Semiconductor resistors. Cheat sheet: Semiconductor diodes and transistors, their areas of application Moscow Mining State University

Prepared

Student of class 10 "A"

School No. 610

Ivchin Alexey

Abstract on the topic:

“Semiconductor diodes and transistors, their areas of application”

1. Semiconductors: theory and properties

2. Basic semiconductor devices (Structure and application)

3. Types of semiconductor devices

4. Production

5. Scope of application

1. Semiconductors: theory and properties

First you need to get acquainted with the conduction mechanism in semiconductors. And to do this, you need to understand the nature of the bonds that hold the atoms of a semiconductor crystal near each other. For example, consider a silicon crystal.

Silicon is a tetravalent element. This means that in the external

the shell of an atom has four electrons, relatively weakly bound

with a core. The number of nearest neighbors of each silicon atom is also equal to

four. The interaction of a pair of neighboring atoms is carried out using

paionoelectronic bond called covalent bond. In education

this bond from each atom involves one valence electron, co-

which are split off from atoms (collectivized by the crystal) and when

in their movement they spend most of their time in the space between

neighboring atoms. Their negative charge holds the positive silicon ions near each other. Each atom forms four bonds with its neighbors,

and any valence electron can move along one of them. Having reached a neighboring atom, it can move on to the next one, and then further along the entire crystal.

Valence electrons belong to the entire crystal. The pair-electron bonds of silicon are quite strong and do not break at low temperatures. Therefore, silicon at low temperatures does not conduct electric current. The valence electrons involved in the bonding of atoms are firmly attached to the crystal lattice, and the external electric field does not have a noticeable effect on their movement.

Electronic conductivity.

When silicon is heated, the kinetic energy of the particles increases, and

individual connections are broken. Some electrons leave their orbits and become free, like electrons in a metal. In an electric field, they move between lattice nodes, forming an electric current.

The conductivity of semiconductors due to the presence of free metals

electrons electrons is called electron conductivity. As the temperature increases, the number of broken bonds, and therefore free electrons, increases. When heated from 300 to 700 K, the number of free charge carriers increases from 10.17 to 10.24 1/m.3. This leads to a decrease in resistance.

Hole conductivity.

When a bond is broken, a vacant site with a missing electron is formed.

It's called a hole. The hole has an excess positive charge compared to other, normal bonds. The position of the hole in the crystal is not constant. The following process occurs continuously. One

from the electrons that ensure the connection of atoms, jumps to the place of exchange

formed holes and restores the pair-electronic bond here.

and where this electron jumped from, a new hole is formed. So

Thus, the hole can move throughout the crystal.

If the electric field strength in the sample is zero, then the movement of holes, equivalent to the movement of positive charges, occurs randomly and therefore does not create an electric current. In the presence of an electric field, an ordered movement of holes occurs, and thus, the electric current associated with the movement of holes is added to the electric current of free electrons. The direction of movement of holes is opposite to the direction of movement of electrons.

So, in semiconductors there are two types of charge carriers: electrons and holes. Therefore, semiconductors have not only electronic but also hole conductivity. Conductivity under these conditions is called the intrinsic conductivity of semiconductors. The intrinsic conductivity of semiconductors is usually low, since the number of free electrons is small, for example, in germanium at room temperature ne = 3 per 10 in 23 cm in –3. At the same time, the number of germanium atoms in 1 cubic cm is about 10 in 23. Thus, the number of free electrons is approximately one ten-billionth of the total number of atoms.

An essential feature of semiconductors is that they

in the presence of impurities, along with intrinsic conductivity,

additional - impurity conductivity. Changing concentration

impurities, you can significantly change the number of charge carriers

or other sign. Thanks to this, it is possible to create semiconductors with

predominant concentration is either negative or positive

strongly charged carriers. This feature of semiconductors has been discovered

provides ample opportunities for practical application.

Donor impurities.

It turns out that in the presence of impurities, for example arsenic atoms, even at very low concentrations, the number of free electrons increases in

many times. This happens for the following reason. Arsenic atoms have five valence electrons, four of which are involved in creating a covalent bond between this atom and surrounding atoms, for example, with silicon atoms. The fifth valence electron appears to be weakly bound to the atom. It easily leaves the arsenic atom and becomes free. The concentration of free electrons increases significantly, and becomes a thousand times greater than the concentration of free electrons in a pure semiconductor. Impurities that easily donate electrons are called donor impurities, and such semiconductors are n-type semiconductors. In an n-type semiconductor, electrons are the majority charge carriers and holes are the minority charge carriers.

Acceptor impurities.

