Transistor device and application in simple language. How does a bipolar transistor work?

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.

– common devices in which the electrical flow is controlled by means of 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 field effect transistor, but more subtle. The signal transmitter is one or more electrons. This transistor is also called a nano- and quantum transistor. Using this technology, in the future they hope to create transistors with a size less than 10 nm, based on graphene.

What are transistors used for?

Transistors are used in amplification circuits, lamps, electric motors and other devices where necessary rapid change amperage or position onoff. The transistor can limit the current or smoothly, or by method pulse—pause. The second one is more often used for -control. Using a powerful power source, it conducts it through itself, regulating it 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 fully digital amplifiers based on. 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 is difficult to imagine at least one electronic device without at least one transistor.

In this article we will try to describe operating principle The most common type of transistor is bipolar. Bipolar transistor is one of the main active elements of radio-electronic devices. Its purpose is to work to amplify the power of the electrical signal arriving at its input. Power amplification is carried out using an external energy source. A transistor is a radio-electronic component with three terminals

Design feature of a bipolar transistor

To produce a bipolar transistor, you need a semiconductor of hole or electronic conductivity type, which is obtained by diffusion or alloying with acceptor impurities. As a result, regions with polar types of conductivities are formed on both sides of the base.

Bipolar transistors are of two types based on conductivity: n-p-n and p-n-p. The operating rules that govern a bipolar transistor having n-p-n conductivity (for p-n-p it is necessary to change the polarity of the applied voltage):

1. The positive potential at the collector is more important compared to the emitter.
2. Any transistor has its maximum valid parameters Ib, Ik and Uke, exceeding which is in principle unacceptable, since this can lead to destruction of the semiconductor.
3. The base-emitter and base-collector terminals function like diodes. As a rule, the diode in the base-emitter direction is open, and in the base-collector direction it is biased in the opposite direction, that is, the incoming voltage interferes with the flow of electric current through it.
4. If steps 1 to 3 are completed, then the current Ik is directly proportional to the current Ib and has the form: Ik = he21*Ib, where he21 is the current gain. This rule characterizes the main quality of the transistor, namely that the low base current controls the powerful collector current.

For different bipolar transistors of one series, the he21 indicator can fundamentally vary from 50 to 250. Its value also depends on the flowing collector current, the voltage between the emitter and the collector, and on the ambient temperature.

Let's study rule No. 3. It follows from this that the voltage applied between the emitter and the base should not be significantly increased, since if the base voltage is 0.6...0.8 V greater than the emitter (forward voltage of the diode), then extremely high current. Thus, in a working transistor, the voltages at the emitter and base are interconnected according to the formula: Ub = Ue + 0.6V (Ub = Ue + Ube)

Let us remind you once again that all these points apply to transistors with n-p-n conductivity. For p-n-p type everything should be reversed.

You should also pay attention to the fact that the collector current has no connection with the conductivity of the diode, since, as a rule, the collector-base diode receives reverse voltage. In addition, the current flowing through the collector depends very little on the potential on the collector (this diode is similar to a small current source)

When the transistor is turned on in amplification mode, the emitter junction is open and the collector junction is closed. This is achieved by connecting power supplies.

Since the emitter junction is open, the emitter current will pass through it, resulting from the transition of holes from the base to the emitter, as well as electrons from the emitter to the base. Thus, the emitter current contains two components - hole and electron. The injection ratio determines the efficiency of the emitter. Charge injection is the transfer of charge carriers from the zone where they were the majority to the zone where they become minority.

In the base, electrons recombine, and their concentration in the base is replenished from the plus of the EE source. As a result of this, in electrical circuit a rather weak current will flow from the base. The remaining electrons that did not have time to recombine in the base, under the accelerating influence of the field of the locked collector junction, as minority carriers, will move into the collector, creating a collector current. The transfer of charge carriers from the zone where they were minority to the zone where they become majority is called extraction electric charges.

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 this important element, like a transistor. We'll tell you what it is, what it does, and how the transistor works.

What is a transistor?

Transistor – semiconductor device, designed to control electric current.

Where are transistors used? Yes everywhere! Almost no modern electrical circuit can do without transistors. 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!

Prefix 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.

There are 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. P-n-p transistors are called forward conduction transistors, and n-p-n transistors are called reverse conduction transistors.

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 emitter, electrons from the collector will begin to be attracted to the positive, but there will be no current between the collector and 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. You need to calculate a power amplifier based on bipolar transistors overnight, or perform 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.

Do not hesitate to seek professional help in 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.

At one time, transistors replaced electronic tubes. This is due to the fact that they have smaller dimensions, high reliability and lower production costs. Now, bipolar transistorsare the basic elements in all amplification circuits.

It is a semiconductor element having a three-layer structure, which forms two electron-hole junctions. Therefore, the transistor can be represented as two back-to-back diodes. Depending on what will be the main charge carriers, they distinguish p-n-p And n-p-n transistors.

Base– a semiconductor layer, which is the basis of the transistor design.

Emitter called a semiconductor layer whose function is to inject charge carriers into the base layer.

Collector called a semiconductor layer, the function of which is to collect charge carriers passing through the base layer.

