Switching circuits for bipolar transistors. What is a transistor and how does it work

A transistor belongs to the category of semiconductor devices. In electrical engineering it is used as a generator and amplifier of electrical oscillations. The basis of the device is a crystal located in the housing. To make a crystal, a special semiconductor material is used, whose properties are in an intermediate position between an insulator and a conductor. The transistor is used in radio and electronic circuits. These devices can be... Each of them has its own parameters and characteristics.

Features of bipolar transistors

The electric current in bipolar transistors is formed by electric charges that have positive and negative polarity. Holes carry positive polarity and electrons carry negative polarity. For this type of device, germanium or silicon crystals are used, which have individual characteristics that are taken into account when creating electronic circuits.

The crystal is based on ultrapure materials. Special impurities are added to them in precise dosages. They influence the occurrence of electron or hole conductivity in the crystal. They are designated respectively as n- or p-conductivity. A base is formed, which is one of the electrodes. Special impurities introduced into the crystalline surface change the conductivity of the base to the opposite value. As a result, n-p-n or p-n-p zones are formed, to which the terminals are connected. Thus, a transistor is created.

The source of charge carriers is called an emitter, and the collector of charge carriers is a collector. Between them there is a zone that acts as a base. The terminals of the device are named according to the connected electrodes. When an input signal in the form of a small electrical voltage arrives at the emitter, current will flow in the circuit between it and the collector. The shape of this current coincides with the input signal, but its value increases significantly. This is precisely the amplifying properties of the transistor.

Operation of a field effect transistor

In field-effect transistors, the directional movement of electrons or holes is formed under the influence of an electric field, which is created at the third electrode by the applied voltage. Carriers come out of one electrode, which is why it is called a source. The second electrode, which receives charges, is called the drain. The third electrode, which controls the movement of particles, is called a gate. The conductive section bounded by the drain and source is called a channel, therefore these devices are also known as channel devices. The channel resistance changes under the influence of voltage generated at the gate. This factor affects the electric current flowing through the channel.

The type of charge carriers affects the characteristics. The directional movement of electrons occurs in the n-channel, and holes move in the p-channel. Thus, the current appears under the influence of carriers with only one sign. This is the main difference between field-effect and bipolar transistors.

The operating principle of each field effect transistor is unipolar current, requiring a constant voltage to provide the initial bias. The polarity value depends on the type of channel, and the voltage is related to a particular type of device. In general, they are reliable in operation, can operate in a wide frequency range, and have a high input impedance.

The necessary explanations have been given, let's get to the point.

Transistors. Definition and history

Transistor- an electronic semiconductor device in which the current in a circuit of two electrodes is controlled by a third electrode. (transistors.ru)

Field-effect transistors were the first to be invented (1928), and bipolar transistors appeared in 1947 at Bell Labs. And it was, without exaggeration, a revolution in electronics.

Very quickly, transistors replaced vacuum tubes in various electronic devices. In this regard, the reliability of such devices has increased and their size has decreased significantly. And to this day, no matter how “sophisticated” the microcircuit is, it still contains many transistors (as well as diodes, capacitors, resistors, etc.). Only very small ones.

By the way, initially “transistors” were resistors whose resistance could be changed using the amount of applied voltage. If we ignore the physics of processes, then a modern transistor can also be represented as a resistance that depends on the signal supplied to it.

What is the difference between field-effect and bipolar transistors? The answer lies in their very names. In a bipolar transistor, charge transfer involves And electrons, And holes (“encore” - twice). And in the field (aka unipolar) - or electrons, or holes.

Also, these types of transistors differ in application areas. Bipolar ones are used mainly in analog technology, and field ones - in digital technology.

And finally: the main area of application of any transistors- strengthening of a weak signal due to an additional power source.

Bipolar transistor. Principle of operation. Main characteristics

A bipolar transistor consists of three regions: emitter, base and collector, each of which is supplied with voltage. Depending on the type of conductivity of these areas, n-p-n and p-n-p transistors are distinguished. Typically the collector area is wider than the emitter area. The base is made of a lightly doped semiconductor (which is why it has high resistance) and is made very thin. Since the emitter-base contact area is significantly smaller than the base-collector contact area, it is impossible to swap the emitter and collector by changing the connection polarity. Thus, the transistor is an asymmetrical device.