If indium, whose atoms are trivalent, is used as an impurity, then the nature of the conductivity of the semiconductor changes. Now, to form normal pair-electronic bonds with its neighbors, the indium atom does not

gets an electron. As a result, a hole is formed. The number of holes in the crystal

talle is equal to the number of impurity atoms. This kind of impurity is

are called acceptor (receiving). In the presence of an electric field

the holes mix across the field and hole conduction occurs. By-

semiconductors with a predominance of hole conduction over electron-

They are called p-type semiconductors (from the word positiv - positive).

2. Basic semiconductor devices (Structure and application)

There are two basic semiconductor devices: the diode and the transistor.

The current-voltage characteristic for forward and reverse connections is shown in Figure 2.

They replaced lamps and are very widely used in technology, mainly for rectifiers; diodes have also found application in various devices.

Transistor.

The left pn junction is direct and separates the base from the p-type region called the 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 the alternating voltage source

resistance) and circuit resistance, including low direct resistance

One-way conduction of contacts between two semiconductors (or metal to semiconductor) is used to rectify and convert alternating currents. If there is one electron-hole transition, then its action is similar to the action of two

electrode lamp - diode. Therefore, a semiconductor device containing one p-n junction is called semiconductor (crystalline) diode. Semiconductor diodes by design they are divided into point And planar. If a short-term current pulse is passed through a diode in the forward direction, a layer with p-conductivity is formed. A pn junction with a high rectification coefficient is formed at the boundary of this layer. Due to the low capacitance of the contact layer, point diodes are used as detectors (rectifiers) of high-frequency oscillations up to the centimeter wavelength range.

p-n junctions not only have excellent rectifying properties, but can also be used for amplification, and if feedback is introduced into the circuit, then for generating electrical oscillations. Devices intended for these purposes are

got the name semiconductor triodes or transistors. Germanium and silicon are used for the manufacture of transistors, as they are characterized by great mechanical strength, chemical resistance and greater

semiconductors, mobility of current carriers. Semiconductor triodes are divided into point And planar. The former significantly increase the voltage, but their output powers are low due to the danger of overheating (for example, the upper limit of the operating

The temperature of a point germanium triode lies in the range of 50 - 80 °C). Planar triodes are more powerful. They might be like p-p-p and type p-p-p depending on the alternation of areas with different conductivity. Transistor comprises bases (middle part of the transistor), emitter And collector (areas adjacent to the base on both sides with a different type of conduction)

bridges). A constant forward bias voltage is applied between the emitter and the base, and a constant reverse bias voltage is applied between the base and collector. The amplified alternating voltage supplies -

to the input impedance , and the amplified one is removed from the output resistance. Current flow in the emitter circuit

is caused mainly by the movement of holes (they are the main current carriers) and is accompanied by their injection - injection - to the base area. The holes that penetrate the base diffuse towards the collector, and with a small thickness

Not at the base, a significant portion of the injected holes reaches the collector. Here the holes are captured by the field acting inside the junction (attracted to the negatively charged collector), as a result of which the collector current changes. Therefore, all

Some change in current in the emitter circuit causes a change in current in the collector circuit. A transistor, like a vacuum tube,

gives an increase in both voltage and power.

25.(Lorentz force. Work of the Lorentz force. Hall effect)

Force acting on an electric charge Q, moving in a magnetic field with speed V , called Lorentz force and is expressed by the formula, where IN- induction of the magnetic field in which the charge moves.

Lorentz force modulus , where α is the angle between v And IN. The Lorentz force is always perpendicular to the speed of motion of a charged particle, so it only changes the direction of this speed, without changing its modulus. Hence, Lorentz force

doesn't do any work. In other words, a constant magnetic field does not do work on a charged particle moving in it, and the kinetic energy of this particle does not change when moving in a magnetic field. If on a moving electric

charge in addition to the magnetic field with induction IN there is also an electric field with the intensity E, then the resultant force F, applied to the charge is equal to the vector sum of forces - the force acting from the electric field and the Lorentz force: The direction of the Lorentz force and the direction of the deflection of a charged particle in a magnetic field caused by it depend on the sign of the charge Q particles.

Hall effect (1879) is the occurrence in a metal (or semiconductor) with a current density j, placed in a magnetic field IN, electric field in a direction perpendicular to IN Toj. Let us place a metal plate with a current density j to magnetic

field IN, perpendicular to j.For a given direction j the speed of current carriers in the metal - electrons - is directed from right to left. The electrons experience the Lorentz force, which in this case is directed upward. Thus, at the upper edge of the plate there will be an increased concentration of electrons (it will be negatively charged), and at the lower edge there will be a lack of electrons (it will be charged positively). As a result, an additional transverse electric field will arise between the edges of the plate Ev, directed from bottom to top. When tension Ev This transverse field reaches such a value that its action on the charges will balance the Lorentz force, then a stationary distribution of charges in the transverse direction will be established.

