Field electron emission

In the field emission by a sufficiently strong electric field ( more than 109 V / m) electrons are solved with a very small energy width of a ( negatively charged ) cathode. Classically considered, it is impossible to leave for a particle with a certain average thermal energy that is smaller than the height of the work function, the cathode material. Considered quantum mechanically but there is a certain probability that individual electrons escape from the solid. These are then sucked through the high external field. This effect is called generally also tunnel effect. Thus, the electron tunneling through the potential barrier, which was tilted by the external electric field - this particular type of tunneling is also called " Fowler- Nordheim tunneling " (named after Ralph Howard Fowler and Lothar Nordheim ).

History

The unsatisfactory explainable by the means of classical physics leakage of electrons from a solid state was one of the first research topics of quantum mechanics. Erwin Wilhelm Müller invented the field emission microscope, with the processes at the atomic level could be studied on metal surfaces for the first time. Also, the tunnel effect is based on quantum mechanical model similar ideas. It was first observed in 1897 in a vacuum in the field emission of electrons in an experiment by Robert William Wood, however, could not indicate this effect. In 1928 he was described by Ralph H. Fowler and Lothar Nordheim theory for the first time.

Applications

In modern electron beam generation systems distributed field emission cathodes are used, since the high spatial and temporal coherence feldemittierter electrons brings benefits to the electron- optical imaging with it. Field emission displays are an application of field emission by the Japanese company (Field Emission Technologies Inc.) was developed and brought to market. And vacuum fluorescent displays can be produced according to the principle of field emission, but are not in use due to the high operating voltage. In contrast to thermionic emission cathode remains cold during field emission. Therefore, it is more energy efficient in certain applications.

In electron tubes for high-voltage field-emission is undesirable and must be avoided due to the smooth, clean and accurate electrode surfaces. Is essential to keep the radius of curvature at the edges as large as possible, because only by the field strength can be maintained sufficiently small ( at a given voltage ) (see also corona ring).

The generated by field emission in vacuum free electrons are in the now largely detached from other electron field emission microscope used directly to an image to create for example a tungsten tip. Both bumps are (surveys lead to a stronger field) as well as regional, crystal structural differences in the work function visible.

Calculation

The current density of the field emission is calculated from general ( Fowler -Nordheim equation for field emission ):

With

  • : Planck's constant
  • : Charge of the tunneling particle
  • : Effective mass in the dielectric
  • : Effective mass of the charge carriers
  • : Electric field strength
  • ,: Weak material-dependent parameters - " constants"
  • : Work function

The tunneling current density according to Fowler and Nordheim is thus caused by the external electric field, the tunnel current per cross sectional area in amperes per square meter ( A / m ) of. In order to obtain the actual current in amperes, the above expression must still with the cross-sectional area through which the current flows, multiply.

329578
de