Before considering the physics of how a transistor operates, let's outline the general problem.

It is as follows: a strong current flows between the emitter and collector ( collector current), and between the emitter and base there is a weak control current ( base current). The collector current will change depending on the change in base current. Why?
Let's consider the p-n junctions of the transistor. There are two of them: emitter-base (EB) and base-collector (BC). In the active mode of operation of the transistor, the first of them is connected with forward bias, and the second with reverse bias. What happens at the p-n junctions? For greater certainty, we will consider an n-p-n transistor. For p-n-p everything is similar, only the word “electrons” needs to be replaced with “holes”.

Since the EB junction is open, electrons easily “run across” to the base. There they partially recombine with holes, but O Most of them, due to the small thickness of the base and its low doping, manage to reach the base-collector transition. Which, as we remember, is reverse biased. And since electrons in the base are minority charge carriers, the electric field of the transition helps them overcome it. Thus, the collector current is only slightly less than the emitter current. Now watch your hands. If you increase the base current, the EB junction will open more strongly, and more electrons will be able to slip between the emitter and collector. And since the collector current is initially greater than the base current, this change will be very, very noticeable. Thus, the weak signal received at the base will be amplified. Once again, a large change in collector current is a proportional reflection of a small change in base current.

I remember that the principle of operation of a bipolar transistor was explained to my classmate using the example of a water tap. The water in it is the collector current, and the base control current is how much we turn the knob. A small force (control action) is enough to increase the flow of water from the tap.

In addition to the processes considered, a number of other phenomena can occur at the p-n junctions of the transistor. For example, with a strong increase in voltage at the base-collector junction, avalanche charge multiplication may begin due to impact ionization. And coupled with the tunnel effect, this will give first an electrical breakdown, and then (with increasing current) a thermal breakdown. However, thermal breakdown in a transistor can occur without electrical breakdown (i.e., without increasing the collector voltage to breakdown voltage). One excessive current through the collector will be enough for this.

Another phenomenon is due to the fact that when the voltages on the collector and emitter junctions change, their thickness changes. And if the base is too thin, then a closing effect may occur (the so-called “puncture” of the base) - a connection between the collector junction and the emitter junction. In this case, the base region disappears and the transistor stops working normally.

The collector current of the transistor in the normal active mode of operation of the transistor is greater than the base current by a certain number of times. This number is called current gain and is one of the main parameters of the transistor. It is designated h21. If the transistor is turned on without load on the collector, then at a constant collector-emitter voltage the ratio of the collector current to the base current will give static current gain. It can be equal to tens or hundreds of units, but it is worth considering the fact that in real circuits this coefficient is smaller due to the fact that when the load is turned on, the collector current naturally decreases.

The second important parameter is transistor input resistance. According to Ohm's law, it is the ratio of the voltage between the base and emitter to the control current of the base. The larger it is, the lower the base current and the higher the gain.

The third parameter of a bipolar transistor is voltage gain. It is equal to the ratio of the amplitude or effective values of the output (emitter-collector) and input (base-emitter) alternating voltages. Since the first value is usually very large (units and tens of volts), and the second is very small (tenths of volts), this coefficient can reach tens of thousands of units. It is worth noting that each base control signal has its own voltage gain.

Transistors also have frequency response, which characterizes the transistor’s ability to amplify a signal whose frequency approaches the cut-off amplification frequency. The fact is that as the frequency of the input signal increases, the gain decreases. This is due to the fact that the time of occurrence of the main physical processes (the time of movement of carriers from the emitter to the collector, the charge and discharge of capacitive barrier junctions) becomes commensurate with the period of change of the input signal. Those. the transistor simply does not have time to react to changes in the input signal and at some point simply stops amplifying it. The frequency at which this happens is called boundary.

Also, the parameters of the bipolar transistor are:

reverse current collector-emitter
on time
reverse collector current
maximum permissible current

The symbols for n-p-n and p-n-p transistors differ only in the direction of the arrow indicating the emitter. It shows how current flows in a given transistor.

