Resistors, semiconductor diodes, transistors. Semiconductor resistors. Moscow Mining State University

04.11.2021

Moscow Mining State University

Essay

in the subject CIRCUIT ENGINEERING

Semiconductor devices.

(diode, transistor, field effect transistor)

Art. gr. CAD-1V-96

Tsarev A.V.

Moscow 1999

Contents

Semiconductor diodes.

Semiconductor transistors.

Field-effect MOS transistors.

Literature.

Semiconductor diodes

A diode is a semiconductor device that transmits electricity only one direction and having two terminals for inclusion in an electrical circuit.

Semiconductor diode is a semiconductor device with a p-n junction. The working element is a germanium crystal, which has n-type conductivity due to a small addition of a donor impurity. To create p-n junctions in it, indium is melted into one of its surfaces. Due to the diffusion of indium atoms deep into the germanium single crystal, a p-type region is formed at the germanium surface. The rest of the germanium is still n-type. A pn junction occurs between these two regions. To prevent harmful effects of air and light, the germanium crystal is placed in a sealed housing. device and schematic representation of a semiconductor diode:

The advantages of semiconductor diodes are small size and weight, long service life, high mechanical strength; The disadvantage is that their parameters depend on temperature.

The volt-ampere characteristic of the diode (at high voltage the current reaches its greatest value - saturation current) is nonlinear, therefore the properties of the diode are assessed by the slope of the characteristic:

Semiconductor transistors

The properties of a pn junction can be used to create an electrical amplifier called a semiconductor triode or transistor.

In a semiconductor triode, the two p-regions of the crystal are separated by a narrow n-region. Such a triode is conventionally designated p-n-p. You can also make an n-p-n triode, i.e. separate two n-regions of the crystal with a narrow p-region (Fig.).

A p-n-p type triode consists of three regions, the outermost of which have hole conductivity, and the middle one has electronic conductivity. Independent contacts e, b and k are made to these three regions of the triode, which allows different voltages to be applied to the left p-n junction between contacts e and b and to the right n-p junction between contacts b and k.

If reverse voltage is applied to the right junction, it will be locked and very little reverse current will flow through it. Let us now apply direct voltage to the left p-n junction, then a significant forward current will begin to pass through it.

One of the regions of the triode, for example the left one, usually contains hundreds of times more amount of p-type impurity than the amount of n-impurity in the n-region. Therefore, the forward current through the pn junction will consist almost exclusively of holes moving from left to right. Once in the n-region of the triode, holes undergoing thermal movement diffuse towards the n-p junction, but partially have time to undergo recombination with free electrons of the n-region. But if the n-region is narrow and there are not too many free electrons in it (not a pronounced n-type conductor), then most of the holes will reach the second transition and, having entered it, will be moved by its field to the right p-region. In good triodes, the flux of holes penetrating into the right p-region is 99% or more of the flux penetrating from the left into the n-region.

If, in the absence of voltage between the points g and b, the reverse current in the n-p junction is very small, then after the voltage appears at the terminals g and b, this current is almost as large as the forward current in the left junction. In this way, you can control the current strength in the right (locked) n-p junction using the left p-n junction. By blocking the left junction, we stop the flow through the right junction; by opening the left junction, we obtain current in the right junction. By changing the magnitude of the forward voltage at the left junction, we will thereby change the current strength in the right junction. This is the basis for the use of a pnp triode as an amplifier.

When the triode (Fig) is operating, load resistance R is connected to the right junction and, using battery B, a reverse voltage (tens of volts) is applied, which blocks the junction. In this case, a very small reverse current flows through the junction, and the entire voltage of battery B is applied to the n-p junction. At the load, the voltage is zero. If you now apply a small forward voltage to the left junction, then a small forward current will begin to flow through it. Almost the same current will begin to flow through the right junction, creating a voltage drop across the load resistance R. The voltage at the right n-p junction decreases, since now part of the battery voltage drops across the load resistance.

As the forward voltage at the left junction increases, the current through the right junction increases and the voltage across the load resistance R increases. When the left p-n junction is open, the current through the right n-p junction becomes so large that a significant part of the voltage of battery B drops at the load resistance R.

Thus, by applying a direct voltage equal to fractions of a volt to the left junction, it is possible to obtain a large current through the load, and the voltage across it will be a significant part of the voltage of battery B, i.e. tens of volts. By changing the voltage supplied to the left junction by hundredths of a volt, we change the voltage at the load by tens of volts. In this way, voltage gain is obtained.

