How transistors work. Transistors

A transistor is a device that operates on semiconductors with electronic filling. It is designed to transform and amplify electrical signals. There are two types of devices: a unipolar transistor, or a field effect transistor.

If two types of charge carriers work simultaneously in a transistor - holes and electrons, then it is called bipolar. If only one type of charge works in a transistor, then it is unipolar.

Imagine the operation of an ordinary water tap. Turn the valve - the flow of water increased, turn it the other way - the flow decreased or stopped. In practice, this is the principle of operation of a transistor. Only instead of water, a stream of electrons flows through it. The principle of operation of a bipolar type transistor is characterized by the fact that through this electronic device There are two types of current. They are divided into large, or main, and small, or manager. Moreover, the power of the control current affects the power of the main one. Let's consider the principle of its operation is different from others. Only one passes through it, which depends on the surrounding

A bipolar transistor is made from 3 layers of semiconductor, and also, most importantly, from two PN junctions. It is necessary to distinguish between PNP and NPN junctions, and, therefore, transistors. These semiconductors alternate between electron and hole conductivity.

A bipolar transistor has three contacts. This is the base, the contact coming out of the central layer, and two electrodes at the edges - the emitter and the collector. Compared to these outer electrodes, the base layer is very thin. At the edges of the transistor, the semiconductor region is not symmetrical. For proper operation For this device, the semiconductor layer located on the collector side should be, albeit slightly, thicker compared to the emitter side.

The operating principles of a transistor are based on physical processes. Let's work with the PNP model. The operation of the NPN model will be similar, except for the voltage polarity between the basic elements such as the collector and emitter. It will be directed in the opposite direction.

A P-type substance contains holes or positively charged ions. N-type substance consists of negatively charged electrons. In the transistor we are considering, the number of holes in the P region is much greater than the number of electrons in the N region.

When a voltage source is connected between parts such as the emitter and collector, the principles of operation of the transistor are based on the fact that holes begin to be attracted to the pole and collect near the emitter. But the current doesn't flow. The electric field from the voltage source does not reach the collector due to the thick semiconductor layer of the emitter and the semiconductor layer of the base.
Then we will connect the voltage source with a different combination of elements, namely between the base and emitter. Now the holes are directed towards the base and begin to interact with electrons. The central part of the base is saturated with holes. As a result, two currents are formed. Large - from emitter to collector, small - from base to emitter.

As the base voltage increases, there will be even more holes in the N layer, the base current will increase, and the emitter current will increase slightly. This means that with a small change in the base current, the emitter current increases quite seriously. As a result, we get an increase in the signal in the bipolar transistor.

Let's consider the principles of operation of a transistor depending on its operating modes. There are normal active mode, inverse active saturation mode, and cutoff mode.
At active mode operation, the emitter junction is open and the collector junction is closed. In inversion mode, everything happens the other way around.

Conventionally, a bipolar transistor can be drawn as a semiconductor wafer with changing regions different conductivity, consisting of two p-n junctions. Moreover, the outer regions of the plate have conductivity of one type, and the middle region of the opposite type, each of the regions has its own personal output.

Depending on the alternation of these areas, transistors are p-n-p and n-p-n conductivity, respectively.

And if we take and cover any one part of the transistor, then we get a semiconductor with one p-n junction or diode. This suggests the conclusion that a bipolar transistor can be conventionally represented as two semiconductors with one common zone, connected back to back.

The part of the transistor, the purpose of which is to inject charge carriers into the base, is called the emitter, and the corresponding p-n junction is the emitter, and that part of the element, the purpose of which is to remove or extract charge carriers from the base, is called the collector, and the p-n junction is the collector. The general area was called the base.

The difference in the designations of different structures is only in the direction of the emitter arrow: in p-n-p it is directed towards the base, and in n-p-n, on the contrary, away from the base.

