What is the purpose of a transistor in an electrical circuit? The principle of operation of the transistor, the internal structure and the main characteristics of transistors. What are transistors for?

Any electronic device consists of radio elements. They can be passive, not requiring a power source, and active, the operation of which is possible only when voltage is applied. Semiconductors are called active elements. One of the most important semiconductor devices is the transistor. This radio element replaced lamp devices and completely changed the circuitry of devices. All microelectronics and the operation of any microcircuit is based on it.

The name "transistor" comes from the fusion of two English words: transfer - transferable, and resistor - resistance. In the generally accepted concept, this is a semiconductor element with three terminals. In it, the magnitude of the current at two terminals depends on the third, when the current or voltage changes at which the current value of the output circuit is controlled. Bipolar devices are controlled by current variation, and field devices are controlled by voltage.

The first developments of the transistor were started in the 20th century. In Germany, the scientist Julius Edgar Lilienfeld described the principle of the transistor, and already in 1934, the physicist Oscar Hale registered a device later called the transistor. Such a device worked on the electrostatic field effect.

Physicists William Shockley, Walter Brattain, together with scientist John Bardeen, made the first model of a point transistor in the late 40s. WITH opening n-p transition, the production of a point transistor ceased, and instead, the development of planar devices from germanium began. The working prototype of the transistor was officially presented in December 1947. On this day, the first bipolar transistor appeared. In the summer of 1948, devices made on a transistor basis began to be sold. From that moment on, the vacuum tubes (triodes) common at that time began to become a thing of the past.

In the mid-1950s, the first junction transistor was mass-produced by Texas Instruments, silicon was used as the material for its manufacture. At that time, in the production of the radioelement, a lot of marriage came out, but this did not prevent the technological development of the device. In 1953, a circuit used in hearing aids was made on transistors, and a year later, American physicists received the Nobel Prize for their discovery.

March 1959 was marked by the creation of the first silicon planar device, its developer was a physicist from Switzerland, Jean Erni. A pair of transistors has been successfully placed on a single silicon chip. From that moment on, the development of integrated circuitry began. Today, there are more than a billion transistors in a single chip. For example, on the popular 8-core computer processor Core i7-5960X their number is 2.6 billion pieces.

In parallel with the improvements in the bipolar transistor, in the 60s, the development of a device based on the connection of a metal with a semiconductor began. Such a radio element was called a MOS (metal-oxide-semiconductor) transistor, today better known under the designation "mosfet".

Initially, the term "transistor" referred to resistance, the value of which was controlled by voltage, since a transistor can be thought of as a kind of resistor that is regulated by the applied potential at one terminal. For field-effect transistors, the comparison with which is more correct, it is the potential at the gate, and for bipolar transistors, the potential at the base or the base current.

The basis of the device is abilities n-p junction to pass current in one direction. When a voltage is applied at one junction, its direct drop occurs, and at the other, it reverses. The transition zone with forward voltage has low resistance, and with reverse voltage it has high resistance. A small control current flows between the base and emitter. The value of this current changes the resistance between the collector and the emitter. There are two types of bipolar device:

• p-n-p;
• n-p-n.

The difference lies only in the main charge carriers, i.e., the direction of the current.

If you connect two semiconductors different type between themselves, then a region or, as is commonly called, a p-n junction, appears at the connection boundary. The type of conduction depends on atomic structure material, namely how strong the bonds in the material. Atoms in a semiconductor are arranged in a lattice, and in itself such a material is not a conductor. But if atoms of another material are added to the lattice, then the physical properties of the semiconductor change. The admixed atoms form, depending on their nature, free electrons or holes.

The formed free electrons form a negative charge, and holes form a positive one. There is a potential barrier in the transition area. It is formed by a contact potential difference, and its height does not exceed tenths of a volt, preventing charge carriers from flowing deep into the material. If the transition is under direct voltage, then the value of the potential barrier decreases, and the value of the current passing through it increases. When a reverse voltage is applied, the size of the barrier increases and the resistance of the barrier to the passage of current increases. realizing work p-n transition, you can figure out how the transistor works.

