Lesson No. 41-169 Electric current in semiconductors. semiconductor diode. Semiconductor devices.
A semiconductor is a substance that resistivity can vary over a wide range and decreases very quickly with increasing temperature, which means that electrical conductivity increases. It is observed in silicon, germanium, selenium and in some compounds.
Conduction mechanism in semiconductors
Semiconductor crystals have an atomic crystal lattice, where outer electrons are bound to neighboring atoms by covalent bonds. At low temperatures, pure semiconductors have no free electrons and it behaves like a dielectric. If the semiconductor is pure (without impurities), then it has its own conductivity (small).
There are two types of intrinsic conduction:
1) electronic (conductivity " P"-type) At low temperatures in semiconductors, all electrons are associated with nuclei and the resistance is large; As the temperature increases, the kinetic energy of the particles increases, bonds break and free electrons appear - the resistance decreases.
Free electrons move opposite to the electric field vector. The electronic conductivity of semiconductors is due to the presence of free electrons.
2) hole (p-type conductivity). With an increase in temperature, the covalent bonds between atoms, carried out by valence electrons, are destroyed and places with a missing electron are formed - a “hole”. It can move throughout the crystal, because. its place can be replaced by valence electrons. Moving a "hole" is equivalent to moving a positive charge. The hole moves in the direction of the electric field strength vector.
Gap covalent bonds and the occurrence of intrinsic conductivity of semiconductors can be caused by heating, lighting (photoconductivity) and the action of strong electric fields.
Dependence of R on illumination: Photoresistor - photorelay - emergency switches |
The total conductivity of a pure semiconductor is the sum of the "p" and "n" -type conductivities and is called electron-hole conductivity.
Semiconductors in the presence of impurities
They have their own and impurity conductivity. The presence of impurities greatly increases the conductivity. When the impurity concentration changes, the number of carriers changes electric current- electrons and holes. The ability to control the current underlies the widespread use of semiconductors. There are the following impurities:
1) donor impurities (donating) - are additional
suppliers of electrons to semiconductor crystals, easily donate electrons and increase the number of free electrons in the semiconductor. These are conductors "n" - type, i.e. semiconductors with donor impurities, where the main charge carrier is electrons, and the minority is holes. Such a semiconductor has electronic impurity conductivity (an example is arsenic).
2) acceptor impurities (receiving) create "holes", taking electrons into themselves. These are semiconductors "p" - type, i.e. semiconductors with acceptor impurities, where the main charge carrier is
holes, and the minority electrons. Such a semiconductor has
hole impurity conductivity (an example is indium).
Electrical properties "p-n » transitions.
"pn" transition (or electron-hole transition) - the contact area of two semiconductors, where the conductivity changes from electronic to hole (or vice versa).
In a semiconductor crystal, such regions can be created by introducing impurities. In the contact zone of two semiconductors with different conductivities, mutual diffusion of electrons and holes will take place and a blocking barrier will form.
electrical layer. The electric field of the barrier layer prevents
further transition of electrons and holes through the boundary. The barrier layer has an increased resistance compared to other areas of the semiconductor.
An external electric field affects the resistance of the barrier layer. In the direct (transmission) direction of the external electric field, the current passes through the boundary of two semiconductors. Because electrons and holes move towards each other to the interface, then the electrons,
crossing the border, fill the holes. The thickness of the barrier layer and its resistance are continuously decreasing.
When locking ( reverse direction external electric field) no current will pass through the contact area of the two semiconductors. Because electrons and holes move from the boundary in opposite directions, then the blocking layer
thickens, its resistance increases.
Thus, the electron-hole transition has one-sided conduction.
semiconductor diode- a semiconductor with one "rn" junction.
Semiconductor diodes are the main elements of AC rectifiers.
When an electric field is applied: in one direction, the resistance of the semiconductor is high, in the opposite direction, the resistance is low.
Transistors.(from English words transfer - transfer, resistor - resistance)
Consider one of the types of germanium or silicon transistors with donor and acceptor impurities introduced into them. The distribution of impurities is such that a very thin (on the order of a few micrometers) n-type semiconductor layer is created between two p-type semiconductor layers (see Fig.).
This thin layer is called basis or base. The crystal has two R-n -transitions, the direct directions of which are opposite. Three outputs from areas with different types of conductivity allow you to include a transistor in the circuit shown in the figure. With this inclusion, the left R-n -jump is direct and separates the base from a p-type region called emitter. If there was no right R-n -transition, in the emitter - base circuit there would be a current depending on the voltage of the sources (batteries B1 and an AC voltage source) and circuit resistance, including the low resistance of the direct emitter-base junction.
