Thermionic valves or vacuum tubes come in many forms including the diode, triode, tetrode, pentode, heptode and many more. These tubes have been manufactured by the millions in years gone by and even today the basic technology finds applications in today's electronics scene. It was the vacuum tube that first opened the way to what we know as electronics today, enabling first rectifiers and then active devices to be made and used.
Although vacuum tube technology may appear to be dated in the highly semiconductor orientated electronics industry, many thermionic valves or vacuum tubes are still used today in applications ranging from vintage wireless sets to high power radio transmitters.
However the most widely used thermionic device today is the cathode ray tube that is still manufactured by the million for use in television sets, computer monitors, oscilloscopes and a variety of other electronic equipment.
The simplest form of vacuum tube is the diode. It is ideal to use this as the first building block for explanations of the technology. It consists of two electrodes: a cathode and anode held within an evacuated glass bulb, connections being made to them through the glass envelope.
If a cathode is heated, it is found that electrons from the cathode become increasingly active and as the temperature increases they can actually leave the cathode and enter the surrounding space.
When an electron leaves the cathode it leaves behind a positive charge, equal but opposite to that of the electron. In fact there are many millions of electrons leaving the cathode. As unlike charges attract, this means that there is a force pulling the electrons back to the cathode. Unless there are any further influences the electrons would stay in the vicinity of the cathode, leaving the cathode as a result of the energy given to them as a result of the temperature, but being pulled back by the positive charge on the cathode.
Concept of thermionic emission
In a diode vacuum tube there is also another electrode called the anode. If a positive potential is applied to this electrode, the electrons will be attracted by this potential and will move towards it if it is at a higher potential than the cathode.
For the optimum performance the space between the cathode and the anode should be a vacuum. If there are any gas molecules in the space in which the electrons travel, collisions will occur and this will impede the flow of electrons. If an appreciable amount of gas is present, the electrons will ionise the gas, giving rise to a blue glow between the electrodes. In the early days of valves, it was thought that a certain amount of gas was necessary in the envelope. Later this was discovered that this was not the case and new "hard" valves were made that had a superior performance to the older "soft" valves.
The electrons flowing between the cathode and the anode form a cloud which is known as the "space charge". It can tend to repel electrons leaving the cathode, but if the potential applied to the anode is sufficiently high then it will be overcome, and electrons will flow toward the anode. In this way the circuit is completed and current flows.
As the potential is increased on the anode, so the current increases until a point is reached where the space change is completely neutralised and the maximum emission from the cathode is reached. At this point the emission can only be increased by increasing the cathode temperature to increase the energy of the electrons and allow further electrons to leave the cathode.