Field Electron Emission - Early History of Field Electron Emission

Early History of Field Electron Emission

Field electron emission has a long, complicated and messy history. This section covers the early history, up to the derivation of the original Fowler–Nordheim-type equation in 1928.

In retrospect, it seems likely that the electrical discharges reported by Winkler in 1744 were started by CFE from his wire electrode. However, meaningful investigations had to wait until after J.J. Thomson's identification of the electron in 1897, and until after it was understood – from thermal emission and photo-emission work – that electrons could be emitted from inside metals (rather than from surface-adsorbed gas molecules), and that – in the absence of applied fields – electrons escaping from metals had to overcome a work function barrier.

It was suspected at least as early as 1913 that field-induced emission was a separate physical effect. However, only after vacuum and specimen cleaning techniques had significantly improved, did this become well established. Lilienfeld (who was primarily interested in electron sources for medical X-ray applications) published in 1922 the first clear account in English of the experimental phenomenology of the effect he had called "autoelectronic emission". He had worked on this topic, in Leipzig, since about 1910. Kleint describes this and other early work.

After 1922, experimental interest increased, particularly in the groups led by Millikan at the California Institute in Pasadena, and by Gossling at the General Electric Company in London. Attempts to understand autoelectronic emission included plotting experimental current-voltage (i - V) data in different ways, to look for a straight-line relationship. Current increased with voltage more rapidly than linearly, but plots of type (log(i) vs. V) were not straight. Schottky suggested in 1923 that the effect might be due to thermally induced emission over a field-reduced barrier. If so, then plots of type (log(i) vs. V1/2) should be straight; but they were not. Nor is Schottky's explanation compatible with the experimental observation of only very weak temperature dependence in CFE – a point initially overlooked.

A breakthrough came when Lauritsen (and Oppenheimer independently) found that plots of type (log(i) vs. 1/V) yielded good straight lines. This result, published by Millikan and Lauritsen in early 1928, was known to Fowler and Nordheim.

Oppenheimer had predicted that the field-induced tunneling of electrons from atoms (the effect now called field ionization) would have this i(V) dependence, had found this dependence in the published experimental field emission results of Millikan and Eyring, and proposed that CFE was due to field-induced tunneling of electrons from atomic-like orbitals in surface metal atoms. An alternative Fowler-Nordheim theory explained both the Millikan-Lauritsen finding and the very weak dependence of current on temperature. Fowler-Nordheim theory predicted both to be consequences if CFE were due to field-induced tunneling from free-electron-type states in what we would now call a metal conduction band, with the electron states occupied in accordance with Fermi–Dirac statistics.

In fact, Oppenheimer (although right in principle about the theory of field ionization) had mathematical details of his theory seriously incorrect. There was also a small numerical error in the final equation given by Fowler-Nordheim theory for CFE current density: this was corrected in the 1929 paper of (Stern, Gossling & Fowler 1929).

Strictly, if the barrier field in Fowler-Nordheim 1928 theory is exactly proportional to the applied voltage, and if the emission area is independent of voltage, then the Fowler-Nordheim 1928 theory predicts that plots of the form (log(i/V2) vs. 1/V) should be exact straight lines. However, contemporary experimental techniques were not good enough to distinguish between the Fowler-Nordheim theoretical result and the Millikan-Lauritsen experimental result.

Thus, by 1928 basic physical understanding of the origin of CFE from bulk metals had been achieved, and the original Fowler-Nordheim-type equation had been derived.

The literature often presents Fowler-Nordheim work as a proof of the existence of electron tunneling, as predicted by wave-mechanics. Whilst this is correct, the validity of wave-mechanics was largely accepted by 1928. The more important role of the Fowler-Nordheim paper was that it was a convincing argument from experiment that Fermi–Dirac statistics applied to the behavior of electrons in metals, as suggested by Sommerfeld in 1927. The success of Fowler-Nordheim theory did much to support the correctness of Sommerfeld's ideas, and greatly helped to establish modern electron band theory. In particular, the original Fowler-Nordheim-type equation was one of the first to incorporate the statistical-mechanical consequences of the existence of electron spin into the theory of an experimental condensed-matter effect. The Fowler-Nordheim paper also established the physical basis for a unified treatment of field-induced and thermally induced electron emission. Prior to 1928 it had been hypothesized that two types of electrons, "thermions" and "conduction electrons", existed in metals, and that thermally emitted electron currents were due to the emission of thermions, but that field-emitted currents were due to the emission of conduction electrons. The Fowler-Nordheim 1928 work suggested that thermions did not need to exist as a separate class of internal electrons: electrons could come from a single band occupied in accordance with Fermi–Dirac statistics, but would be emitted in statistically different ways under different conditions of temperature and applied field.

The ideas of Oppenheimer, Fowler and Nordheim were also an important stimulus to the development, by Gurney and Condon, later in 1928, of the theory of the radioactive decay of nuclei (by alpha particle tunneling).