Then where A- width of the plate; ∆f - transverse (Hall) potential difference.

Considering that the current strength I = jS =nevS (S- cross-sectional area of ​​the plate thickness d, n- electron concentration, v - average speed of ordered movement of electrons, j-current density = env), we obtain i.e. Hall transverse potential difference is proportional to magnetic induction IN, current strength / and is inversely proportional to the thickness of the plate d.

- Hall constant, depending on the substance. By the measured value of the Hall constant can be: 1) determined

concentration of current carriers in the conductor (with the known nature of conductivity and charge of carriers); 2) judge the nature of the conductivity of semiconductors, since the sign of the Hall constant coincides with the sign of the charge e of current carriers. Therefore the effect

Hall effect is the most effective method for studying the energy spectrum of current carriers in metals and semiconductors.

The main element of most semiconductor elements is the p-n junction.

A p-n junction is the region at the boundary of p and n type semiconductors.

Conventionally, a pn junction can be shown as follows:

Experiment 14.3. Semiconductor diode.

Goal of the work:

Study the principle of operation of a semiconductor diode.

Equipment:

1. Adjustable AC voltage source

2. Oscilloscope

3. Stand with a diagram

Progress.

1. The installation consists of a source of adjustable alternating voltage, an oscilloscope and a stand with a circuit. Alternating voltage from the source is supplied to the input of the stand. A sinusoid is observed on the oscilloscope screen. If you increase or decrease the applied voltage, the amplitude of the sinusoidal signal visible on the oscilloscope screen increases or decreases accordingly.

2. Let's study the nature of the current flowing through the diode. The voltage entering the stand is applied to the edges of a chain consisting of a resistor and a diode connected in series. As a result, it is no longer alternating current that flows through the chain, but pulsating current, since the diode rectifies the current. It allows current to pass in one direction and not in the other. In the diagram, the diode is depicted in such a way that the tip of the triangle, at this stage it is directed upward, indicates the direction of the current passing through the diode. In order to find out what the nature of the current passing through the diode is, a voltage is applied to the vertical amplifier, which is removed from the ends of the resistance. This voltage is proportional to the current flowing through the resistance. It is observed that the current through the diode actually flows in only one direction. There is no current for half a period - horizontal sections, for half a period the current flows. These are halves of sinusoids that look down. But if you change the voltage supplied to the input of the stand, the amount of current flowing through the diode will also change. If you rotate the diode 180 degrees, the tip of the triangle in the diagram will be directed downward, i.e. the direction of the current flowing through the diode will change. The signal on the oscilloscope screen disappeared. The diode is removed from the stand, and the signal appears on the oscilloscope screen again. However, now those half-cycles that correspond to the flow of current through the diode are displayed as halves of a sine wave directed upward.

3. Current-voltage characteristic of a diode - the relationship between the current flowing through the diode and the voltage supplied to the diode. The current flowing through the diode is still proportional to the voltage at the ends of the resistors. This voltage is supplied to the vertical input of the oscilloscope, and the horizontal input is supplied by the voltage from the ends of this chain; it is proportional to the voltage on the diode. As a result, the current-voltage characteristic of the diode is observed on the oscilloscope screen. There is no half-period of current, this is a horizontal section of this characteristic, and half-period the current flows. Ohm's law is fulfilled here to a certain extent. The amount of current flowing through the diode is proportional to the voltage applied to the diode. If you increase or decrease the voltage applied to the diode, the current flowing through the diode increases or decreases accordingly.

Conclusion:

The one-way conductivity of the pn junction makes it possible to create a rectifying semiconductor device, the so-called semiconductor diode.

1. The sign of conductivity corresponds to the sign of the source, then holes will move to the left, electrons to the right. Through р-n transition, an electric current consisting of electrons and holes will flow.

2. The sign of conductivity is opposite to the sign of the source, then charge carriers move to the poles without crossing the semiconductor contact boundary, no current occurs through the p-n junction, therefore, the p-n junction has one-way conductivity.

pn junction is used in semiconductor diodes.

A transistor is a semiconductor device that consists of two pn junctions connected back to back. The emitter is the area of ​​the transistor where charge carriers come from. A collector is an area where charge carriers flow. The base performs a role similar to that of the control grid in a lamp.

Transistors serve to amplify electrical signals because a small change in voltage between the emitter and base results in a large change in the voltage across the load connected in the collector circuit.

Experience 14.4 Transistor DC amplifier

Equipment:

1. Transistor on a stand;

2. Photodiode on a stand;

3. Current source V-24;

4. Connecting wires;

5. Light bulb;

6. Two demonstration galvanometers;

Installation diagram:

When the photocell is darkened, the current is small. If the photocell is illuminated, the current increases in section G2.

