Valve & Digital Electronics Chapter 30 PDF
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This document presents an overview of valve and digital electronics. It discusses different types of electronics, the concept of work function and important topics like Fermi energy and electron emission. The chapter appears to provide a foundational introduction to the field.
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Valve & Digital Electronics 169 P G Do You Know. (1) Electronics can be devided in two categories E3 60 K (i) Valve electronics (ii) Semiconductor electronics ID (2) Free electron in metal experiences a barrier on surface due to attractive Coulombian force. (4) Fermi energy (Ef) U (3) When kinetic e...
Valve & Digital Electronics 169 P G Do You Know. (1) Electronics can be devided in two categories E3 60 K (i) Valve electronics (ii) Semiconductor electronics ID (2) Free electron in metal experiences a barrier on surface due to attractive Coulombian force. (4) Fermi energy (Ef) U (3) When kinetic energy of electron becomes greater than barrier potential energy (or binding energy E b ) then electron can come out of the surface of metal. D YG Is the maximum possible energy possessed by free electron in metal at 0K temperature (i) In this energy level, probability of finding electron is 50%. (ii) This is a reference level and it is different for different metals. (5) Threshold energy (or work function W0) Is the minimum energy required to take out an electron from the surface of metal. Also W0 = Eb – Ef U Work function for different materials = 4.5 eV (W0)Throated tungsten = 2.6 eV W0 Eb ST (W0)Pure tungsten (W0)Oxide coated tungsten = 1 eV Ionised energy level Ef Fermi energy level W0 = Eb – Ef Ground energy level W0 Temp (6) Electron emission Four process of electron emission from a metal are (i) Thermionic emission (ii) Photoelectric emission (iii) Field emission (iv) Secondary emission Thermionic Emission and Emitters. (1) Thermionic emission 170 Valve & Digital Electronics (i) 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. (ii) Thermions have different velocities. 60 (iii) This was discovered by Edison (iv) Richardson – Dushman equation for current density (i.e. electric current emitted per unit area of metal surface) is given as J AT 2 e W0 / kT AT 2 e qV kT AT 2 e 11600 V T E3 where A = emission constant = 12 10 4 amp/ m2-K2 , k = Boltzmann’s constant, T = Absolute temp and W0 = work function. (v) The number of thermions emitted per second per unit area (J) depends upon following : J/T J 2 ID (b) J e W0 (a) J T 2 W1 W2 loge (J / T 2 ) U (W1
