4 . Forbidden Band This band is completely empty . The minimum energy required to shift an electron from valence band to conduction band is called band gap ( E g ).
Thermionic Emission
Thermionic emission occurs when a metal is heated to a high temperature , the free electrons in the metal gain kinetic energy sufficient to escape through the surface of the metal .
Thermionic Diode
The thermionic diode is a two electrode ( cathode and plate ) device based on thermionic emission .
A diode allows unidirectional flow of electrons , i . e ., only when the plate is positive with respect to cathode . Hence , it is also called a valve .
The triode value consists of three electrodes , e . g ., cathode . plate and grid enclosed in an evacuated glass bulb .
Grid influences the space charge and controls the flow of plate current .
[ When the grid is given a negative potential with respect to cathode . It repels the electrons escaping from the cathode and Increases the effect of space charge , at sufficientLy negative grid potential is known as cut-off grid bias .
If the grid is given a positive potential with respect to cathode , it attracts the electrons and decreases the effect of space charge . The increasing the plate current . In this case a current flow into the circuit , thus grid modifies the function of valve .]
Grid is always kept at small negative potential with respect to cathode .
Triode can be used as an amplifier , oscillator modulator and demodulator .