What is the difference between PNP and NPN transistors? In this video I tried to show the difference in the operation of two types of bipolar transistors. I used readily available radio components such as an LED (and a resistor for protection) to demonstrate the operation. For example, I used transistors like 2n2907 and bc337. I regulated the voltage using a variable resistor (potentiometer).

In the initial period of development of semiconductor electronics, they were made only from germanium using the technology of fusing impurities, which is why they were called alloy. For example, the base is a germanium crystal and I melt small pieces of indium into it.

Indium atoms penetrate the body of a germanium crystal, creating two regions in it - a collector and an emitter. Between them there remains a very thin layer of a few microns of semiconductor of the opposite type - the base. And to hide the crystal from light, it is hidden in a housing.

The figure shows that a crystal holder is welded to the metal disk, which is the output of the base, and at the bottom of the disk there is its outer wire output.

The internal terminals of the collector and emitter are welded to the conductors of the external electrodes.

With the development of electronics, they began processing silicon crystals and invented silicon devices, which almost completely retired germanium transistors.

They are able to work with more high temperatures, they have a lower reverse current value and a higher breakdown voltage.

The main manufacturing method is planar technology. For such transistors, p-n junctions are located in the same plane. The principle of the method is based on the diffusion or fusion of an impurity into a silicon wafer, which can be in a gaseous, liquid or solid component. When heated to a strictly fixed temperature, diffusion occurs impurity elements into silicon.

IN in this case one of the balls creates a thin base region, and the other creates an emitter region. As a result, two p-n junctions are formed in silicon. Using this technology, the most common types of silicon transistors are produced in factories.

In addition, they are widely used for the manufacture of transistor structures. combined methods: fusion and diffusion or various options diffusion, for example, two-way or double one-way.

Let's carry out a practical experiment, for this we will need any transistor and an incandescent light bulb from an old flashlight and a little bit of mounting wire so that we can assemble this circuit.

Transistor Operation Practical Experience for Beginners

The light bulb lights up because the collector junction receives forward voltage bias, which unlocks the collector junction and the collector current Ik flows through it. Its value depends on the resistance of the lamp filament and internal resistance batteries or power supply.

Now let’s present this diagram in structural form:

Since in the N region the main charge carriers are electrons, they pass through the potential p-n barrier transition, fall into the p-type hole region and become minority charge carriers, where they begin to be absorbed by the majority charge carriers by holes. In the same way, holes from the collector tend to get into the base area and are absorbed by the main charge carriers, electrons.

Since the base is to the minus of the power source, many electrons will flow to it, compensating for losses from the base area. And the collector, connected to the plus through the lamp filament, is capable of receiving the same number, so the concentration of holes will be restored.

The conductivity of the pn junction will increase significantly and the collector current will begin to flow through the collector junction Ik. And the higher it is, the stronger the incandescent light bulb will burn.

Similar processes occur in the emitter junction circuit. The figure shows the circuit connection option for the second experiment.

Let's carry out another practical experiment and connect the base of the transistor to the plus of the power supply. The light bulb does not light up, since we connected the p-n junction of the transistor in the opposite direction and the resistance of the junction increased sharply and only a very small reverse collector current Ikbo flows through it, which is not capable of igniting the light bulb filament.

Let's carry out another interesting experiment: connect a light bulb in accordance with the picture. The light doesn't light up, let's figure out why.

If voltage is applied to the emitter and collector, then for any polarity of the power source, one of the transitions will be forward and the other will be reverse, and therefore no current will flow and the light bulb will not light.

From the block diagram it is very clear that the emitter junction is forward biased and open and awaits the reception of free electrons. The collector junction, on the contrary, is connected in the opposite direction and prevents electrons from entering the base. A potential barrier is formed between the collector and the base, which will provide great resistance to the current and the lamp will not light.

Let's add just one jumper to our circuit, which will connect the emitter and base, but the light bulb still doesn't light up.

Here, in principle, everything is clear: when the base and emitter are short-circuited with a jumper, the collector junction turns into a diode, which receives a reverse bias voltage.

