Resistors semiconductor diodes transistors. Semiconductor resistors
semiconductor diode is called a two-electrode device with one-sided conductivity. Its design is based on the equilibrium R-n transition. According to the nature of the transition formation, diodes are divided into point and planar.
Semiconductor triodes have found wide application for converting, amplifying and generating electrical oscillations - transistors. For the transistor to work, it is necessary to have two electron-hole junctions; germanium is often used as a semiconductor.
In transistors using n-p-n junction, semiconductor R-type located between semiconductors n-type, The device of a planar bipolar transistor is shown in Figure 2.7.
Rice. 2.7. The principle of the device of the transistor and the image of transistors in the diagrams.
In this transistor n-p-n type, there is a middle region with hole conductivity, and two extreme regions with electronic conductivity. The middle region of the transistor is called - base, one extreme region is emitter , another - collector. Thus, the transistor has two n-r transition: emitter between emitter and base collector between base and collector. The distance between them should be very small, no more than a few micrometers, i.e. the base area should be very thin. This is a condition for good transistor performance. In addition, the concentration of impurities in the base is always much less than in the collector and emitter. On the schematic images of transistors, the arrow shows the direction of the current (conditional, from plus to minus) in the emitter wire with a forward voltage at the emitter junction.
Consider the operation of the transistor in no-load mode, when only sources of constant supply voltages E 1 and E 2 are turned on (Fig. 2.8).
Their polarity is such that the voltage at the emitter junction is direct, and at the collector junction it is reversed. Therefore, the resistance of the emitter junction is small, and to obtain a normal current in this junction, a voltage E 1 of tenths of a volt is sufficient. The resistance of the collector junction is high, and the voltage E 2 is usually units or tens of volts.
Rice. 2.8. The movement of electrons and holes in an n-p-n type transistor.
The principle of operation of the transistor is that the direct voltage of the emitter junction, i.e., the base-emitter section, significantly affects the collector current: the higher this voltage, the greater the emitter and collector currents. In this case, the changes in the collector current are only slightly less than the changes in the emitter current. Thus, the voltage between the base and the emitter E 1, i.e. input voltage, controls the collector current. The amplification of electrical oscillations with the help of a transistor is based precisely on this phenomenon.
Physical processes in the transistor occur as follows. With an increase in the direct input voltage E 1, the potential barrier in the emitter junction decreases and, accordingly, the current through this junction increases - the emitter current i uh. The electrons of this current are injected from the emitter into the base and, due to diffusion, penetrate through the base into the collector junction, increasing the collector current. Since the collector junction operates at a reverse voltage, space charges appear in this junction, shown in the figure by circles with the signs "+" and "-". Between them there is an electric field. It promotes the advancement (extraction) through the collector junction of electrons that have come here from the emitter, i.e. draws electrons into the collector junction region.
If the base thickness is sufficiently small and the concentration of holes in it is low, then the majority of electrons, having passed through the base, do not have time to recombine with the base holes and reach the collector junction. Only a small fraction of electrons recombine with holes in the base. As a result of recombination, a base current flows in the base wire. Indeed, in the steady state, the number of holes in the base should be unchanged. As a result of recombination, some holes disappear every second, but the same number of new holes arise due to the fact that the same number of electrons leave the base in the direction of the source E 1 pole. In other words, many electrons cannot accumulate in the base.
If the base had a considerable thickness and the concentration of holes in it was high, then most of the electrons of the emitter current, diffusing through the base, would recombine with holes and would not reach the collector junction.
Under the action of the input voltage, a significant emitter current arises, electrons are injected into the base region from the emitter side, which are minority carriers for this region. Not having time to recombine with holes during diffusion through the base, they reach the collector junction. The greater the emitter current, the more electrons come to the collector junction and the lower its resistance becomes. Accordingly, the collector current increases. In other words, with an increase in the emitter current in the base, the concentration of minority carriers injected from the emitter increases, and the more these carriers, the greater the collector junction current, i.e. collector current i to .
It should be noted that the emitter and collector can be interchanged (the so-called inverse mode). But on transistors, as a rule, the collector junction is made with a much larger area than the emitter junction, since the power dissipated in the collector junction is much greater than the power dissipated in the emitter. Therefore, if you use the emitter as a collector, then the transistor will work, but it can only be used at a much lower power, which is impractical. If the junction areas are made the same (transistors in this case are called symmetrical), then any of the extreme regions can equally well work as an emitter or collector.
We have considered the physical phenomena in the n-p-n type transistor. Similar processes occur in a p-n-p transistor, but in it the roles of electrons and holes change, and they also change to reverse voltage polarities and current directions.
The three most common ways to turn on transistors are:
- common base circuit when the emitter input and collector output
connected to a common base;
- in a common emitter circuit collector output circuit
connects to the emitter instead of the base;
- common collector circuit, otherwise called the emitter repeater.
Conclusion: 1. The presence of impurities in semiconductors causes a violation of the equality between the number of holes and electrons, and the electric current will be created mainly by charges of the same sign, depending on what prevails in the semiconductor.
2. The design of any semiconductor device is based on equilibrium R-n transitions.
Prepared
A student of 10 "A" class
School No. 610
Ivchin Alexey
Abstract on the topic:
"Semiconductor diodes and transistors, areas of their application"
1. Semiconductors: theory and properties
2. Basic semiconductor devices (Structure and application)
3. Types of semiconductor devices
4. Production
5. Scope
1. Semiconductors: theory and properties
First you need to get acquainted with the mechanism of conduction in semiconductors. And for this you need to understand the nature of the bonds that hold the atoms of a semiconductor crystal next to each other. For example, consider a silicon crystal.
