JEE Main & Advanced Physics Semiconducting Devices Thermionic Emission

(1) The phenomenon of ejection of electrons from a metal surface by the application of heat is called thermionic emission and emitted electrons are called thermions and current flowing is called thermion current.

(2) Thermions have different velocities.

(3) This was discovered by Edison

(4) Richardson - Dushman equation for current density (i.e. electric current emitted per unit area of metal surface) is given as \[J=A{{T}^{2}}{{e}^{-\,{{W}_{0}}/kT}}=A{{T}^{2}}{{e}^{-\frac{qV}{kT}}}=A{{T}^{2}}{{e}^{-\,\,\frac{11600V}{T}}}\] where A = emission constant \[=12\times {{10}^{4}}\] amp/ \[{{m}^{2}}-{{K}^{2}}\], k = Boltzmann?s constant, T = Absolute temp and \[{{W}_{0}}=\] work function.

(5) The number of thermions emitted per second per unit area (J) depends upon following : (i) \[J\propto {{T}^{2}}\]  (ii) \[J\propto {{e}^{-{{W}_{0}}}}\] Types of thermionic emitters  

Directly heated emitter	Indirectly heated emitter
Cathode is directly heated by passing current.	Cathode is indirectly heated.
Thermionic current is less.	Thermionic current is more.
Energy consumption and life is small.	Energy consumption and life is more.