Thermionic emission
Thomas Edison in 1880 while inventing his light bulb noticed this current, so subsequent scientists referred to the current as the Edison effect, though it wasn't until after the 1897 discovery of the electron that scientists understood that electrons were emitted and why.
[6] Other early contributors included Johann Wilhelm Hittorf (1869–1883),[7][8][9][10][11][12] Eugen Goldstein (1885),[13] and Julius Elster and Hans Friedrich Geitel (1882–1889).
[14][15][16][17][18] Thermionic emission was observed again by Thomas Edison in 1880 while his team was trying to discover the reason for breakage of carbonized bamboo filaments[19] and undesired blackening of the interior surface of the bulbs in his incandescent lamps.
This projected carbon was deemed "electrical carrying" and initially ascribed to an effect in Crookes tubes where negatively-charged cathode rays from ionized gas move from a negative to a positive electrode.
To try to redirect the charged carbon particles to a separate electrode instead of the glass, Edison did a series of experiments (a first inconclusive one is in his notebook on 13 February 1880) such as the following successful one:[20] This effect had many applications.
Edison found that the current emitted by the hot filament increased rapidly with voltage, and filed a patent for a voltage-regulating device using the effect on 15 November 1883,[21] notably the first US patent for an electronic device.
Visiting British scientist William Preece received several bulbs from Edison to investigate.
Following J. J. Thomson's identification of the electron in 1897, the British physicist Owen Willans Richardson began work on the topic that he later called "thermionic emission".
He received a Nobel Prize in Physics in 1928 "for his work on the thermionic phenomenon and especially for the discovery of the law named after him".
The minimum amount of energy needed for an electron to leave a surface is called the work function.
The work function is characteristic of the material and for most metals is on the order of several electronvolts (eV).
In the period 1911 to 1930, as physical understanding of the behaviour of electrons in metals increased, various theoretical expressions (based on different physical assumptions) were put forward for AG, by Richardson, Saul Dushman, Ralph H. Fowler, Arnold Sommerfeld and Lothar Wolfgang Nordheim.
In fact, by about 1930 there was agreement that, due to the wave-like nature of electrons, some proportion rav of the outgoing electrons would be reflected as they reached the emitter surface, so the emission current density would be reduced, and λR would have the value 1 − rav.
Thus, one sometimes sees the thermionic emission equation written in the form: However, a modern theoretical treatment by Modinos assumes that the band-structure of the emitting material must also be taken into account.
To avoid misunderstandings, the meaning of any "A-like" symbol should always be explicitly defined in terms of the more fundamental quantities involved.
Because of the exponential function, the current increases rapidly with temperature when kT is less than W.[further explanation needed] (For essentially every material, melting occurs well before kT = W.) The thermionic emission law has been recently revised for 2D materials in various models.
Without the field, the surface barrier seen by an escaping Fermi-level electron has height W equal to the local work-function.
The electric field lowers the surface barrier by an amount ΔW, and increases the emission current.
For electric field strengths higher than 108 V⋅m−1, so-called Fowler–Nordheim (FN) tunneling begins to contribute significant emission current.
[37] Photon-enhanced thermionic emission (PETE) is a process developed by scientists at Stanford University that harnesses both the light and heat of the sun to generate electricity and increases the efficiency of solar power production by more than twice the current levels.