First of all, such devices are divided into single and composite. There are also so-called complex radioelements. They have three conclusions and are made as a whole. Such assemblies contain both the same type and transistors of different types. The main division of devices occurs according to the following features:

The general definition for a radio element can be formulated as follows: a transistor is a semiconductor element designed to convert electrical quantities. Its main application is to amplify the signal or work in key mode.

The principle of operation of a transistor for a "teapot" is easier to describe by analogy with a water pipe. The element itself can be represented as a valve. The tap with a slight turn allows you to adjust the flow of water (current strength). If you turn the handle a little, water will flow through the pipe (conductor), if you open the tap even more, the water flow will also increase. Thus, the output of a stream of water is proportional to its input, multiplied by a certain value. This value is called the gain.

A bipolar transistor has three terminals: emitter, base, collector. The emitter and collector have the same type of conductivity, which is different from the base. Hole-type transistors consist of two regions of p-type conductivity, and one n-type. The electronic type is the opposite. Each area has its own output.

When a signal of the required conductivity is applied to the emitter, the current in the base region increases. The main charge carriers move to the base zone, which leads to an increase in current in the reverse connection area. There is a bulk charge. Electric field begins to draw carriers of a different sign into the reverse connection zone. In the base, a partial recombination (destruction) of charges of the opposite sign occurs, due to which the base current arises.

The emitter is the area of ​​the device that serves to transfer charge carriers to the base. A collector is a zone designed to extract charge carriers from the base. And the base is the area for the emitter to transfer the opposite amount of charge. The main characteristic of the device is volt-ampere characteristics, whose function describes the relationship between current and voltage.

In the diagram, the device is signed in Latin letters VT or Q. It looks like a circle with an arrow inside, where the arrow indicates the direction of current flow. For PNP (forward conduction) the arrow is inward, and for NPN (reverse conduction) the arrow is outward. In order to make a transistor, germanium or silicon is used. These materials differ in the working area of ​​the voltage of the base transition. For germanium, it lies in the range of 0.1-0.4 V, and for silicon, from 0.4 to 1.2 V. Silicon is usually used.

The difference between a field-effect transistor and a bipolar one is that in it the voltage value applied to the controlled contact is responsible for the passage of current.

The main purpose of mosfets is associated with their good switching speed with very little power applied to the control pin. The field element has three outputs: gate, drain, source. When operating the mosfet with manager n-p transition potential at the gate is either equal to zero (the device is open), or has a certain value greater than zero (the device is closed). When the reverse voltage reaches a certain level, the barrier layer opens and the device goes into cutoff mode.

In a mosfet with a p-n junction, the control electrode (gate) is a semiconductor layer with p-type conductivity, and an n-type channel with the opposite conductivity.

Its image on the diagram is similar to a bipolar device, only all lines are straight, and the arrow inside emphasizes the type of device. The principle of operation of MOS devices is based on the effect of a change in the conductivity of a semiconductor at the boundary of a region with a dielectric when exposed to an electric field. Field devices depending on the controlled p-n junction can be:

Each species can have both p-type and n-type conductivity. In a general sense, the principle of operation does not depend on conductivity, only the polarity of the voltage source changes.

A transistor is a complex device, the physical processes taking place in which are difficult for beginner radio amateurs (dummies) to understand. How a transistor works can be explained as follows: a transistor is an electronic switch, the degree of opening of which depends on the level of current or voltage applied to its controlled output (base or gate).

Why a transistor is needed can be described in a generalized form. For example, the base (shutter) of the device is a door. It is opened by an external influence, i.e., by a voltage of the same polarity as the collector (source). The greater the voltage, the more the door will open. There is a queue of people (charge carriers) in front of the door who want to run through it (collector-emitter or source-drain). The greater the impact on the door, the more it is open, which means that more people will run through.