Silicon is a tetravalent element. This means that in the outer
The shell of an atom has four electrons, relatively weakly bound to the nucleus. The number of nearest neighbors of each silicon atom is also four. The interaction of a pair of neighboring atoms is carried out using a paonoelectronic bond, called a covalent bond. In the formation of this bond from each atom, one valence electron participates, which are split off from the atoms (collectivized by the crystal) and, during their movement, spend most of their time in the space between neighboring atoms. Their negative charge keeps 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 the neighboring atom, it can move on to the next, 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 does not conduct electricity at low temperatures. The valence electrons participating in the bonding of atoms are firmly attached to the crystal lattice, and the external electric field does not have a noticeable effect on their movement.
electronic conductivity.
When silicon is heated, the kinetic energy of the particles increases, and individual bonds break. Some electrons leave their orbits and become free, like electrons in a metal. In an electric field, they move between lattice sites, forming an electric current.
The conductivity of semiconductors due to the presence of free electrons in metals is called electronic conductivity. As the temperature rises, the number of broken bonds, and hence the number of 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 V3. This leads to a decrease in resistance.
hole conduction.
When the bond is broken, a vacancy is formed with the missing electron.
It's called a hole. The hole has an excess positive charge compared to the rest of the normal bonds. The position of the hole in the crystal is not fixed. The following process is continuously going on. One of the electrons providing the bond between the atoms jumps to the place of the formed holes and restores the pair-electron bond here. and where the electron jumped from, a new hole is formed. Thus, the hole can move throughout the crystal.
If the electric field strength in the sample is zero, then the movement of holes, equivalent to the movement of positive charges, occurs randomly and therefore does not create an electric current. In the presence of an electric field, an ordered movement of holes occurs, and, thus, 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 by 10 in 23 cm in -3. At the same time, the number of germanium atoms in 1 cubic cm is about 10–23. Thus, the number of free electrons is approximately one ten-billionth of the total number of atoms.
An essential feature of semiconductors is that in the presence of impurities, along with their own conductivity, an additional one arises - impurity conductivity. By changing the impurity concentration, one can significantly change the number of charge carriers of one sign or another. This makes it possible to create semiconductors with a predominant concentration of either negatively or positively charged carriers. This feature of semiconductors opens up wide possibilities for practical applications.
donor impurities.
It turns out that in the presence of impurities, such as arsenic atoms, even at very low concentrations, the number of free electrons increases many times over. This happens for the following reason. Arsenic atoms have five valence electrons, four of which are involved in creating a covalent bond of a given atom with the surrounding ones, for example, with silicon atoms. The fifth valence electron is 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 donate electrons easily 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 minor ones.
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-electron bonds with neighbors, the indium atom lacks an electron. As a result, a hole is formed. The number of holes in a crystal is equal to the number of impurity atoms. Such impurities are called acceptor (accepting) impurities. In the presence of an electric field, the holes move along the field and hole conduction occurs. Semiconductors with a predominance of hole conduction over electron conduction are called p-type semiconductors (from the word positiv - positive).
2.Basic semiconductor devices (Structure and application)
There are two main semiconductor devices: diode and transistor.
Diode.
At present, semiconductor diodes are increasingly used 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 generated by heating the cathode. In the 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 the rectified current are more miniature than lamp ones.
The current-voltage characteristic for direct and reverse connection is shown in Figure 2.
They replaced the lamps, they are very widely used in technology, mainly for rectifiers, and diodes have also found application in various devices.
Transistor.
Let us consider one of the types of a 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 a few micrometers) n-type semiconductor layer is created between two p-type semiconductor layers (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 leads from regions with different types of conductivity allow the transistor to be included in the circuit shown in Figure 3. With this inclusion, the left p-n 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, in the emitter-base circuit there would be a current depending on the voltage of the sources (battery B1 and AC voltage source) and the resistance of the circuit, including the low resistance of the direct emitter-base junction. Battery B2 is connected so that the right pn junction in the circuit (see Fig. 3) is reversed. It separates the base from the right p-type region called the collector. If there were no left p-n-junction, the current strength and the collector circuit would be close to zero. Since the resistance of the reverse transition is very high. If there is a current in the left p-n junction, a current also appears in the collector circuit, and the current in the collector is only slightly less than the current 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, gdr they are already major 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 the electrons of the base and penetrate into the collector due to diffusion. The right p-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 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.
The resistance of the resistor R has little effect on the collector current, and this resistance can be made quite large. By controlling the emitter current with an AC voltage source included in its circuit, we will get a synchronous change in the voltage across the resistor. With a large resistance of the resistor, the voltage change across it can be tens of thousands of times greater than the signal change in the emitter circuit. This means voltage amplification. Therefore, at the load R, it is possible to obtain electrical signals whose power is many times greater than the power entering the emitter circuit. They replace vacuum tubes and are widely used in technology.
3. Types of semiconductor devices.
In addition to planar diodes in Fig. 8 and transistors, there are also point diodes in Fig. 4. Point transistors (see the structure in the figure) are shaped before use, i.e. pass a current of a certain magnitude, as a result of which a region with hole conductivity is formed under the tip of the wire. Transistors are p-n-p and n-p-n types. Designation and general view in Figure 5.
There are photo and thermal resistors and varistors, see 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, as a result of which a film with electronic conductivity is formed, as a result of which a rectifying current transition is formed. The larger the area, the greater the rectified current.
4. Production
The manufacturing technology of the diode is as follows. A piece of indium is melted on the surface of a square plate with an area of 2-4 cm2 and a thickness of a few fractions of a millimeter, cut from a semiconductor crystal with electronic conductivity. Indium fuses strongly with the plate. At the same time, indium atoms penetrate
(diffuse) into the thickness of the plate, forming in it a region with a predominance of hole conductivity. The thinner the semiconductor wafer. the lower the resistance of the diode in the forward direction, the greater the current rectified by the diode. The contacts of the diode are a drop of indium and a metal disk or rod with lead wires.
After assembling the transistor, it is mounted in a case, an email is connected. terminals to the contact plates of the crystal and the output of the package and seal the package.
5. Scope
Diodes are very reliable, but the limit of their use is from -70 to 125 C. Since. for a point diode, the contact area is very small, so the currents that such diodes can rectify are not more than 10-15 mA. And they are used mainly for modulating high-frequency oscillations and for measuring instruments. For any diode, there are some maximum allowable limits for forward and reverse current, depending on the forward and reverse voltage and determining its rectifying and strength properties.