There is no current amplification with this triode connection scheme, since the current flowing through the right junction is even slightly less than the current flowing through the left junction. But due to voltage amplification, power amplification occurs here. Ultimately, the power gain occurs due to the energy of source B.

The action of a transistor can be compared to the action of a dam. With the help of a constant source (river flow) and a dam, a difference in water levels is created. By spending very little energy on the vertical movement of the valve, we can control a high-power flow of water, i.e. manage the energy of a powerful constant source.

A transition connected in the through direction (left in the figures) is called emitter, and a transition connected in the blocking direction (right in the figures) is called collector. The middle region is called the base, the left is the emitter, and the right is the collector. The thickness of the base is only a few hundredths or thousandths of a millimeter.

The service life of semiconductor triodes and their efficiency are many times longer than that of electronic tubes. Due to this, transistors have found wide application in microelectronics - television, video, audio, radio equipment and, of course, in computers. They replace vacuum tubes in many electrical circuits of scientific, industrial and household equipment.

The advantages of transistors over vacuum tubes are the same as those of semiconductor diodes - the absence of a heated cathode, which consumes significant power and takes time to warm up. In addition, transistors themselves are many times smaller in mass and size than electric lamps, and transistors are able to operate at lower voltages.

But along with positive qualities, triodes also have their disadvantages. Like semiconductor diodes, transistors are very sensitive to temperature increases, electrical overloads and highly penetrating radiation (to make the transistor more durable, it is packaged in a special “case”).

The main materials from which triodes are made are silicon and germanium.

Field-effect MOS transistors.

A field-effect transistor (FET) is a three-electrode semiconductor device in which the electric current is created by the main charge carriers under the action of a longitudinal electric field, and the current is controlled by a transverse electric field created by the voltage on the control electrode.

In recent years, devices using phenomena in the surface layer of a semiconductor have occupied a large place in electronics. The main element of such devices is the Metal-Dielectric-Semiconductor (MDS) structure. An oxide layer, such as silicon dioxide, is often used as a dielectric layer between the metal and the semiconductor. Such structures are called MOS structures. The metal electrode is usually applied to the dielectric by vacuum sputtering. This electrode is called a gate.

PTs are unipolar semiconductor devices, since their operation is based on the drift of charge carriers of the same sign in a longitudinal electric field through a controlled n- or p-type channel. The current through the channel is controlled by a transverse electric field, and not by current, as in bipolar transistors. Therefore, such transistors are called field-effect transistors.

Field effect transistors with gate in p-n form Depending on the channel, transitions are divided into PTs with a p-type and n-type channel. The p-type channel has hole conductivity, and the n-type channel has electronic conductivity.

If a certain bias voltage relative to the semiconductor is applied to the gate, then a space charge region appears at the surface of the semiconductor, the sign of which is opposite to the sign of the charge on the gate. In this region, the concentration of current carriers can differ significantly from their volume concentration.

Charging of the near-surface region of a semiconductor leads to the appearance of a potential difference between it and the volume of the semiconductor and, consequently, to the curvature of energy bands. With a negative charge on the gate, the energy bands bend upward, since when an electron moves from the bulk to the surface, its energy increases. If the gate is positively charged, the zones bend downward.

The figure shows the band structure of an n-semiconductor with a negative charge on the gate and shows the designations of the main quantities characterizing the surface; potential difference between the surface and volume of the semiconductor; bending of zones near the surface; middle of the bandgap. It can be seen from the figure that in the bulk of the semiconductor the distance from the bottom of the conduction band to the Fermi level is less than the distance from the Fermi level to the top of the valence band. Therefore, the equilibrium electron concentration is greater than the hole concentration: as it should be for n-semiconductors. In the surface layer of the space charge, the bands are bent and the distance from the bottom of the conduction band to the Fermi level continuously increases as one moves towards the surface, and the distance from the Fermi level to the top of the valence band continuously decreases.

The bending of zones near the surface is often expressed in units of kT and denoted Ys. Then, during the formation of the near-surface region of a semiconductor, three important cases may occur: depletion, inversion, and enrichment of this region with charge carriers. These cases for n- and p-type semiconductors are presented in Fig.