Therefore, representing the door in the form of transition resistance, we can conclude: the greater the impact on the base (gate), the lower the resistance to the main charge carriers (people) in the case of direct polarity. If the polarity changes (the door is locked), then there will be no movement of charges (people).

This is such a tricky 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 made direct. Or when it is necessary to separate the reverse voltage from the direct one. Look at the programmer circuit (where there was an example with a divider). You see there are diodes, what do you think, why? And everything is simple. For the microcontroller, the logic levels are 0 and 5 volts, and for the COM port, one is minus 12 volts, and zero is plus 12 volts. Here the diode cuts off this minus 12, forming 0 volts. And since the conductivity of the diode in the forward direction is not ideal (it generally depends on the applied forward voltage, the larger it is, the better the diode conducts current), then about 0.5-0.7 volts will drop on its resistance, the remainder, being divided by resistors in two, will be approximately 5.5 volts, which is within the limits of the controller.
The terminals of a diode are called an anode and a cathode. Current flows from the anode to the cathode. It is very simple to remember where which conclusion is: on the symbol, the arrow and the stick from the side To as if drawing a letter TO here look - TO|—. K = Cathode! And on the part, the cathode is indicated by a strip 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 conventional diode, but in the reverse direction it breaks off at some voltage, for example, at 3.3 volts. Similar to a steam boiler pressure relief valve that opens when pressure is exceeded and bleeds off excess steam. Zener diodes are used when they want to get a voltage of a given value, regardless of the input voltages. This can 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 put a 5.5 volt zener diode to power the controller, so that if something happens, if the voltage jumps sharply, this zener diode bleeds 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 nutrition protection.

Transistor.

A terrible thing, as a child I could not understand how it works, but everything turned out to be simple.
In general, a transistor can be compared to a controlled valve, where we control the most powerful flow with a tiny effort. He turned the handle a little and tons of shit rushed through the pipes, opened it harder and now everything around was choked 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.
The bipolar transistor has emitter, collector And base(see picture symbol). It has an emitter with an arrow, the base is designated as a straight platform between the emitter and the collector. There is a large payload current between emitter and collector, the direction of the current is determined by the arrow on the emitter. But between the base and the 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 one in that in it the channel resistance between the source and drain is no longer determined by the current, but by the gate voltage. Recently, field-effect transistors have gained immense popularity (all microprocessors are built on them), because. microscopic currents flow in them, voltage plays a decisive role, which means that losses and heat generation are minimal.

In short, the transistor will allow you a weak signal, for example from the foot of the microcontroller,. If the amplification 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 MOSFET transistor. Look, for example, as in the diagrams cell phones vibrating alert is controlled. There, the output from the processor goes to the gate of the power MOSFET key.

A transistor, otherwise called a semiconductor triode, is an electronic device based on semiconductor materials. The main purpose of the device is the ability, by changing the low current in the control circuit, to obtain an amplified signal at the output. The semiconductor triode is one of the main components of the circuits of the set electronic devices from radio to computer.

The definition of "transistor" is closely related to the etymology of this word. It is formed from two English words: transfer (transfer) and resistor (resistance). Indeed, the principle of operation of the device is associated with the transfer (change) of resistance in electrical circuit.

• bipolar;
• field (unipolar).

Each class, in turn, is divided into several varieties.

Bipolar:

Both of these types of triodes can be used in the same electronic circuit. Therefore, in order not to confuse which part should be used in a particular place in the circuit, p-n-p images and n-p-n triodes are different from each other.

Field:

• unipolar with p-n junction;
• MIS transistors with an insulated gate.

The principle of operation of the device

In electronics, semiconductors with electronic (n) or hole (p) conductivity are used. These designations indicate that in the first case, negatively charged electrons predominate in the semiconductor, in the second, positively charged holes.