Transistors, like diodes, are sensitive to temperature and overload and penetrating radiation. Transistors, unlike radio tubes, burn out from improper connection.
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Figure 2
Picture 1
Figure 3
Figure 4
Figure 5
Figure 4
SEMICONDUCTOR DIODES
Semiconductor diodes are semiconductor devices with one electrical junction and two terminals. They are used for rectifying alternating current, detecting variable oscillations, converting microwave oscillations into intermediate frequency oscillations, stabilizing voltage in DC circuits, etc. According to their purpose, semiconductor diodes are divided into rectifier, high-frequency, varicaps, zener diodes, etc.
rectifier diodes. Rectifier semiconductor diodes are designed to convert AC to DC.
The basis of modern rectifier diodes is the electron-hole junction (EHP), which is obtained by fusion or diffusion. The material used is germanium or silicon.
To obtain large values of rectified currents in rectifier diodes, EAFs with a large area are used, since for normal operation of the diode, the current density through the junction should not exceed 1-2 A / mm 2.
Such diodes are called planar. The design of a low power planar semiconductor diode is shown in fig. 2.1, a. To improve heat dissipation in diodes of medium And high power, a screw is welded to their case, with which the diodes are attached to a special radiator or chassis (Fig. 2.1, b).
The main characteristic of a rectifier diode is its current-voltage characteristic (CVC). The type of CVC depends on the semiconductor material and temperature (Fig. 2.2, a and b).
The main parameters of rectifier semiconductor diodes are:
constant forward voltage U np at a given forward current;
the maximum allowable reverse voltage U o 6 p max at which the diode can still work normally for a long time;
direct reverse currentflowing through the diode at a reverse voltage equal to U o 6 p max ;
average rectified current, which can pass through the diode for a long time at an acceptable temperature for its heating;
maximum allowable power dissipated by the diode, at which ensures the specified reliability of the diode.
According to the maximum allowable value of the average rectified current, diodes are divided into low-power (), medium power ( 
) and high power (). High power rectifier diodes are called power diodes.
Low-power rectifier elements, which are series-connected rectifier semiconductor diodes, are called rectifier poles. Rectifier units are also produced, in which rectifier diodes are connected according to a specific (for example, bridge) circuit.
Rectifier semiconductor diodes are capable of operating at frequencies of 50 ... 10 5 Hz (power diodes - at frequencies of 50 Hz), i.e., they are low-frequency.
high frequency diodes. High-frequency diodes include semiconductor diodes capable of operating at frequencies up to 300 MHz. Diodes operating at frequencies above 300 MHz are called microwave diodes.
With increasing frequency, the shunting of the differential resistance of the reverse-biased EAF by the charging capacitance increases. This leads to a decrease in reverse resistance and a deterioration in the rectifier properties of the diode. Since the value of the charging capacity is proportional to the area of the EAF, to reduce it, it is necessary to reduce the area of the EAF.
Microalloy diodes have a small junction area, but they. the disadvantage is the accumulation in the base of minor charge carriers injected into it when the diode is directly turned on. This limits the speed (frequency range) of microalloy diodes.
Point diodes capable of operating in the microwave range have the best speed and, therefore, higher frequencies. In their design, a metal spring with a diameter of about 0.1 mm is pressed with a tip against a semiconductor crystal. The material of the spring is chosen so that the work function of the electrons from it is greater than from the semiconductor. In this case, a barrier layer is formed at the metal-semiconductor interface, called the Schottky barrier - after the name of the German scientist who studied this phenomenon. Diodes whose operation is based on the use of the properties of the Schottky barrier are called Schottky diodes. In them, the electric current is carried by the main charge carriers, as a result of which there are no injection and accumulation of minor charge carriers.
High-frequency and microwave diodes are used for rectification of high-frequency oscillations (rectifier), detection (detector), power level control (switching), frequency multiplication (multiplier) and other non-linear transformations of electrical signals.
Varicaps. Varicaps are called semiconductor diodes, the action of which is based on the use of the dependence of capacitance on reverse voltage. Varicaps are used as an element with an electrically controlled capacitance.
The character of dependence is shown in fig. 2.3, a. This dependence is called the capacitance-voltage characteristic of the varicap. Main parameters
varicaps are:
rated capacitance measured at a given reverse voltage ;
capacitance overlap coefficient Kc, determined by the ratio of varicap capacitances at two reverse voltage values;
maximum allowable reverse voltage ;
quality factor Q B defined as the ratio of varicap reactance to loss resistance.
Semiconductor zener diodes. A semiconductor zener diode is a semiconductor diode, the voltage on which is maintained with a certain accuracy when the current passing through it changes in a given range. It is designed to stabilize the voltage in DC circuits.
The CVC of the zener diode is shown in fig. 2.4, a, and the symbol - in fig. 2.4, b.
If an EAF is created on both sides of a silicon wafer, then a zener diode with a symmetrical CVC will be obtained - a symmetrical zener diode (Fig. 2.4, c).
The working section of the zener diode is the section of electrical breakdown. When changing the current flowing through the zener diode, from a value to a value. the voltage on it differs little from the value. The use of zener diodes is based on this property.
The principle of operation of a voltage stabilizer on a silicon zener diode (Fig. 2.4, d) is that when the voltage U VX changes, the current flowing through the zener diode changes, and the voltage on the zener diode and the load R connected in parallel to it practically does not change.
The main parameters of silicon zener diodes are:
stabilization voltage U st;
minimum and maximum stabilization currents;
maximum allowable power dissipation
differential resistance in the stabilization area 
;
temperature coefficient of voltage in the stabilization section
For modern zener diodes, the stabilization voltage ranges from 1 to 1000 V at stabilization currents from 1 mA to 2 A. To stabilize voltages less than 1 V, a direct I–V characteristic of a silicon diode, called a stabistor, is used. At stabistors B. By connecting zener diodes (or stabistors) in series, you can get any required stabilization voltage.
