The field electron emission (FEE) is a unique quantum-mechanical effect of electrons tunneling from a condensed matter (solid or liquid) into vacuum. The efficiency of this emission process is tens of millions of times higher than in other known emission processes. The extremely high current density in FEE and the fact that no energy is consumed by the emission process proper afford exceptionally wide possibilities for practical application of this effect.

Currently, the field electron emission is being infused with new life due to emergence of vacuum microelectronics, a principally new branch of the micro- and nanoelectronics.

Vacuum microelectronics (VME) is a new field in micro- and nanoelectonics that has been developed during the recent few decades. VME is base on the employment of electrons in vacuum at the dimensions of active elements in tenths and hundredths of a micrometer. Practically, a field electron emitter is used as active element in VME systems. With employing microscopic anode-to-cathode gaps, one can build devices with a field emission cathode controlled with a voltage in tens of volt.

Small dimensions of a field electron emitter provides an opportunity of bringing the size of active elements down to a few tens of atomic dimensions (i.e., to 10^-6 - 10^-7 cm). Some up-to-date experimental data indicate at the existence of elective elements of a size on the atomic scale (1 -3 Å) on such special materials as, e.g., carbon nanotubes.

The small dimensions of field electron emitters also permit achieving of a density of active elements of up to 10⁸ - 10¹⁰ cm^-2, and even of up to 10¹² cm^-2, if one employs such "self-organizing" systems as fullerenes, nanotubes, etc.

One of the fundamental problems of the modern vacuum microelectronics is fabrication of high-efficiency, low-voltage field electron emitters. Such field emitters should meet the following basic requirements: large working area, efficient emission at low voltages (from a few to tens of volts) and fields (1 - 10 V/micrometer), high density of emission centers, a controllable and reproducible fabrication procedure. The traditional method of fabricating field emitters is based on the use of multi-needles field emission cathodes and precision technologies based on photo- and electron lithography techniques. For this purpose, usually metals (W, Mo) and semiconductors (mainly Si) are used which, regrettably, have rather high work function (4 to 5 eV).

An alternative direction in vacuum microelectronics is a relatively inexpensive technology of planar field emission cathodes based on materials with low electron affinity or having high surface microroughness to ensure high local amplification factor of the electric field.. These materials include films and coats of various allotropic forms of carbon such as nanoclusters: diamond-like structures, fullerenes, tubelenes (nanotubes), and their derivatives. Carbon nanoclusters are self-organizing structures and open a principally new way of fabrication of field emission cathodes with unique characteristics such as low average magnitude of the threshold emission fields, high emission stability, uniformity of the emission current distribution over the cathode area and so on. Considerable theoretical and practical interest in the ways of obtaining efficient and stable field electron emission from carbon nanoclusters is reflected in the fact that in the past two years the number of publications on this subject exceeded 700. However, the achievements in this field have not yet been presented in a systematic manner.

The purpose of this review is to summarize the results accumulated in the area of physics of field emission from different kinds of carbon materials with a view to developing field emission cathode arrays with emphasis on problems of vacuum microelectronics. The review includes: 1) a systematic presentation of data on physical-chemical characteristics of the carbon materials used as field emission cathodes; 2) consideration of the methods for producing these materials; 3) analysis of data on the structure and microgeometry of diamond-like films and carbon nanoclusters; 4) analysis of the results of investigations of their field emission characteristics; 5) consideration of the proposed mechanisms of the field electron emission in these materials; 6) analysis of current-voltage characteristics, data of the field electron emission microscopy; data on stability of the field electron emission, and its uniformity across the surface of field emission cathode arrays. In conclusion, the latest data on applications of the field emission cathodes based on carbon materials in various equipment; mainly in low-voltage field emission displays are considered. Special attention is paid to investigations performed in last years; the results of research by the present authors are given as well.