Instead of the jumper, let's install a resistance Rb with a nominal value of 200 - 300 Ohms, and another power source of 1.5 volts. We connect its minus through Rb to the base, and its plus to the emitter. And a miracle happened, the light bulb lit up.

The lamp lit up because we connected additional source power supply between the base and the emitter, and thereby applied a direct voltage to the emitter junction, which led to its opening and a direct current flowed through it, which unlocks the collector junction of the transistor. The transistor opens and a collector current Ik flows through it, many times greater than the emitter-base current. And so this current lit up the light bulb.

If we change the polarity of the additional power source and apply plus to the base, then the emitter junction will close, followed by the collector junction. Reverse Ikbo will flow through the transistor and the light bulb will stop lighting.

The main function of resistor Rb is to limit the current in the base circuit. If all 1.5 volts are supplied to the base, then too much current will flow through the junction, as a result of which thermal breakdown of the junction will occur and the transistor may burn out. For germanium transistors, the gate voltage should be about 0.2 volts, and for silicon ones 0.7 volts.

Let's turn to structural diagram: When additional voltage is applied to the base, the emitter junction opens and free holes from the emitter are mutually absorbed with the base electrons, creating a direct base current Ib.

But not all holes entering the base recombine with electrons. Since the base area is quite narrow, therefore only a small part of the holes is absorbed by the base electrons.

The main volume of emitter holes skips the base and falls under more high level negative voltage in the collector, and together with the holes of the collector flow to its negative terminal, where they are mutually absorbed by electrons from the main power source GB. Resistance collector circuit The emitter-base-collector drops sharply and the forward collector current Ik begins to flow in it, many times greater than the base current Ib of the emitter-base circuit.

The higher the level of the unlocking voltage at the base, the higher the number of holes from the emitter to the base, the higher the current value in the collector. And, conversely, the lower the unlocking voltage at the base, the lower the current in the collector circuit.

In these experiments by a novice radio amateur on the principles of operation of a transistor, it is in one of two states: open or closed. Switching it from one state to another is carried out under the action of an unlocking voltage at the base Ub. This mode of operation of a transistor in electronics is called key mode. It is used in instruments and automation devices.

In amplification mode, the transistor amplifier operates in receiver and amplifier circuits audio frequency(USCH and ULF). During operation, small currents are used in the base circuit that control high currents in the collector. This is the difference between the amplification mode and the switching mode, which only opens or closes the transistor depending on the voltage at the base

A transistor is a very common active radio component that is found in almost all circuits, and very often, especially during experimental courses on learning the basics of electronics, it fails. Therefore, without the skill of checking transistors, it is better not to meddle with electronics. So let's figure out how to check the transistor.

Transistor (transistor, English) is a triode made of semiconductor materials, with three outputs, the main property of which is to control a significant current at the output of the circuit with a relatively low input signal. Field-effect transistors are used in radio components from which modern complex electrical devices are assembled. Their properties make it possible to solve problems of turning off or turning on the current in an electrical circuit printed circuit board, or its strengthening.

What is a field effect transistor

A field effect transistor is a device with three or four contacts in which the current on two contacts is adjustable electric field voltage on the third. That's why they are called field ones.

Contacts:

A field-effect transistor with a n-p junction is a special type of transistor that serves for current control.

It differs from a simple ordinary one in that the current passes through it without crossing the p-n junction zone, the zone formed at the boundaries of these two zones. The size of the p-n zone is adjustable.

Field effect transistors, their types

Field effect transistors with a n-p junction are divided into classes:

1. By type of conductor channel: n or r. The sign, polarity, of the control signal depends on the channel. It should be opposite in sign to the n-zone.
2. According to the structure of the device: diffuse, alloyed along the p-n junction, with a shutter, thin-film.
3. By number of contacts: 3 and 4-pin. In the case of a 4-pin device, the substrate also acts as a gate.
4. According to the materials used: germanium, silicon, gallium arsenide.