The differential resistance in the stabilization section is approximately constant and for most zener diodes is 0.5 ... 200 ohms. The voltage temperature coefficient can be positive (for zener diodes with) and negative (for zener diodes with U CT< 6 В) и для большинства стабилитронов находится в пределах (- 0,5... + 0,2) %/°С.
BIPOLAR TRANSISTORS
A bipolar transistor (BT) or simply a transistor is a semiconductor device with two interacting EHPs and three or more leads, the amplifying properties of which are due to the phenomena of injection and extraction of minor charge carriers.
Electron-hole transitions are formed between three regions of a semiconductor with different types of electrical conductivity. In accordance with the order of alternation of p- and n-regions, BTs are divided into transistors of the p-p-p type and transistors of the p-p-p type (Fig. 2.5).
The middle region of the transistor is called the base, one extreme region is the emitter (E), and the other is the collector (K). Usually the concentration of impurities in the emitter is greater than in the collector. In BT type p - p - p, the base has p-type electrical conductivity, and the emitter and collector are n-type.
The EAF formed between the emitter and the base is called the emitter, and between the base and the collector - the collector.
Transistor operating modes. Depending on how the emitter and collector EAFs are connected to power sources, the bipolar transistor can operate in one of four modes: cutoff, saturation, active, and inverse.
The emitter and collector EHP in the cutoff mode (Fig. 2.6, a) are shifted in the opposite direction, and in the saturation mode (Fig. 2.6, 6) - in the forward direction. The collector current in these modes is practically independent of the emitter voltage and current.
Cut-off and saturation modes are used when operating BT in pulsed and key devices.
When the transistor is in active mode, its emitter junction is shifted in the forward direction, and the collector junction in the opposite direction (Fig. 2.6, c).
Under the action of a forward voltage 11eb, a current flows in the emitter circuit, creating collector and base currents, so that
The collector current contains two components: controlled, proportional to the emitter current, and uncontrolled, created by the drift of minority carriers through a reverse-biased collector junction. The coefficient of proportionality is called the static current transfer coefficient of the emitter. For most modern BT 
and more.
The base current includes a recombination component due to electrons entering the base to compensate for the positive charge of holes recombining in the base, and an uncontrolled collector current component , so that 
When using BT as an amplifying element, one of the conclusions must be common to the input and output circuits. In the diagram shown in fig. 2.6, c, the common electrode is the base. Such a BT switching circuit is called a common base (CB) circuit and is usually depicted as shown in Fig. 2.7, a. In addition to the OB circuit, circuits with a common emitter (OE) and a common collector (OC) are also used in practice.
In the OE circuit (Fig. 2.7, b), the relationship between the output and input currents is determined by the equation
The coefficient is called the static current transfer coefficient of the base. It is related to the ratio
At values are within 19...99.
The component is the reverse (uncontrolled) collector current in the OE circuit. This current is related to the reverse current in the circuit
ABOUT ratio
From relation (2.4) it follows that the collector reverse current in the OE circuit is much greater than in the OB circuit. This means that the change in temperature in the OE circuit has a greater effect on the change in currents (and hence on the change in static characteristics and parameters) than in the OB circuit. This is one of the disadvantages of turning on BT according to the OE scheme.
When you turn on the BT according to the OK scheme. (Fig. 2.7, c) the relationship between the output and input currents is determined by the relation
From a comparison of expressions (2.2) and (2.5) it follows that the dependences between the input and output currents of the BT in the OE and OK circuits are approximately the same. This makes it possible to use the same characteristics and parameters for the calculation of OE and OK schemes.
The inverse mode differs from the active mode by the opposite polarity of the voltages applied to the emitter and collector EHFs.
Static characteristics. Static characteristics express complex relationships between currents and voltages.
transistor electrodes and depend on the way it is turned on.
 |
On fig. 2.8, a shows a family of input characteristics of the BT type n - p - n, included according to the OE scheme, which express the dependence at . When the input characteristic is
direct branch of the CVC of the emitter EHP. With a positive collector voltage, the input characteristic shifts to the right.
Output characteristics (Fig. 2.8, b) reflect the dependence at. The steep part of the characteristics corresponds to the saturation mode, and the flat part corresponds to the active mode. The relationship between the collector and base currents in the flat area is determined by expression (2.2).
Low-signal static mode parameters. When a transistor operates in an amplifying mode, its properties are determined by small-signal parameters, for which the transistor can be considered a linear element. In practice, small-signal hybrid or h-parameters have received the greatest use. Currents and voltages at small amplitudes of variable components in the system of h-parameters are related by the following relationships:
- input impedance;
- voltage feedback factor
- current transfer coefficient;
- output conductivity.
The parameters and are measured in the short circuit mode of the output circuit, and the parameters and are measured in the idle mode of the input circuit. These modes are easy to implement. The values of h-parameters depend on the way the transistor is turned on and at low frequencies can be determined from static characteristics. In this case, the amplitudes of small currents and voltages are replaced by increments. So, for example, when the transistor is turned on according to the circuit with OE, the formulas for the parameters and determined by the input characteristics at point A (Fig. 2.8, a) are written as:
The parameters and are determined by the output (Fig. 2.8, b) characteristics according to the formulas:
Similarly, -parameters are determined when the transistor is turned on according to the circuit with OB.
Small-signal parameters and are respectively called the transfer coefficients of the emitter current and the base current. They characterize the current amplifying properties of the transistor for variable signals, and their values depend on the operating mode of the transistor and on the frequency of the amplified signals. So, with increasing frequency, the modulus of the base current transfer coefficient decreases
The frequency at which it decreases by a factor of compared to its value at a low frequency is called the limiting frequency of the base current transmission and is denoted. The frequency at which it decreases to 1 is called the cutoff frequency of the BT and is denoted by . According to the value of the cutoff frequency, transistors are divided into low-frequency, medium-frequency, high-frequency and superhigh-frequency.
