Semiconductors are materials that are dielectrics under normal conditions, but become conductors with increasing temperature. That is, in semiconductors with increasing temperature, the resistance decreases.
Semiconductor structure on the example of a silicon crystal
Consider the structure of semiconductors and the main types of conductivity in them. Consider a silicon crystal as an example.
Silicon is a tetravalent element. Consequently, in its outer shell there are four electrons that are weakly bound to the nucleus of the atom. Each of them has four more atoms in the neighborhood.
Atoms interact with each other and form covalent bonds. One electron from each atom participates in such a bond. The silicon device schematic is shown in the following figure.
picture
Covalent bonds are strong enough and do not break at low temperatures. Therefore, silicon has no free charge carriers, and at low temperatures it is an insulator. There are two types of conductivity in semiconductors: electronic and hole.
Electronic conduction
When silicon is heated, additional energy will be imparted to it. The kinetic energy of the particles increases and some covalent bonds are broken. This creates free electrons.
In an electric field, these electrons move between the sites of the crystal lattice. This will create an electric current in the silicon.
Since the main charge carriers are free electrons, this type of conductivity is called electronic conductivity. The number of free electrons depends on the temperature. The more we heat silicon, the more covalent bonds will break, and therefore, more free electrons will appear. This leads to a decrease in resistance. And silicon becomes a conductor.
Hole conductivity
When the covalent bond breaks, a vacant place is formed in the place of the escaped electron, which can be occupied by another electron. This place is called the hole. There is an excess positive charge in the hole.
The position of the hole in the crystal is constantly changing, any electron can take this position, and the hole will move to where the electron jumped from. If electric field no, then the movement of holes is disorderly, and therefore no current arises.
If it is present, an orderliness of the movement of holes arises, and in addition to the current that is created by free electrons, there is also a current that is created by the holes. The holes will move in the opposite direction to the direction of the electrons.
Thus, in semiconductors, the conductivity is electron-hole. The current is created both with the help of electrons and with the help of holes. This type of conductivity is also called intrinsic conductivity, since the elements of only one atom are involved.
Semiconductor is a substance in which the resistivity can vary over a wide range and decreases very quickly with increasing temperature, which means that the electrical conductivity (1 / R) increases.
- observed in silicon, germanium, selenium and some compounds.
Conduction mechanism in semiconductors
Semiconductor crystals have an atomic crystal lattice, where the outer electrons are bonded to neighboring atoms by covalent bonds.
At low temperatures, pure semiconductors have no free electrons and it behaves like a dielectric.
Pure semiconductors (no impurities)
If the semiconductor is pure (no impurities), then it has own conductivity, which is small.
Intrinsic conductivity is of two types:
1 electronic(conductivity "n" - type)
At low temperatures in semiconductors, all electrons are bound to nuclei and the resistance is large; as the temperature rises, the kinetic energy of the particles increases, bonds break down 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 of free electrons.
2. hole(conductivity "p" - type)
As the temperature rises, the covalent bonds between the atoms are destroyed, carried out by the valence electrons, and places with the missing electron are formed - a "hole".
She 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.
In addition to heating, the breaking of covalent bonds and the onset of intrinsic conductivity of semiconductors can be caused by illumination (photoconductivity) and the action of strong electric fields.
The total conductivity of a pure semiconductor is the sum of the "p" and "n" -types
and is called electron-hole conductivity.
Semiconductors in the presence of impurities
They have intrinsic + impurity conductivity
The presence of impurities greatly increases the conductivity.
With a change in the concentration of impurities, the number of carriers of electric current - electrons and holes - changes.
The ability to control current is at the heart of the widespread use of semiconductors.
Exists:
1)donor impurities (giving off)
They are additional suppliers of electrons to semiconductor crystals, easily donate electrons and increase the number of free electrons in the semiconductor.
These are the guides "n" - type, i.e. semiconductors with donor impurities, where the main charge carrier is electrons, and the minor one is holes.
Such a semiconductor has electronic impurity conductivity.
For example - arsenic.
2. acceptor impurities (host)
They create "holes" by taking in electrons.
These are semiconductors "p" - like, those. semiconductors with acceptor impurities, where the main charge carrier is holes, and the minor one is electrons.
Such a semiconductor possesses impurity hole conductivity.
For example - indium.
Electrical properties of the "p-n" junction
"p-n" junction(or electron-hole junction) - the region of contact between two semiconductors, where the conductivity changes from electron 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 will take place. electrons and holes and a blocking electrical layer is formed. The electric field of the blocking layer prevents further transition of electrons and holes across the boundary. The blocking layer has increased resistance compared to other areas of the semiconductor.
