Name: Vacuum Tubes and Semiconductors
Uploaded: 2019-07-19
Duration: 8 min 14 s
Description: Vacuum tubes were the first devices where the electronic discharge was realised. Further work on them gave birth to semiconductor electronics. Let's discuss...

Question

Draw VI characteristics of a vacuum diode and explain the reasons for the difference in its nature in different regions.

Accepted Answer

Vacuum diode works on the principle of thermionic emission.

This non-linear curves that can be subdivided into two regions:
1.Space charge limited region ( OA and OC )
2. Saturation reason ( AB and CD )

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 T^oC, one for temperature more than T^oC and one for temperature less than T^oC. 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.

Draw VI characteristics of a vacuum diode and explain the reasons for the difference in its nature in different regions.

Important Questions on Electron Emission

Learn and Prepare for any exam you want !