Operating modes of a bipolar transistor

The option discussed above represents the normal active mode of operation of the transistor. However, there are several more combinations of open/closed p-n junctions, each of which represents a separate mode of operation of the transistor.

Inverse active mode. Here the BC transition is open, but on the contrary, the EB is closed. The amplification properties in this mode, of course, are worse than ever, so transistors are used very rarely in this mode.
Saturation mode. Both crossings are open. Accordingly, the main charge carriers of the collector and emitter “run” to the base, where they actively recombine with its main carriers. Due to the resulting excess of charge carriers, the resistance of the base and p-n junctions decreases. Therefore, a circuit containing a transistor in saturation mode can be considered short-circuited, and this radio element itself can be represented as an equipotential point.
Cut-off mode. Both transitions of the transistor are closed, i.e. the current of the main charge carriers between the emitter and collector stops. Flows of minority charge carriers create only small and uncontrollable thermal currents of transitions. Due to the poverty of the base and transitions with charge carriers, their resistance increases greatly. Therefore, it is often believed that a transistor operating in cutoff mode represents an open circuit.
Barrier mode In this mode, the base is directly or through a low resistance connected to the collector. A resistor is also included in the collector or emitter circuit, which sets the current through the transistor. This creates the equivalent of a diode circuit with a resistor in series. This mode is very useful, as it allows the circuit to operate at almost any frequency, over a wide temperature range and is undemanding to the parameters of the transistors.

Switching circuits for bipolar transistors

Since the transistor has three contacts, in general, power must be supplied to it from two sources, which together produce four outputs. Therefore, one of the transistor contacts has to be supplied with a voltage of the same sign from both sources. And depending on what kind of contact it is, there are three circuits for connecting bipolar transistors: with a common emitter (CE), a common collector (OC) and a common base (CB). Each of them has both advantages and disadvantages. The choice between them is made depending on which parameters are important to us and which can be sacrificed.

Connection circuit with common emitter

This circuit provides the greatest gain in voltage and current (and hence in power - up to tens of thousands of units), and therefore is the most common. Here the emitter-base junction is turned on directly, and the base-collector junction is turned on reversely. And since both the base and the collector are supplied with voltage of the same sign, the circuit can be powered from one source. In this circuit, the phase of the output AC voltage changes relative to the phase of the input AC voltage by 180 degrees.

But in addition to all the goodies, the OE scheme also has a significant drawback. It lies in the fact that an increase in frequency and temperature leads to a significant deterioration in the amplification properties of the transistor. Thus, if the transistor must operate at high frequencies, then it is better to use a different switching circuit. For example, with a common base.

Connection diagram with a common base

This circuit does not provide significant signal amplification, but is good at high frequencies, since it allows more full use of the frequency response of the transistor. If the same transistor is connected first according to a circuit with a common emitter, and then with a common base, then in the second case there will be a significant increase in its cutoff frequency of amplification. Since with such a connection the input impedance is low and the output impedance is not very high, transistor cascades assembled according to the OB circuit are used in antenna amplifiers, where the characteristic impedance of the cables usually does not exceed 100 Ohms.

In a common-base circuit, the signal phase does not invert, and the noise level at high frequencies is reduced. But, as already mentioned, its current gain is always slightly less than unity. True, the voltage gain here is the same as in a circuit with a common emitter. The disadvantages of a common base circuit also include the need to use two power supplies.

Connection diagram with a common collector

The peculiarity of this circuit is that the input voltage is completely transmitted back to the input, i.e. the negative feedback is very strong.

Let me remind you that negative feedback is such feedback in which the output signal is fed back to the input, thereby reducing the level of the input signal. Thus, automatic adjustment occurs when the input signal parameters accidentally change

The current gain is almost the same as in the common emitter circuit. But the voltage gain is small (the main drawback of this circuit). It approaches unity, but is always less than it. Thus, the power gain is equal to only a few tens of units.

In a common collector circuit, there is no phase shift between the input and output voltage. Since the voltage gain is close to unity, the output voltage matches the input voltage in phase and amplitude, i.e., repeats it. That is why such a circuit is called an emitter follower. Emitter - because the output voltage is removed from the emitter relative to the common wire.

