What is a transistor and how does it work. Types of field effect transistors

The principle of semiconductor control of electric current was known at the beginning of the twentieth century. Even though electronics engineers knew how a transistor worked, they continued to design devices based on vacuum tubes. The reason for such distrust of semiconductor triodes was the imperfection of the first point-point transistors. The family of germanium transistors did not have stable characteristics and were highly dependent on temperature conditions.

Monolithic silicon transistors began to seriously compete with vacuum tubes only in the late 50s. Since that time, the electronics industry began to develop rapidly, and compact semiconductor triodes actively replaced energy-intensive lamps from electronic device circuits. With the advent integrated circuits, where the number of transistors can reach into the billions, semiconductor electronics has won a landslide victory in the fight to miniaturize devices.

What is a transistor?

In its modern meaning, a transistor is a semiconductor radio element designed to change the parameters of an electric current and control it. A conventional semiconductor triode has three terminals: a base, which receives control signals, an emitter, and a collector. There are also high power composite transistors.

The scale of sizes of semiconductor devices is striking - from several nanometers (unpackaged elements used in microcircuits) to centimeters in diameter for powerful transistors intended for power plants and industrial equipment. Reverse voltages of industrial triodes can reach up to 1000 V.

Device

Structurally, the triode consists of semiconductor layers enclosed in a housing. Semiconductors are materials based on silicon, germanium, gallium arsenide and others. chemical elements. Today, research is being conducted to prepare certain types of polymers, and even carbon nanotubes, for the role of semiconductor materials. Apparently in the near future we will learn about new properties of graphene field-effect transistors.

Previously, semiconductor crystals were located in metal cases in the form of caps with three legs. This design was typical for point-point transistors.

Today, the designs of most flat, including silicon semiconductor devices made on the basis of a single crystal doped in certain parts. They are pressed into plastic, metal-glass or metal-ceramic cases. Some of them have protruding metal plates for heat dissipation, which are attached to the radiators.

The electrodes of modern transistors are arranged in one row. This arrangement of the legs is convenient for automatic board assembly. The terminals are not marked on the housings. The type of electrode is determined from reference books or by measurements.

For transistors, semiconductor crystals with different structures, such as p-n-p or n-p-n, are used. They differ in the polarity of the voltage on the electrodes.

Schematically, the structure of a transistor can be represented as two semiconductor diodes, separated by an additional layer. (See Figure 1). It is the presence of this layer that allows you to control the conductivity of the semiconductor triode.

Rice. 1. Structure of transistors

Figure 1 schematically shows the structure of bipolar triodes. There is also a class of field-effect transistors, which will be discussed below.

Basic operating principle

At rest, no current flows between the collector and emitter of a bipolar triode. Electric current is prevented by the resistance of the emitter junction, which arises as a result of the interaction of the layers. To turn on the transistor, you need to apply a small voltage to its base.

Figure 2 shows a diagram explaining the working principle of a triode.

Rice. 2. Operating principle

By controlling the base currents, you can turn the device on and off. If you apply to the base analog signal, then it will change the amplitude of the output currents. In this case, the output signal will exactly repeat the oscillation frequency at the base electrode. In other words, the electrical signal received at the input will be amplified.

Thus, semiconductor triodes can operate in electronic switch mode or in input signal amplification mode.

Operation of the device in mode electronic key can be understood from Figure 3.

Rice. 3. Triode in switch mode

Designation on diagrams

Common designation: "VT" or "Q", followed by a positional index. For example, VT 3. On earlier diagrams you can find outdated designations: “T”, “PP” or “PT”. The transistor is depicted as symbolic lines indicating the corresponding electrodes, circled or not. The direction of current in the emitter is indicated by an arrow.

Figure 4 shows a ULF circuit in which transistors are designated in a new way, and Figure 5 shows schematic images of different types of field-effect transistors.

Rice. 4. Example ULF circuits on triodes

Types of transistors

Based on their operating principle and structure, semiconductor triodes are distinguished:

• field;
• bipolar;
• combined.

These transistors perform the same functions, but there are differences in the principle of their operation.