An external electric field affects the resistance of the barrier layer. With the forward (throughput) direction of the external electric field, the electric current passes through the border 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.
Pn transition throughput mode:
With the blocking (reverse) direction of the external electric field, the electric 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.
Locking pn transition mode.
Lesson number 41-169 Electric current in semiconductors. Semiconductor diode. Semiconductor devices.
A semiconductor is a substance in which the 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 some compounds. Conduction mechanism in semiconductors
Semiconductor crystals have an atomic crystal lattice, where the outer electrons are bonded 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 (no impurities), then it has its own conductivity (low). Intrinsic conductivity is of two types: 1) electronic (conductivity " P"-type) At low temperatures in semiconductors, all electrons are bound to the nuclei and the resistance is large; As the temperature increases, the kinetic energy of particles increases, bonds break and free electrons appear - the resistance decreases. Free electrons move opposite to the vector of the electric field strength. The electronic conductivity of semiconductors is due to the presence free electrons 2) hole ("p" -type conductivity) When the temperature rises, the covalent bonds between the atoms are destroyed, and places with the 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 vector of the electric field strength. The breaking of covalent bonds and the appearance of intrinsic conductivity of semiconductors can be caused by heating, illumination m (photoconductivity) and the action of strong electric fields. 
R (t) dependence: thermistor
- remote measurement of t; - fire alarm
1) donor impurities (giving off) - are additional suppliers of electrons to semiconductor crystals, easily donate electrons and increase the number of free electrons in the semiconductor. These are the guides " n "- type, ie semiconductors with donor impurities, where the main charge carrier is electrons, and the minor one is holes. Such a semiconductor has electronic impurity conductivity (for example, arsenic). 2
) acceptor impurities (receiving) create "holes", taking in electrons. These are "p" - type semiconductors, i.e. semiconductors with acceptor impurities, where the main charge carrier is holes, and the minor one is electrons. Such a semiconductor has hole impurity conductivity (for example, indium). Electrical properties "p-
n"transitions."pn" junction (or electron-hole junction) is the region of contact between two semiconductors, where the conductivity changes from electron to hole (or vice versa). V 
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 electrical layer. The electric field of the barrier layer preventsfurther transition of electrons and holes across the boundary. The blocking layer has increased resistance compared to other areas of the semiconductor. V 
The external electric field influences the resistance of the barrier layer. With the forward (throughput) direction of the external electric field, the current passes through the interface of two semiconductors. Because electrons and holes move towards each other to the interface, then electrons, crossing the border, fill the holes. The thickness of the barrier layer and its resistance are continuously decreasing.P 
With a blocking (opposite direction of the external electric field) current will not pass through the contact area of 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 junction has one-sided conductivity.
Semiconductor diode- a semiconductor with one "pn" 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 small.
Transistors.(from 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 several micrometers) layer of an n-type semiconductor is created between two layers of a p-type semiconductor (see Fig.). 
This thin layer is called basis or base. Two R-n-jumps, the direct directions of which are opposite. Three leads from regions with different types of conductivity allow the transistor to be included in the circuit shown in the figure. With this switch on, the left R-n-jump is direct and separates the base from the p-type region called emitter. If there was no right R-n-junction, in the emitter-base circuit there would be a current that depends on the voltage of the sources (batteries B1 and an alternating voltage source) and the resistance of the circuit, including the low resistance of the direct junction of the emitter - base. 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 the resistance of the reverse junction is very high. If there is a current in the left R-n-junction, a current 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 minority carriers. Since the thickness of the base is very small and the number of majority carriers (electrons) in it is small, the holes trapped in it hardly combine (do not recombine) with the electrons of the base 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 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 figure above) plane is much less than the cross-section in the vertical plane.
The current in the collector, almost equal to the current in the emitter, changes with the current in the emitter. Resistor R has little effect on the collector current, and this resistance can be made large enough. By controlling the emitter current with an AC voltage source included in its circuit, we get a synchronous change in 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 signal voltage in the emitter circuit. This means voltage amplification. Therefore, at the load R you can receive electrical signals, the power of which is many times greater than the power supplied to the emitter circuit.
Application of transistors Properties R-n-junction in semiconductors are used to amplify and generate electrical oscillations.