Classes are divided according to the principle of operation:

• device controlled by p-n junction;
• insulated gate or Schottky barrier device.

Field effect transistor, operating principle

In simple terms, how does a field-effect transistor work with manager transition, we can say this: the radio component consists of two zones: p - transition and n - transition. Flows through the zone electricity. Zone p is an overlapping zone, a kind of valve. If you press it hard, it blocks the area for current passage and it passes less. Or, if the pressure is reduced, more will pass. This pressure is carried out by increasing the voltage at the gate contact located in the river zone.

A device with a control p-n channel junction is a semiconductor wafer with electrical conductivity of one of these types. Contacts are connected to the ends of the plate: drain and source, in the middle there is a gate contact. The action of the device is based on the variability of thickness p-n spaces transition. Since there are almost no mobile charge carriers in the blocking region, it conductivity is zero. In the semiconductor wafer, in the area not under the influence of the blocking layer, a current-conducting channel is created. When a negative voltage is applied relative to the source, a flow is created at the gate through which charge carriers flow out.

In the case of an insulated gate, there is a thin layer of dielectric on it. This type of device works on the principle of electric field. A small amount of electricity is enough to destroy it. Therefore, to protect against static voltage, which can reach thousands of volts, special device housings are created - they help minimize the impact of viral electricity.

Why do you need a field effect transistor?

Considering the job difficult electronic technology, how the operation of a field-effect transistor (as one of the components of an integrated circuit) is difficult to imagine, that main directions of his work five:

1. High frequency amplifiers.
2. Bass amplifiers.
3. Modulation.
4. DC amplifiers.
5. Key devices (switches).

On simple example The operation of a transistor, like a switch, can be thought of as a microphone with a light bulb. The microphone picks up the sound, which generates an electric current. It goes to a locked field-effect transistor. By its presence, the current turns on the device, turns on electrical circuit to which the light bulb is connected. The light comes on when the microphone picks up sound, but it lights up due to a power source that is not connected to the microphone and is more powerful.

Modulation applied to control the information signal. The signal controls the frequency of the oscillation. Modulation is used for high-quality sound signal in radio, for transmitting sound in television broadcasts, broadcasting color and TV signal High Quality. It is used wherever work with high quality material is required.

Like an amplifier a field-effect transistor works in a simplified way: graphically, any signal, in particular an audio series, can be represented as a broken line, where its length is time, and the height of the breaks is the sound frequency. To amplify the sound, a powerful voltage is supplied to the radio component, which acquires the necessary frequencies, but with higher values, due to the supply weak signal to the control contact. In other words, the device proportionally redraws the original line, but with higher peak values.

Application of field effect transistors

The first device to go on sale using a field-effect transistor with manager p-n transition was hearing aid . Its appearance was recorded in the fifties of the last century. On an industrial scale they were used in telephone exchanges.

IN modern world, devices are used in all electrical engineering. Due to the small size and variety of characteristics of the field-effect transistor, it can be found in kitchen appliances, audio and television equipment, computers and electronic children's toys. They are used in alarm systems of both security mechanisms and fire alarms.

Transistor equipment is used in factories for machine power regulators. In transport, from the operation of equipment on trains and locomotives, to the fuel injection systems of private cars. In housing and communal services from dispatch systems to street lighting control systems.

One of the most important applications of transistors is processor production. In fact, the entire processor consists of many miniature radio components. But when moving to operating frequencies above 1.5 GHz, they begin to consume energy like an avalanche. Therefore, processor manufacturers have taken the path of multi-cores rather than increasing clock speeds.

Pros and cons of field effect transistors

Field effect transistors with their characteristics left far behind other species devices. They have found wide application in integrated circuits as switches.

• a cascade of parts consumes little energy;
• the gain is higher than that of other species;
• high noise immunity is achieved by the absence of current flow in the gate;
• more high speed on and off - they can operate at frequencies inaccessible to other transistors.