Tests for lecture No. 14

Test 14.1.What conclusions can be drawn from the results of the experiment demonstrating the dependence of semiconductor resistance on temperature?

£ As the temperature of a semiconductor increases, its resistance increases

£ The resistance of a semiconductor does not depend on its temperature

£ As the temperature of a semiconductor increases, its resistance decreases

£ The resistance of a semiconductor does not depend significantly on its temperature

Test 14.2.What is the name of a material whose electrical properties strongly depend on the concentration of chemical impurities in it and external conditions?

£ superconductor.

£ magnetoelectric.

£ ferroelectric.

£ semiconductor.

Test 14.3.What is the name of a quasiparticle whose charge in modulus is equal to the charge of an electron, and whose mass is equal to the mass of the electron?

£ neutron

£ "hole"

£ α-particle

£ positron

Test 14.4.What is the name of a semiconductor device that consists of two pn junctions connected back to back?

£transistor

£ collector

£ galvanometer

£ thyristor

Test 14.5.What is the name of the transistor region, where do they come from?

charge carriers?

£collector

emitter

£photocell

£zener diode

Tests for chapter No. 3.

Test 1. What is meant by third-party forces?

£ Forces of non-electrostatic origin.

£ Forces caused only by chemical processes.

£ Only mechanical forces (forces applied to rotate the generator rotor).

£ Forces of electrical origin.

Test 2. A physical quantity characterized by a charge passing through a conductor area of ​​unit area per unit time is...

£ current strength.

£ current density.

£ voltage.

£ electrical resistivity.

Test 3. When two conductors are connected in series to a DC network, the current strength in the network is 6.25 times less than when the same conductors are connected in parallel. How many times do the resistances of the conductors differ?

Test 4. What does the polarization vector in a dielectric depend on?

dielectric composition

£ dielectric size

£electrical induction

£field strength in the dielectric

£presence of free charges in the dielectric

Test 5. Select the correct conclusions following from the experiment demonstrating the dependence of conductor resistance on temperature?

Conductor resistance does not depend on temperature

As the temperature of the conductor increases, its resistance increases

As the temperature of the conductor decreases, its resistance increases

As the temperature of the conductor increases, its resistance decreases

As the temperature of the conductor decreases, its resistance decreases

Test 6. In what year was the phenomenon of superconductivity discovered by Kamerling - Oness?

Test 7. If there are N-nodes in a branched chain, for how many nodes can independent equations be drawn up? .

Test 8.

When connecting conductors in parallel, the following is the same for them:

Test 9.

Highlight the formulas for series connection of conductors:

£

£

£

£

£

Test 10. The formulation “the phenomenon of direct conversion of heat into electricity in solid or liquid conductors, as well as the reverse phenomenon of direct heating and cooling of the junctions of two conductors by passing current” is the definition ...

£thermoelectricity

£thermo-EMF

£Faraday effect

Hall effect

Test 11. What determines the value of thermo-EMF of a thermocouple?

£from the junction temperature difference+

£from the specific thermo-EMF of both conductors

£from voltage difference

£from potential difference

Test 12. The formulation “The difference in electrical potentials that arises between contacting bodies under conditions of thermodynamic equilibrium” is a definition...

£contact voltage difference.

£ contact resistance difference.

£contact difference of ions.

£contact potential difference .

£contact current difference

Test 13 . Solutions of salts, alkalis, acids are...

£ electrolytes

£ semielectrolytes

£ dielectrics

£ quasi-electrolytes

£ semiconductors

Test 14.Which of the following metals are noble?

Test 15. Faraday's first law for electrolysis states:

The electrochemical equivalent of a substance is directly proportional to its chemical equivalent.

£ the mass of the substance released on the electrodes is directly proportional to the square of the charge flowing through the electrolyte

The mass of the substance released on the electrodes is directly proportional to the charge flowing through the electrolyte.+

£ the mass of the substance released on the electrodes is directly proportional to the square root of the amount of charge flowing through the electrolyte

The mass of the substance released on the electrodes is inversely proportional to the charge flowing through the electrolyte

Test 16. What physical factors have an ionizing effect on gas?

£ heating

£ electric field

£ increase in gas volume.

£ exposure to radiation.

£ decrease in atmospheric pressure.

Test 17. If you examine the gas discharge tube during the discharge, you will notice that the discharge is not uniform. The following areas are distinguished:

£ Aston's Dark Space; cathode film; smoldering glow; negative column.

£ Aston's Dark Space; anode film; cathode dark space; smoldering glow; Faraday dark space; negative column.

£ Aston's Dark Space; cathode film; cathode dark space; smoldering glow; Faraday dark space; positive column.