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Electric current in semiconductors The purpose of the lesson: to form an idea of free carriers of electric charge in semiconductors and the nature of electric current in semiconductors. Lesson type: lesson in learning new material. LESSON PLAN Knowledge control 5 min. 1. Electric current in metals. 2. Electric current in electrolytes. 3. Faraday's law for electrolysis. 4. Electric current in gases Demonstrations 5 min. Fragments of the video "Electric current in semiconductors" Study of new material 28 min. 1. Charge carriers in semiconductors. 2. Impurity conductivity of semiconductors. 3. Electron-hole transition. 4. Semiconductor diodes and transistors. 5. Integrated microcircuits Securing the studied material 7 min. 1. Qualitative questions. 2. Learning to solve problems STUDYING NEW MATERIAL 1. Carrying charges in semiconductors Resistivity of semiconductors at room temperature have values that are in a wide range, ie. from 10-3 to 107 Ohm · m, and occupy an intermediate position between metals and dielectrics. Semiconductors are substances whose resistivity decreases very quickly with increasing temperature. Semiconductors include many chemical elements (boron, silicon, germanium, phosphorus, arsenic, selenium, tellurium, etc.), a huge amount of minerals, alloys and chemical compounds. Almost all inorganic substances of the surrounding world are semiconductors. For sufficiently low temperatures and the absence of external influences of lighting or heating), semiconductors do not conduct electric current: under these conditions, all electrons in semiconductors are bound. However, the bond between electrons and their atoms in semiconductors is not as strong as in dielectrics. And in the case of an increase in temperature, as well as in bright light, some electrons are detached from their atoms and become free charges, that is, they can move throughout the sample. Due to this, negative charge carriers appear in semiconductors - free electrons. electrons are called electronic. When an electron is detached from an atom, the positive charge of this atom becomes uncompensated, i.e. an extra positive appears in this place. This positive charge is called a "hole". An atom near which a hole has formed can take away a bound electron from a neighboring atom, and the hole will move to the neighboring atom, and that atom, in turn, can “transfer” the hole further. Such "estafetine" movement of bound electrons can be considered as the movement of holes, that is, positive charges. The conductivity of a semiconductor due to motion (for example, charge. The conductivity of a semiconductor due to the movement of holes is called hole conductivity. pure semiconductor (no impurities), electric current creates the same number of free electrons and holes. This conductivity is called the intrinsic conductivity of semiconductors. silicon lattice, but in some lattice sites instead of silicon atoms there will be atoms of arsenic. Arsenic, as you know, is a pentavalent element. Chotirivalent electrons form paired electron bonds with neighboring silicon atoms. There will not be enough bonding for this electron, and it will be so weakly bonded to the Arsenic atom that it easily becomes free. As a result, each impurity atom will give one free electron. Impurities whose atoms easily donate electrons are called donor impurities. Electrons from silicon atoms can become free, forming a hole, therefore, impurities that "capture" the electrons of atoms can simultaneously exist in the crystal, and free electrons and holes are also called. However, there will be many times more free electrons than holes. Semiconductors in which electrons are the main charge carriers are called n-type semiconductors. If a small amount of trivalent indium is added to silicon, then the nature of the semiconductor's conductivity will change. Since indium has three valence electrons, it can only establish a covalent bond with three neighboring atoms. An electron is not enough to establish a bond with the fourth atom. Indium will "lend" an electron to neighboring atoms, as a result, each atom of India forms one vacant place - a hole. crystal lattice of semiconductors, acceptor. In the case of an acceptor impurity by the majority charge carriers, there are holes during the passage of an electric current through a semiconductor. Semiconductors in which holes are the main charge carriers are called p-type semiconductors. Almost all semiconductors contain both donor and acceptor impurities. The type of conductivity of a semiconductor is determined by an impurity with a higher concentration of charge carriers - electrons and holes. 3. Electron-hole junction Among the physical properties inherent in semiconductors, the properties of contacts (pn junction) between semiconductors with different types of conductivity are most widely used. In an n-type semiconductor, electrons participate in thermal motion and diffuse across the interface into a p-type semiconductor, where their concentration is much lower. Likewise, holes will diffuse from a p-type semiconductor into an n-type semiconductor. This happens in the same way as the atoms of a solute diffuse from a strong solution to a weak solution in the event of their collision. As a result of diffusion, the near-contact area is depleted in the majority of charge carriers: in an n-type semiconductor, the concentration of electrons decreases, and in a p-type semiconductor, the concentration of holes. Therefore, the resistance of the near-contact area is very significant. The diffusion of electrons and holes through the pn junction leads to the fact that the n-type semiconductor, from which the electrons come, is charged positively, and the p-type - negatively. An electric double layer is formed, which creates