Field

This type of triode is also called unipolar, due to its electrical properties - they carry current of only one polarity. Based on their structure and type of control, these devices are divided into 3 types:

1. Transistors with manager p-n transition (Fig. 6).
2. With an insulated gate (available with a built-in or induced channel).
3. MIS, with structure: metal-dielectric-conductor.

A distinctive feature of an insulated gate is the presence of a dielectric between it and the channel.

Parts are very sensitive to static electricity.

Circuits of field triodes are shown in Figure 5.

Rice. 5. Field effect transistors
Rice. 6. Photo of a real field-effect triode

Pay attention to the names of the electrodes: drain, source and gate.

Field effect transistors consume very little power. They can work for more than a year on a small battery or rechargeable battery. Therefore, they have found wide application in modern electronic devices such as remote controls remote control, mobile gadgets and so on.

Bipolar

Much has been said about this type of transistor in the subsection “ Basic principle work." Let us only note that the device received the name “Bipolar” because of its ability to pass charges of opposite signs through one channel. Their feature is low output impedance.

Transistors amplify signals and act as switching devices. A fairly powerful load can be connected to the collector circuit. Due to the high collector current, the load resistance can be reduced.

Let's look at the structure and principle of operation in more detail below.

Combined

In order to achieve certain electrical parameters From the use of a single discrete element, transistor developers are inventing combined designs. Among them are:

• with embedded resistors and their circuit;
• combinations of two triodes (same or different structures) in one package;
• lambda diodes - a combination of two field-effect triodes forming a section with negative resistance;
• designs in which a field-effect triode with an insulated gate controls a bipolar triode (used to control electric motors).

Combined transistors are, in fact, an elementary microcircuit in one package.

How does a bipolar transistor work? Instructions for dummies

The operation of bipolar transistors is based on the properties of semiconductors and their combinations. To understand the principle of operation of triodes, let's understand the behavior of semiconductors in electrical circuits.

Semiconductors.

Some crystals, such as silicon, germanium, etc., are dielectrics. But they have one feature - if you add certain impurities, they become conductors with special properties.

Some additives (donors) lead to the appearance of free electrons, while others (acceptors) create “holes”.

If, for example, silicon is doped with phosphorus (donor), we obtain a semiconductor with an excess of electrons (n-Si structure). By adding boron (an acceptor), the doped silicon will become a hole-conducting semiconductor (p-Si), that is, its structure will be dominated by positively charged ions.

One-way conduction.

Let's conduct a thought experiment: connect two different types of semiconductors to a power source and supply current to our design. Something unexpected will happen. If you connect the negative wire to an n-type crystal, the circuit will be completed. However, when we reverse the polarity, there will be no electricity in the circuit. Why is this happening?

As a result of connecting crystals with different types of conductivity, a region with a p-n junction is formed between them. Some electrons (charge carriers) from an n-type crystal will flow into a crystal with hole conductivity and recombine holes in the contact zone.

As a result, uncompensated charges arise: in the n-type region - from negative ions, and in the p-type region from positive ions. The potential difference reaches values ​​from 0.3 to 0.6 V.

The relationship between voltage and impurity concentration can be expressed by the formula:

φ= V T*ln( Nn* Np)/n 2 i , where

V T – value of thermodynamic stress, Nn And Np – the concentration of electrons and holes, respectively, and n i denotes the intrinsic concentration.

When connecting a plus to a p-conductor and a minus to an n-type semiconductor, the electric charges will overcome the barrier, since their movement will be directed against the electric field inside p-n junction. IN in this case the passage is open. But if the poles are reversed, the transition will be closed. Hence the conclusion: the p-n junction forms one-way conductivity. This property is used in the design of diodes.

From diode to transistor.

Let's complicate the experiment. Let's add another layer between two semiconductors with the same structures. For example, between p-type silicon wafers we insert a conductivity layer (n-Si). It is not difficult to guess what will happen in the contact zones. By analogy with the process described above, regions with p-n junctions are formed that will block the movement of electrical charges between the emitter and collector, regardless of the polarity of the current.

The most interesting thing will happen when we apply a slight voltage to the layer (base). In our case, we will apply a current with a negative sign. As in the case of a diode, an emitter-base circuit is formed through which current will flow. At the same time, the layer will begin to become saturated with holes, which will lead to hole conduction between the emitter and collector.