£ Aston's Dark Space; cathode film; smoldering glow; negative column; positive column

£ cathode film; cathode dark space; smoldering glow; Faraday dark space; positive column

Test 18.Which category is used mainly for lighting and advertising purposes?

£ crown.

£ arc.

£ smoldering.

£spark

£shimmering

Test 19. What types of plasma are there according to the method of production?

£ gas discharge

£ high voltage

£ high temperature

£ magnetic-electronic

Test 20. What types of magnetic traps exist?

£ betatron

£ stellate

£ stellator

£ tokamak

£ plasma torch

Test 21. What property is the main one for plasma?

£ good electrical conductivity

£ polarizability

£ ionizability

£ quasi-neutrality

£ lifetime

Test 22. What is the contact zone of semiconductors with different types of conductivity called?

£prohibited area

£conduction band

£p-n junction

£valence band

Test 23. What is the name of the region of the transistor where charge carriers enter?

emitter

£collector

£photocell

£microchip

Test 24.What is the peculiarity of semiconductors?

£hard dipole moment of molecules of a substance

£high operating temperature

£presence of free carriers of negative charges

£there are two types of electric charge carriers+

£presence of free carriers of positive charges

Magnetic field in vacuum and matter

15. Interaction of currents. A magnetic field. Induction and magnetic field strength. A coil with current in a magnetic field. Biot-Savart-Laplace law. Magnetic field of direct, circular and solenoidal currents.

16. Vortex nature of the magnetic field. Circulation of the magnetic field induction vector. Magnetic flux. Ampere power. The work of moving a current-carrying conductor in a magnetic field. Lorentz force. Determination of the specific charge of an electron.

17. Magnetics. Magnetization. Relationship between induction and magnetic field strength in a magnet. Magnetic permeability and susceptibility. Magneto-mechanical phenomena.

18. The concept of dia-, para- and ferromagnets. Domain structure of ferromagnets. Magnetic hysteresis. Stoletov's works. Curie point. Magnetic materials and their applications.

Interaction of currents. A magnetic field. Induction and magnetic field strength. A coil with current in a magnetic field. Biot-Savart-Laplace law. Magnetic field of direct, circular and solenoidal currents

15.1. Interaction of currents

15.2. A magnetic field. Induction and magnetic field strength

15.3. A coil with current in a magnetic field

15.4. Biot-Savart-Laplace law. Magnetic field of direct, circular and solenoidal currents

The study of the nature of magnetic phenomena began with the consideration of natural magnetism. This interaction of natural magnets also occurred with some substances that belong to the class of ferromagnets. In the future, we will see that the interaction remains the same if one of the natural magnets is replaced by a conductor with current (Oersted's experiment), and, finally, this phenomenon can be observed if two conductors with current interact (Ampere's experiment).

Experience 15.1 Oersted's experience.

Equipment:

Rice. 15.1.

1. Magnetic needle;

2. Current source V-24;

3. Conductor;

Installation diagram:

The arrow is initially parallel to the conductor. When the current source is turned on, the arrow is set perpendicular to the conductor. When the power source is turned off, the arrow returns to its original position.

Conclusion: There is a magnetic field around the current-carrying conductor, i.e. Where there are moving electric charges, a magnetic field exists.

Experience 15.2 Interaction of two conductors with current.

Equipment:

1. Two flexible foil tapes;

2. Current source V-24;

3. Conductor;

Installation diagram:

The currents are directed in the opposite direction - the conductors repel each other.

The currents are co-directed - and the conductors attract each other.

Conclusion: When two conductors interact with current, forces arise that repel or attract the conductors.

The study of magnetic phenomena has shown that magnetic interaction is observed when there is a movement of electric charges in relation to the observer (or recording device). Since all phenomena associated with the relative motion of objects are called relativistic (from the English word “relative” - relative), they say that magnetism is a relativistic effect.

Semiconductor diode called a two-electrode device with one-way conductivity. Its design is based on an equilibrium R-n transition. Based on the nature of junction formation, diodes are divided into point and planar.

Semiconductor triodes are widely used for converting, amplifying and generating electrical oscillations - transistors. For a transistor to operate, it is necessary to have two electron-hole junctions; germanium is often used as a semiconductor.

In transistors using n-р-n junction, semiconductor R-type located between semiconductors n-type, The design of a planar bipolar transistor is shown in Figure 2.7.

Rice. 2.7. The principle of the transistor device and the image of transistors in the diagrams.

In this transistor n-р-n type there is a middle region with hole conductivity, and two outer regions with electronic conductivity. The middle region of the transistor is called - base, one extreme area – emitter , another - collector. Thus, the transistor has two n-r transition: emitter– between emitter and base and collector- between the base and the collector. The distance between them should be very small, no more than a few micrometers, i.e. The base area should be very thin. This is a condition for good operation of the transistor. In addition, the concentration of impurities in the base is always significantly less than in the collector and emitter. In schematic images of transistors, the arrow shows the direction of the current (conditional, from plus to minus) in the emitter wire with forward voltage at the emitter junction.

