Classification of semiconductor devices and their purpose.

Industrial electronics uses a large number of different types of semiconductor devices, which can be divided into several main groups: 1) semiconductor resistors; 2) semiconductor diodes; 3) bipolar transistors; 4) field-effect transistors; 5) thyristors.

Semiconductor resistors and diodes are two-electrode devices, bipolar and field-effect transistors are three-electrode devices. Thyristors can be either two-electrode or three-electrode.

Semiconductor resistors use an isotropic (homogeneous) semiconductor material, the electrical properties of which determine the electrical characteristics of the resistor. Semiconductor diodes use semiconductors with different types of electrical conductivity, forming one p-n junction. The electrical characteristics of the diode are determined mainly by the electrical properties of the p-n junction.

In bipolar transistors, semiconductors with different types of electrical conductivity form two p-n junctions. The electrical characteristics of bipolar transistors are determined by the electrical properties of these pn junctions and significantly depend on their interaction. Field-effect transistors are based on semiconductors with different types of electrical conductivity, which form one p-n junction. But unlike diodes and bipolar transistors, the electrical characteristics of field-effect transistors depend on the interaction of an isotropic semiconductor channel with a p-n junction.

Thyristors use semiconductors with different types of electrical conductivity, which form three or more p-n junctions. The main electrical characteristics of thyristors are determined by the interaction of these p-n junctions.

Semiconductor diodes

A semiconductor diode is an electrically converting semiconductor device with one electrical junction and two terminals.

The classification and conventional graphic designation of semiconductor diodes are given in table. 2.2. As can be seen from the table, all semiconductor diodes are divided into two classes: point and planar.

A point diode uses a germanium or silicon plate with n-type electrical conductivity, 0.1-0.6 mm thick and 0.5-1.5 mm 2 in area; A sharpened steel wire comes into contact with the plate (Fig. 2.5), forming a p-n junction at the point of contact.

The current-voltage characteristics of a point diode at various temperatures are shown in Fig.

Due to the small contact area, the forward current and interelectrode capacitance of such diodes are relatively small, which allows them to be used in the region of very high frequencies (microwave diodes). Point diodes serve mainly to rectify alternating current (rectifier diodes).

In planar diodes, the pn junction is formed by two semiconductors with different types of electrical conductivity, and the junction area of ​​different types of diodes ranges from hundredths of a square millimeter (microplanar diodes) to several tens of square centimeters (power diodes).

According to the method of introducing impurities, diodes are divided into alloy and diffusion.

The electrical characteristics of a planar diode are determined by the characteristics of the pn junction. Depending on the purpose of the diode, it uses certain characteristics of the p-n junction.

Let's take a closer look at the types and characteristics of various planar diodes.

A rectifying diode is a semiconductor device that, like a point diode, uses the rectifying properties of a p-n junction.

The design of a powerful rectifier diode is shown in Fig. 2.7. Low-power rectifier diodes, as well as rectifier diodes designed for operation in high-frequency and pulse circuits, usually have a design similar to point diodes.

The current-voltage characteristic of a powerful rectifying diode is shown in Fig. 2.8.

Due to their large junction area, planar diodes are designed for high forward current. Typically, the forward voltage of the diode does not exceed 1-2 V, while the current density in the semiconductor reaches 1-10 A/mm2, which causes a slight increase in its temperature. To maintain the performance of a germanium diode, its temperature should not exceed 85-100° C. Silicon diodes can operate at temperatures of 150-200° C.

When a reverse voltage is applied to a semiconductor diode, a slight reverse current appears in it (Fig. 2.8), caused by the movement of minority charge carriers through the p-n junction.

As the temperature of the pn junction increases, the number of minority charge carriers increases due to the transition of some electrons from the valence band to the conduction band and the formation of electron-hole charge carrier pairs. Therefore, the reverse current of the diode increases.

When a reverse voltage of several hundred volts is applied to the diode, the external electric field in the blocking layer becomes so strong that it can pull electrons from the valence band into the conduction band (Zener effect). In this case, the reverse current increases sharply, which causes heating of the diode, a further increase in current and, finally, thermal breakdown (destruction) of the p-n junction. Most diodes can operate reliably at reverse voltages not exceeding (0.7-0.8) U samples. Even a short-term increase in reverse voltage above the breakdown voltage, as a rule, leads to breakdown of the p-n junction and failure of the diode.

