Electronics is rich in all kinds of details. Each of these elements fulfills a specific role assigned to it. The transistor is characterized by its versatility and ability to perform various tasks. To understand what distinguishes it from other radio components, it is necessary to consider the device and principle of operation of bipolar transistors.
Content
- Transistor structure
- Use of p- and p-type semiconductors
- Base, collector and emitter assignment
- Modes and wiring diagrams
- Application area
Transistor structure
The bipolar transistor belongs to semiconductors - materials that conduct electricity worse than conductors, but are not dielectrics either. But if its temperature is brought to absolute zero, it becomes a dielectric. On the other hand, as the temperature rises, the conductivity of the device will increase. This makes it vulnerable to overheating. The increase in conductivity increases the current, which can damage the device.
For clarity, we can cite a diamond (adamant) as an example. In natural conditions, it is a semiconductor, but if you put it in a vacuum or inert gas and heat it, it turns into graphite, which is a good conductor. For industrial purposes, materials such as silicon, germanium and others are widely used for the production of transistors. By material used
transistors are:- germanium;
- silicon;
- gallium arsenide.
The semiconductor itself is very sensitive to external influences (deformation, radiation and temperature), internal defects and impurities. Under natural conditions, it behaves like a variable resistor, the resistance of which changes with temperature (used to manufacture varistors). When adding impurities, the properties of the semiconductor change dramatically, and it turns into a conductor. Impurities are divided into:
- donor;
- acceptor.
Donors, such as arsenic, easily donate their electrons, converting the semiconductor into a negatively charged material. The letter "p" is used to designate such material. Trivalent indium is an acceptor type. When combined with silicon, which has a tetravalent bond, one electron is missing, so a so-called "hole" is formed. Such material is designated by the letter "p".
Use of p- and p-type semiconductors
When conductors of different types are connected, a pn junction is formed between them, due to which electrons can only move in one direction. By connecting the "p" region to the minus of the power supply, and the "p" region to the plus, you can create a direct connection in which the electrons move freely. If the polarity of the source is reversed, then the electrons from the electronic region will go to the source, and the device will close, because the pn boundary will not allow electrons to pass through the hole region.
A radioelement consisting of two opposite types of conduction is called a diode. Its peculiarity is that it passes current in only one direction and does not regulate the flow of electrons. To get a bipolar transistor, add a third region "p" or "p" - this is what distinguishes the device of the transistor. As a result, a crystal is obtained with three regions and two pn junctions. Taking into account how the areas follow each other, distinguish the following types:
- p-p-p;
- p-p-p.
The first area is called the emitter, the second (middle) is the base, and the third is the collector. The base always has the opposite sign of the emitter and collector.
Base, collector and emitter assignment
The base controls the current between the collector and emitter. It will be clearer if the transistor is divided into two separate sections: emitter - base and base - collector. Since the base is in the middle part, we get two diodes directed towards each other (p-p-p type) or in the opposite direction (p-p-p type). Since the power supply is connected to the emitter and collector, and the control voltage is applied to the base, then one of the diodes is connected in the opposite direction, i.e. closed. Almost always it is the transition from the base to the collector. The task of the base is to gradually open this passage.
Most of all impurities are added to the emitter region, because it is an injector or generator of the main carriers. The collector, on the other hand, is minimally diluted with impurities, so as not to pass current in the closed state.
The base must meet stringent requirements. Firstly, how quickly the transistor works depends directly on the thickness of the base - the thinner it is, the higher the speed. Secondly, the thinner the base, the lower the voltage can be applied to the transistor, otherwise an electrical breakdown will occur between the emitter and the collector.
To understand how a bipolar transistor works, an example can be given. The water tap is located horizontally. Nearby (along the course of the tap, and not under it) there is a gutter through which water can go to the desired place. When they start to open the tap, the water pressure is small, it flows vertically without falling on the gutter. With further opening of the tap, the pressure will increase until the moment comes for water to enter the tray.
Just as the distance between the tap and the drain determines the water pressure that will help reach the gutter, so the thickness of the base affects the saturation voltage, after which the transistor turns on. Of course, the example is rough and is needed only in order to roughly understand the principle of operation of the transistor. But it is the voltage between the emitter and the base that will be the very "tap" that opens the transistor.
After the crystal is formed, metal leads are attached to each of its regions, everything is carefully insulated and protected by the case. There are unpackaged and transparent transistors, as well as those designed to work with a radiator.
Modes and wiring diagrams
Thanks to the development of science and technology, new transistors are being developed aimed at eliminating harmful factors. These include both physical (for example, chips and impurities) and electrical (material resistance). The elimination of such disadvantages leads to an increase in performance, a decrease in power consumption, and a number of other advantages. But transistor tasks remain the same:
- signal amplification;
- generation;
- switching.
Before breaking down each of these tasks, it is important to understand how a transistor works. Take a pnp-type transistor as an example. A plus is connected to the emitter, and a minus to the collector. Such inclusion is called normal, the opposite is called inverse. The emitter is saturated with holes, which are actually atoms that cannot move because they lack electrons. The area in which there are not enough electrons increases. In the collector, on the contrary, there is a supersaturation of electrons.
The base becomes an insulator between these areas, since the electrons in it go through the emitter to the power source. When a negative voltage appears on the base, electrons gradually turn it into a conductor. This leads to the fact that the transistor opens, and a current appears between the emitter and collector. From this we can conclude that the operating modes of the transistor - closed state, saturation and open state - are completely dependent on the voltage at the base.
The next thing you need to pay attention to is which connection scheme is used. Let's take a battery and a light bulb as an example. Each of these elements has two outputs, that is, there are four of them. This rule applies to direct (pulsed) current. A transistor is a converter, in other words, it has an input and an output. In this case, it should have not three, but four pins. In practice, however, it usually has three conclusions. It turns out that one of its conclusions should be common for both input and output. Based on this, there are the following connection types:
- with a common emitter (there is an increase in voltage and current, it is used more often than other types);
- with a common base (amplifies only the current, rarely used);
- with a common collector (amplifies the voltage, often used to match stages with different resistances).
Application area
A transistor is used to amplify continuous signals of various shapes in analog circuits. Human speech is a prime example. Noise is easily interwoven into such signals and filters are used to eliminate them. Analog circuits are the opposite of discrete digital signals.
In the oscillator circuit, the transistor generates signals of various shapes. For example, in television, transistor generators allow you to create an intermediate frequency with which video and sound are transmitted. In the TVs themselves, such generators help create an image on the screen, setting the frequency by line and frame.
In switching circuits, you need to quickly disconnect and connect the load, performing the relay task, for example, connecting more powerful thyristors, contactors, etc. Particularly stringent requirements are imposed on circuits with inert loads in the form of inductance. They are used to amplify digital signals in computers and other equipment.
Nowadays, transistors have almost completely replaced vacuum tubes. These devices have a number of advantages, including should be highlighted:
-
small size; - lighter production, which leads to lower costs;
- much less voltage is needed to control;
- do not require warming up, which leads to lower energy consumption and shorter set-up time;
- high resistance to mechanical stress;
- increased service life.
The best materials are used today for the production of transistors. Some metals (for example, germanium) have already been abandoned in the manufacture of devices. But even modern transistors have their drawbacks and limitations. To them include the following:
- silicon devices cannot operate at voltages above 1 kV;
- the creation of powerful transmitters requires very complex coordination;
- strong sensitivity to radiation and electromagnetic interference.
Recent developments have come close to solving many problems. In addition, today new possibilities and directions of using bipolar devices are being revealed.