THYRISTORS
A thyristor is a bistable semiconductor device that has three or more transitions and can switch from closed to open and vice versa.
Thyristors with two leads are called diode or dinistors, and with three leads - triode or trinistors.
Dinistors. The structure of a dinistor consists of four semiconductor regions with alternating types of electrical conductivity. 
between which three EHPs are formed. The extreme EAFs are emitter, and the middle one is collector. The area is called the emitter or anode, the area is called the cathode.
Connecting the dinistor anode to the positive pole of an external source, and the cathode to the negative one, corresponds to the direct connection of the dinistor. When the polarity of the source voltage is reversed, reverse switching takes place.
With direct connection, the dinistor can be represented as a combination of two transistors p - n - p and n - p - n (Fig. 2.9, a) with emitter current transfer coefficients and.
The current flowing through the dinistor contains the hole injection component of the transistor, the electronic injection component of the transistor and the reverse current of the collector junction, i.e.
For now, the dinistor is closed. At 
processes develop in the dinistor, leading to an avalanche-like increase in the injection components of the current and switching the collector junction to the forward direction. In this case, the resistance of the dinistor decreases sharply and the voltage drop across it does not exceed 1-2 V. The rest of the source voltage drops on the limiting resistor (Fig. 2.9, b).
When the dinistor is turned on again, a small reverse current flows through it.
Trinistors. The trinistor differs from the dinistor in the presence of an additional control output from the base region (Fig. 2.10, a). The conclusion can be made from any base. A source connected to this pin creates
control current, which adds up to the main current. As a result, the switching of the trinistor from the closed state to the open state occurs at a lower value of U a (Fig. 2.10, b).
In five layer structures 
by appropriately performing the extreme areas, you can get a symmetrical CVC (Fig. 2.10, c). Such a thyristor is called symmetrical. It can be diode (diac) or triode (triac).
Turning off the thyristor is carried out by reducing (or interrupting) the anode current or changing the polarity of the anode voltage.
The considered thyristors are called non-lockable. There are also lockable thyristors, which can be switched from open to closed by changing the current of the control electrode. They differ from non-lockable designs.
Thyristor parameters. The main parameters of thyristors are:
switch-on voltage;
unlocking control current ;
breaking current ;
residual voltage U np ;
turn-on time t on;
off time ;
delay time t 3 ;
maximum slew rates of forward voltage (du/dt) max and forward current (di/dl) max .
Thyristors are widely used in controlled rectifiers, DC-to-AC converters (inverters), voltage stabilizers,
as proximity switches, in electric drives, automation devices, telemechanics, computer technology, etc.
Conventional graphic designations of thyristors are shown in fig. 2.11.
FIELD TRANSISTORS
A field-effect transistor (FET) is a semiconductor device, the amplifying properties of which are due to the flow of the main charge carriers of the same sign, flowing through a conducting channel, and which is controlled by an electric field.
A control electrode isolated from the channel is called a gate. According to the method of gate isolation, field-effect transistors are divided into three types:
1) with a control p - n-junction, or with a p - t-shutter;
2) with a metal-semiconductor gate, or with a Schottky gate;
3) with an insulated gate.
Field-effect transistors with p-n -shutter. In a field-effect transistor with a p-n-gate (Fig. 2.12), the n-type channel is isolated from the substrate and the p-n-gate
moves, which, due to the fulfillment of the condition, are formed mainly in the channel. When the channel thickness is greatest, and its resistance is minimal. If a negative voltage is applied to the gate with respect to the source, then the p-n junctions will expand, the channel thickness will decrease, and its resistance will increase. Therefore, if a voltage source is connected between the source and drain, then the current I c flowing through the channel can be controlled by changing the channel resistance using the voltage applied to the gate. On this principle, the operation of a FET with a p - n-shutter is based.
The main static characteristics of a FET with a p-n-gate are the transfer (drain-gate) and output (drain) characteristics (Fig. 2.13).
The gate voltage, at which the channel is completely blocked, and the drain current decreases to tenths of a microampere, is called the cut-off voltage and denoted by .
The drain current at U 3I = 0 is called the initial drain current.
The output characteristics contain steep, or ohmic, and flat regions. The flat region is also called the saturation region or the channel overlap region.
The drain current, flowing through the channel, creates a voltage drop across its distributed resistance, which increases the channel-gate and channel-substrate reverse voltages, which leads to a decrease in the channel thickness. The reverse stresses reach their greatest value at the boundary with the drain, and in this area the narrowing of the channel turns out to be maximum (Fig. 2.12). At a certain voltage value, both p-n junctions close in the drain region and the channel overlaps. This drain voltage is called the overlap voltage or saturation voltage (). When a reverse voltage is applied to the gate, an additional narrowing of the channel occurs, and its overlap occurs at a lower voltage value .
Field-effect transistors with a Schottky gate. IN Fri With a Schottky gate, the channel resistance is controlled by changing, under the action of the gate voltage, the thickness of the rectifying junction formed at the interface between the metal and the semiconductor. Compared with the p - n-junction, the rectifying metal-semiconductor junction allows you to significantly reduce the channel length: up to 0.5 ... 1 μm. At the same time, the dimensions of the entire FET structure are also significantly reduced, as a result of which FETs with a Schottky barrier are able to operate at higher frequencies - up to 50...80 GHz.
Field-effect transistors with an insulated gate. These transistors have a metal - dielectric - semiconductor structure and are briefly called MIS transistors. If silicon oxide is used as the dielectric, then they are also called MOSFETs.
There are two types of MOS transistors: with induced and with built-in channels.
In MIS transistors with an induced p-type channel (Fig. 2.14), the p-type drain and source regions form two oppositely opposite substrates with the n-region of the substrate
switched on EAF, and when a source of any polarity is connected to them, there will be no current in the circuit. If, however, a negative voltage is applied to the gate relative to the source and substrate, then at a sufficient value of this voltage in the near-surface layer of the semiconductor located under the gate, an inversion of the type of electrical conductivity will occur and the p-regions of the drain and source will be connected by a p-type channel. This gate voltage is called the threshold voltage and denoted by . With an increase in the negative gate voltage, the depth of penetration of the inversion layer into the semiconductor increases, which corresponds to an increase in the channel thickness and a decrease in its resistance.