The main parameters of point and planar rectifier diodes are: forward current of the diode I pr, which is normalized at a certain forward voltage (usually 1-2 V). The maximum permissible forward current of the diode I pr max, the maximum permissible reverse voltage of the diode U rev max; reverse diode current I rev, which is normalized at the maximum reverse voltage U rev max. The parameters of various rectifier diodes are given in table.

Semiconductor Zener diode- a semiconductor diode, the voltage on which in the region of electrical breakdown weakly depends on the current and which is used to stabilize the voltage.

A semiconductor zener diode operates in the area of ​​electrical breakdown of the p-n junction. To prevent thermal breakdown, the zener diode design ensures effective heat removal from the p-n junction. The most common material for zener diodes is silicon. The current-voltage characteristic of a semiconductor zener diode is shown in Fig.

As can be seen from the figure, in the breakdown region, the voltage on the zener diode U CT changes only slightly with large changes in the stabilization current I CT. This characteristic of the zener diode is used to obtain a stable voltage, for example, in parametric voltage stabilizers.

The main parameters of a semiconductor zener diode are: stabilizing voltage U CT; dynamic resistance in the stabilization section Rd = d U CT / dI CT ; minimum zener diode current Ist min; maximum zener diode current Ist max; temperature coefficient of voltage in the stabilization section TKU = d U CT /dT 100%.

The stabilizing voltage of modern zener diodes lies in the range of 1-1000 V and depends on the thickness of the blocking layer of the p-n junction.

Tunnel diode- a semiconductor diode based on a degenerate semiconductor, in which the tunnel effect leads to the appearance of a section of negative differential conductivity on the current-voltage characteristics at forward voltage (see figure).

Direct branch c is used as a working branch. A. X.

The material for tunnel diodes is heavily doped germanium or gallium arsenide.

The main parameters of a tunnel diode are: peak current Ip (curve 1 in Fig.) and the ratio of peak current to valley current Ip/Ib. For diodes produced by the domestic industry, Ip = 0.1-100 mA, and Ip / Iv = 5 - 20.

Tunnel diodes are high-speed semiconductor devices and are used in high-frequency oscillators and high-speed pulse switches.

Reversed diode- a type of tunnel diode with a peak current Iп = 0 (curve 2 in the figure). If forward voltage Upr is applied to the reversed diode< 0,3 В, то пряой ток диода Iпр = 0, в то же время даже при небольшом обратном напряжении (порядка десятков милливольт) обратный ток диода достигает нескольких миллиампер в результате туннельного пробоя. Таким образом, обращенный диод обладает вентильными свойствами при малых напряжениях именно в той области, где обычные выпрямительные диоды этими свойствами не обладают. При этом направлением наибольшей проводимости является направление, соответствующее обратному току.

Reversed diodes are used, like tunnel diodes, in pulsed devices, and also as signal converters (mixers and detectors) in radio devices.

Varicap- a semiconductor diode that uses capacitance dependence

p-n junction from reverse voltage and which is intended for use as an element with an electrically controlled capacitance value. The semiconductor material for the manufacture of varicaps is silicon. The dependence of the varicap capacity on the reverse voltage is shown in Fig.

The main parameters of the varicap are: the total capacity of the varicap C, which is usually fixed at a small reverse voltage Uo6p = 2-5 V; capacitance overlap coefficient Ks = Cmax/Cmin. For most varicaps, Sv = 10-500 pF, and capacitance overlap coefficient Ks = 5-20.

Varicaps are used in remote control systems and in parametric amplifiers with low noise levels.

F o. d i o d, semiconductor photocell, LED- semiconductor diodes that use the effect of interaction of radiation (visible, infrared or ultraviolet) with charge carriers (electrons and holes) in the blocking layer of the p-n junction.

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 conductivity 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.