Look at Figure 7. It shows that positive ions have filled the entire space of our conditional structure and now nothing interferes with the conduction of current. We got a visual model bipolar transistor p-n-p structures.

Rice. 7. Principle of operation of the triode

When the base is de-energized, the transistor very quickly returns to its original state and the collector junction closes.

The device can also operate in amplification mode.

The collector current is directly proportional to the base current : ITo= ß* IB , Where ß – current gain, IB – base current.

If you change the value of the control current, the intensity of hole formation on the base will change, which will entail a proportional change in the amplitude of the output voltage, while maintaining the signal frequency. This principle is used to amplify signals.

By applying weak pulses to the base, at the output we get the same amplification frequency, but with a much larger amplitude (set by the voltage applied to the collector-emitter circuit).

They work in a similar way npn transistor s. Only the polarity of the voltages changes. Devices with n-p-n structure have direct conductivity. Transistors of the pnp type have reverse conductivity.

It remains to add that semiconductor crystal reacts similarly to the ultraviolet spectrum of light. By turning the photon flow on and off, or adjusting its intensity, you can control the operation of a triode or change the resistance of a semiconductor resistor.

Bipolar transistor connection circuits

Circuit engineers use the following connection diagrams: with common base, common emitter electrodes and connection with a common collector (Fig. 8).

Rice. 8. Connection diagrams for bipolar transistors

Amplifiers with a common base are characterized by:

• low input impedance, which does not exceed 100 Ohms;
• good temperature properties and frequency characteristics of the triode;
• high permissible voltage;
• two different power sources are required.

Schemes with common emitter have:

• high current and voltage gain;
• low power gain;
• inversion of the output voltage relative to the input.

With this connection, one power source is sufficient.

The connection diagram based on the “common collector” principle provides:

• high input and low output resistance;
• low voltage gain factor (< 1).

How does a field effect transistor work? Explanation for dummies

The structure of a field-effect transistor differs from a bipolar one in that the current in it does not cross the p-n junction zone. The charges move through a controlled area called the gate. Bandwidth The gate is voltage controlled.

Space p-n zones decreases or increases under the influence of an electric field (see Fig. 9). The number of free charge carriers changes accordingly - from complete destruction to extreme saturation. As a result of this effect on the gate, the current at the drain electrodes (contacts that output the processed current) is regulated. The incoming current flows through the source contacts.

Figure 9. Field-effect transistor with p-n junction

Field triodes with built-in and induced channels operate on a similar principle. You saw their diagrams in Figure 5.

Field-effect transistor connection circuits

In practice, connection diagrams are used by analogy with a bipolar triode:

• with a common source - produces a large gain in current and power;
• common gate circuits provide low input impedance and negligible gain (has limited use);
• common-drain circuits that operate in the same way as common-emitter circuits.

Figure 10 shows various schemes inclusions.

Rice. 10. Image of field triode connection diagrams

Almost every circuit is capable of operating at very low input voltages.

Videos explaining the principle of operation of the transistor in simple language

Greetings, dear friends! Today we will talk about bipolar transistors and the information will be useful primarily to beginners. So, if you are interested in what a transistor is, its operating principle and in general what it is used for, then take a more comfortable chair and come closer.

Let's continue, and we have content here, it will be more convenient to navigate the article :)

Types of transistors

Transistors are mainly of two types: bipolar transistors and field-effect transistors. Of course, it was possible to consider all types of transistors in one article, but I don’t want to cook porridge in your head. Therefore, in this article we will look exclusively at bipolar transistors, and I will talk about field-effect transistors in one of the following articles. Let's not lump everything together, but pay attention to each one individually.

Bipolar transistor

The bipolar transistor is a descendant of tube triodes, those that were in televisions of the 20th century. Triodes went into oblivion and gave way to more functional brothers - transistors, or rather bipolar transistors.

With rare exceptions, triodes are used in equipment for music lovers.

Bipolar transistors may look like this.

As you can see, bipolar transistors have three terminals and structurally they can look completely different. But on electrical diagrams they look simple and always the same. And all this graphic splendor looks something like this.

This image of transistors is also called UGO (Conventional graphic symbol).