• lower destruction temperature than other species;
• at a frequency of 1.5 GHz, energy consumption begins to increase sharply;
• sensitivity to static electricity.

The characteristics of semiconductor materials, taken as the basis for field-effect transistors, made it possible use devices in everyday life and production. Based on spitting transistors, they created household appliances in the usual way modern man form. Processing high-quality signals, producing processors and other high-precision components is impossible without the achievements of modern science.

Electronics surround us everywhere. But almost no one thinks about how this whole thing works. It's actually quite simple. This is exactly what we will try to show today. Let's start with such an important element as the transistor. We'll tell you what it is, what it does, and how the transistor works.

What is a transistor?

Transistor– a semiconductor device designed to control electric current.

Where are transistors used? Yes everywhere! Almost no modern technology can do without transistors. electrical diagram. They are widely used in production computer technology, audio and video equipment.

Times when Soviet microcircuits were the largest in the world, have passed, and the size of modern transistors is very small. Thus, the smallest devices are on the order of a nanometer in size!

Console nano- denotes a value of the order of ten to the minus ninth power.

However, there are also giant specimens that are used primarily in the fields of energy and industry.

Exist different types transistors: bipolar and polar, direct and reverse conduction. However, the operation of these devices is based on the same principle. A transistor is a semiconductor device. As is known, in a semiconductor the charge carriers are electrons or holes.

The region with excess electrons is indicated by the letter n(negative), and the region with hole conductivity is p(positive).

How does a transistor work?

To make everything very clear, let's look at the work bipolar transistor (the most popular type).

(hereinafter referred to simply as a transistor) is a semiconductor crystal (most often used silicon or germanium), divided into three zones with different electrical conductivities. The zones are named accordingly collector, base And emitter. The device of the transistor and its schematic representation are shown in the figure below

Separate forward and reverse conduction transistors. Pnp transistors are called forward conduction transistors, and npn transistors– from the reverse.

Now let's talk about the two operating modes of transistors. The operation of the transistor itself is similar to the operation of a water tap or valve. Only instead of water there is electric current. There are two possible states of the transistor - operating (transistor open) and rest state (transistor closed).

What does it mean? When the transistor is turned off, no current flows through it. In the open state, when a small control current is applied to the base, the transistor opens and a large current begins to flow through the emitter-collector.

Physical processes in a transistor

And now more about why everything happens this way, that is, why the transistor opens and closes. Let's take a bipolar transistor. Let it be n-p-n transistor.

If you connect a power source between the collector and the emitter, the collector's electrons will begin to be attracted to the positive, but there will be no current between the collector and the emitter. This is hampered by the base layer and the emitter layer itself.

If you connect an additional source between the base and emitter, electrons from the n region of the emitter will begin to penetrate into the base region. As a result, the base area will be enriched with free electrons, some of which will recombine with holes, some will flow to the plus of the base, and some (most) will go to the collector.

Thus, the transistor turns out to be open, and the emitter-collector current flows in it. If the base voltage is increased, the collector-emitter current will also increase. Moreover, with a small change in the control voltage, a significant increase in the current through the collector-emitter is observed. It is on this effect that the operation of transistors in amplifiers is based.

That, in a nutshell, is the essence of how transistors work. It is necessary to calculate the power amplifier for bipolar transistors overnight, or complete laboratory work to study the operation of a transistor? This is not a problem even for a beginner if you use the help of our student service specialists.

Feel free to contact for professional help such important issues like studying! And now that you already have an idea about transistors, we suggest you relax and watch the video by Korn “Twisted transistor”! For example, you decide to contact the Correspondence Student.

Transistor belongs to the category 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 own parameters and characteristics.

Features of bipolar transistors

Electric current in bipolar transistors is formed electric charges having 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 appearance of electronic 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, zones n-p-n or pnp 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 arrives at the emitter in the form of a small electrical voltage, 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 where the amplifying properties of the transistor lie.

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.

How each one works field effect transistor consists of a unipolar current, requires DC voltage to provide the initial offset. 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.