Transistor - a device that runs on semiconductors with electronic filling. It is designed to convert and amplify electrical signals. There are two types of devices: a bipolar transistor and a unipolar transistor, or field.
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 the transistor, then it is unipolar.
Imagine the work of an ordinary water tap. They turned the valve - the flow of water intensified, turned the other way - the flow decreased or stopped. In fact, this is the principle of operation of the transistor. Only instead of water does a stream of electrons flow through it. The principle of operation of a bipolar transistor is characterized by the fact that two types of current go through this electronic device. 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. Consider a field effect transistor. The principle of its operation is different from others. Only one current passes in it , the power of which depends on the surrounding electromagnetic field.
A bipolar transistor is made of 3 layers of a semiconductor, and also, most importantly, of two PN junctions. It should be distinguished PNP and NPN junctions, and, therefore, transistors. In these semiconductors there is an alternation of electron and hole conductivity.
The bipolar transistor has three pins. This is a base, a contact emerging from the central layer, and two electrodes at the edges - an emitter and a collector. Compared to these extreme electrodes, the base layer is very thin. At the edges of the transistor, the semiconductor region is not symmetrical. For the correct operation of this device, the semiconductor layer located on the collector side should be slightly, but thicker, in comparison with the emitter side.
The principles of operation of the transistor are based on physical processes. Let's work with the PNP model. The operation of the NPN model will be similar, with the exception of voltage polarity between such basic elements as the collector and emitter. She 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 under consideration, the number of holes in region P is much larger than the number of electrons in region N.
When connecting a voltage source between parts such as the emitter and collector, the principles of the transistor are based on the fact that holes begin to attract to the pole and collect near the emitter. But the current does not go. The electric field from the voltage source does not reach the collector due to the thick layer of the emitter semiconductor and the base semiconductor layer.
Then we will connect the voltage source with another combination of elements, namely between the base and the emitter. Now the holes are sent to 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 the emitter to the collector, small - from the base to the emitter.
As the voltage in the base increases, there will be even more holes in the N layer, the base current will increase, and the emitter current will slightly increase. This means that with a small change in the base current, the emitter current amplifies quite seriously. As a result, we get a signal growth in a bipolar transistor.
Consider the principles of operation of the transistor, depending on the modes of its operation. Distinguish between normal active mode, inverse active mode, saturation mode, cutoff mode.
In active mode, the emitter junction is open, and the collector junction is closed. In inverse mode, everything happens the other way around.