Cheat sheet: Semiconductor diodes and transistors, their areas of application. Semiconductor devices - types, overview and use Resistors, semiconductor diodes, transistors

Moscow Mining State University

Abstract

in the subject CIRCUIT ENGINEERING

Semiconductor devices.

(diode, transistor, field effect transistor)

Art. gr. CAD-1V-96

Tsarev A.V.

Moscow 1999

Contents

Semiconductor diodes.

Semiconductor transistors.

Field-effect MOS transistors.

Literature.

Semiconductor diodes

A diode is a semiconductor device that passes electric current in only one direction and has two terminals for inclusion in an electrical circuit.

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

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

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

Semiconductor transistors

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Field-effect MOS transistors.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Literature:

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

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

Semiconductor resistors, diodes, transistors

LECTURE 11

Emission properties (transition of charges from one area to another).

Semiconductor resistors are semiconductor devices with two terminals, which R depends on U, t° C, illumination, deformation, etc.

1. Linear resistor − R= const, used in ICs (lightly doped silicon or gallium arsenide).

2. Varistor, R(U) is made from silicon carbide mixed with clay.

Varistor current-voltage characteristic

Nonlinearity coefficient λ = R/Rg = (U/I)/(dU/dI) » const for various types. Used to protect electrical circuits from overvoltages.

3. Thermistor

Thermistor Characteristics

1 - thermistor, its R decreases with growth tº

2 - posistor, it R increases with growth tº

Main parameter − temperature coefficient resistance: α = dRt*100/(dT R) is the percentage change R when changing T by 1 degree.

For the thermistor α = − 0.3 ÷ 0.66.

Thermistors are used in temperature control systems, thermal protection, and fire alarm systems.

4. Photoresistor

Resistance R depends on the lighting. A film of photoactive material is applied to a ceramic or glass substrate.

The internal photoelectric effect is used. When illuminated, electrons are excited, they move to a higher energy level, and the concentration of free electrons changes.

Photoresistor connection diagram

Photoresistor characteristics

At Ф = 0 I f 0 – dark current. In the presence of lighting I f increases. The difference in current is called light current or photocurrent.

S = I f/F - sensitivity; dark resistance R T= 10 2 − 10 9 Ohm. U slave = 100 V.

5. Strain gauge , R depends on the deformation of the working fluid. Main characteristic – deformation characteristic – dependence ∆ R/R = f(∆l/l), Where l− length of the working fluid.

Characteristics of strain gauge

Basic parameters R nom = 100 − 500 Ohm; K = ∆ R/R: ∆l/l(−150 ÷ + 150) – strain sensitivity coefficient. Used to measure the deformation of solids.

Classification of semiconductor devices and their purpose.

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

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

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

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

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

Semiconductor diodes

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

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

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

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

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

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

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

The electrical characteristics of a planar diode are determined by the characteristics of the pn junction. Depending on the purpose of the diode, it uses one or another p-n characteristics–transition.

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

A rectifier diode is a semiconductor device in which, just like a point diode, rectifiers are used. p-n properties-transition.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Varicap- a semiconductor diode that uses capacitance dependence

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

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

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

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

The rapid development and expansion of areas of application of electronic devices is due to the improvement of the element base, the basis of which is semiconductor devices. Therefore, to understand the functioning of electronic devices, it is necessary to know the structure and operating principle of the main types of semiconductor devices.

Transistors

A transistor is a semiconductor device designed to amplify, generate and convert electrical signals, as well as switch electrical circuits.

A distinctive feature of the transistor is the ability to amplify voltage and current - the voltages and currents acting at the input of the transistor lead to the appearance of significantly higher voltages and currents at its output.

With the spread of digital electronics and pulse circuits, the main property of a transistor is its ability to be in open and closed states under the influence of a control signal.

The transistor gets its name from the abbreviation of two English words tran(sfer) (re)sistor - controlled resistor. This name is not accidental, since under the influence of the input voltage applied to the transistor, the resistance between its output terminals can be adjusted within a very wide range.

The transistor allows you to regulate the current in the circuit from zero to the maximum value.

Transistor classification:

Based on the operating principle: field (unipolar), bipolar, combined.

According to the value of power dissipation: low, medium and high.

According to the limiting frequency value: low-, medium-, high- and ultra-high-frequency.

According to the operating voltage: low and high voltage.

By functional purpose: universal, amplifier, key, etc.

By design: unframed and packaged, with rigid and flexible leads.

Depending on the functions performed, transistors can operate in three modes:

1) Active mode - used to amplify electrical signals in analog devices. The resistance of the transistor changes from zero to the maximum value - they say the transistor “opens slightly” or “closes slightly”.