Moreover, bipolar transistors can have different types of conductivity. There are NPN type and PNP type transistors.

The difference between an n-p-n transistor and a p-n-p transistor is only that it is a “carrier” electric charge(electrons or “holes”). Those. For a pnp transistor, electrons move from the emitter to the collector and are driven by the base. For an n-p-n transistor, electrons go from the collector to the emitter and are controlled by the base. As a result, we come to the conclusion that in order to replace a transistor of one conductivity type with another in a circuit, it is enough to change the polarity of the applied voltage. Or stupidly change the polarity of the power source.

Bipolar transistors have three terminals: collector, emitter and base. I think that it will be difficult to get confused with the UGO, but in a real transistor it’s easier than ever to get confused.

Usually where which output is determined is from the reference book, but you can simply. The terminals of the transistor sound like two diodes connected at a common point (in the area of ​​the base of the transistor).

On the left is a picture for a p-n-p type transistor; when testing, you get the feeling (through multimeter readings) that in front of you are two diodes that are connected at one point by their cathodes. For transistor n-p-n type The diodes at the base point are connected by their anodes. I think after experimenting with a multimeter it will be more clear.

The principle of operation of a bipolar transistor

Now we will try to figure out how a transistor works. I won't go into details internal structure transistors since this information only confuses. Better take a look at this drawing.

This image best explains the working principle of a transistor. In this image, a person controls the collector current using a rheostat. He looks at the base current; if the base current increases, then the person also increases the collector current, taking into account the gain of the transistor h21E. If the base current drops, then the collector current will also decrease - the person will correct it using a rheostat.

This analogy has nothing to do with the actual operation of a transistor, but it makes it easier to understand the principles of its operation.

For transistors, rules can be noted to help make things easier to understand. (These rules are taken from the book).

1. The collector has a more positive potential than the emitter
2. As I already said, the base-collector and base-emitter circuits work like diodes
3. Each transistor is characterized by limiting values ​​such as collector current, base current and collector-emitter voltage.
4. If rules 1-3 are followed, then the collector current Ik is directly proportional to the base current Ib. This relationship can be written as a formula.

From this formula we can express the main property of a transistor - a small base current controls a large collector current.

Current gain.

It is also denoted as

Based on the above, the transistor can operate in four modes:

1. Transistor cut-off mode— in this mode the base-emitter junction is closed, this can happen when the base-emitter voltage is insufficient. As a result, there is no base current and therefore there will be no collector current either.
2. Transistor active mode- This normal mode transistor operation. In this mode, the base-emitter voltage is sufficient to cause the base-emitter junction to open. The base current is sufficient and the collector current is also available. The collector current is equal to the base current multiplied by the gain.
3. Transistor saturation mode - The transistor switches to this mode when the base current becomes so large that the power of the power source is simply not enough to further increase the collector current. In this mode, the collector current cannot increase following an increase in the base current.
4. Inverse transistor mode— this mode is used extremely rarely. In this mode, the collector and emitter of the transistor are swapped. As a result of such manipulations, the gain of the transistor suffers greatly. The transistor was not originally designed to operate in such a special mode.

To understand how a transistor works, you need to look at specific circuit examples, so let's look at some of them.

Transistor in switch mode

A transistor in switch mode is one of the cases of transistor circuits with a common emitter. The transistor circuit in switching mode is used very often. This transistor circuit is used, for example, when it is necessary to control a powerful load using a microcontroller. The controller leg is not capable of pulling a powerful load, but the transistor can. It turns out that the controller controls the transistor, and the transistor controls a powerful load. Well, first things first.

The main idea of ​​this mode is that the base current controls the collector current. Moreover, the collector current is much more current bases. Here you can see with the naked eye that the current signal is amplified. This amplification is carried out using the energy of the power source.

The figure shows a diagram of the operation of a transistor in switching mode.

For transistor circuits, voltages do not play a big role, only currents matter. Therefore, if the ratio of the collector current to the base current is less than the gain of the transistor, then everything is okay.

In this case, even if we have a voltage of 5 volts applied to the base and 500 volts in the collector circuit, then nothing bad will happen, the transistor will obediently switch the high-voltage load.

The main thing is that these voltages do not exceed the limit values ​​for a specific transistor (set in the transistor characteristics).