an electric field that prevents further diffusion of free current carriers through the semiconductor contact. For some voltage between the double charged layer, further impoverishment of the near-contact area by the main carriers stops. If now the semiconductor is connected to the current source so that its electronic region is connected to the negative pole of the source, and the hole region to the positive pole, then the electric field created by the current source will be directed so that it moves the main current carriers in each section of the semiconductor with p- n-junction. Upon contact, the section will be enriched by the main current carriers, and its resistance will decrease. A noticeable current will flow through the contact. The direction of the current in this case is called throughput, or direct. If you connect an n-type semiconductor to the positive, and p-type to the negative pole of the source, then the near-contact section expands. The area's resistance is greatly increased. The current through the transition layer will be very small. This direction of the current is called closing, or reverse. 4. Semiconductor diodes and transistors Therefore, through the interface between n-type and p-type semiconductors, the electric current flows in only one direction - from the p-type semiconductor to the n-type semiconductor. This is used in devices called diodes. Semiconductor diodes are used to rectify alternating current (this is called alternating current), as well as for the manufacture of LEDs. Semiconductor rectifiers have high reliability and long service life. devices: Semiconductor diodes are widely used in radio-technical radios, video recorders, televisions, computers. An even more important application of semiconductors has become the transistor. It consists of three layers of semiconductors: along the edges are semiconductors of one type, and between them - a thin layer of semiconductor of another type. The widespread use of transistors is due to the fact that they can be used to amplify electrical signals. Therefore, the transistor has become the main element of many semiconductor devices. 5. Integrated Circuits Semiconductor diodes and transistors are the building blocks of very complex devices called integrated circuits. Microcircuits work today in computers and televisions, mobile phones and artificial satellites, in cars, airplanes, and even in washing machines. The integrated circuit is fabricated on a silicon wafer. The size of the plate is from a millimeter to a centimeter, and one such plate can accommodate up to a million components - tiny diodes, transistors, resistors, etc. Important advantages of integrated circuits are high speed and reliability, as well as low cost. It is thanks to this that, on the basis of integrated circuits, it was possible to create complex, but many devices, computers and modern household appliances are available. QUESTION TO STUDENTS DURING THE PRESENTATION OF THE NEW MATERIAL First level 1. What substances can be classified as semiconductor? 2. The movement of which charged particles creates a current in semiconductors? 3. Why is the resistance of semiconductors very much dependent on the presence of impurities? 4. How is the p-n-junction formed? What property does a p-n-junction have? 5. Why can't free charge carriers pass through the p-n-junction of a semiconductor? Second level 1. After the introduction of arsenic impurities into germanium, the concentration of conduction electrons increased. How did the hole concentration change in this case? 2. With the help of what experience can you be convinced of the one-sided conductivity of a semiconductor diode? 3. Is it possible to obtain a pn junction by fusing tin into germanium or silicon? ATTACHMENT OF THE STUDIED MATERIAL 1). Qualitative questions 1. Why are the requirements for the purity of semiconductor materials very high (in some cases, the presence of even one impurity atom per million atoms is not allowed)? 2. After the introduction of arsenic impurities into germanium, the concentration of conduction electrons increased. How did the hole concentration change in this case? 3. What happens in the contact of two semiconductors of n- and p-type? 4. A semiconductor diode and a rheostat are in a closed box. The end of the devices are brought out and connected to the terminals. How to determine which terminals belong to a diode? 2). Learning to solve problems 1. What conductivity (electron or hole) does silicon have with an impurity of gallium? india? phosphorus? antimony? 2. What conductivity (electron or hole) will be in silicon if phosphorus is added to it? boron? aluminum? arsenic? 3. How will the resistance of a silicon sample with an impurity of phosphorus change if an impurity of gallium is introduced into it? The concentration of Phosphorus and Gallium atoms is the same. (Answer: will increase) WHAT WE LEARNED IN THE LESSON · Semiconductors are substances whose resistivity decreases very quickly with increasing temperature. · The conductivity of a semiconductor, due to the movement of electrons, is called electronic. · The conductivity of a semiconductor due to the movement of holes is called hole conductivity. · Impurities whose atoms easily donate electrons are called donor impurities. · Semiconductors in which electrons are the main charge carriers are called n-type semiconductors. · Impurities that "capture" the electrons of the atoms of the crystal lattice of semiconductors are called acceptor. · Semiconductors in which holes are the main charge carriers are called p-type semiconductors. · The contact of two semiconductors with different types of conductivity has the ability to conduct current well in one direction and much worse in the opposite direction, i.e. has one-sided conductivity. Homework 1. §§ 11, 12.