The transfer and output characteristics of an MOS transistor with an induced p-type channel are shown in Fig. 2.15. The voltage drop across the channel resistance reduces the gate-to-gate voltage
and channel and channel thickness. The greatest narrowing of the channel will be at the drain, where the voltage is the least 
.
In MOS transistors with a built-in channel between the drain and source regions, a thin near-surface layer (channel) with the same type of electrical conductivity as the drain and source is already created at the manufacturing stage. Therefore, in such transistors, the drain current, called the initial current, also flows at.
The static output and transfer characteristics of an MIS transistor with a built-in p-type channel are shown in fig. 2.16.
Differential parameters of PT. In addition to the parameters discussed above, the properties of the FET are characterized by differential parameters: the steepness of the transfer characteristic, or the steepness of the FET; differential resistance and static gain.
The steepness of the FET at characterizes the amplifying properties of the transistor and for low-power transistors is usually a few mA/V.
The differential resistance at is the resistance of the FET channel to alternating current.
The steepness of the FET can be determined from the static output or transfer characteristics (Fig. 2.16) based on the expression
and the differential resistance - according to the output characteristics in accordance with the expression
Static Gain 
at is usually calculated by the formula .
Conventional graphic designations of field-effect transistors are shown in fig. 2.17.
Field-effect transistors are used in amplifiers with high input resistance, key and logic devices, as well as in controlled attenuators as an element whose resistance changes under the influence of a control voltage.
Similar information.
Moscow Mining State University
Essay
on the subject CIRCUIT ENGINEERING
Semiconductor devices.
(diode, transistor, field effect transistor)
Art. gr. CAD-1V-96
Tsarev A.V.
Moscow 1999
Table of Contents
semiconductor diodes.
semiconductor transistors.
Field MIS transistors.
Literature.
Semiconductor diodes
Diode - a semiconductor device that passes electric current in only one direction and has two terminals for inclusion in an electrical circuit.
A 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 near the germanium surface. The rest of the germanium is still n-type. A p-n junction occurs between these two regions. To prevent the harmful effects of air and light, the germanium crystal is placed in a hermetic case. 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 the dependence of their parameters on temperature.
The volt-ampere characteristic of the diode (at a high voltage, the current strength reaches its maximum value - saturation current) is non-linear, therefore, the properties of the diode are estimated by the steepness of the characteristic:
Semiconductor transistors
The properties of the p-n junction can be used to create an amplifier of electrical oscillations, 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. to 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 areas of the triode, which allows you to apply different voltages to the left p-n junction between contacts e and b and to the right n-p junction between contacts b and k.
If a reverse voltage is applied to the right junction, then it will be locked and a very small reverse current will flow through it. Now let's apply a direct voltage to the left p-n-junction, then a significant forward current will begin to flow through it.
One of the regions of the triode, for example, the left one, usually contains hundreds of times more p-type dopant than the amount of n-impurity in the n-region. Therefore, the forward current through the p-n junction will consist almost exclusively of holes moving from left to right. Once in the n-region of the triode, holes that perform thermal motion 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 got into it, will move 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 the left into the n-region.
If, in the absence of voltage between points h and b, the reverse current in the n-p junction is very small, then after the voltage appears at the terminals h and b, this current is almost as large as the direct current in the left transition. In this way, you can control the current strength in the right (locked) n-p junction using the left p-n junction. By closing the left transition, we stop the current through the right transition; opening the left junction, we get the current in the right junction. By changing the value 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 p-n-p triode as an amplifier.
During the operation of the triode (Fig.), the load resistance R is connected to the right junction, and with the help of battery B, a reverse voltage (tens of volts) is applied, blocking 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. On load, the voltage is zero. If we 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.
With an increase in the forward voltage at the left junction, 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 across the load resistance R.
Thus, by applying a forward voltage equal to fractions of a volt to the left junction, you can get a large current through the load, and the voltage on 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. voltage gain is obtained in this way.
Current gain with this triode switching circuit is not obtained, 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, power amplification 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 permanent source (river flow) and a dam, a water level difference is created. By spending very little energy on the vertical movement of the shutter, we can control the flow of water of great power, i.e. control the energy of a powerful constant source.
The junction switched on in the flow direction (left in the figures) is called emitter, and the junction switched in the blocking direction (right in the figures) is called collector. The middle region is called the base, the left one is the emitter, and the right one 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 greater than those of vacuum tubes. Due to which transistors are widely used 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 compared to vacuum tubes are the same as those of semiconductor diodes - the absence of a hot cathode that consumes significant power and takes time to heat it up. In addition, transistors themselves are many times smaller in mass and size than electric lamps, and transistors are capable of operating at lower voltages.
But along with positive qualities, triodes also have their drawbacks. Like semiconductor diodes, transistors are very sensitive to temperature rise, electrical overloads, and highly penetrating radiation (to make the transistor more durable, it is packed in a special “case”).
The main materials from which triodes are made are silicon and germanium.
Field MIS transistors.
A field-effect transistor (FET) is a three-electrode semiconductor device in which an 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 a voltage on the control electrode.
In recent years, a large place in electronics has been occupied by devices that use phenomena in the near-surface layer of a semiconductor. The main element of such devices is the Metal-Dielectric-Semiconductor /MDP/ structure. An oxide layer, such as silicon dioxide, is often used as a dielectric layer between a metal and a semiconductor. Such structures are called MOS structures. The metal electrode is usually applied to the dielectric by vacuum sputtering. This electrode is called the gate.
FETs 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 a gate in the form of a p-n junction, depending on the channel, are divided into FETs 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 is applied to the gate relative to the semiconductor, then a space charge region appears near 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 bulk concentration.
