Let us discuss how does vacuum tube diode work
In November 16, 1904, first vacuum diode was invented by Sir John Ambrose Fleming
and it is also called the Fleming valve. This is the first thermionic valve
In those days, there was no existence of p-n junction and electronics field
Here the vacuum diode works mostly like 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 the voltage source
The anode is applied with positive voltage with respect to the cathode
Generally, it works on principle of thermionic emission. This cathode is heated by a filament
Hence, electrons get emitted from cathode and attracted towards anode. If the positive voltage applied at the anode is not sufficient enough
the electrons emitted from cathode due to hot filament are not sufficiently attracted by anode
As a result, one cloud of electrons that is negative charge is 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 Due to this space charge, the further emitted electrons get repealed and come back to the cathode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 weakened space for further emitted electrons
So, with increase of voltage across anode and cathode, the emission rate of electrons will be increased
At the same time, the space charge is gradually vanished that is neutralized on the anode
Once for certain applied voltage between anode and cathode, the entire space charge is vanished
There is no more obstruction for emission of electrons from the cathode
Then a beam of electrons start flowing freely from cathode to anode through the 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 to the anode
Hence, no virtual emission takes place. Hence, no current flows in the circuit
So, vacuum diode allows current to flow in one direction only