>> Physics: Electric current in semiconductors
What is the main difference between semiconductors and conductors? What structural features of semiconductors gave them access to all radio devices, televisions and computers?
The difference between conductors and semiconductors is especially evident when analyzing the dependence of their electrical conductivity on temperature. Studies show that for a number of elements (silicon, germanium, selenium, etc.) and compounds (PbS, CdS, GaAs, etc.), the resistivity does not increase with increasing temperature, as in metals ( fig.16.3), but, on the contrary, decreases extremely sharply ( fig. 16.4). Such substances are called semiconductors.
From the graph shown in the figure, it can be seen that at temperatures close to absolute zero, the resistivity of semiconductors is very high. This means that at low temperatures, the semiconductor behaves like an insulator. As the temperature rises, its resistivity decreases rapidly.
Semiconductor structure... In order to turn on the transistor receiver, you do not need to know anything. But in order to create it, one had to know a lot and have an extraordinary talent. Understanding in general terms how a transistor works is not so difficult. First, you need to get acquainted with the mechanism of conduction in semiconductors. And for this you have to delve into nature of connections holding the atoms of the semiconductor crystal near each other.
For example, consider a silicon crystal.
Silicon is a tetravalent element. This means that in the outer shell of its atom there are four electrons, relatively weakly bound to the nucleus. The number of nearest neighbors of each silicon atom is also four. A schematic diagram of the silicon crystal structure is shown in Figure 16.5.
The interaction of a pair of neighboring atoms is carried out using a pair-electron bond, called covalent bond... In the formation of this bond, one valence electron is involved from each atom, which are separated from the atom to which they belong (are collectivized by the crystal) and spend most of their time in space between neighboring atoms during their movement. Their negative charge keeps the positive silicon ions close to each other.
Do not think that the collectible pair of electrons belongs to only two atoms. Each atom forms four bonds with neighboring ones, and any valence electron can move along one of them. Having reached a neighboring atom, it can move on to the next, and then further along the entire crystal. Valence electrons belong to the entire crystal.
Pair-electronic bonds in a silicon crystal are strong enough and do not break at low temperatures. Therefore, silicon does not conduct electric current at low temperatures. The valence electrons participating in the bond of atoms are like a "cementing solution" that holds the crystal lattice, and the external electric field does not have a noticeable effect on their movement. A germanium crystal has a similar structure.
Electronic conductivity. When silicon is heated, the kinetic energy of the particles increases and individual bonds break. Some electrons leave their "beaten path" and become free, like electrons in a metal. In an electric field, they move between the nodes of the lattice, creating an electric current ( fig. 16.6).
The conductivity of semiconductors, due to the presence of free electrons in them, is called electronic conduction... As the temperature rises, the number of broken bonds, and hence 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 between the semiconductor atoms is broken, a vacant place with a missing electron is formed. He is called hole... The hole has an excess positive charge compared to the rest of the unbroken bonds (see Fig. 16.6).
The position of the hole in the crystal is not unchanged. The following process is continuously going on. One of the electrons, providing the connection of atoms, jumps to the place of the hole formed 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, which is 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 orderly 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 ( fig. 16.7).
In the absence of an external field, there is one hole (+) for one free electron (-). When a field is applied, a free electron is displaced against the field strength. One of the bound electrons also moves in this direction. It looks like moving the hole in the direction of the field.
So, in semiconductors there are two types of charge carriers: electrons and holes. Therefore, semiconductors have not only electronic, but also hole conduction.
We examined the conduction mechanism of pure semiconductors. The conductivity under these conditions is called intrinsic conductivity semiconductors.
The conductivity of pure semiconductors (intrinsic conductivity) is carried out by the movement of free electrons (electronic conductivity) and the movement of bound electrons to the vacant places of pair-electron bonds (hole conductivity).
???
1. What bond is called covalent?
2. What is the difference between the temperature dependence of the resistance of semiconductors and metals?
3. What mobile charge carriers are there in a pure semiconductor?
4. What happens when an electron meets a hole?
G.Ya. Myakishev, B.B. Bukhovtsev, N.N. Sotsky, Physics Grade 10
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