This connection is used to match transistor stages or when the input signal source has a high input impedance (for example, a piezoelectric pickup or a condenser microphone).

Two words about cascades

It happens that you need to increase the output power (i.e. increase the collector current). In this case, parallel connection of the required number of transistors is used.

Naturally, they should be approximately the same in characteristics. But it must be remembered that the maximum total collector current should not exceed 1.6-1.7 of the maximum collector current of any of the cascade transistors.
However (thanks for the note), it is not recommended to do this in the case of bipolar transistors. Because two transistors, even of the same type, are at least slightly different from each other. Accordingly, when connected in parallel, currents of different magnitudes will flow through them. To equalize these currents, balanced resistors are installed in the emitter circuits of the transistors. The value of their resistance is calculated so that the voltage drop across them in the operating current range is at least 0.7 V. It is clear that this leads to a significant deterioration in the efficiency of the circuit.

There may also be a need for a transistor with good sensitivity and at the same time good gain. In such cases, a cascade of a sensitive but low-power transistor (VT1 in the figure) is used, which controls the power supply of a more powerful fellow (VT2 in the figure).

Other applications of bipolar transistors

Transistors can be used not only in signal amplification circuits. For example, due to the fact that they can operate in saturation and cutoff modes, they are used as electronic keys. It is also possible to use transistors in signal generator circuits. If they operate in the key mode, then a rectangular signal will be generated, and if in the amplification mode, then a signal of arbitrary shape, depending on the control action.

Marking

Since the article has already grown to an indecently large volume, at this point I will simply give two good links, which describe in detail the main marking systems for semiconductor devices (including transistors): http://kazus.ru/guide/transistors/mark_all .html and .xls file (35 kb).

Helpful comments:
http://habrahabr.ru/blogs/easyelectronics/133136/#comment_4419173

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A radio-electronic element made of semiconductor material, using an input signal, creates, amplifies, and changes pulses in integrated circuits and systems for storing, processing and transmitting information. A transistor is a resistance whose functions are regulated by the voltage between the emitter and base or source and gate, depending on the type of module.

Types of transistors

Converters are widely used in digital and analog IC manufacturing to null static consumer current and achieve improved linearity. The types of transistors differ in that some are controlled by changing the voltage, while others are controlled by changing the current.

Field modules operate with increased DC resistance; high-frequency transformation does not increase energy costs. If we say what a transistor is in simple words, then it is a module with a high gain limit. This characteristic is greater in field species than in bipolar types. The former do not have resorption of charge carriers, which speeds up the work.

Field-field semiconductors are used more often due to their advantages over bipolar types:

powerful input resistance at constant current and high frequency, this reduces energy loss for control;
lack of accumulation of minority electrons, which speeds up the operation of the transistor;
transfer of mobile particles;
stability under temperature deviations;
small noise due to lack of injection;
low power consumption during operation.

The types of transistors and their properties determine their purpose. Heating a bipolar type converter increases the current along the path from the collector to the emitter. They have a negative resistance coefficient, and moving carriers flow to the collecting device from the emitter. The thin base is separated by p-n junctions, and the current arises only when mobile particles accumulate and are injected into the base. Some charge carriers are captured by the neighboring p-n junction and accelerated, this is how the transistor parameters are calculated.

Field-effect transistors have one more type of advantage that needs to be mentioned for dummies. They are connected in parallel without equalizing the resistance. Resistors are not used for this purpose, since the indicator increases automatically when the load changes. To obtain a high value of switching current, a set of modules is assembled, which is used in inverters or other devices.

It cannot be connected in parallel; the determination of functional parameters leads to the detection of a thermal breakdown of an irreversible nature. These properties are related to the technical qualities of simple p-n channels. The modules are connected in parallel using resistors to equalize the current in the emitter circuits. Depending on the functional features and individual specifics, the classification of transistors includes bipolar and field-effect types.

Bipolar transistors

Bipolar designs are manufactured as semiconductor devices with three conductors. Each electrode contains layers with hole p-conductivity or impurity n-conductivity. The choice of layer configuration determines the production of p-n-p or n-p-n types of devices. When the device is turned on, different types of charges are simultaneously transferred by holes and electrons, and 2 types of particles are involved.