Let's consider how a transistor is arranged using the example of a bipolar semiconductor triode. Externally, the device looks like a small part in a metal or plastic case with three leads. Inside - a kind of sandwich of three layers of semiconductor. If the central layer is p-type, then the surrounding layers are n-type. It turns out an n-p-n triode. If the center, also called the base, is n-type, then the plates are made of a semiconductor with hole conductivity, and the structure of the device is p-n-p. One of the outer layers is called an emitter, the other a collector. Each of these three parts of the device is connected to a corresponding conclusion.

A brief explanation of how a transistor works, for "dummies" looks like this. Let's take for example NPN transistor, where the emitter and collector are layers with predominantly electronic conductivity, and the base is with hole.

We connect the emitter to the negative terminal of the electric battery, and the base and collector to the positive terminal. For a novice electronics enthusiast, you can imagine that A triode is made up of two diodes., moreover, the emitter-base diode is turned on in the forward direction, and current flows through it, and the base-collector diode is turned on in the opposite direction, and there is no current.

Suppose we have included a variable resistor in the base circuit, with which we can regulate the voltage supplied to the base. What effect will we get when the voltage is reduced to zero? The current in the emitter-base circuit will stop flowing. Let's increase the voltage a bit. Electrons from the n - emitter region will rush to the base connected to the battery plus.

An important detail - the base is made as thin as possible. Therefore, the mass of electrons passes through this layer and ends up in the collector under the influence of the positive pole of the battery, to which it is attracted. Thus, the current begins to pass not only between the emitter and the base, but also between the emitter and the collector. In this case, the collector current is much greater than the base current.

Another important circumstance: A small change in the base current causes a much larger change in the collector current. Thus, a semiconductor triode serves to amplify various signals. Typically, bipolar triodes are more commonly used in analog technology.

FETs

This type of triode differs from the bipolar one not in properties or functions, but in the principle of operation. In a field triode, current flows from a terminal called the source to a terminal called the drain through a semiconductor of one type of conductivity, for example, p. And the control of the strength of this current is carried out by changing the voltage at the third output - the gate.

Such a structure more accurately meets the requirements of modern digital technology, where field triodes are mainly used. Today's technological capabilities make it possible to place several billion MOS elements with an insulated gate on a semiconductor chip with an area of ​​1–2 square centimeters. This is how the central processing units of personal computers are created.

Prospects for the development of devices

Prospects lie, first of all, in the field of further miniaturization of devices. So, American scientists are developing today the so-called single-molecule transistor. The main element of such a device is a benzene molecule, to which three electrodes are attached.

If the idea justifies itself, it will be possible to create super-powerful computing systems. After all, the size of the molecule is much smaller than the size of today's MOS triodes on a silicon chip chip.

Transistors are at the heart of most electronic devices. It can be in the form of separate radio components, or as part of microcircuits. Even the most complex microprocessor consists of a great many tiny transistors packed tightly into its mighty crystal.

Transistors are different:
The two main groups are bipolar and field. The bipolar transistor is indicated on the diagram, as shown in Figure 1. It can be direct (p-p-p) and reverse (p-p-p) conductivity. The structure of the transistor, and the physical processes occurring in it, are studied at school, so we will not talk about it here - so to speak, closer to practice. In essence, the difference is that p-p-p transistors are connected so that a positive voltage potential arrives at their emitter, and a negative voltage potential goes to their collector. For n-p-p transistors, the opposite is true, a negative potential is given to the emitter, and a positive potential to the collector.

Why do you need a transistor?
It is mainly used to amplify current, signals, voltage. And amplification occurs due to the power supply. I will try to explain the principle of work "on the fingers". The car has a vacuum brake booster. When the driver presses the brake pedal, his membrane moves and a valve opens through which the car's engine sucks in this membrane, adding force to it. As a result, a weak force on the brake pedal leads to a strong force on the brake pads. And the addition of force occurs due to the power of the running motor of the machine.