As far as we know, the current value is a characteristic of the load.

We don't know the resistance of the light bulb, but we know the operating current of the light bulb is 100 mA. In order for the transistor to open and allow such current to flow, you need to select the appropriate base current. We can adjust the base current by changing the value of the base resistor.

Because minimum value transistor gain is 10, then to open the transistor the base current must become 10 mA.

The current we need is known. The voltage across the base resistor will be This voltage value across the resistor is due to the fact that 0.6V-0.7V is dropped at the base-emitter junction and we must not forget to take this into account.

As a result, we can easily find the resistance of the resistor

All that remains is to choose a specific value from a number of resistors and it’s done.

Now you're probably thinking that transistor switch will it work as needed? That when the base resistor is connected to +5 V the light bulb lights up, when it is turned off the light bulb goes out? The answer may or may not be yes.

The thing is that there is a small nuance here.

The light bulb will go out when the resistor potential is equal to the ground potential. If the resistor is simply disconnected from the voltage source, then everything is not so simple. The voltage on the base resistor can miraculously arise as a result of interference or some other otherworldly evil spirits :)

To prevent this effect from happening, do the following. Another resistor Rbe is connected between the base and emitter. This resistor is chosen with a value at least 10 times larger than the base resistor Rb (In our case, we took a 4.3 kOhm resistor).

When the base is connected to any voltage, the transistor works as it should, the resistor Rbe does not interfere with it. This resistor consumes only a small portion of the base current.

In the case when voltage is not applied to the base, the base is pulled up to the ground potential, which saves us from all kinds of interference.

So, in principle, we have figured out the operation of the transistor in the key mode, and as you can see, the key mode of operation is a kind of voltage amplification of the signal. After all, we controlled a voltage of 12 V using a low voltage of 5V.

Emitter follower

An emitter follower is a special case of common-collector transistor circuits.

A distinctive feature of a circuit with a common collector from a circuit with a common emitter (option with a transistor switch) is that this circuit does not amplify the voltage signal. What went in through the base came out through the emitter, with the same voltage.

Indeed, let’s say we applied 10 volts to the base, while we know that at the base-emitter junction somewhere around 0.6-0.7V is dropped. It turns out that at the output (at the emitter, at the load Rн) there will be a base voltage of minus 0.6V.

It turned out 9.4V, in a word, almost as much as went in and out. We made sure that this circuit will not increase the voltage for us.

“What is the point then of turning on the transistor like this?” you ask. But it turns out that this scheme has something very different important property. The circuit for connecting a transistor with a common collector amplifies the signal in terms of power. Power is the product of current and voltage, but since voltage does not change, power increases only due to current! The load current is the sum of the base current plus the collector current. But if you compare the base current and the collector current, the base current is very small compared to the collector current. This results in load current equal to current collector And the result is this formula.

Now I think it’s clear what the essence of the emitter follower circuit is, but that’s not all.

The emitter follower has another very valuable quality - high input impedance. This means that this transistor circuit consumes almost no input current and creates no load on the signal source circuit.

To understand the principle of operation of a transistor, these two transistor circuits will be quite sufficient. And if you experiment with a soldering iron in your hands, then the epiphany simply won’t keep you waiting, because theory is theory and practice is personal experience hundreds of times more valuable!

Where can I buy transistors?

Like all other radio components, transistors can be purchased at any nearby radio parts store. If you live somewhere on the outskirts and have not heard of such stores (like I did before), then the last option remains - order transistors from an online store. I myself often order radio components through online stores, because something may simply not be available in a regular offline store.

However, if you are assembling a device purely for yourself, then you can not worry about it, but extract it from the old one, and, so to speak, breathe new life into the old radio component.

Well friends, that’s all for me. I told you everything that I planned today. If you have any questions, then ask them in the comments, if you don’t have any questions, then write comments anyway, your opinion is always important to me. By the way, don’t forget that everyone who leaves a comment for the first time will receive a gift.

Also, be sure to subscribe to new articles, because a lot of interesting and useful things await you further.

I wish you good luck, success and a sunny mood!

From n/a Vladimir Vasiliev

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13. Design and principle of operation of transistors

Depending on the principle of operation and design features, transistors are divided into two large classes: bipolar and field-effect.