Carriers move due to the diffusion mechanism. Atoms and molecules of a substance penetrate into the intermolecular lattice of the neighboring material, after which their concentration is equalized throughout the entire volume. Transport occurs from areas of high density to areas of low density.

Electrons also propagate under the influence of a force field around particles when alloying additives are unevenly included in the base mass. To speed up the operation of the device, the electrode connected to the middle layer is made thin. The outermost conductors are called the emitter and collector. The reverse voltage characteristic of the transition is not important.

Field effect transistors

The field effect transistor controls the resistance using the electric transverse field generated by the applied voltage. The place from which electrons move into the channel is called the source, and the drain appears as the final point of entry of charges. The control voltage passes through a conductor called the gate. Devices are divided into 2 types:

with control p-n junction;
MIS transistors with insulated gate.

Devices of the first type contain a semiconductor wafer that is connected to a controlled circuit using electrodes on opposite sides (drain and source). A place with a different type of conductivity appears after connecting the plate to the gate. A constant bias source inserted into the input circuit produces a blocking voltage at the junction.

The source of the amplified pulse is also located in the input circuit. After changing the voltage at the input, the corresponding indicator at the p-n junction is transformed. The thickness of the layer and the cross-sectional area of the channel transition in the crystal, which transmits a flow of charged electrons, are modified. The channel width depends on the space between the depletion region (under the gate) and the substrate. The control current at the start and end points is controlled by changing the width of the depletion region.

The MIS transistor is characterized by the fact that its gate is separated by insulation from the channel layer. Doped sites with the opposite sign are created in the semiconductor chip, called the substrate. Conductors are installed on them - drain and source, between which a dielectric is located at a distance of less than a micron. A metal electrode – a gate – is applied to the insulator. Due to the resulting structure containing a metal, a dielectric layer and a semiconductor, transistors are given the abbreviation MIS.

Design and principle of operation for beginners

Technologies operate not only with the charge of electricity, but also with a magnetic field, light quanta and photons. The principle of operation of a transistor is the states between which the device switches. Opposite small and large signals, open and closed states - this is the dual operation of the devices.

Together with the semiconductor material in the composition, used in the form of a single crystal, doped in some places, the transistor has in its design:

metal leads;
dielectric insulators;
transistor housing made of glass, metal, plastic, metal ceramics.

Before the invention of bipolar or polar devices, electronic vacuum tubes were used as active elements. The circuits developed for them, after modification, are used in the production of semiconductor devices. They could be connected like a transistor and used, since many of the functional characteristics of the lamps are suitable for describing the operation of field species.

Advantages and disadvantages of replacing lamps with transistors

The invention of transistors is a stimulating factor for the introduction of innovative technologies in electronics. The network uses modern semiconductor elements; compared to old tube circuits, such developments have advantages:

small dimensions and light weight, which is important for miniature electronics;
the ability to apply automated processes in the production of devices and group stages, which reduces costs;
the use of small-sized current sources due to the need for low voltage;
instantaneous switching on, no cathode heating required;
increased energy efficiency due to reduced power dissipation;
strength and reliability;
smooth interaction with additional elements on the network;
resistance to vibration and shock.

Disadvantages are manifested in the following provisions:

silicon transistors do not function at voltages above 1 kW, lamps are effective at values above 1-2 kW;
when using transistors in high-power radio broadcasting networks or microwave transmitters, matching of low-power amplifiers connected in parallel is required;
vulnerability of semiconductor elements to the effects of electromagnetic signals;
sensitive reaction to cosmic rays and radiation, requiring the development of radiation-resistant microcircuits in this regard.

Connection schemes

To operate in a single circuit, a transistor requires 2 outputs at the input and output. Almost all types of semiconductor devices have only 3 connection points. To get out of a difficult situation, one of the ends is assigned as common. This leads to 3 common connection schemes:

for bipolar transistor;
polar device;
with an open drain (collector).

The bipolar module is connected to a common emitter for both voltage and current (CE) amplification. In other cases, it matches the pins of a digital chip when there is a high voltage between the external circuit and the internal connection plan. This is how a connection with a common collector works, and only an increase in current is observed (OK). If an increase in voltage is needed, then the element is introduced with a common base (CB). This option works well in compound cascade circuits, but is rarely used in single-transistor projects.