Battery B2 turned on so that the right R-n -transition in the circuit (see fig.) is reverse. It separates the base from the right p-type region called collector. If there was no left R-n -junction, the current in the collector circuit would be close to zero, since
reversal resistance is very high. In the presence of a current in the left R-n -junction current also appears in the collector circuit, and the current in the collector is only slightly less than the current in the emitter (if a negative voltage is applied to the emitter, then the left R-n -transition will be reversed and the current in the emitter circuit and in the collector circuit will be practically absent). 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 minor carriers. Since the thickness of the base is very small and the number of majority carriers (electrons) in it is small, the holes that have fallen into it hardly combine (do not recombine) with base electrons and penetrate into the collector due to diffusion. Right R-n -transition is closed for the main charge carriers of the base - electrons, but not for holes. In the collector holes are carried away electric field and close the circuit. The strength of the current branching into the emitter circuit from the base is very small, since the base cross-sectional area in the horizontal (see Fig. above) plane is much smaller than the cross-sectional area 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 resistance R has little effect on the current in the collector, and this resistance can be made sufficiently large. By controlling the emitter current using an AC voltage source included in its circuit, we get a synchronous change in the voltage across the resistor R .
With a large resistance of the resistor, the change in voltage across it can be tens of thousands of times greater than the change in the signal voltage in the emitter circuit. This means increased voltage. Therefore, on the load R it is possible to obtain electrical signals whose power is many times greater than the power entering the emitter circuit.
Application of transistors Properties R-n-junctions in semiconductors are used to amplify and generate electrical oscillations.
Semiconductors occupy an intermediate place in electrical conductivity between conductors and non-conductors of electric current. The group of semiconductors includes many more substances than the groups of conductors and non-conductors taken together. The most characteristic representatives of semiconductors that have found practical use in technology, are germanium, silicon, selenium, tellurium, arsenic, cuprous oxide and a huge number of alloys and chemical compounds. Almost all inorganic substances of the world around us are semiconductors. The most common semiconductor in nature is silicon, which makes up about 30% of the earth's crust.
The qualitative difference between semiconductors and metals is manifested primarily in the dependence of resistivity on temperature. With decreasing temperature, the resistance of metals decreases. In semiconductors, on the contrary, as the temperature decreases, the resistance increases and near absolute zero they practically become insulators.
In semiconductors, the concentration of free charge carriers increases with increasing temperature. The mechanism of electric current in semiconductors cannot be explained within the free electron gas model.
Germanium atoms have four weakly bound electrons per outer shell.
They are called valence electrons. In a crystal lattice, each atom is surrounded by four nearest neighbors. The bond between atoms in a germanium crystal is covalent, that is, it is carried out by pairs of valence electrons. Each valence electron belongs to two atoms. The valence electrons in a germanium crystal are much more strongly bonded to atoms than in metals; therefore, the concentration of conduction electrons at room temperature in semiconductors is many orders of magnitude lower than in metals. Near absolute zero temperature in a germanium crystal, all electrons are engaged in the formation of bonds. Such a crystal does not conduct electricity.
As the temperature rises, some of the valence electrons can gain enough energy to break covalent bonds. Then free electrons (conduction electrons) will appear in the crystal. At the same time, vacancies that are not occupied by electrons are formed at the sites of bond breaking. These vacancies are called "holes".
At a given semiconductor temperature, a certain number of electron-hole pairs are formed per unit time. At the same time, the reverse process is going on - when a free electron meets a hole, the electronic bond between germanium atoms is restored. This process is called recombination. Electron-hole pairs can also be produced when a semiconductor is illuminated due to the energy of electromagnetic radiation.
If a semiconductor is placed in an electric field, then not only free electrons are involved in the ordered movement, but also holes, which behave like positively charged particles. Therefore, the current I in a semiconductor is the sum of the electronic I n and hole I p currents: I = I n + I p.
The concentration of conduction electrons in a semiconductor is equal to the concentration of holes: n n = n p . The electron-hole mechanism of conduction manifests itself only in pure (i.e., without impurities) semiconductors. It is called intrinsic electrical conductivity of semiconductors.
