.
Checking the assimilation of the studied material.
Overview of the topic "Laws" direct current"- supporting summary.
Electric current in semiconductors.
2.2.1 Structure of semiconductors.
2.2.2 Electronic conductivity.
2.2.3 Hole conductivity.
2.2.4 Impurity conductivity.
2.2.5 Donor impurities.
2.2.6 Acceptor impurities.
The survey of students is carried out using reference notes.
2.2.7 Physical dictation.
1. What is the intrinsic conductivity of semiconductors?
2. Under what conditions do pure semiconductors become electrically conductive?
3. How does the conductivity of semiconductors depend on temperature?
4. What conductivity of semiconductors is called electronic?
5. How do “holes” appear in a pure semiconductor?
6. What is the nature of current in a semiconductor?
7. How does the presence of impurities in them affect the conductivity of semiconductors?
8. Under what condition in impurity semiconductor does electronic conductivity occur?
9. Under what condition does hole conduction occur in an impurity semiconductor?
10. What are the names of semiconductors in which the main charge carriers are electrons?
11. What are the names of semiconductors in which the main charge carriers are holes?
Learning new material .
3.1 Electric current through a semiconductor contactp And ntypes (according to the supporting summary)
3.1.1 Electrical properties of "p-n" junctions.
"p-n" junction (or electron-hole junction) is the area of contact of two semiconductors where the conductivity changes from electronic to hole (or vice versa).
Such regions can be created in a semiconductor crystal by introducing impurities. In the contact zone of two semiconductors with different conductivities, mutual diffusion will take place. electrons and holes and a blocking electric layer is formed. The electric field of the blocking layer prevents the further passage of electrons and holes across the boundary. The blocking layer has increased resistance compared to other areas of the semiconductor.
The external electric field affects the resistance of the barrier layer.
In the forward (through) direction of the external electric field, the electric current passes through the boundary of two semiconductors.
Because electrons and holes move towards each other towards the interface. Electrons, crossing the boundary, fill the holes. The thickness of the barrier layer and its resistance are continuously decreasing.
With a blocking (reverse direction of the external electric field), electric current will not pass through the contact area of the two semiconductors.
Because electrons and holes move from the boundary in opposite directions. The blocking layer thickens, its resistance increases.
3.2 Semiconductor diode (reference summary).
A semiconductor with one "p - n" junction is called a semiconductor diode.
When applying el. fields in one direction, the resistance of the semiconductor is high,
in the opposite direction - there is little resistance. 
Semiconductor diodes are the main elements of AC rectifiers.
3.3 Scope of application of semiconductor diodes .
The explanation of the material is accompanied by a demonstration of semiconductor diodes. Presentation slide.
....................
Fixing the material.
Supporting notes.
Computers - Open Physics program.
Homework assignment : $73,74.
Summarizing.
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 high 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. Switching diodes are a type of high-frequency diode and are designed for use as key elements in high-speed switching 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 large quantity 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 influence during normal 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).
Ticket 19
1.TRANSISTOR, semiconductor device, designed to amplify and control electric 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.
The use of nuclear energy in the modern world turns out to be so important that if we woke up tomorrow and the energy from the 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.
Modern man wants 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 update. 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.
Ticket 22
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 big system- 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.
Ticket 25
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.