Nowadays, diodes are increasingly used in semiconductors to rectify electric current in radio circuits, along with two-electrode lamps, since they have a number of advantages. In a vacuum tube, charge carriers electrons are created 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. 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 the indium atoms did not penetrate, still has n-type conductivity. A p-n junction occurs between the regions. In a semiconductor diode, germanium 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 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.

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 Fig. 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 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

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 p-type region called the collector. If there were no left pn junction, the current and collector circuit would be close to zero. Since the reverse junction resistance 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. When a voltage is created between the emitter and the base, the main carriers of the p-type semiconductor - holes penetrate the base, GDR they are already the main 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. 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. 3) plane is much smaller than the cross-section in the vertical plane. The current strength in the collector, almost equal to the current strength in the emitter, changes along with the current in the emitter. Resistor R has little effect on the collector current, and this resistance can be made quite large. By controlling the emitter current using an alternating voltage source connected to its circuit, we obtain a synchronous change in the voltage across the resistor. 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 entering 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 transistors (structure see in the figure) are molded before use, i.e. 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 view 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 diode manufacturing technology is as follows. 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 with 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. leads to the contact plates of the crystal and the lead of the package and seal the package.

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 properties.

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

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 conductivity 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 that are relatively weakly bound to the nucleus. The number of nearest neighbors of each silicon atom is also four. The interaction of a pair of neighboring atoms is carried out using a polyelectronic bond, called a covalent bond. In the formation of this bond, one valence electron from each atom participates, which are split off from the atoms (collectivized by the crystal) and during their movement spend most of the 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 bonds break. 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 electrons in metals is called electronic 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 of the electrons that ensures the connection of atoms jumps to the place of the formed holes and restores the pair-electron bond here. and where this electron jumped from, a new hole is formed. 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 in the presence of impurities, along with their own conductivity, an additional one appears - impurity conductivity. By changing the impurity concentration, you can significantly change the number of charge carriers of one or another sign. Thanks to this, it is possible to create semiconductors with a predominant concentration of either negatively or positively charged carriers. This feature of semiconductors opens up wide possibilities for practical applications.

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 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 lacks an electron. As a result, a hole is formed. The number of holes in the crystal is equal to the number of impurity atoms. Impurities of this kind are called acceptor impurities. In the presence of an electric field, holes move around the field and hole conduction occurs. Semiconductors with a predominance of hole conductivity over electron conductivity 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.

Diode.
Nowadays, diodes are increasingly used in semiconductors to rectify electric current in radio circuits, along with two-electrode lamps, since they have a number of advantages. In a vacuum tube, charge carriers electrons are created 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.

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.
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 Fig. 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 include a transistor in the circuit shown in Figure 3. With this connection, the left p-n junction is direct and separates the base from the area with p-type conductivity, called the emitter. If there were no right p –n
-transition, in the emitter-base circuit there would be a current depending on the voltage of the sources (battery B1 and the alternating voltage source) and the resistance of the circuit, including the low resistance of the direct emitter-base junction. 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 p-type region called the collector. If there were no left pn junction, the current strength in the collector circuit would be close to zero. Since the reverse junction resistance 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. When a voltage is created between the emitter and the base, the main carriers of the p-type semiconductor - holes penetrate the base, GDR they are already the main 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. 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. 3) plane is much smaller than the cross-section in the vertical plane. The current strength in the collector, almost equal to the current strength in the emitter, changes along 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 entering 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 transistors (structure see in the figure) are molded before use, i.e. 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 view 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 diode manufacturing technology is as follows. 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 with the plate. In this case, indium atoms penetrate
(diffuse) into the thickness of the plate, forming in it a region with a predominance of 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. leads to the contact plates of the crystal and the lead of the package and seal the package.

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 properties.

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

-----------------------

Figure 2

Picture 1

Figure 3

Figure 4

Figure 5

Figure 4

Semiconductor diode called a non-signal amplifying electronic element with one electron-hole junction and two leads from the anode and cathode.

Diodes are used in electronic circuits to convert the parameters of electrical signals (rectification, stabilization). Diodes differ in design ( point, planar) and according to the symbol on the diagrams (depending on the functional purpose).

Operating principle diode illustrates it volt-ampere characteristics, those. dependence of the current on the applied voltage, (Fig. 1), from which it is clear that the diode has one-way conductivity(passes current in the forward direction and practically does not pass it in the reverse direction).