The depletion region appears when the gate charge coincides in sign with the sign of the main current carriers. The band bending caused by such a charge leads to an increase in the distance from the Fermi level to the bottom of the conduction band in an n-type semiconductor and to the top of the valence band in a p-type semiconductor. An increase in this distance is accompanied by a depletion of the near-surface region in major carriers. At a high gate charge density, the sign of which coincides with the sign of the charge of the majority carriers, as one approaches the surface, the distance from the Fermi level to the top of the valence band in an n-type semiconductor turns out to be less than the distance to the bottom of the conduction band. As a result, the concentration of non-majority charge carriers (holes) at the surface of the semiconductor becomes higher than the concentration of the majority carriers and the type of conductivity of this region changes, although there are few electrons and holes here, almost like in the own semiconductor. At the surface itself, however, there may be as many or even more non-majority carriers than the majority carriers in the bulk of the semiconductor. Such highly conductive layers near the surface with the type of conductivity opposite to the bulk layer are called inversion layers. Adjacent to the inversion layer deep from the surface is a depletion layer.

If the sign of the gate charge is opposite to the sign of the charge of the main current carriers in the semiconductor, then under its influence the main carriers are attracted to the surface and enrich the near-surface layer with them. Such layers are called enriched.

In integrated electronics, MIS structures are widely used to create transistors and various integrated circuits based on them. In Fig. The structure of an insulated gate MOS transistor is schematically shown. The transistor consists of a silicon crystal (for example, n-type), at the surface of which p-regions are formed by diffusion (or ion implantation) into windows in the oxide, as shown in Fig. One of these areas is called the source, the other - the drain. Ohmic contacts are applied on top of them. The gap between the regions is covered with a film of metal, isolated from the surface of the crystal by a layer of oxide. This electrode of the transistor is called the gate. At the boundary between the p- and n-regions, two p-n junctions appear - the source and the drain, which are shown in the figure. shown by shading.

In Fig. The diagram for connecting a transistor to a circuit is shown: the plus is connected to the source, the minus of the voltage source is connected to the drain, and the minus of the source is connected to the gate. For simplicity of consideration, we will assume that there is no contact potential difference, no charge in the oxide, and no surface states. Then the properties of the surface region, in the absence of voltage on the gate, are no different from the properties of semiconductors in the bulk. The resistance between drain and source is very high, since the drain pn junction is reverse biased. Applying a negative bias to the gate first leads to the formation of a depletion region under the gate, and at a certain voltage called threshold, to the formation of an inversion region connecting the p-regions of the source and drain with a conducting channel. At higher gate voltages, the channel becomes wider and the drain-source resistance is lower. The structure in question is thus a controlled resistor.

However, the channel resistance is determined only by the gate voltage only at small drain voltages. With an increase, carriers from the channel move to the drain region, the depletion layer at the drain n-p junction expands and the channel narrows. The dependence of the current on the drain voltage becomes nonlinear.

As the channel narrows, the number of free current carriers under the gate decreases as it approaches the drain. In order for the current in the channel to be the same in any cross section, the electric field along the channel must, in this case, be non-uniform, its strength must increase as it approaches the drain. In addition, the emergence of a concentration gradient of free current carriers along the channel leads to the appearance of a diffusion component of the current density.

At a certain voltage at the drain, the channel at the drain closes; with an even greater displacement, the channel is shortened towards the source. Blocking the channel, however, does not lead to the disappearance of the drain current, since in the depletion layer that blocked the channel, the electric field pulls holes along the surface. When current carriers from the channel enter this region due to diffusion, they are picked up by the field and transferred to the drain. Thus, as the voltage at the drain increases, the purely drift mechanism of movement of current carriers along the channel is replaced by a diffusion-drift mechanism.

The mechanism of current flow in a MIS transistor with a closed channel has some common features with the current flow in a reverse-biased n-p junction. Recall that in an n-p junction, minority current carriers enter the space charge region of the junction due to diffusion and are then picked up by its field.

As theory and experiment show, after blocking the channel, the drain current is practically saturated. The value of the saturation current depends on the gate voltage; the higher the voltage, the wider the channel and the greater the saturation current. This is a typical transistor effect - the gate voltage (in the input circuit) can control the drain current (current in the output circuit). A characteristic feature of MOS transistors is that its input is a capacitor formed by a metal gate isolated from the semiconductor.