Field semiconductor devices of MIS varieties and using a p-n junction are included in the circuit:

with a common emitter (COM) - a connection similar to a bipolar-type OE module
with a single exit (OS) - OK type plan;
with a joint shutter (OZ) - a similar description of OB.

In open-drain plans, the transistor is connected with a common emitter as part of the microcircuit. The collector terminal is not connected to other parts of the module, and the load goes to the external connector. The choice of voltage intensity and collector current is made after installation of the project. Open-drain devices operate in circuits with powerful output stages, bus drivers, and TTL logic circuits.

What are transistors used for?

The area of application is delimited depending on the type of device - bipolar module or field. Why are transistors needed? If low current is required, for example, in digital plans, field types are used. Analog circuits achieve high gain linearity over a wide range of supply voltages and output parameters.

Application areas for bipolar transistors are amplifiers, their combinations, detectors, modulators, transistor logistics circuits and logic type inverters.

The places where transistors are used depend on their characteristics. They work in 2 modes:

in amplification order, changing the output pulse with small deviations of the control signal;
in the key regulation, controlling the power supply to loads at low input current, the transistor is completely closed or open.

The type of semiconductor module does not change its operating conditions. The source is connected to a load, for example, a switch, an audio amplifier, a lighting device, it can be an electronic sensor or a powerful adjacent transistor. With the help of current, the load device begins to operate, and the transistor is connected to the circuit between the installation and the source. The semiconductor module limits the amount of energy supplied to the unit.

The resistance at the output of the transistor is transformed depending on the voltage on the control conductor. The current and voltage at the beginning and end of the circuit changes and increases or decreases and depends on the type of transistor and how it is connected. Control of the controlled power supply leads to an increase in current, a power pulse or an increase in voltage.

Transistors of both types are used in the following cases:

In digital regulations. Experimental designs of digital amplification circuits based on digital-to-analog converters (DACs) have been developed.
In pulse generators. Depending on the type of unit, the transistor operates in switch or linear order to reproduce square or arbitrary signals, respectively.
In electronic hardware devices. To protect information and programs from theft, illegal hacking and use. The operation takes place in key mode, the current strength is controlled in analog form and adjusted using the pulse width. Transistors are used in electric motor drives and pulse voltage stabilizers.

Monocrystalline semiconductors and loop opening and closing modules increase power but function only as switches. In digital devices, field-effect transistors are used as cost-effective modules. Manufacturing technologies in the concept of integrated experiments involve the production of transistors on a single silicon chip.

Miniaturization of chips leads to faster computers, lower power consumption, and less heat generation.

This is how a diode works

This is such a cunning thing that passes current only in one direction. It can be compared to a nipple. It is used, for example, in rectifiers, when alternating current is converted into direct current. Or when you need to separate the reverse voltage from the forward voltage. Look at the programmer circuit (where there was an example with a divider). You see there are diodes, why do you think? It's simple. For a microcontroller, the logical levels are 0 and 5 volts, and for the COM port, one is minus 12 volts, and zero is plus 12 volts. So the diode cuts off this minus 12, forming 0 volts. And since the diode’s conductivity in the forward direction is not ideal (it generally depends on the applied forward voltage; the higher it is, the better the diode conducts current), then its resistance will drop approximately 0.5-0.7 volts, the remainder, being divided in half by the resistors, will be approximately 5.5 volts, which is within the controller’s normal limits.
The leads of the diode are called the anode and cathode. Current flows from the anode to the cathode. It’s very easy to remember where each conclusion is: on the symbol there is an arrow and a stick on the side To it's like drawing a letter TO here look - TO|—. K= Cathode! And on the part the cathode is indicated by a stripe or a dot.