Let's consider the operation of the transistor in no-load mode, when only the sources of constant supply voltages E 1 and E 2 are turned on (Figure 2.8).

Their polarity is such that at the emitter junction the voltage is forward, and at the collector junction it is reverse. Therefore, the resistance of the emitter junction is low and to obtain a normal current in this junction, a voltage E 1 of tenths of a volt is sufficient. The resistance of the collector junction is high, and the voltage E2 is usually a few or tens of volts.

Rice. 2.8. The movement of electrons and holes in an n-p-n transistor.

The principle of operation of the transistor is that the forward voltage of the emitter junction, i.e., the base-emitter section, significantly affects the collector current: the greater this voltage, the greater the emitter and collector currents. In this case, changes in the collector current are only slightly less than changes in the emitter current. Thus, the voltage between the base and emitter E 1, i.e. input voltage controls the collector current. The amplification of electrical oscillations using a transistor is based precisely on this phenomenon.

Physical processes in the transistor occur as follows. As the forward input voltage E1 increases, the potential barrier in the emitter junction decreases and, accordingly, the current through this junction increases - the emitter current i uh. Electrons of this current are injected from the emitter into the base and, due to diffusion, penetrate through the base into the collector junction, increasing the collector current. Since the collector junction operates at reverse voltage, space charges appear in this junction, shown in the figure by circles with the signs “+” and “–”. An electric field arises between them. It promotes the movement (extraction) through the collector junction of electrons that came here from the emitter, i.e. draws electrons into the region of the collector junction.

If the thickness of the base is small enough and the concentration of holes in it is low, then most of the electrons, having passed through the base, do not have time to recombine with the holes of the base and reach the collector junction. Only a small fraction of the electrons recombine with holes in the base. As a result of recombination, a base current flows in the base wire. Indeed, in a steady state, the number of holes in the base should remain unchanged. Due to recombination, a number of holes disappear every second, but the same number of new holes appear due to the fact that the same number of electrons leaves the base towards the pole of the source E 1. In other words, many electrons cannot accumulate in the base.

If the base had a significant thickness and the concentration of holes in it was high, then most of the electrons of the emitter current, diffusing through the base, would recombine with holes and would not reach the collector junction.

Under the influence of the input voltage, a significant emitter current arises; electrons are injected into the base region from the emitter side, which are minority carriers for this region. Without having time to recombine with holes during diffusion through the base, they reach the collector junction. The higher the emitter current, the more electrons come to the collector junction and the lower its resistance becomes. The collector current increases accordingly. In other words, with an increase in the emitter current in the base, the concentration of minority carriers injected from the emitter increases, and the more of these carriers, the greater the collector junction current, i.e. collector current i to .

It should be noted that the emitter and collector can be swapped (so-called inverse mode). But on transistors, as a rule, the collector junction is made with a much larger area than the emitter junction, since the power dissipated in the collector junction is much greater than the power dissipated in the emitter junction. Therefore, if you use the emitter as a collector, the transistor will work, but it can only be used at significantly lower power, which is impractical. If the junction areas are made identical (transistors in this case are called symmetrical), then any of the extreme regions can work as an emitter or collector with equal success.

We examined the physical phenomena in an n-p-n transistor. Similar processes occur in a p-n-p transistor, but in it the roles of electrons and holes change, and the voltage polarities and current directions change to reverse.

The three most common ways to turn on transistors are:

- common base circuit, when the emitter input and collector output

connected to a common base;

- in a common emitter circuit collector output circuit

connects to the emitter instead of the base;

- common collector circuit, otherwise called emitter repeat.

Conclusion: 1. The presence of impurities in semiconductors causes a violation of the equality between the number of holes and electrons, and the electric current will be created predominantly by charges of the same sign, depending on what predominates in the semiconductor.

2. The design of any semiconductor device is based on equilibrium R-n transitions.

Prepared

Student of class 10 "A"

School No. 610

Ivchin Alexey

Abstract on the topic:

“Semiconductor diodes and transistors, their areas of application”

2. Basic semiconductor devices (Structure and application)

3.Types of semiconductor devices

4.Production

5. Application area

1. Semiconductors: theory and properties

First you need to get acquainted with the mechanism of conductivity in semiconductors. And to do this, you need to understand the nature of the bonds that hold the atoms of a semiconductor crystal near each other. For example, consider a silicon crystal.