2) Saturation mode - the transistor resistance tends to zero. In this case, the transistor is equivalent to a closed relay contact.

3) Cut-off mode - the transistor is closed and has a high resistance, i.e. it is equivalent to an open relay contact.

Saturation and cutoff modes are used in digital, pulse and switching circuits.

Bipolar transistor is a semiconductor device with two p-n junctions and three terminals that provides power amplification of electrical signals.

In bipolar transistors, the current is caused by the movement of charge carriers of two types: electrons and holes, which determines their name.

On diagrams, transistors can be depicted both in a circle and without it (Fig. 3). The arrow indicates the direction of current flow in the transistor.

Figure 3 - Graphic symbols n-p-n transistors(a) and p-n-p (b)

The basis of the transistor is a semiconductor wafer, in which three sections are formed with alternating types of conductivity - electronic and hole. Depending on the alternation of layers, two types of transistor structure are distinguished: n-p-n (Fig. 3, a) and p-n-p (Fig. 3, b).

Emitter (E) - a layer that is a source of charge carriers (electrons or holes) and creates a device current;

Collector (K) – a layer that receives charge carriers coming from the emitter;

Base (B) - the middle layer that controls the transistor current.

When a transistor is connected to an electrical circuit, one of its electrodes is the input (the source of the input alternating signal is turned on), the other is the output (the load is turned on), and the third electrode is common with respect to the input and output. In most cases, a common emitter circuit is used (Figure 4). A voltage of no more than 1 V is supplied to the base, and more than 1 V to the collector, for example +5 V, +12 V, +24 V, etc.

Figure 4 – Connection circuits for a bipolar transistor with a common emitter

The collector current occurs only when the base current Ib flows (determined by Ube). The more Ib, the more Ik. Ib is measured in units of mA, and the collector current is measured in tens and hundreds of mA, i.e. IbIk. Therefore, when an alternating signal of small amplitude is supplied to the base, small Ib will change, and large Ik will change in proportion to it. When a load resistance is connected to the collector circuit, a signal will be emitted on it, repeating the shape of the input, but with a larger amplitude, i.e. amplified signal.

The maximum permissible parameters of transistors primarily include: the maximum permissible power dissipated at the collector Pk.max, the voltage between the collector and the emitter Uke.max, the collector current Ik.max.

To increase the maximum parameters, transistor assemblies are produced, which can number up to several hundred parallel-connected transistors enclosed in one housing.

Bipolar transistors are now used less and less, especially in pulsed power engineering. Their place is taken MOSFET field effect transistors and combined IGBT transistors, which have undoubted advantages in this area of electronics.

In field-effect transistors, the current is determined by the movement of carriers of only one sign (electrons or holes). Unlike bipolar ones, the transistor current is controlled by an electric field, which changes the cross-section of the conducting channel.

Since there is no current flow in the input circuit, the power consumption from this circuit is practically zero, which is undoubtedly an advantage of the field-effect transistor.

Structurally, the transistor consists of an n- or p-type conducting channel, at the ends of which there are areas: a source that emits charge carriers and a drain that receives charge carriers. Electrode used for regulation cross section channel is called a gate.

Field effect transistor is a semiconductor device that regulates the current in a circuit by changing the cross-section of the conductive channel.

There are field-effect transistors with a gate in the form of a p-n junction and with an insulated gate.

Field-effect transistors with an insulated gate have an insulating layer of dielectric between the semiconductor channel and the metal gate - MOS transistors (metal - dielectric - semiconductor), special case- silicon oxide - MOS transistors.

An MOS transistor with a built-in channel has an initial conductivity, which in the absence of an input signal (Uzi = 0) is approximately half of the maximum. In MOS transistors with an induced channel, at voltage Uzi = 0, there is no output current, Ic = 0, since there is initially no conducting channel.

Induced channel MOS transistors are also called MOSFET transistors. They are mainly used as key elements, for example in switching power supplies.

Key elements on MOS transistors have a number of advantages: the signal circuit is not galvanically connected to the source of control action, the control circuit does not consume current, and has bidirectional conductivity. Field-effect transistors, unlike bipolar transistors, are not afraid of overheating.

Transistors

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

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

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

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

Emitter Base Collector

Iuh ITo

EntranceExit

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

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

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

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

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

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

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

Transistor characteristics:

Input, output and transient characteristics, fig. 3,

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

Gain (transmission) in general view, by voltage, current, power

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

Transistor AC input impedance

R = ΔU ВХ / ΔI ВХ.

Collector power loss

P K = U K * I K .

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

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