There is another interesting type of diode - zener diode. I used it in one of the previous articles. Its peculiarity is that in the forward direction it works like a regular diode, but in the reverse direction it breaks down at some voltage, for example 3.3 volts. Similar to the limit valve of a steam boiler, which opens when the pressure is exceeded and releases excess steam. Zener diodes are used when they want to obtain a voltage of a given value, regardless of the input voltages. This could be, for example, a reference value against which the input signal is compared. They can cut the incoming signal to the desired value or use it as protection. In my circuits, I often use a 5.5-volt zener diode to power the controller, so that if something happens, if the voltage suddenly jumps, this zener diode will bleed off the excess through itself. There is also such a beast as a suppressor. The same zener diode, only much more powerful and often bidirectional. Used for power protection.

Transistor.

It’s a terrible thing, as a child I couldn’t understand how it worked, but it turned out to be simple.
In general, a transistor can be compared to a controlled valve, where with a tiny effort we control a powerful flow. He turned the handle a little and tons of shit rushed through the pipes, he opened it harder and now everything around was drowning in sewage. Those. The output is proportional to the input multiplied by some value. This value is the gain.
These devices are divided into field and bipolar.
A bipolar transistor has emitter, collector And base(see picture of the symbol). The emitter has an arrow, the base is designated as a straight area between the emitter and the collector. There is a large payload current between the emitter and collector, the direction of the current is determined by the arrow on the emitter. But between the base and emitter there is a small control current. Roughly speaking, the magnitude of the control current affects the resistance between the collector and emitter. Bipolar transistors are of two types: p-n-p And n-p-n the fundamental difference is only in the direction of the current through them.

A field-effect transistor differs from a bipolar transistor in that in it the resistance of the channel between the source and drain is determined not by the current, but by the voltage at the gate. Recently, field-effect transistors have gained enormous popularity (all microprocessors are built on them), because the currents flow in them are microscopic, voltage plays a decisive role, which means losses and heat generation are minimal.

In short, the transistor will allow you to receive a weak signal, for example from the leg of a microcontroller, . If the gain of one transistor is not enough, then they can be connected in cascades - one after the other, more and more powerful. And sometimes one mighty field man is enough MOSFET transistor. Look, for example, at how the vibration alert is controlled in cell phone circuits. There the output from the processor goes to the power gate MOSFET key

Transistors are active components and are used throughout electronic circuits as amplifiers and switching devices (transistor switches). As amplification devices, they are used in high and low frequency devices, signal generators, modulators, detectors and many other circuits. In digital circuits, switching power supplies and controlled electric drives, they serve as switches.

Bipolar transistors

This is the name of the most common type of transistor. They are divided into npn and pnp types. The material most often used for them is silicon or germanium. At first, transistors were made from germanium, but they were very sensitive to temperature. Silicon devices are much more resistant to its fluctuations and are cheaper to produce.

Various bipolar transistors are shown in the photo below.

Low-power devices are located in small plastic rectangular or metal cylindrical cases. They have three terminals: for the base (B), emitter (E) and collector (K). Each of them is connected to one of three layers of silicon with conductivity of either n-type (the current is generated by free electrons) or p-type (the current is generated by the so-called positively charged “holes”), which make up the structure of the transistor.

How does a bipolar transistor work?

The principles of operation of a transistor need to be studied, starting with its design. Consider the structure of an NPN transistor, which is shown in the figure below.

As you can see, it contains three layers: two with n-type conductivity and one with p-type conductivity. The type of conductivity of the layers is determined by the degree of doping of various parts of the silicon crystal with special impurities. The n-type emitter is very heavily doped to provide many free electrons as the majority current carriers. The very thin p-type base is lightly doped with impurities and has high resistance, and the n-type collector is very heavily doped to give it low resistance.

Transistor operating principles

The best way to get to know them is through experimentation. Below is a diagram of a simple circuit.

It uses a power transistor to control the light bulb. You will also need a battery, a small flashlight bulb of approximately 4.5 V/0.3 A, a variable resistor potentiometer (5K) and a 470 ohm resistor. These components must be connected as shown in the figure to the right of the diagram.

Turn the potentiometer slider to its lowest position. This will lower the base voltage (between base and ground) to zero volts (U BE = 0). The lamp does not light, which means there is no current flowing through the transistor.