In the presence of impurities, the electrical conductivity of semiconductors changes greatly. For example, adding impurities phosphorus into crystal silicon in the amount of 0.001 atomic percent reduces the resistivity by more than five orders of magnitude.
A semiconductor in which an impurity is introduced (i.e., part of the atoms of one type is replaced by atoms of another type) is called doped or doped.
There are two types of impurity conduction, electron and hole conduction.
Thus, when doping a four-valent germanium (Ge) or silicon (Si) pentavalent - phosphorus (P), antimony (Sb), arsenic (As) an extra free electron appears at the location of the impurity atom. In this case, the impurity is called donor .
When doping four valent germanium (Ge) or silicon (Si) trivalent - aluminum (Al), indium (Jn), boron (B), gallium (Ga)
- there is a line hole. Such impurities are called acceptor
.
In the same sample of a semiconductor material, one section may have p-conductivity, and the other n-conductivity. Such a device is called a semiconductor diode.
The prefix "di" in the word "diode" means "two", it indicates that the device has two main "details", two semiconductor crystals closely adjacent to each other: one with p-conductivity (this is the zone R), the other - with n - conductivity (this is the zone P). In fact, a semiconductor diode is one crystal, in one part of which a donor impurity is introduced (zone P), into another - acceptor (zone R).
If a constant voltage is applied from the battery to the diode "plus" to the zone R and "minus" to the zone P, then free charges - electrons and holes - will rush to the boundary, rush to the pn junction. Here they will neutralize each other, new charges will approach the boundary, and a D.C.. This is the so-called direct connection of the diode - the charges move intensively through it, a relatively large forward current flows in the circuit.
Now we will change the polarity of the voltage on the diode, we will carry out, as they say, its reverse inclusion - we will connect the “plus” of the battery to the zone P,"minus" - to the zone R. Free charges will be pulled away from the boundary, electrons will go to the "plus", holes - to the "minus" and as a result, the pn - junction will turn into a zone without free charges, into a pure insulator. This means that the circuit will break, the current in it will stop.
Not a large reverse current through the diode will still go. Because, in addition to the main free charges (charge carriers) - electrons, in the zone P, and holes in the p zone - in each of the zones there is also an insignificant amount of charges of the opposite sign. These are their own minority charge carriers, they exist in any semiconductor, appear in it due to the thermal movements of atoms, and it is they who create the reverse current through the diode. There are relatively few of these charges, and the reverse current is many times less than the direct one. The magnitude of the reverse current is highly dependent on: temperature environment, semiconductor material and area pn transition. With an increase in the transition area, its volume increases, and, consequently, the number of minority carriers appearing as a result of thermal generation and the thermal current increase. Often CVC, for clarity, is presented in the form of graphs.
Yeryutkin Evgeny Sergeevich
physics teacher of the highest qualification category, secondary school №1360, Moscow
If you make a direct connection, then external field neutralizes the blocking, and the current will be made by the main charge carriers.
Rice. 9. p-n junction with direct connection ()
In this case, the current of minority carriers is negligible, it is practically non-existent. Therefore, the p-n junction provides one-way conduction of electric current.
Rice. 10. Atomic structure of silicon with increasing temperature
The conduction of semiconductors is electron-hole, and such conduction is called intrinsic conduction. And unlike conductive metals, as the temperature increases, the number of free charges just increases (in the first case, it does not change), so the conductivity of semiconductors increases with increasing temperature, and the resistance decreases
A very important issue in the study of semiconductors is the presence of impurities in them. And in the case of the presence of impurities, one should speak of impurity conductivity.
The small size and very high quality of transmitted signals have made semiconductor devices very common in modern electronic technology. The composition of such devices may include not only the aforementioned silicon with impurities, but also, for example, germanium.
One of these devices is a diode - a device that can pass current in one direction and prevent it from passing in the other. It is obtained by implanting another type of semiconductor into a p- or n-type semiconductor crystal.
Rice. 11. The designation of the diode on the diagram and the diagram of its device, respectively
Another device, now with two pn junctions called a transistor. It serves not only to select the direction of current flow, but also to convert it.
Rice. 12. Scheme of the structure of the transistor and its designation on wiring diagram respectively ()
It should be noted that modern microcircuits use many combinations of diodes, transistors and other electrical devices.
In the next lesson, we will look at the propagation of electric current in a vacuum.