The diode is connected in the forward direction when the positive pole of the current source is connected to the anode A, and the negative pole of the current source is connected to the cathode K. This corresponds to the characteristic branch in the first quadrant. A large forward current passes through the diode.

When connected to reverse direction (plus - to the cathode, minus - to the anode), the reverse current I OBR passing through the diode is very small (mkA).

In this case, the direct current, as can be seen from Fig. 1, depends significantly on temperature environment (increases with increasing temperature).

Rice. 1. Current-voltage characteristic of the diode.

Diode characteristics:

In addition to the considered current-voltage, the main characteristics of the diode include:

Maximum forward current I ETC ;

Temperature resistance t 0 max ;

Maximum reverse voltage U KP .

DC resistance R 0 = U ETC / I ETC ;

AC Resistance R i = Δ U ETC / Δ I ETC ;

Slope of current-voltage characteristic S = Δ I ETC / Δ U ETC ;

Power loss at the anode P A = U ETC I ETC ;

Area of ​​use of diodes: AC rectification; voltage stabilization; work in photovoltaic devices; work in microwave circuits, etc.

Transistors

Transistors – semiconductor devices with two r-p transitions allowing enhance electrical signal and usually having three terminals. Divided into two groups - bipolar and unipolar(field). Basic circuits for connecting a bipolar transistor - with a common base, with a common emitter and with a common collector. The type of switching circuit determines by what parameter the transistor amplifies the signal (voltage, current, etc.).

Bipolar transistor is a semiconductor device with a three-layer structure with alternating types of conductivity and two r-p transitions, allowing amplification of electrical signals and having three outputs. Distinguish direct (p-n-p) and reverse (n-p-n) transistors, the difference between which is polarity connecting power supplies.

The components of a transistor correspond to its layers and are named: emitter– charge emitter, base– base and collector– charge collector. Layers have

different conductivity: extreme (emitter and collector) - holep, and the base located between them is electronicn(Fig. 2).

Emitter Base Collector

Iuh ITo

EntranceExit

Rice. 2. Bipolar p- n- p transistor connected according to a common base circuit

Let's consider the principle of operation of a transistor. As can be seen in Fig. 2, the transistor has two junctions: p- n And n- p. First transition ( p- n) included in direct direction, i.e. minus k n-areas, and plus to R– areas - to the emitter. Therefore, direct current will flow through this junction. Second transition ( n- p) included in reverse direction, i.e. plus to base ( n- area), and minus to R– areas - to the collector. If you open the emitter (input) circuit, this junction, located under reverseU K when turned on, it will be practically closed.

If you close the emitter circuit (apply an input signal), through the first (open) p- n junction, a direct current will flow, formed by the injection of holes into the base. Since the thickness of the base is small, and the semiconductors from which the emitter and base are made are selected with different concentrations of the main carriers, i.e. the concentration of holes in the emitter is significantly higher than the concentration of electrons in the base, there will be so many holes in the base that only a small part of them will find in the base the electrons necessary for recombination. Therefore, incoming holes that have not recombined with electrons begin to move to those regions of the base that are adjacent to the collector. Positive holes approaching the collector junction, experiencing the action of a strong accelerating field from a powerful collector battery U K, pass into the collector and recombine with electrons coming into the collector from the negative pole of the battery. As a result, collector current will begin to flow through the collector junction I K, despite the fact that reverse voltage is applied to the junction. This collector current will be 90 - 95% of the emitter current (due to the small number of recombining holes remaining in the base). But the most important thing is that the magnitude of the collector current will depend on the magnitude of the emitter current and will change in proportion to its change. Indeed, the greater the current through the emitter junction, i.e., the more holes the emitter injects into the base, the greater the collector current, which depends on the number of these holes. This leads to a practically important conclusion:

By controlling the emitter current of the transistor, you can thereby control the collector current, and in this case an amplification effect takes place.