Typically, the emitter contains much more main charges than the base. This is the main condition for the operation of the transistor, because in this case, when the emitter junction is forward biased, the current will be determined by the main carriers of the emitter. The emitter will be able to perform its main function - injecting carriers into the base layer. They usually try to make the emitter reverse current as small as possible. An increase in emitter majority carriers is achieved using a high dopant concentration.

Make the base as thin as possible. This is due to the lifetime of the charges. The charge carriers must cross the base and recombine as little as possible with the main carriers of the base in order to reach the collector.

In order for the collector to be able to more fully collect media passing through the base, they try to make it wider.

Transistor operating principle

Let's look at example p-n-p transistor.

In the absence of external voltages, a potential difference is established between the layers. Potential barriers are installed at crossings. Moreover, if the number of holes in the emitter and collector is the same, then the potential barriers will be the same width.

In order for the transistor to work correctly, the emitter junction must be forward biased and the collector junction must be reverse biased.. This will match active mode transistor operation. In order to make such a connection, two sources are needed. A source with voltage Ue is connected with the positive pole to the emitter, and the negative pole to the base. A source with voltage Uк is connected with the negative pole to the collector, and the positive pole to the base. Moreover, Ue< Uк.

Under the influence of voltage Ue, the emitter junction is biased in the forward direction. As is known, when the electron-hole transition is forward biased, the external field is directed opposite to the transition field and therefore reduces it. The majority carriers begin to pass through the transition; in the emitter there are 1-5 holes, and in the base there are 7-8 electrons. And since the number of holes in the emitter is greater than the number of electrons in the base, the emitter current is mainly due to them.

The emitter current is the sum of the hole component of the emitter current and the electronic component of the base.

Since only the hole component is useful, they try to make the electronic component as small as possible. Qualitative characteristics emitter junction is injection ratio.

They try to bring the injection coefficient closer to 1.

Holes 1-5 that have passed into the base accumulate at the boundary of the emitter junction. Thus, a high concentration of holes is created near the emitter and a low concentration near the collector junction, as a result of which the diffusion movement of holes from the emitter to the collector junction begins. But near the collector junction, the hole concentration remains zero, because as soon as the holes reach the junction, they are accelerated by its internal field and are extracted (pulled) into the collector. Electrons are repelled by this field.

While the holes cross the base layer, they recombine with the electrons located there, for example, like hole 5 and electron 6. And since holes come constantly, they create an excess positive charge, therefore, electrons must also come in, which are drawn through the base terminal and form a base current Ibr. This important condition transistor operation – the concentration of holes in the base should be approximately equal to the concentration of electrons. In other words The electrical neutrality of the base must be ensured.

The number of holes reaching the collector is less than the number of holes leaving the emitter by the amount of recombined holes in the base. That is, The collector current differs from the emitter current by the amount of the base current.

From here it appears transfer coefficient carriers, which they also try to bring closer to 1.

The collector current of the transistor consists of the hole component Icr and the reverse collector current.

The reverse collector current arises as a result of the reverse bias of the collector junction, so it consists of minority carriers of hole 9 and electron 10. Precisely because the reverse current is formed by minority carriers, it depends only on the thermal generation process, that is, on temperature. Therefore it is often called thermal current.

The quality of the transistor depends on the magnitude of the thermal current; the smaller it is, the better the transistor.

The collector current is connected to the emitter current transfer coefficient.

The currents in the transistor can be represented as follows

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 modern name consists of two words. The first word is “transfer” (here “transformer” immediately comes to mind) means transmitter, converter, carrier. And the second half of the word resembles the word “resistor” - a part 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 electrical engineering formulas. Therefore, the word “transistor” can be interpreted as a resistance converter. In much the same way as in hydraulics, changes in fluid flow are regulated by a valve. In a transistor, such a “latch” changes the amount of electrical charges that create an electric current. This change is nothing more than a change internal resistance semiconductor device.

Gain electrical signals

The most common operation performed transistors, is amplification of electrical signals. But this is not an entirely correct expression, because weak signal from the microphone it 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 enough to produce sound or an image on a screen. And this is already a power of several tens, and in some cases hundreds of watts. Therefore, the amplification process comes down to using additional sources of energy received from the power supply to obtain a powerful copy of the weak input signal. In other words, a low-power input influence controls powerful energy flows.

Strengthening in other areas of technology and nature

Such examples can be found not only in electrical diagrams. For example, when you press the gas pedal, the speed of the car increases. At the same time, you don’t have to press the gas pedal very hard - compared to the power of the engine, the power you press on the pedal is negligible. To reduce the speed, you will have to release the pedal a little and weaken the input influence. In this situation, gasoline is a powerful source of energy.

The same effect can be observed in hydraulics: very little energy is used to open an electromagnetic valve, for example in a machine tool. And the oil pressure on the piston of the mechanism can create a force of several tons. This force can be adjusted if an adjustable valve is provided in the oil line, as in a regular kitchen faucet. Closed it a little - the pressure dropped, the force decreased. If I opened it more, the pressure increased.

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

Electrical Signal Amplifiers