Silicon is a tetravalent element. This means that in the external

the shell of an atom has four electrons, relatively weakly bound

with a core. The number of nearest neighbors of each silicon atom is also equal to

four. The interaction of a pair of neighboring atoms is carried out using

paionoelectronic bond called covalent bond. In education

This bond from each atom involves a monovalent electron, which

which are split off from atoms (collectivized by the crystal) and when

in their movement they spend most of their time in the space between

neighboring atoms. Their negative charge holds the positive silicon ions near each other. Each atom forms four bonds with its neighbors,

and any valence electron can move along one of them. Having reached a neighboring atom, it can move on to the next one, and then further along the entire crystal.

Valence electrons belong to the entire crystal. Pair-electronic bonds of silicon are quite strong and cannot be broken at low temperatures. Therefore, silicon does not conduct electric current at low temperatures. The valence electrons involved in the bonding of atoms are firmly attached to the crystal lattice, and the external electric field does not have a noticeable effect on their movement.

Electronic conductivity.

When silicon is heated, the kinetic energy of the particles increases, and

individual connections are broken. Some electrons leave their orbits and become free, like electrons in a metal. In an electric field, they move between lattice nodes, forming an electric current.

Conductivity of semiconductors due to the presence of free metals

electrons electrons is called electron conductivity. As the temperature increases, the number of broken bonds, and therefore free electrons, increases. When heated from 300 to 700 K, the number of free charge carriers increases from 10.17 to 10.24 1/m.3. This leads to a decrease in resistance.

Hole conductivity.

When a bond is broken, a vacant position is created by the missing electron.

It's called a hole. The hole has an excess positive charge compared to other, normal bonds. The position of the hole in the crystal is not constant. The following process occurs continuously. One

from the electrons that ensure the connection of atoms, jumps to the place of exchange

formed holes and restores the pair-electronic connection here.

and where this electron jumped from, a new hole is formed. So

Thus, the hole can move throughout the crystal.

If the electric field strength in the sample is zero, then the movement of holes, equivalent to the movement of positive charges, occurs randomly and therefore does not create an electric current. In the presence of an electric field, an ordered movement of holes occurs, and thus, an electric current associated with the movement of holes is added to the electric current of free electrons. The direction of movement of holes is opposite to the direction of movement of electrons.

So, in semiconductors there are two types of charge carriers: electrons and holes. Therefore, semiconductors have not only electronic but also hole conductivity. Conductivity under these conditions is called the intrinsic conductivity of semiconductors. The intrinsic conductivity of semiconductors is usually low, since the number of free electrons is small, for example, in germanium at room temperature ne = 3 per 10 in 23 cm in –3. At the same time, the number of germanium atoms in 1 cubic cm is about 10 in 23. Thus, the number of free electrons is approximately one ten-billionth of the total number of atoms.

An essential feature of semiconductors is that they

in the presence of impurities, along with intrinsic conductivity,

additional - impurity conductivity. Changing concentration

impurities, you can significantly change the number of charge carriers

or other sign. Thanks to this, it is possible to create semiconductors with

predominant concentration is either negative or positive

strongly charged carriers. This feature of semiconductors has been discovered

provides ample opportunities for practical application.

Donor impurities.

It turns out that in the presence of impurities, for example arsenic atoms, even at very low concentrations, the number of free electrons increases in

many times. This happens for the following reason. Arsenic atoms have five valence electrons, four of which are involved in creating a covalent bond between this atom and surrounding atoms, for example, with silicon atoms. The fifth valence electron is weakly bonded to the atom. It easily leaves the arsenic atom and becomes free. The concentration of free electrons increases significantly, and becomes a thousand times greater than the concentration of free electrons in a pure semiconductor. Impurities that easily donate electrons are called donor impurities, and such semiconductors are n-type semiconductors. In an n-type semiconductor, electrons are the majority charge carriers and holes are the minority charge carriers.

Acceptor impurities.

If indium, whose atoms are trivalent, is used as an impurity, then the nature of the conductivity of the semiconductor changes. Now, for the formation of normal pair-electronic bonds with neighbors, the indium atom does not

gets an electron. As a result, a hole is formed. Number of holes in the crystal

talle is equal to the number of impurity atoms. This kind of impurity

are called acceptor (receiving). In the presence of an electric field

the holes mix across the field and hole conductivity occurs. By-

semiconductors with predominant hole conduction over electrons

They are called p-type semiconductors (from the word positiv - positive).

2. Basic semiconductor devices (Structure and application)

There are two basic semiconductor devices: the diode and the transistor.