Bipolar transistors are semiconductor devices with two or more interacting electrical p-n junctions and three or more terminals, the amplifying properties of which are due to the phenomena of injection and extraction of minority charge carriers.

Currently, bipolar transistors with two p-n junctions are widely used, to which this term is most often referred. They consist of alternating regions (layers) of a semiconductor having electrical conductivities of different types. Depending on the type of electrical conductivity of the outer layers, transistors are distinguished p-p-p and n-p-n types.

Transistors in which p-n junctions are created at the contact surfaces of the semiconductor layers are called planar

Bipolar transistor is a semiconductor crystal consisting of three layers with alternating conductivity and equipped with three leads (electrodes) for connection to an external circuit.

In Fig. 1.5, a and b shows the circuit designation of two types of transistors p-p-p-type And p-p-p- type . The outermost layers are called issuerum(E) and collector(K), between them is base(B). The three-layer structure has two p-n junctions: emitter junction between emitter and base and collector junction between the base and the collector. Germanium or silicon is used as the starting material for transistors.

When manufacturing a transistor, two conditions must be met:

base thickness (distance between emitter and number)

lecture transitions) should be small compared to the free path of charge carriers;

2) the concentration of impurities (and main charge carriers) in the emitter must be significantly greater than in the base (N a >> N D V p-p-p transistor).

Let's consider the principle of operation p-p-p transistor.

The transistor is connected in series with the load resistance Rк in the circuit of the collector voltage source E To . A control EMF is supplied to the input of the transistor E B", as shown in Fig. 1.6, a. Turning on the transistor when the input ( E B , R B ) and day off ( E TO , R TO ) circuits have a common point - the emitter, is the most common and is called inclusion with common emitter(OE).

In the absence of voltage (E B =0, E TO=0) the emitter and collector junction are in a state of equilibrium, the currents through them are zero. Both transitions have a double electrical layer consisting of impurity ions and a potential barrier  o, different at each of the transitions. The potential distribution in the transistor in the absence of voltage is shown in Fig. 1.6, b with a dashed line.

Polarity of external sources E B and E TO is chosen such that there is a forward voltage at the emitter junction (minus the source E B supplied to the base, plus to the emitter), and at the collector junction - reverse voltage (minus source E TO- to the collector, plus - to the emitter), and the voltage |Uke|> |Ube| (voltage at the collector junction Ukb = Uke-Ube) With such inclusion of sources E B and E TO the potential distribution in the transistor has the form shown in Fig. .1.6, b solid line. The potential barrier of the forward-biased emitter junction decreases, while the potential barrier at the collector junction increases. As a result of applying a forward voltage to the emitter junction, enhanced diffusion (injection) of holes from the emitter to the base begins. The electronic component of the diffusion current through the emitter junction can be neglected, since R R >>p P , since the condition was stated above N A >>N D . Thus, the emitter current I E = I Edif R. Under the influence of diffusion forces as a result of a concentration difference along the base, holes move from the emitter to the collector. Since the base in the transistor is performed thin, the main part of the holes injected by the emitter reaches the collector junction without getting into the recombination centers. These holes are captured by the field of the collector junction, biased in the opposite direction, since this field is accelerating for minority carriers - holes in the n-type base. The current of holes entering the collector from the emitter is closed through an external circuit, the source E TO . When the emitter current increases by I E, the collector current will increase by I K = I O. Due to the low probability of recombination in thin base emitter current transfer coefficient  =I K /I E =0.9-0.99.

A small part of the holes injected by the emitter enters the recombination centers and disappears, recombining with electrons. The charge of these holes remains in the base, and to restore the charging neutrality of the base from the external circuit at the expense of the source Ev electrons enter the base. Therefore, the base current represents the recombination current I rec =I E (1-) In addition to the indicated main components of the transistor current, it is necessary to take into account the possibility of the transition of minority carriers arising in the base and collector as a result of carrier generation through the collector junction, to which a reverse voltage is applied. This small current (transition of holes from base to collector and electrons from collector to base) is similar reverse current r-p transition, also called reverse current of the collector junction or thermal current and is designated I kbo (Fig. 1.6, a)

field effect transistors- semiconductor devices that practically do not consume current from the input circuit.