This property determined the area of ​​​​use of transistors in amplifier circuits. So, for example, the considered circuit for connecting a transistor with a common base will give voltage and power gain input signal, since the output load resistance Rn with appropriate selection of battery voltage UTo may be significantly greater than the resistance at the amplifier input, i.e. R H >> R VX, and the input (emitter I E) and output (collector I TO) the currents are approximately equal. Hence the voltage and power supplied to the input U VX = I VX * R VX ; Pinput= I 2 input * Rinput less than the corresponding values ​​of voltage and power at the output, i.e. in the load U = I TO * R N ; Pn = I K 2 * RN. There is no current gain in this case (since I E ~ = I TO).

More often, however, another transistor connection circuit is used - common emitter circuit, at which, in addition to power amplification, there is also current amplification. Connection diagram with common collector used when operating on a low-resistance load or from a high-resistance sensor. The gain of such a circuit in terms of current and power is several tens of units, and in terms of voltage - about one.

To correctly understand the principle of operation of transistor circuits, it is necessary to have a good understanding of the features of the operation of a transistor as an amplifier, which are as follows: unlike a vacuum tube, the transistor has a low input resistance in most switching circuits, as a result of which it is believed that the transistor is controlled by the input current, and not by the input current. tension; the low input resistance of transistor amplifiers leads to a noticeable consumption of power (current) from the source of amplified oscillations, therefore, in these amplifiers, the main importance is not voltage gain, but current or power gain; power gain k is determined by the ratio of the power allocated at the output of the amplifier in the payload to the power expended at the input impedance of the amplifier; The parameters and characteristics of the transistor are highly dependent on the temperature and the selected mode, which is a disadvantage.

Transistor characteristics:

Input, output and transient characteristics, fig. 3,

Rice. 3. Transistor characteristics: a – input, b – output, c – transition

Gain (transmission) in general terms, voltage, current, power

k=ΔΧ OUT /ΔΧ IN;ΔU OUT /ΔU IN;ΔI OUT /ΔI IN;ΔP OUT /ΔP IN.

Transistor AC input impedance

R = ΔU ВХ / ΔI ВХ.

Collector power loss

P K = U K * I K .

Advantages of transistors: small dimensions, high sensitivity, inertia-free; durability; flaws: significant influence of external factors (temperature, e/m fields, radioactive radiation, etc.).

Area of ​​use transistors: Wired and radio communications; TV; radar; radio navigation; automation and telemechanics; Computer Engineering; measuring technology; amplifier circuits; memory chips for digital devices, etc.

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 a two-electrode lamp - a diode (see § 105). Therefore, a semiconductor device containing one р-n-transition is called semiconductor (crystalline) diode. Semiconductor diodes according to their design are divided into point And planar.

Rice. 339 Fig. 340

As an example, consider a point germanium diode (Fig. 339), in which a thin tungsten wire 1 is pressed against n- germanium 2 with an aluminum-coated tip. If a short-term current pulse is passed through a diode in the forward direction, then the diffusion of A1 in Ge sharply increases and a germanium layer is formed, enriched in aluminum and having R- conductivity. At the boundary of this layer a р-n- a junction with a high rectification coefficient. 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.

The schematic diagram of a planar copper oxide (cuproxy) rectifier is shown in Fig. 340. Using chemical treatment, a layer of copper oxide Cu 2 O is built up on a copper plate, which is covered with a layer of silver. The silver electrode serves only to connect the rectifier to the circuit. The part of the Cu 2 O layer adjacent to Cu and enriched with it has electronic conductivity, and the part of the Cu 2 O layer adjacent to Ag and enriched (during the manufacturing process of the rectifier) ​​with oxygen has hole conductivity. Thus, a barrier layer is formed in the thickness of cuprous oxide with the flow direction of the current from Cu 2 O to Cu ().

The manufacturing technology of a germanium planar diode is described in § 249 (see Fig. 325). Selenium diodes and diodes based on gallium arsenide and silicon carbide are also common. The considered diodes have a number of advantages compared to electron tubes (small overall dimensions, high efficiency and service life, constant readiness for work, etc.), but they are very sensitive to temperature, so the range of their operating temperatures is limited (from –70 to +120°C). p-n- Transitions not only have excellent rectifying properties, but can also be used for amplification, and if feedback is introduced into the circuit, then also for generating electrical oscillations. Devices designed for these purposes are called semiconductor triodes or transistors(the first transistor was created in 1949 by American physicists D. Bardeen, W. Brattain and W. Shockley; Nobel Prize 1956).

