The devices in which a controlled flow of current can be obtained is the basic building blocks of all the electronic circuits.
Before the discovery of transistors (semiconductor devices) in 1948, generally vacuum tubes are used in place of transistors.
Its use is now very limited but was previously used in almost all electronic devices.
The basic working principle of a vacuum tube is a phenomenon called thermionic emission. When the cathode is heated the electrons are supplied by the heated cathode and the controlled flow of these electrons in a vacuum is obtained by varying the voltage between its different electrodes. In these devices, the electrons can flow only from the cathode to the anode (i.e. only in one direction)
Earlier vacuum tubes were used as diodes, which were generally used for rectification. (i.e. converting from an A.C to a D.C). But around 1907, the first triode tube was invented, which had three electrodes. These third electrodes called the control grid, enable the vacuum tube to be used not just as a rectifier but as an amplifier of electrical signals.
Similarly, tetrode and pentode (respectively with 4 and 5 electrodes) were invented in which a vacuum tube was used.
These vacuum tube devices are bulky, consume high power, operate generally at high voltages (~ 100 V) and have limited life and low reliability. These are the basic drawbacks of vacuum tubes.
When semiconductors were invented, it was seen that some solid-state semiconductors and their junctions offer the possibility of controlling the number and the direction of flow of charge carriers through them. Note that the supply and flow of charge carriers in the semiconductor devices are within the solid itself.
No external heating or large evacuated space is required by the semiconductor devices. They are small in size, consume low power, operate at low voltages and have a long life and high reliability.
Despite this, the vacuum tube did not disappear completely.
vacuum tubes are still used in high power RF transmitters, as they can generate more power than modern semiconductors equivalents. For this reason, we will find vacuum tubes in a particle accelerator, MRI scanners, and even microwave ovens. In the musical community, equipment used by a vacuum tube is still preferred today. Vacuum tube being used in a modern guitar amplifier.