Electron Emission. Introduction
The reader is familiar with current conduction (i.e. flow of electrons)through a conductor. Examples are: current through power lines, windings of transformers and motors etc. Many electronic devices depend for what their operation on the movement of electrons in an evacuated space. For this purpose, the free electrons must be ejected from the surface of metallic conductor by supplying sufficient energy from some external source. This is known as electron emission. The emitted electrons can be made to move in vacuum under the influence of an electric field, thus constituting electric current in vacuum.
Metals are most suitable substances for electron emission because they contain a large number of free electrons. However, metal can not emit electrons from their surface under ordinary conditions. This is due to the fact that the electrons in the metal are free only to the extend that they may be transferred from one atom to another within the metal but they cannot leave metal surface to provide electron emission. However, if sufficient external energy is given to free electrons, their kinetic energy is increased and thus electrons will leave the metal surface. The additional energy required to emit electrons from a metallic surface is known as work function of the metal. The work function of pure metals varies roughly from 2eV to 6eV. It may be noted that it is desirable that metal used for electron emission should have low work function so that a small amount of energy is required to cause emission of electrons.
Types of Electron Emission. In order to liberate electrons from a metallic surface, external energy equal to work function of the metal must be supplied. The external energy supplied to the metal may be in several forms such as heat energy, energy stored in electric field, light energy or kinetic energy of the electric charges bombarding the metal surface. Accordingly, the following are the four principal methods of obtaining electron emission from the surface of a metal:
(1) Thermionic emission (2) Field emission
(3) Photo-electric emission (4) Secondary emission
The most commonly used type of emission is the thermionic emission. In this case, the emission from the metallic surface is caused by supplying thermal energy. The commonly used substances for thermionic emission are tungsten, thoriated tungsten, oxide-coated cathode ect. Although thermionic emitters should have the desirable characteristic of low work function, high melting point and high mechanical strength, yet their choice for a particular situation would depend upon the service requirements.
JOHNPAOLO R. NAPIZA
JEWEL R. MARCELLA
BSE TLE III
A.Y. 2013 - 2014
ENGR. AQUILINO NOCEDA
A vacuum tube is an electron device in which the flow of electrons is through vacuum. It usually contains cathode which is the electron emitter; an anode (anode called plate) which is the electron collector and one or more electrodes (called grids) for controlling the flow of electrons between cathode and plate.
These electrodes are housed in highly evacuated glass envelope. The plate is held at positive potential w.r.t. cathode so that the emitted electrons are attracted to plate to provide current in vacuum. The ability of vacuum tubes to conduct current in vacuum enables them to perform such functions as rectification, amplification etc.
The vacuum tubes are generally classified according to the number of electrodes present. There are two principal electrodes, namely cathode and plate present in every tube. The other electrodes, if any, called grids. It may be noted counted as electrodes because it is merely incandescent filament to heat the cathode electrically.
A vacuum diode is the simplest tube and contains cathode and anode(or plate) enclosed in a highly evacuated glass envelope. The plate is held at some high positive potential w.r.t. cathode. When the cathode is heated by passing electric current through the heater, it emits a large number current “Ib” . The plate current in a diode depends upon plate voltage and cathode temperature. Is used as rectifier to changed the alternating current to direct current. The major limitation of a vacuum diode is that plate current cannot be changed easily.