Germanium and silicon are used for the manufacture of transistors, as they are characterized by high mechanical strength, chemical stability and greater mobility of current carriers than in other semiconductors. Semiconductor triodes are divided into point And planar. The former significantly increase the voltage, but their output powers are low due to the risk of overheating (for example, the upper limit of the operating temperature of a point-type 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.

For example, consider the operating principle of a planar triode p-p-p, i.e. triode based n-semiconductor (Fig. 341). The working “electrodes” of the triode, which are base(middle part of the transistor), emitter And collector(areas adjacent to the base on both sides with a different type of conductivity) are included in the circuit using non-rectifying contacts - metal conductors. 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 is applied to the input resistance, and the amplified one is removed from the output resistance

The flow of current in the emitter circuit is mainly due to 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 of the base, a significant part 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) and change the collector current. Consequently, any change in current in the emitter circuit causes a change in current in the collector circuit.

By applying an alternating voltage between the emitter and the base, we obtain an alternating current in the collector circuit, and an alternating voltage at the output resistance. The amount of gain depends on the properties p-n-transitions, load resistances and battery voltage Bk. Usually >>, therefore significantly higher than the input voltage (gain can reach 10,000). Since the alternating current power released in can be greater than that consumed in the emitter circuit, the transistor also provides power amplification. This amplified power comes from a current source connected to the collector circuit.

From what has been discussed it follows that a transistor, like a vacuum tube, provides amplification of both voltage and power. If in a lamp the anode current is controlled by the grid voltage, then in a transistor the collector current corresponding to the anode current of the lamp is controlled by the base voltage.

Transistor operating principle p-p-p-type is similar to that discussed above, but the role of holes is played by electrons. There are other types of transistors, as well as other circuits for connecting them. Due to its advantages over vacuum tubes (small dimensions, high efficiency and service life, absence of an incandescent cathode and therefore less power consumption, no need for a vacuum, etc.), the transistor revolutionized the field of electronic communications and ensured the creation of high-speed computers with large amounts of memory.

Control questions

• What is the essence of the adiabatic approximation and the self-consistent field approximation?
• How do the energy states of electrons in an isolated atom and a crystal differ? What are forbidden and permitted energy zones?
• How do semiconductors and dielectrics differ according to band theory? metals and dielectrics?
• When, according to band theory, is a solid a conductor of electric current?
• How to explain the increase in conductivity of semiconductors with increasing temperature?
• What determines the conductivity of intrinsic semiconductors?
• Why is the Fermi level in an intrinsic semiconductor located in the middle of the band gap? Prove this position.
• What is the mechanism of electronic impurity conductivity in semiconductors? hole impurity conductivity?
• Why does intrinsic conductivity predominate in impurity semiconductors at sufficiently high temperatures?
• What is the mechanism of intrinsic photoconductivity? impurity photoconductivity? What is the red limit of photoconductivity?
• According to the band theory, what are the mechanisms of fluorescence and phosphorescence?
• What are the reasons for the occurrence of contact potential difference?
• What is the essence of thermoelectric phenomena? How to explain their occurrence?
• When a blocking contact layer occurs when a metal comes into contact with a semiconductor n-like? with semiconductor R-like? Explain the mechanism of its formation.
• How to explain one-way conduction r-p-transition?
• What is the current-voltage characteristic p-n-transition? Explain the occurrence of forward and reverse current.
• Which direction in a semiconductor diode allows current to pass?
• Why does a semiconductor diode carry current (albeit weak) even at the cut-off voltage?

Tasks

31.1. The germanium sample is heated from 0 to 17°C. Taking the band gap of silicon to be 0.72 eV, determine how many times its specific conductivity will increase. [2.45 times]

31.2. A small admixture of boron is introduced into pure silicon. Using the Periodic Table of D.I. Mendeleev, determine and explain the type of conductivity of impurity silicon.

31.3. Determine the wavelength at which photoconductivity is still excited in the impurity semiconductor.