Critical Absorption Energy - Applications

Applications

An important implication of the work function is that there will be spatial variations in the vacuum electrostatic potential inside a vacuum chamber, if it is not lined with a material of uniform work function. These equilibrium variations in ϕ (known as Volta potentials, contact potential differences, patch fields, or patch potentials) can disrupt sensitive apparatus that rely on a perfectly uniform vacuum. For example, the Gravity Probe B experiment was significantly impacted by variations in W (and ϕ) over the surface of a free spinning gyroscope, as the resulting electric dipole torques resulted in precession and slowing by eddy currents. Critical apparatus may be lined with molybdenum which shows low variations in work function between different crystal faces.

In thermionic electron gun, the work function and temperature of the hot cathode are critical parameters in determining the amount of current that can be emitted. Tungsten, the common choice for vacuum tube filaments, can survive to high temperatures but its emission is somewhat limited due to its relatively high work function (approximately 4.5 eV). Various surface coatings (e.g., thorium or barium oxide) can substantially reduce the work function and thereby give enhanced emission at lower temperatures, prolonging the lifetime of the filament (for more information, see hot cathode).

The work function is also important parameter in the design of solid state electronics, even though these devices incorporate no vacuum. In insulated gate electronic devices (such as MOSFET) the work function difference between the semiconductor and gate influences the threshold voltage required to form an inversion layer. In junctions between different conductors, there are heuristic rules that predict the degree of band bending based on the thought experiment of two materials coming together in vacuum, such that the materials must equalize their work function upon contact. For a semiconductor-semiconductor junction this is known as Anderson's rule and is reasonably accurate. For a metal-semiconductor junction this is known as the Schottky-Mott rule and gives poor accuracy, due to a phenomenon known as Fermi level pinning.