Charging the near-surface region of the semiconductor leads to the appearance of a potential difference between it and the volume of the semiconductor and, consequently, to the curvature of the energy bands. With a negative charge on the gate, the energy bands bend upward, since when an electron moves from the volume to the surface, its energy increases. If the gate is positively charged, then the zones bend down.
The figure shows the band structure of an n-semiconductor with a negative charge on the gate and the designations of the main quantities characterizing the surface are given; potential difference between the surface and volume of the semiconductor; bending of zones near the surface; middle of the forbidden zone. It can be seen from the figure that, in the bulk of a 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 concentration of electrons is greater than the concentration of holes: as it should be in 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 it moves to the surface, and the distance to the Fermi level from the top of the valence band continuously decreases.
Often, the bending of the zones near the surface is expressed in units of kT and denoted by Ys. Then, during the formation of the near-surface region of the semiconductor, three important cases can occur: depletion, inversion, and enrichment of this region with charge carriers. These cases for n- and p-type semiconductors are shown in Figs.
The depletion region appears when the sign of the gate charge coincides with the sign of the majority 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 depletion of the near-surface region by 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-major charge carriers /holes/ at the surface of the semiconductor becomes higher than the concentration of the majority carriers and the type of conduction in this region changes, although there are few electrons and holes, almost like in the intrinsic semiconductor. Near the surface, however, there may be as many or even more non-majority carriers as there are majority carriers in the bulk of the semiconductor. Such well-conducting layers near the surface with the type of conductivity opposite to the bulk one are called inversion layers. A depletion layer adjoins the inversion layer deep from the surface.
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 the near-surface layer is enriched by them. Such layers are called enriched.
In integrated electronics, MIS structures are widely used to create transistors and various integrated microcircuits based on them. On fig. schematically shows the structure of an MIS transistor with an insulated gate. 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 is called the drain. Ohmic contacts are applied on top of them. The gap between the regions is covered with a metal film isolated from the crystal surface by an oxide layer. This transistor electrode is called the gate. On the border between p- and n-regions, two p-n-junctions appear - source and drain, which are shown in the figure. shown with hatching.
On fig. a diagram of the inclusion of a transistor in a circuit is shown: a plus is connected to the source, a minus of the voltage source to the drain, and a minus of the source to the gate. For simplicity, 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, do not differ in any way from the properties of semiconductors in the bulk. The resistance between the drain and the source is very high, since the drain p-n junction is under reverse bias. 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 the threshold, to the formation of an inversion region connecting the source and drain p-regions with a conducting channel. At higher gate voltages, the channel becomes wider, and the drain-source resistance becomes smaller. The structure under consideration is thus a controlled resistor.
However, the channel resistance is determined only by the gate voltage only at low drain voltages. With an increase, the carriers leave the channel to the sink 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 non-linear.
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 of its sections, 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 appearance 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 is blocked, with an even greater offset, the channel is shortened towards the source. Closing 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 drain voltage increases, the purely drift mechanism of current carrier motion along the channel is replaced by a diffusion-drift mechanism.
The mechanism of current flow in an MIS transistor with a closed channel has some common features with the flow of current in a reverse-biased n-p junction. Recall that in the 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 the channel is closed, the drain current is practically saturated. The saturation current value depends on the gate voltage; the higher, the wider the channel and the greater the saturation current. This is a typical transistor effect - the gate voltage (in the input circuit) can be controlled by the drain current (current in the output circuit). A characteristic feature of MOS transistors is that their 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 of the order of the lattice constant. The reason for the appearance of the considered states is the imperfection of the semiconductor-dielectric (oxide) interface. At real interfaces, there is always a certain number of dangling bonds and the stoichiometry of the composition of the dielectric oxide film is violated. The density and nature of the states of the interface essentially depend on the technology of creating a dielectric film.
The presence of surface states at the semiconductor-dielectric interface adversely affects the parameters of the MIS transistor, since part of the charge induced under the gate in the semiconductor is captured by these states. Success in the creation of field-effect transistors of the type under consideration was achieved after the development of the technology for creating a film on a silicon surface with a low density of states of the interface.
In silicon oxide itself, there is always a positive "built-in" charge, the nature of which has not yet been fully elucidated. The value of this charge depends on the oxide fabrication 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 near its surface even at zero gate bias. Such transistors are called BUILT-IN CHANNEL transistors. The channel in them is preserved even when some negative bias is applied to the gate. In contrast to them, in transistors fabricated on an n-substrate, in which too much oxide charge is required for the formation of an inversion layer, a channel appears only when a voltage exceeding a certain threshold voltage is applied to the gate. This gate bias must be negative in sign for transistors with an n-substrate and positive in the case of a p-substrate.
At high drain voltages of the MOS transistor, the space charge region from the drain region can spread so strongly that the channel disappears altogether. Then the carriers from the heavily doped source region will rush to the drain, just like when the base of a bipolar transistor is "pierced".
Literature:
"Solid State Electronics" G.I.Epifanov, Yu.A.Moma.
“Electronics and Microcircuit Engineering” V.A. Skarzhepa, A.N. Lutsenko.
Prepared
A student of 10 "A" class
School No. 610
Ivchin Alexey
Abstract on the topic:
"Semiconductor diodes and transistors, areas of their application"
1. Semiconductors: theory and properties
2. Basic semiconductor devices (Structure and application)
3. Types of semiconductor devices
4. Production
5. Scope
1. Semiconductors: theory and properties
First you need to get acquainted with the mechanism of conduction in semiconductors. And for this you need to understand the nature of the bonds that hold the atoms of a semiconductor crystal next to each other. For example, consider a silicon crystal.
Silicon is a tetravalent element. This means that in the outer
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
paonoelectronic bond, called a covalent bond. In education
this bond from each atom involves one valence electron, which
which are split off from atoms (collectivized by the crystal) and
spend most of their time in the space between
neighboring atoms. Their negative charge keeps 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 the neighboring atom, it can move on to the next, 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 does not conduct electricity at low temperatures. The valence electrons participating 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
ties are broken. Some electrons leave their orbits and become free, like electrons in a metal. In an electric field, they move between lattice sites, forming an electric current.