- Tikhomirova S.A., Yavorsky B.M. Physics ( a basic level of) M.: Mnemosyne. 2012
- Gendenstein L.E., Dick Yu.I. Physics grade 10. M.: Ileksa. 2005
- Myakishev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics M.: 2010
- Principles of operation of devices ().
- Encyclopedia of Physics and Technology ().
- What causes conduction electrons in a semiconductor?
- What is intrinsic conductivity of a semiconductor?
- How does the conductivity of a semiconductor depend on temperature?
- What is the difference between a donor impurity and an acceptor impurity?
- * What is the conductivity of silicon with an admixture of a) gallium, b) indium, c) phosphorus, d) antimony?
According to the value of electrical resistivity semiconductors occupy intermediate place between conductors and dielectrics. Many semiconductors are chemical elements(germanium, silicon, selenium, tellurium, arsenic, etc.), a huge number of alloys and chemical compounds.
The qualitative difference between semiconductors and metals is manifested primarily in the dependence of resistivity on temperature. With decreasing temperature, the resistance of metals decreases. In semiconductors, on the contrary, as the temperature decreases, the resistance increases and near absolute zero they practically become insulators.
Resistivity ρ of a pure semiconductor as a function of absolute temperature T.
Semiconductorsare called substances whose resistivity decreases with increasing temperature.
Such a behavior of the dependence ρ(T) shows that the concentration of free charge carriers in semiconductors does not remain constant, but increases with increasing temperature. The mechanism of electric current in semiconductors cannot be explained within the free electron gas model. An explanation of the phenomena observed in conductors is possible on the basis of the laws of quantum mechanics. Let us consider qualitatively the mechanism of electric current in semiconductors using germanium (Ge) as an example.
Germanium atoms have four loosely bound electrons in their outer shell. They are called valence electrons. In a crystal lattice, each atom is surrounded by four nearest neighbors. The bond between atoms in a germanium crystal is covalent, that is, carried out by pairs of valence electrons. Each valence electron belongs to two atoms.
The valence electrons in a germanium crystal are much more strongly bonded to atoms than in metals; therefore, the concentration of conduction electrons at room temperature in semiconductors is many orders of magnitude lower than in metals. Near absolute zero temperature in a germanium crystal, all electrons are engaged in the formation of bonds. Such a crystal does not conduct electricity. As the temperature rises, some of the valence electrons can gain enough energy to break covalent bonds. Then the crystal will havefree electrons(conduction electrons). At the same time, vacancies that are not occupied by electrons are formed at the sites of bond breaking.
Vacancies that are not occupied by electrons are called holes.
A vacant place can be occupied by a valence electron from a neighboring pair, then the hole will move to a new place in the crystal. At a given semiconductor temperature, a certain amount of electron-hole pairs.
At the same time, the reverse process is going on - when a free electron meets a hole, the electronic bond between germanium atoms is restored. This process is called recombination.
Recombination -restoration of the electronic bond between atoms.
Electron-hole pairs can also be produced when a semiconductor is illuminated due to the energy of electromagnetic radiation.
In the absence of an electric field, conduction electrons and holes participate in chaotic thermal motion.
If a semiconductor is placed in an electric field, then not only free electrons are involved in the ordered movement, but also holes, which behave like positively charged particles. Therefore, the current I in a semiconductor is made up of an electronic I n and hole Ip currents: I = I n + Ip
Electric current in semiconductorscalled the directed movement of electrons to the positive pole, and holes to the negative.
The concentration of conduction electrons in a semiconductor is equal to the concentration of holes: n n = np. The electron-hole mechanism of conduction is manifested only in pure (that is, without impurities) semiconductors. It is called own electrical conductivity semiconductors.
Own electrical conductivity semiconductors is called the electron-hole conductivity mechanism, which manifests itself only in pure (that is, without impurities) semiconductors.
In the presence of impurities, the electrical conductivity of semiconductors changes greatly.
impurity conductivitycalled the conductivity of semiconductors in the presence of impurities.
A necessary condition for a sharp decrease in the resistivity of a semiconductor upon the introduction of impurities is the difference in the valence of the impurity atoms from the valence of the main atoms of the crystal.
There are two types of impurity conduction - electronic and hole conductivity.
- Electronic conductivity occurs when a semiconductor crystal is injected an admixture with a higher valency.
For example, pentavalent arsenic atoms, As, are introduced into a germanium crystal with tetravalent atoms.