At the semiconductor-dielectric interface, in the band gap of the semiconductor, there are energy states called surface or, more precisely, interface states. The wave functions of electrons in these states are localized near the interface in regions on the order of the lattice constant. The reason for the occurrence of the states under consideration is the imperfection of the semiconductor-dielectric (oxide) interface. At real interfaces there is always a certain number of broken bonds and the stoichiometry of the composition of the dielectric oxide film is violated. The density and nature of the interface states significantly depend on the technology for creating the dielectric film.

The presence of surface states at the semiconductor-dielectric interface negatively affects the parameters of the MOS transistor, since part of the charge induced under the gate in the semiconductor is captured by these states. Success in creating field-effect transistors of the type under consideration was achieved after developing the technology for creating a film on the silicon surface with a low density of states at the interface.

In silicon oxide itself there is always a positive “built-in” charge, the nature of which is still not fully understood. The value of this charge depends on the oxide manufacturing technology and often turns out to be so large that if p-type silicon is used as a substrate, then an inversion layer is formed at its surface even at zero gate bias. Such transistors are called transistors with a BUILT-IN CHANNEL. The channel in them is maintained even when some negative bias is applied to the gate. In contrast, in transistors made on an n-substrate, in which too much oxide charge is required to form an inversion layer, a channel appears only when a voltage exceeding a certain threshold voltage is applied to the gate. The sign of this gate bias should be negative for transistors with an n-substrate and positive in the case of a p-substrate.

At high voltages at the drain of the MIS transistor, the space charge region from the drain region can spread so strongly that the channel disappears altogether. Then carriers from the heavily doped source region will rush to the drain, just as when the base of a bipolar transistor is “punctured”.

Literature:

"Solid-state electronics" G.I.Epifanov, Yu.A.Moma.

“Electronics and Microcircuitry” V.A. Skarzhepa, A.N. Lutsenko.

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 crystal lattice, and the external electric field does not have a noticeable effect on their movement.

Electronic conductivity.

When heating silicon kinetic energy 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 the bond is broken, it forms vacant place 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 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 strength in the collector is almost equal to strength current in the emitter, changes 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. 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 pnp 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 current-rectifying junction is formed. larger area, the greater the rectified 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 properties.

Transistors, like diodes, are sensitive to temperature and overload and to 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 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

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.

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 in the collector, which is almost equal to the current 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.

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 12.3. Semiconductor diode.

Goal of the work: Study the principle of operation of a semiconductor diode.

Equipment:

Adjustable AC voltage source
Oscilloscope
Stand with diagram

Progress.

1. The installation consists of a source of adjustable alternating voltage, an oscilloscope and a stand with a circuit. AC 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.

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 no longer goes through the chain alternating current, but pulsating, 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. The diode is removed from the stand (the signal on the oscilloscope screen has disappeared). If you rotate the diode 180 degrees, the tip of the triangle in the diagram will be directed downward, i.e. the direction of current flowing through the diode will change. After installing the diode on the stand, the signal again appears on the oscilloscope screen, but 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 to the voltage from the ends of this chain; it is proportional to the voltage across 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 p-n junction makes it possible to create a rectifying semiconductor device - a 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 the 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 р-n transitions, included counter. 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.

Experiment 12.4. 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 (Fig. 117):

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

Control questions to § 12.

1) Define semiconductors?

2) Elements of which groups of the periodic table belong to semiconductors?

3) Name two types of electric charge carriers present in a semiconductor.

4) List the equipment in experiment 12.1 “Action of a semiconductor photocell.”

5) List the equipment in experiment 12.2 “Electron-hole conductivity of semiconductors.”

6) Give p-n definition transition?

7) List the equipment in experiment 12.3 “Semiconductor diode”.

8) Define the current-voltage characteristic?

9) Define a transistor?

10) List the equipment in experiment 12.4 “DC transistor amplifier.”

11) Define emitter?

12) Define a collector?

13) Why can a transistor be used to amplify electrical signals?

14) How does the electronic conductivity of germanium occur?

15) How does hole conductivity in germanium occur?

16) Describe the structure of a selenium photocell.

17) Which semiconductor device uses one-way conduction of a p-n junction?

18) Describe the structure of a semiconductor diode.

19) How much p-n junctions exists in a transistor?

20) Describe the structure of a transistor.

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)

The force acting on electric charge Q, moving in a magnetic field with speed V , called Lorentz force and is expressed by the formula, where IN- induction 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's place a metal plate with a current density j to magnetic

field IN, perpendicular j .At in this 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.

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