It's the same with the transistor. A weak current is applied to the base (Fig. 2). Under the influence of this current, the conductivity of the collector - emitter increases and a much stronger current flows from the power source through the collector. The weak base current changes, and the strong collector current changes accordingly. Ideally, the collector current graph looks like an enlarged copy of the base current graph.
This difference between a weak base current and a strong collector current is called the current gain of the transistor, and is denoted by I21e. It is defined as follows: h21e \u003d Ik / I6 (collector current divided by base current). The larger this parameter, the better the amplifying properties of the transistor.
But this is all ideal. In fact, the dependence of the collector current on the base voltage is not so linear. It should be remembered BAX diode, where at the very bottom of the characteristics of the current is very small, and begins to rise sharply when the voltage reaches a certain value. Since the transistor is based on the same physical processes, there is a similar “defect” here.

If we assemble the amplifier circuit shown in Figure 3 and speak into the microphone, there will be no sound in the speaker. Because the voltage at the microphone is very low, it is below the threshold for opening the transistor. Here, not only will there be no amplification, but on the contrary, there will be signal attenuation.

In order for the transistor to work as an amplifier, you need to increase the voltage at its base. This can be done somehow by increasing the voltage at the output of the microphone. But then the meaning of the amplifier is lost. Or you need to apply some constant voltage to the base of the transistor (Fig. 4) through a resistor, such that the transistor opens slightly. And apply a weak alternating voltage to the base of this transistor through a capacitor. Now the most important thing is that a weak alternating voltage will add up with a constant voltage at the base. The voltage at the base will change in time with a weak alternating voltage. But since the DC voltage has shifted the operating point of the transistor to a steep linear section of the characteristic, amplification occurs.
Simply put, a weak voltage did not have the strength to open the transistor, and we added a constant voltage to help it, which slightly opened the transistor.

The constant voltage that is applied to the base of a transistor to shift its mode of operation to a region with a steeper and more linear characteristic is called the bias voltage. By changing this voltage, we can even adjust the gain of the amplifier stage.

Transistors are not always used with bias voltage. For example, in the amplifying stages of transmitters, the bias voltage may not be applied to the bases of transistors, since the amplitude of the input alternating voltage there is quite enough to “build up” the transistor.

If the transistor is used not as an amplifier, but as a key, then the bias voltage is also not given to the base. Simply, when the key must be closed, the voltage at the base is zero, and when it must be open, voltage is applied to the base sufficient to open the transistor. This is usually used in digital electronics, where there are only zeros (no voltage) and ones (voltage) and no intermediate values.

Figure 5 shows a practical diagram of how to make a computer speaker out of a radio speaker. You need a simple single-program speaker with only one plug for connecting to the radio network (multi-program has a second plug for the mains). There is no need to make any changes to the speaker circuit. It is connected to the collector of the transistor in the same way as to the radio network.

Inside the single-program loudspeaker there is a speaker, a variable resistor for volume control and a transformer. All this is needed, and it remains. When you open the case of the loudspeaker, solder the collector of the transistor and the plus of the power source to the places to which its wire with the plug is soldered. The wire itself can be removed.

To connect to a computer, you need a shielded wire with an appropriate plug at the end. Or a regular two-wire wire. If the wire is shielded, connect the braid to the emitter of the transistor, and the central core to the capacitor C1.
The signal from the computer sound card is fed through the plug to the capacitor C1. The supply voltage is supplied from the mains power supply. The best power supply game console to the TV, such as "Dandy", "Kanga". In general, any power supply with an output voltage from 7V to 12V is suitable. To connect to the power supply, you need an appropriate socket; you need to install it on the loudspeaker case by drilling a hole for it. Although, of course, you can solder the wires from the power supply and directly to the circuit. When connecting the power supply, the polarity must be observed. In principle, the VD1 diode is not needed, but it protects the circuit from failure if you confuse the plus and minus of the power supply. Without it, if the power is connected incorrectly, the transistor can be burned, and with a diode, if you confuse the poles of the power supply, the circuit will simply not turn on.