/>Nowadays, semiconductor diodes are increasingly used to rectify electrical current in radio circuits, along with two-electrode lamps, since they have a number of advantages. In a vacuum tube, charge carriers electrons are generated by heating the cathode. In a p-n junction, charge carriers are formed when an acceptor or donor impurity is introduced into the crystal. Thus, there is no need for an energy source to obtain charge carriers. In complex circuits, the energy savings resulting from this turn out to be very significant. In addition, semiconductor rectifiers, with the same values ​​of rectified current, are more miniature than tube rectifiers.

/> Semiconductor diodes are made from germanium and silicon. selenium and other substances. Let's consider how a p-n junction is created when using a bottom impurity; this junction cannot be obtained by mechanically connecting two semiconductors of different types, because this results in too large a gap between the semiconductors and the semiconductors. This thickness should be no greater than the interatomic distances. Therefore, indium is melted into one of the surfaces of the sample. Due to the diffusion of indium atoms deep into the germanium single crystal, a region with p-type conductivity is transformed at the germanium surface. The rest of the germanium sample, into which indium atoms have not penetrated, still has n-type conductivity. A p-n junction occurs between the regions. In a semiconductor, diodegermanium serves as the cathode, and indium serves as the anode. Figure 1 shows the direct (b) and reverse (c) connection of the diode.

The current-voltage characteristic for direct and reverse connections is shown in Figure 2.

They replaced lamps and are very widely used in technology, mainly for rectifiers; diodes have also found application in various devices.

Transistor.

/> Let's consider one type of transistor 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) layer of n-type semiconductor is created between two layers of p-type semiconductor. 3. This thin layer is called the base 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 connect the transistor to the circuit shown in Figure 3. With this connection

The left pn junction is direct and separates the base from the region with p-type conductivity, called the 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 the alternating voltage source).

resistance) and circuit resistance, including low direct resistance

/>emitter-base transition. Battery B2 is connected so that the right pn junction in the circuit (see Fig. 3) is reverse. It separates the base from the right-hand region with p-type conductivity, called the collector. If there were no left pn junction, the current strength of the collector circuit would be close to zero, since the resistance of the reverse junction is very high. When there is a current 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. When a voltage is created between the emitter and the base, the main carriers of the p-type semiconductor - holes penetrate into the base, where they are already the main carriers carriers. 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. The holes in the collector 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. 3) plane is much smaller than the cross-section in the vertical plane. The current in the collector, which is almost equal to the current in the emitter, varies with the current in 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 the resistor. If the resistance of the resistor is large, the change in voltage across it can be tens of thousands of times greater than the change in the signal in the emitter circuit. This means an increase in voltage. Therefore, using a load R, it is possible to obtain electrical signals whose power is many times greater than the power supplied to the emitter circuit. They replace vacuum tubes and are widely used in technology.

3.Types of semiconductor devices.

/>In addition to planar diodes (Fig. 8) and transistors, there are also point diodes (Fig. 4). Point-point transistors (see figure for structure) are molded before use, i.e. They pass a current of a certain magnitude, as a result of which an area with hole conductivity is formed under the tip of the wire. Transistors come in p-n-p and n-p-n types. Designation and general are visible in Figure 5.

There are photo- and thermistors and varistors as shown in the figure. Planar diodes include selenium rectifiers. The basis of such a diode is a steel washer, coated on one side with a layer of selenium, which is a semiconductor with hole conductivity (see Fig. 7). The surface of selenium is coated with a cadmium alloy, resulting in the formation of a film with electronic conductivity, as a result of which a rectifying current transition is formed. The larger the area, the greater the rectifying current.

4. Production

/>The manufacturing technology of diodata is similar. A piece of indium is melted on the surface of a square plate with an area of ​​2-4 cm2 and a thickness of several fractions of a millimeter, cut from a semiconductor crystal with electronic conductivity. Indium is firmly alloyed by the plate. In this case, indium atoms penetrate (diffuse) into the thickness of the plate, forming in it a region with predominant hole conductivity (Fig. 6). This results in a semiconductor device with two regions of different types of conductivity, and a p-n junction between them. The thinner the semiconductor wafer. the lower the resistance of the diode in the forward direction, the greater the current corrected by the diode. The diode contacts are an indium droplet and a metal disk or rod with lead conductors

After assembling the transistor, it is mounted in the housing and the electrical connection is connected. the leads to the contact plates of the crystal and the body lead seal the body.

5. Scope of application

/> Diodes are highly reliable, but the limit of their use is from –70 to 125 C. Because A point diode has a very small contact area, so the currents that such diodes can deliver are no more than 10-15 mA. And they are used mainly for modulating high-frequency oscillations and for measuring instruments. For any diode, there are certain maximum permissible limits of forward and reverse current, depending on the forward and reverse voltage and determining its rectifying and strength characteristics.

Transistors, like diodes, are sensitive to temperature and overload and to penetrating radiation. Transistors, unlike radio tubes, burn out due to improper connection.