The conductivity of semiconductors due to the presence of free metals in metals
electrons of electrons, is called electronic conductivity. As the temperature rises, the number of broken bonds, and hence the number of 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 V3. This leads to a decrease in resistance.
hole conduction.
When the bond is broken, a vacancy is formed with the missing electron.
It's called a hole. The hole has an excess positive charge compared to the rest of the normal bonds. The position of the hole in the crystal is not fixed. The following process is continuously going on. One
from the electrons that provide the connection of atoms, jumps to the place of
developed holes and restores the pair-electron bond here.
and where the 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 by 10 in 23 cm in -3. At the same time, the number of germanium atoms in 1 cubic cm is about 10–23. Thus, the number of free electrons is approximately one ten-billionth of the total number of atoms.
An essential feature of semiconductors is that they
in the presence of impurities, along with intrinsic conductivity,
additional - impurity conductivity. By changing the concentration
impurities, you can significantly change the number of charge carriers in addition
or some other sign. This makes it possible to create semiconductors with
predominant concentration either negatively or positively
strongly charged carriers. This feature of semiconductors is open
offers ample opportunities for practical application.
donor impurities.
It turns out that in the presence of impurities, such as 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 of a given atom with the surrounding ones, for example, with silicon atoms. The fifth valence electron is 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 donate electrons easily 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 minor ones.
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-electron bonds with neighbors, the indium atom does not
gets an electron. As a result, a hole is formed. The number of holes in the crystal
thalle is equal to the number of impurity atoms. This kind of impurities
are called acceptors. In the presence of an electric field
the holes move along the field and hole conduction occurs. By-
semiconductors with a predominance of hole conduction over electron-
Noah is called p-type semi-conductors (from the word positiv - positive).
2.Basic semiconductor devices (Structure and application)
There are two main semiconductor devices: diode and transistor.
At present, semiconductor diodes are increasingly used 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 generated by heating the cathode. In the 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 the rectified current are more miniature than lamp ones.
Semiconductor diodes are made from germanium, silicon. selenium and other substances. Consider how a p-n junction is created using a donor impurity; this junction cannot be obtained by mechanically connecting two semiconductors of various types, because in this case, too large a gap is obtained between the semiconductors. This thickness should not be greater than the interatomic distances. Therefore, indium is melted into one of the surfaces of the sample. Due to the diffusion of indium indium atoms deep into the germanium single crystal, a region with p-type conductivity is transformed near the germanium surface. The rest of the germanium sample, into which indmyanium atoms have not penetrated, 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 direct and reverse connection is shown in Figure 2.
They replaced the lamps, they are very widely used in technology, mainly for rectifiers, and diodes have also found application in various devices.
Transistor.
Let us consider one of the types of a 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 a few micrometers) n-type semiconductor layer is created between two p-type semiconductor layers (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 outputs from regions with different types of conductivity allow you to turn on the transistor in the circuit shown in Figure 3. With this inclusion
the left p-n 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 AC voltage source).
motion) and circuit resistance, including low resistance direct
transition emitter - base. Battery B2 is connected so that the right pn junction in the circuit (see Fig. 3) is reversed. It separates the base from the right p-type region called the collector. If there were no left p-n junction, the current strength and the collector circuit would be close to zero. Since the resistance of the reverse transition is very high. If there is a current in the left p-n junction, a current also appears in the collector circuit, and the current in the collector is only slightly less than the current 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, gdr they are already major 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 the electrons of the base and penetrate into the collector due to diffusion. The right p-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 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 resistance R 
has little effect on the current in the collector, and this resistance can be made sufficiently large. By controlling the emitter current with an AC voltage source included in its circuit, we will get a synchronous change in the voltage across the resistor. With a large resistance of the resistor, the voltage change across it can be tens of thousands of times greater than the signal change in the emitter circuit. This means voltage amplification. Therefore, at the load R, it is possible to obtain electrical signals whose power is many times greater than the power entering the emitter circuit. They replace vacuum tubes and are widely used in technology.
3. Types of semiconductor devices.
In addition to planar diodes in Fig. 8 and transistors, there are also point diodes in Fig. 4. Point transistors (see the structure in the figure) are shaped before use, i.e. pass a current of a certain magnitude, as a result of which a region with hole conductivity is formed under the tip of the wire. Transistors are p-n-p and n-p-n types. Designation and general view in Figure 5.
There are photo and thermal resistors and varistors, see 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, as a result of which a film with electronic conductivity is formed, as a result of which a rectifying current transition is formed. The larger the area, the greater the rectified current.
4. Production
The manufacturing technology of the diode is as follows. A piece of indium is melted on the surface of a square plate with an area of 2-4 cm2 and a thickness of a few fractions of a millimeter, cut from a semiconductor crystal with electronic conductivity. Indium is strongly fused with the plate. At the same time, indium atoms penetrate (diffuse) into the thickness of the plate, forming in it a region with a predominance of hole conductivity. The thinner the semiconductor wafer. the lower the resistance of the diode in the forward direction, the greater the current rectified by the diode. The contacts of the diode are a drop of indium and a metal disk or rod with lead wires.
After assembling the transistor, it is mounted in a case, an email is connected. terminals to the contact plates of the crystal and the output of the package and seal the package.
5. Scope
Diodes are very reliable, but the limit of their use is from -70 to 125 C. Since. for a point diode, the contact area is very small, so the currents that such diodes can rectify are not more than 10-15 mA. And they are used mainly for modulating high-frequency oscillations and for measuring instruments. For any diode, there are some maximum allowable limits for forward and reverse current, depending on the forward and reverse voltage and determining its rectifying and strength properties.
Transistors, like diodes, are sensitive to temperature and overload and penetrating radiation. Transistors, unlike radio tubes, burn out from improper connection.
