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.