History of Vacuum Diode
In November 16, 1904 first vacuum diode was invented by Sir John Ambrose Fleming and it is also called the Fleming valve, the first thermionic valve. In those days there was no existence of p-n junction in electronics field. A conceptual figure of vacuum diode is shown below.
How Does Vacuum Tube Diode Work?
Here the vacuum diode works mostly like a modern diode. But its size is larger. It consists of a vacuum container with cathode and anode inside. This cathode and anode are connected across a voltage source.
The anode is applied with positive voltage with respect to cathode. It works on the principle of thermionic emission. A filament heats this cathode. Hence electrons get emitted from the cathode and attracted towards the anode. If the positive voltage applied at the anode, is not sufficient enough, the anode cannot attract the electrons emitted from the cathode due to hot filament.
As a result, one cloud of electrons gets accumulated in the space between cathode and anode. This is called space charge. Due to this space charge, the further emitted electrons get repealed and come back to the cathode. Hence virtually electron emission stops. No current flows through the circuit.
If the applied voltage between anode and cathode is increased gradually then more and more space charge electrons come to the anode and create vacant space for further emitted electrons. So with the increase of voltage across anode and cathode, we can increase the emission rate of electrons.
At the same time, the space charge gradually vanishes that means it gets neutralised on the anode. Once for certain applied voltage between anode and cathode, the entire space charge vanishes. There is no more obstruction for emission of electrons from the cathode. Then a beam of electrons starts flowing freely from cathode to anode through space. As a result, current flows from the anode to cathode.
On the other hand if anode is made negative with respect to cathode there is no electron emission from it as it is cold not hot. Now the emitted electrons from heated cathode do not come to the anode. Due to repulsion of negative anode strong space charge will be accumulated between anode and cathode. Again due to repulsion of this space charge all further emitted electrons come back to the cathode hence no virtual emission takes place hence no current flows in the circuit. So, vacuum diode allows current to flow in one direction only.
Under reverse bias this vacuum diode does not work. This vacuum tube was the basic component of electronics throughout the first half of the twentieth century. It was available and common in the circuit of radio, television, radar, sound reinforcement, sound recording system, telephone, analog and digital computers, and industrial process control.
V-I Characteristics of Vacuum Diode
The V-I characteristics of a vacuum diode is shown below.
The size of space charge depends upon the emission of electrons from cathode during formation of space charge. The emission of electrons further depends upon the temperature at which the cathode is heated. Hence if temperature is increased the amount of space charge is also increased. So the anode voltage required to neutralize the space charge will also be more.
Thus same vacuum diode will have different V-I characteristic graphs at different cathode temperatures. In the above figure we have shown only three of them. One graph for ToC, one for temperature more than ToC and one for temperature less than ToC. When anode voltage is gradually increased from zero the current from anode to cathode is proportionally increased. Since the space charge limits the emission from cathode, the current is proportionally increased with decrease of space charge strength.
This zone of the characteristics is called space charge limiting region as shown in the figure. After space charge is vanished the electron emission becomes constant and is solely dependent upon temperature of the cathode. Here the current in the vacuum diode becomes saturated. When no voltage is applied to anode there should not be any current in the circuit but actual case is not like that. Because of statistical fluctuation in the velocity, some electrons are energetic enough to reach the anode even there is no voltage at anode. The small current caused by this phenomenon is known as splash current.
Use of Vacuum Tube Diodes
Gradually p-n junction semiconductor has come in the market and vacuum tubes got replaced by them. The most basic structure of vacuum tube is a vacuum diode. Vacuum tubes are still being used widely around the world. The applications for vacuum tubes include:
- Atomic Clocks
- Audio Systems
- Car Dashboards
- Cellular Telephone Satellites
- Computer Monitors
- DVD Players and Recorders
- Electromagnetic Testing
- Electron Microscopes
- Gas Discharge Systems
- Gas Lasers
- Guitar Amplifiers
- Ham Radio
- High-speed Circuit Switching
- Klystron Tubes
- Industrial Heating
- Ion Microscopes
- Ion Propulsion Systems
- Lasers
- LCD Computer Displays
- Lighting
- Microwave Systems
- Microwave Ovens
- Military Systems
- Mobile Phone, Bluetooth and Wi-Fi Microwave Components
- Musical Instrument Amplifiers
- Particle Accelerators
- Photo multiplier Tubes
- Plasma Panel Displays
- Plasma Propulsion Systems
- Professional Audio Equipment
- Radar Systems
- Radio Communications
- Radio Stations
- Recording Studios
- Solar Collectors
- Sonar Systems
- Strobe Lights
- Satellite Ground Stations
- Semiconductor Vacuum Electronic Systems
- TV Stations
- Vacuum Electron Devices
- Vacuum Panel Displays
Types of Vacuum Diodes
The vacuum diode tubes are classified as
- Frequency range wise (audio, radio, microwave)
- Power rating wise (small signal, audio power)
- Cathode/filament type wise (indirectly heated, directly heated)
- Application wise (receiving tubes, transmitting tubes, amplifying or switching)
- Specialized parameters wise (long life, very low micro phonic sensitivity and low noise audio amplification)
- Specialized functions wise (light or radiation detectors, video imaging tubes)
