TABLE OF CONTENTS Under normal conditions electrons are prevented from leaving a metal by a potential step at the surface. In the absence of an electric field, the height of this potential step is called the work function ?. Electrons can, however, escape if they are given enough energy. This energy can be supplied in a number of different ways, giving rise to the various types of electron emission.
When a metal is heated, some electrons with energies near the Fermi level are enabled to escape by acquiring extra thermal energy. An adjacent anode carrying a sufficiently positive potential will collect all the electrons emitted, and the saturated emission current will flow. A further increase of anode potential causes a positive field at the metal surface; this lowers the potential barrier slightly and increases the current. The 'zero field' saturated emission current per unit area of the cathode J, is related to the temperature according to the Richardson-Dushman equation.
where A is a constant, e the magnitude of the electronic charge, k Boltzmann's constant and T the absolute temperature.
For a metal, the theoretical value of A is 1.2 MA m -2. In practice ? usually has a temperature dependence and this results in a different value of A. The work function ? cannot be calculated reliably, but tends to increase with the density of the metal. The observed values of A and ? are shown...