The figure shows a pentavalent arsenic atom in a lattice site of germanium. The four valence electrons of the arsenic atom are involved in the formation of covalent bonds with four neighboring germanium atoms. The fifth valence electron turned out to be superfluous; it easily detaches from the arsenic atom and becomes free. An atom that has lost an electron turns into a positive ion located at a site in the crystal lattice.
Donor impurity- is called an impurity of atoms with a valency exceeding the valency of the main atoms of a semiconductor crystal.
As a result of its introduction, a significant number of free electrons appear in the crystal. This leads to a sharp decrease in the resistivity of the semiconductor - by thousands and even millions of times. The resistivity of a conductor with a high content of impurities can approach the resistivity of a metallic conductor.
In a germanium crystal with an arsenic impurity, there are electrons and holes responsible for the intrinsic conductivity of the crystal. But the main type of free charge carriers are electrons detached from arsenic atoms. In such a crystal n n >> np.
Conductivity in which the majority free charge carriers are electrons is called electronic.
A semiconductor that exhibits electronic conductivity is called n-type semiconductor.
- hole conduction occurs when an impurity with lower valency.
For example, trivalent In atoms are introduced into a germanium crystal.
The figure shows an indium atom that has created covalent bonds with only three neighboring germanium atoms using its valence electrons. The indium atom does not have an electron to form a bond with the fourth germanium atom. This missing electron can be captured by an indium atom from a covalent bond of neighboring germanium atoms. In this case, the indium atom turns into a negative ion located at a site of the crystal lattice, and a vacancy is formed in the covalent bond of neighboring atoms.
Acceptor impurity -called pAn admixture of atoms with a valence less than the valency of the main atoms of a semiconductor crystal capable of capturing electrons.
As a result of the introduction of an acceptor impurity in the crystal, many covalent bonds are broken and vacant sites (holes) are formed. Electrons can jump to these places from neighboring covalent bonds, which leads to random wandering of holes around the crystal.
The presence of an acceptor impurity sharply reduces the resistivity of the semiconductor due to the appearance of a large number of free holes. The concentration of holes in a semiconductor with an acceptor impurity significantly exceeds the concentration of electrons that arose due to the mechanism of intrinsic electrical conductivity of the semiconductor: np >> n n.
Conductivity in which holes are the majority carriers of a free charge is called hole conductivity.
A semiconductor with hole conductivity is called p-type semiconductor.
It should be emphasized that hole conductivity is actually due to the movement of electrons through vacancies from one germanium atom to another, which carry out a covalent bond.
The dependence of the electrical conductivity of semiconductors on temperature and illumination
- For semiconductors with increasing temperature the mobility of electrons and holes decreases, but this does not play a significant role, since when the semiconductor is heated, the kinetic the energy of valence electrons increases and individual bonds break, which leads to an increase in the number of free electrons, i.e., an increase in electrical conductivity.
- When illuminated semiconductor, additional carriers appear in it, whichleads to an increase in its electrical conductivity.This occurs as a result of the fact that light pulls electrons out of the atom and at the same time the number of electrons and holes increases.
Lesson No. 41-169 Electric current in semiconductors. semiconductor diode. Semiconductor devices.
A semiconductor is a substance whose resistivity can vary over a wide range and decreases very quickly with increasing temperature, which means that the electrical conductivity increases. It is observed in silicon, germanium, selenium and in some compounds. Conduction mechanism in semiconductors
Semiconductor crystals have an atomic crystal lattice, where outer electrons are bound to neighboring atoms by covalent bonds. At low temperatures, pure semiconductors have no free electrons and it behaves like a dielectric. If the semiconductor is pure (without impurities), then it has its own conductivity (small). There are two types of intrinsic conductivity: 1) electronic (conductivity " P"-type) At low temperatures in semiconductors, all electrons are associated with nuclei and the resistance is large; As the temperature increases, the kinetic energy of the particles increases, bonds break and free electrons appear - the resistance decreases. Free electrons move opposite to the electric field strength vector. The electronic conductivity of semiconductors is due to the presence free electrons. 2) hole (conductivity "p"-type). As the temperature increases, the covalent bonds carried out by valence electrons between atoms are destroyed and places with a missing electron are formed - a "hole". its place can be replaced by valence electrons.The movement of the "hole" is equivalent to the movement of a positive charge.The movement of the hole occurs in the direction of the electric field strength vector.The rupture of covalent bonds and the occurrence of intrinsic conductivity of semiconductors can be caused by heating, lighting m (photoconductivity) and the action of strong electric fields. 
