Bipolar transistor. The principle of operation, design and connection scheme
A bipolar transistor is a semiconductor type electrical transducer designed to amplify the power of signals and consists of three zones with alternating types of electrical conductivity. Due to its versatility, such devices today are widely used in a variety of amplifiers, pulse devices and generators. The first bipolar transistor was a point type and was characterized by insufficient stability of the work. Currently, bipolar transistors of the flat type are produced and used, which are distinguished by much greater reliability and wide functionality.
Bipolar Transistor Structure
This device consists of three semiconductor areas (electrodes). The two extreme ones have the same type of conductivity, and the middle one is opposite to them. Such a structure is called an npn structure.The bipolar transistor is included in the electrical circuit through the external leads of its electrodes. A characteristic feature of this device are the different sizes of its extreme areas. The smaller of them, called the emitter, performs the function of creating a powerful stream of charge carriers (electrons) that permeates the entire structure of the device. Therefore, the main characteristic of the emitter is a high degree of doping. The other extreme zone of the transistor of this type - the collector - is intended to collect the emitted flow, which makes it large in the transistor structure. The middle area, in turn, is called the base.
Types and principle of operation
The bipolar transistor can be manufactured by various technologies - by the method of fusion, diffusion and others, which largely determines the technical characteristics of the device. As for the material from which such devices are made, silicon or germanium plates are used to create a three-layer structure. The electrode of the base part is always shifted towards the emitter, which gives the device some asymmetry.The width of the middle part, which depends on the frequencies at which the transistor operates, can vary within certain limits. The lower the frequency of the device, the wider its base. The operation of the device is based on controlling the movement of charge carriers in accordance with the voltages applied to its transitions.
The most widely used is such a scheme, which is used to turn on a transistor of this type with a common emitter output. The main elements of this circuit are the power supply, the transistor itself and the resistor component. These elements form a linear cascade circuit where, due to the flow of controlled currents, an amplified voltage of variable nature is formed at the output of the circuit. All other elements of such a chain play a purely auxiliary role. For example, a capacitor performs a separation function. In the absence of this element, the circuit of the source of the incoming signal would be characterized by a direct current from the power source. In most electrical circuits, a bipolar transistor is used as a four-pole component. In other words - as a device that has two input and output pins.Since transistors of this type have only three contacts, to use these devices as quadrupoles, you need to make one of the contacts common. As a rule, this is done by an emitter electrode. Therefore, a common emitter bipolar transistor is often used.