Field-effect transistors are divided into two types, differing from each other in their operating principle: a) with r-p transition; b) MDP type.

. 1.6.1. Field effect transistors withr-p transition have a structure, a section of which is shown in Fig. 1.9, a. A layer with p-type conductivity is called channel, it has two outputs to the external circuit: WITH- drain And AND- source. Layers with conductivity type P, surrounding the channel are interconnected and have an output to an external circuit called shutter 3. Connecting voltage sources to the device is shown in Fig. 1.9, a, in Fig. 1.9.6 shows the circuit designation of a field-effect transistor with r-p junction and p-type channel. There are also field-effect transistors with an n-type channel; their designation is shown in Fig. 1.9, V, the operating principle is similar, but the directions of the currents and the polarity of the applied voltages are opposite.

Let's consider the principle of operation of a field-effect transistor with a p-type channel. In Fig. 1.9, G The family of drain (output) characteristics of this device Iс=f(Uс) at Uз=const is given.

With control voltage Uzi = 0 and a voltage source connected between drain and source Usi a current flows through the channel, which depends on the resistance of the channel. Voltage U applied uniformly along the length of the channel, this voltage causes a reverse bias r-p transition between the p-type channel and the n-layer, with the highest reverse voltage at r-p transition exists in the region adjacent to the drain, and near the source r-p the transition is in an equilibrium state. As the voltage increases Usi electrical double layer region r-p transition, depleted of mobile charge carriers, will expand, as shown in Fig. 1.10, A. The expansion of the junction is especially strong near the drain, where the reverse voltage across the junction is greater. Extension r-p transition leads to a narrowing of the current-carrying channel of the transistor, and the channel resistance increases. Due to the increase in channel resistance with increasing Usi, the drain characteristic of the field-effect transistor has a nonlinear character (Fig. 1.9d). At some voltage Usi borders r-p transitions close (dotted line in Fig. 1.10, a), and the current Ic increases with increasing Ucb stops.

When a positive voltage is applied to the gate Uzi>0 r-p The transition shifts even more towards the reverse voltage region, and the transition width increases, as shown in Fig. 1.10.6. As a result, the current-conducting channel narrows and the current Ic decreases. Thus, increasing the voltage Uzi. it is possible to reduce I c, as can be seen from considering Fig. 1.9, G. At a certain Uzi called cut-off voltage, There is practically no drain current flowing. The ratio of the change in drain current I C to the change in voltage between the gate and source Uzi that caused it when Uс =const is called steepness:S = I C /Uzi at Uс = const

Unlike bipolar transistors, field effect transistors are voltage controlled, and only a small thermal current Iz flows through the gate circuit r-p junction under reverse voltage.

Once upon a time a radio receiver was called a transistor, but in our article we will not talk about a radio receiver. So what is a transistor and how does it work.

There is a class of materials called semiconductors for their properties. Their distinctive feature is conductivity - they can be both conductors of electric current and dielectrics, i.e. insulators and do not carry out electricity.

This is the material used to make the transistor, which is widely used in industry and serves as the basis for almost all modern electronics.

Without touching on manufacturing technology, types of transistors, and their applications, we simply note that there are transistors of different types, for example, an npn transistor. It received this name because of the material used and the type of conductivity. What has been said is enough for now and we will not delve into the manufacturing technology and variety of transistors now.

How does a transistor work? It is designed to control electric current, is structurally manufactured in a metal or plastic case and has three terminals, called emitter, base, collector. The name of the pins already indicates their purpose: the emitter emits electrons, the base controls them, and the collector collects them. All these processes occur inside the transistor.

To understand how a transistor works, consider a much simpler example - a water tap.

It also has three outlets - one where water flows into the faucet, one where it flows out of the faucet, and the third is a valve that controls the operation of the faucet. When the valve is open, water flows freely through the tap; when the valve is closed, water does not flow. This is an imitation of one of the options for how a transistor works. This mode of operation is called a key mode - the transistor is open - it is leaking or closed, then no current flows. To open the transistor, voltage is applied to the base; if there is voltage, then the transistor is open, if not, then it is closed. Everything happens as if it were open - water flows, the valve is closed - there is no water.