If you now turn the handle from its lower position, then U BE gradually increases. When it reaches 0.6 V, current begins to flow into the base of the transistor and the lamp begins to glow. When the handle is moved further, the voltage U BE remains at 0.6 V, but the base current increases and this increases the current through the collector-emitter circuit. If the handle is moved to the up position, the base voltage will increase slightly to 0.75 V, but the current will increase significantly and the lamp will glow brightly.

What if you measure the transistor currents?

If we connect an ammeter between the collector (C) and the lamp (to measure I C), another ammeter between the base (B) and the potentiometer (to measure I B), and a voltmeter between common and base and repeat the whole experiment, we can get some interesting data. When the potentiometer knob is in its lowest position, U BE is 0 V, as are the currents IC and I B. When the handle is moved, these values increase until the light bulb begins to glow, when they are equal: U BE = 0.6 V, I B = 0.8 mA and IC = 36 mA.

As a result, we get from this experiment the following principles of transistor operation: in the absence of a positive (for npn-type) bias voltage at the base, the currents through its terminals are zero, and in the presence of base voltage and current, their changes affect the current in the collector-emitter circuit.

What happens when you turn on the power of a transistor

During normal operation, the voltage applied to the base-emitter junction is distributed such that the potential of the base (p-type) is approximately 0.6 V higher than that of the emitter (n-type). In this case, a forward voltage is applied to this junction, it is biased in the forward direction and is open for current to flow from the base to the emitter.

A much higher voltage is applied across the base-collector junction, with the collector (n-type) potential being higher than that of the base (p-type). So a reverse voltage is applied to the junction and it is reverse biased. This results in the formation of a fairly thick electron-depleted layer in the collector near the base when supply voltage is applied to the transistor. As a result, no current passes through the collector-emitter circuit. The distribution of charges in the junction zones of an npn transistor is shown in the figure below.

What is the role of base current?

How can we make our electronic device work? The principle of operation of the transistor is the influence of the base current on the state of the closed base-collector junction. When the base-emitter junction is forward biased, a small current will flow into the base. Here its carriers are positively charged holes. These combine with electrons coming from the emitter to produce a current I BE. However, due to the fact that the emitter is very heavily doped, many more electrons flow from it into the base than can be combined with holes. This means that there is a large concentration of electrons in the base, and most of them cross it and enter the electron-depleted collector layer. Here they come under the influence of a strong electric field applied to the base-collector junction, pass through the electron-depleted layer and the main volume of the collector to its output.

Changes in the current flowing into the base affect the number of electrons attracted from the emitter. Thus, the operating principles of the transistor can be supplemented by the following statement: very small changes in the base current cause very large changes in the current flowing from the emitter to the collector, i.e. the current increases.

Types of field effect transistors

In English they are designated FETs - Field Effect Transistors, which can be translated as “field effect transistors”. Although there is a lot of confusion about the names for them, there are mainly two main types:

1. With a control pn junction. In English-language literature they are designated JFET or Junction FET, which can be translated as “junction field-effect transistor”. Otherwise they are called JUGFET or Junction Unipolar Gate FET.

2. With an insulated gate (otherwise MOS or MOS transistors). In English they are designated IGFET or Insulated Gate FET.

Outwardly, they are very similar to bipolar ones, as confirmed by the photo below.

Field effect transistor device

All field-effect transistors can be called UNIPOLAR devices, because the charge carriers that form the current through them are of a single type for a given transistor - either electrons or “holes,” but not both at the same time. This distinguishes the principle of operation of a field-effect transistor from a bipolar one, in which the current is generated simultaneously by both of these types of carriers.

Current carriers flow in junction field effect transistors through a layer of silicon without junctions, called a channel, with either n- or p-type conductivity between two terminals called "source" and "drain" - analogues of emitter and collector or , more precisely, the cathode and anode of a vacuum triode. The third terminal - the gate (analogue of the triode grid) - is connected to a layer of silicon with a different type of conductivity than that of the source-drain channel. The structure of such a device is shown in the figure below.

How does a field effect transistor work? Its operating principle is to control the channel cross-section by applying a voltage to the gate-channel junction. It is always reverse biased, so the transistor consumes virtually no current in the gate circuit, whereas a bipolar device requires a certain base current to operate. As the input voltage changes, the gate area can expand, blocking the source-drain channel until it is completely closed, thus controlling the drain current.