KT315 transistor in a rectangular case, which has a bevel on one side (shown in the figure). Now, if you turn it away from you with this bevel, and with the leads up, then the base will be on the left, the emitter on the right, and the collector in the middle. Suitable transistor KT315 with any letter (KT315A, KT315B ...). The transistor must be soldered correctly, without confusing its conclusions. If you make a mistake and turn on the power, he can die. Therefore, after you solder everything, do not be too lazy three times to check the correctness of the installation, whether the terminals of the transistor, capacitors, and diode are correctly soldered. And only when you are 100% sure, turn it on.

Diode VD1 type KD209. It has an anode on it. You can put another diode, for example, 1N4004 or some other. If the diode is soldered incorrectly, the circuit will not work. So, if everything is turned on, but does not work, start by checking the correct connection of the diode.

Capacitors - electrolytic, for a voltage of at least 12V. Our K50-16, K50-35 or imported analogues will do. It should be noted that our capacitors on the case have a plus near the positive terminal, while imported ones have a minus or a wide vertical strip at the negative terminal. Instead of a 10 uF capacitor, you can choose any capacitance from 2 uF to 20 uF. Instead of a 100 uF capacitor, a capacitor of any capacitance of at least 100 uF will do.

The figure below the diagram shows the wiring diagram, on which the soldering points are marked with dots. Do not confuse solder joints with wire crossings. The installation is made by a hinged method, using the conclusions of parts and wiring. It is desirable to place the entire circuit inside the loudspeaker case (there is usually a lot of space there).

If everything works, but it emits a lot of noise, it means that you mixed up the wires going to the sound card. Swap them.

DO NOT power the circuit from a computer power supply!

For the stereo version, you can make two speakers by combining the inputs into one stereo cable for connecting to a sound card, and power both speakers from one power supply.

With one transistor stage, the speaker will not sound loud, but enough to listen in small room. The volume can be adjusted both with the computer regulator and with the knob that the loudspeaker has.

What does the name "transistor" mean?

The transistor did not immediately receive such a familiar name. Initially, by analogy with lamp technology, it was called semiconductor triode. The current name consists of two words. The first word is “transfer”, (here one immediately comes to mind “transformer”) means transmitter, converter, carrier. And the second half of the word resembles the word "resistor" - a detail of electrical circuits, the main property of which is electrical resistance.

It is this resistance that is found in Ohm's law and many other formulas of electrical engineering. Therefore, the word "transistor" can be interpreted as a resistance converter. In much the same way as in hydraulics, the change in fluid flow is controlled by a valve. In a transistor, such a “valve” changes the amount of electric charges that create electricity. This change is nothing but a change in the internal resistance of a semiconductor device.

Amplification of electrical signals

The most common operation performed transistors, is amplification of electrical signals. But this is not quite the right expression, because the weak signal from the microphone remains so.

Amplification is also required in radio and television reception: a weak signal from an antenna with a power of billionths of a watt must be amplified to such an extent that a sound or image is received on the screen. And this is already a power of several tens, and in some cases hundreds of watts. Therefore, the amplification process boils down to using additional sources energy received from the power supply, get a powerful copy of the weak input signal. In other words, a low-power input action controls powerful energy flows.

Strengthening in other areas of technology and nature

Such examples can be found not only in electrical circuits. For example, pressing the gas pedal increases the speed of the car. At the same time, you do not have to press the gas pedal very hard - compared to the engine power, the power of pressing the pedal is negligible. To reduce the speed, the pedal will have to be released somewhat, to weaken the input effect. In this situation, gasoline is a powerful source of energy.

The same effect can be observed in hydraulics: it takes very little energy to open an electromagnetic valve, for example, in a machine tool. And the pressure of the oil on the piston of the mechanism is capable of creating a force of several tons. This force can be adjusted if an adjustable valve is provided in the oil line, as in a conventional kitchen faucet. Slightly covered - the pressure dropped, the effort decreased. If you opened more, then the pressure increased.

Turning the valve also does not require much effort. In this case, the external source of energy is pumping station machine. And similar influences in nature and technology can be seen a great many. But still, we are more interested in the transistor, so we will have to consider further ...

Electrical signal amplifiers