R(t) dependence: thermistor
- remote measurement t; - fire alarm
1) donor impurities (donating) - are additional suppliers of electrons to semiconductor crystals, easily donate electrons and increase the number of free electrons in the semiconductor. These are the conductors n "- type, i.e. semiconductors with donor impurities, where the main charge carrier is electrons, and the minor charge is holes. Such a semiconductor has electronic impurity conductivity (an example is arsenic). 2
) acceptor impurities (receiving) create "holes", taking electrons into themselves. These are "p"-type semiconductors, i.e. semiconductors with acceptor impurities, where the main charge carrier is holes, and the minority - electrons. Such a semiconductor has hole impurity conductivity (an example is indium). Electrical properties "p-
n" transitions."p-n" transition (or electron-hole transition) - the contact area of two semiconductors, where the conductivity changes from electronic to hole (or vice versa). AT 
It is possible to create such regions in a semiconductor crystal by introducing impurities. In the contact zone of two semiconductors with different conductivities, mutual diffusion of electrons and holes will take place and a blocking barrier will form. electrical layer. The electric field of the barrier layer preventsfurther transition of electrons and holes through the boundary. The barrier layer has an increased resistance compared to other areas of the semiconductor. AT 
The external electric field affects the resistance of the barrier layer. In the direct (transmission) direction of the external electric field, the current passes through the boundary of two semiconductors. Because electrons and holes move towards each other to the interface, then the electrons, crossing the border, fill the holes. The thickness of the barrier layer and its resistance are continuously decreasing.P 
With a blocking (reverse direction of the external electric field), the current will not pass through the contact area of the two semiconductors. Because electrons and holes move from the boundary in opposite directions, then the blocking layer thickens, its resistance increases. Thus, the electron-hole transition has one-sided conduction.
semiconductor diode- a semiconductor with one "rn" junction.P 
Semiconductor diodes are the main elements of AC rectifiers.
When an electric field is applied: in one direction, the resistance of the semiconductor is high, in the opposite direction, the resistance is low.
Transistors.(from the English words transfer - transfer, resistor - resistance) Consider one of the types of transistors made of germanium or silicon with donor and acceptor impurities introduced into them. The distribution of impurities is such that a very thin (on the order of a few micrometers) n-type semiconductor layer is created between two p-type semiconductor layers (see Fig.). 
This thin layer is called basis or base. The crystal has two R-n-junctions, the direct directions of which are opposite. Three outputs from areas with different types of conductivity allow you to include a transistor in the circuit shown in the figure. With this inclusion, the left R-n-jump is direct and separates the base from a p-type region called emitter. If there was no right R-n-junction, in the emitter-base circuit there would be a current depending on the voltage of the sources (batteries B1 and an AC voltage source) and circuit resistance, including the low resistance of the direct emitter-base junction. Battery B2 turned on so that the right R-n-junction in the circuit (see fig.) is reverse. It separates the base from the right p-type region called collector. If there was no left R-n-junction, the current in the collector circuit would be close to zero, since the reverse junction resistance is very high. In the presence of a current in the left R-n-junction current also appears in the collector circuit, and the current in the collector is only slightly less than the current in the emitter (if a negative voltage is applied to the emitter, then the left R-n-junction will be reversed and there will be practically no current in the emitter circuit and in the collector circuit). 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 minor carriers. Since the thickness of the base is very small and the number of majority carriers (electrons) in it is small, the holes that have fallen into it hardly combine (do not recombine) with base electrons and penetrate into the collector due to diffusion. Right R The -n-junction is closed for the main charge carriers of the base - electrons, but not for holes. In the collector, the holes are carried away by the electric field and close the circuit. The strength of the current branching into the emitter circuit from the base is very small, since the base cross-sectional area in the horizontal (see Fig. above) plane is much smaller than the cross-sectional area 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 resistance R has little effect on the current in the collector, and this resistance can be made sufficiently large. By controlling the emitter current using an AC voltage source included in its circuit, we get a synchronous change in the voltage across the resistor R .
With a large resistance of the resistor, the change in voltage across it can be tens of thousands of times greater than the change in the signal voltage in the emitter circuit. This means increased voltage. Therefore, on the load R it is possible to obtain electrical signals whose power is many times greater than the power entering the emitter circuit.
Application of transistors Properties R-n-junctions in semiconductors are used to amplify and generate electrical oscillations.
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