The operation of a transistor was discussed above when it is used as a key: either closed or open. However, there are other modes of operation. Let's look at the water faucet again as an example. If we open the valve a little, water from the tap will flow constantly, and the water pressure will be determined by how far we open the tap.

The transistor has approximately the same operating mode. Voltage is applied to its base, it opens, and through it current is flowing. By changing the voltage at the base, you can regulate the amount of current passing through the transistor. Complete analogy with the position of the valve on the tap: more open - more water flows (i.e. current for the transistor); less open - less water flows (current for the transistor). This mode of operation of the transistor is called amplification, when, using a small voltage applied to the base, it is possible to control a significant current removed from the collector.

In conclusion, it should be noted that transistors can be different types, everything is determined by the material used in manufacturing. They can differ in power and can control and pass through significant flows of electric current. Transistors can be of different designs. There are other operating modes of transistors that differ from those discussed. But the basic idea of ​​how a transistor works is given above.

Everything stated is approximate, but still allows you to understand the operation of the transistor. In fact, the operation of a transistor is much more complicated. Eat special parameters, using which you can calculate and set the required operating mode using formulas, but this is a completely different topic for conversation and for another article.

Transistor(transistor) - a semiconductor element with three terminals (usually), one of which ( collector) a strong current is supplied, and the other ( base) served weak ( control current). At a certain strength of the control current, it is as if a valve “opens” and the current from the collector starts to flow on third output ( emitter).

That is, a transistor is a kind of valve, which, at a certain current strength, sharply reduces the resistance and sends the current further (from the collector to the emitter). This happens because under certain conditions, holes that have an electron lose it, accepting a new one, and so on in a circle. If no electric current is applied to the base, the transistor will be in a balanced state and will not pass current to the emitter.

In modern electronic chips, the number of transistors numbers in the billions. They are used primarily for calculations and consist of complex connections.

Semiconductor materials mainly used in transistors are: silicon, gallium arsenide And germanium. There are also transistors carbon nanotubes, transparent for displays LCD And polymer(the most promising).

Types of transistors:

Bipolar– transistors in which charge carriers can be both electrons and “holes”. Current can flow like towards the emitter, so towards the collector. To control the flow, certain control currents are used.

– widespread devices in which the electrical flow is controlled through an electric field. That is, when a larger field is formed, more electrons are captured by it and cannot transfer charges further. That is, this is a kind of valve that can change the amount of transferred charge (if the field-effect transistor is controlled p—n— transition). Distinctive feature These transistors are high input voltage and high voltage gain.

Combined– transistors with combined resistors, or other transistors in one housing. They serve for various purposes, but mainly to increase the current gain.

Subtypes:

Bio-transistors– are based on biological polymers that can be used in medicine and biotechnology without harm to living organisms. Studies have been conducted on metalloproteins, chlorophyll A (derived from spinach), and tobacco mosaic virus.

Single-electron transistors– were first created by Russian scientists in 1996. They could work at room temperature, unlike their predecessors. The operating principle is similar to a field-effect transistor, but more subtle. The signal transmitter is one or more electrons. This transistor also called nano- and quantum transistor. Using this technology, in the future they hope to create transistors with a size less than 10 nm, based graphene.

What are transistors used for?

Transistors are used in amplification circuits, lamps, electric motors and other devices where rapid changes in current or position are required onoff. The transistor can limit the current or smoothly, or by method pulse—pause. The second is more often used for -control. Using powerful source supply, he conducts it through himself, regulating with a weak current.

If the current is not enough to turn on the transistor circuit, then use several transistors with greater sensitivity, connected in a cascade manner.

Powerful transistors connected in one or more packages are used in completely digital amplifiers based . They often need additional cooling . In most schemes, they work in key mode(in switch mode).

Transistors are also used in power systems, both digital and analog ( motherboards , video cards, Power supplies&etc).

Central processors, also consist of millions and billions of transistors, connected in a certain order for specialized calculations.

Each group of transistors encodes the signal in a certain way and transmits it further for processing. All types and ROM memories also consist of transistors.

All achievements of microelectronics would be practically impossible without the invention and use of transistors. It's hard to imagine even one electronic device without at least one transistor.