Electron emission from ferroelectrics ͑FEE͒ is an unconventional electron emission effect. Methods of FEE excitation are quite different compared to classic electron emission from solids. Two kinds of FEE have been observed, ''weak'' and ''strong.'' ''Weak'' electron emission ͑current density 10 Ϫ12-10 Ϫ7 A/cm 2 ͒ occurs from polar surfaces of ferroelectric materials in the ferroelectric phase only. A source of the electric field for ''weak'' FEE excitation is an uncompensated charge, generated by a deviation of macroscopic spontaneous polarization from its equilibrium state under a pyroelectric effect, piezoelectric effect, or polarization switching. The FEE is a tunneling emission current which screens uncompensated polarization charges. It is shown that the FEE is an effective tool for direct domain imaging and studies of electronic properties of ferroelectrics. ''Strong'' FEE, which is 10-12 orders of magnitude higher than ''weak'' FEE, achieves 100 A/cm 2 and is plasma-assisted electron emission. Two modes of the surface flashover plasma formation followed by strong electron emission have been studied. The plasma of ferroelectric origin has been observed only in the ferroelectric phase and it is induced by polarization switching or a field-enforced phase transition, such as antiferroelectric-ferroelectric or relaxor-ferroelectric. The second mode of plasma is conventional surface flashover which may be initiated by a high voltage application in any phase from any dielectric, including ferroelectrics. In this review paper we consider numerous experimental results, as well as mechanisms of both types of electron emission from ferroelectrics. The main stress is placed on the material aspect in order to clarify the influence of ferroelectricity ͑ferroelectric phase transitions, polarization switching, etc.͒ on electron emission. Another aspect which is broadly discussed is the potential applications of these unconventional FEE emitters in various devices for development of high density FEE cathodes for microwave devices, as well as FEE converters of IR irradiation into visible light, x-ray imaging, FEE flat panel displays, etc.

Strong pulsed electron emission has been observed from 12/65/35 lead lanthanum zirconate titanate ceramic composition in two different nonswitched phases at room temperature and at the temperature 100 °C. The electron emission parameters of this composition appear to be independent of phase for the two phases investigated. Fast photography and direct observation show that the strong electron emission occurs from the surface discharge plasma. The new experimental data make it possible to demonstrate the validity of the Child–Langmuir law for this electron emitter. A pulsed plasma lead lanthanum zirconate titanate ceramic cathode with burst frequency up to 100 kHz and collector current density up to 10 A/cm2 is developed.

A flashover plasma has been induced by spontaneous polarization switching on a polar surface of the ferroelectric crystal triglycine sulphate (TGS). The effect has not been observed in the paraelectric phase. The threshold switching voltage for a surface flashover ignition was as low as 100 V for pulsed and ac voltage. A mechanism of plasma initiation on a ferroelectric surface is proposed. It is assumed that the plasma is ignited by electron emission initiated by polarization switching. Subsequent electron avalanching occurs as a result of the high potential gradient along the ferroelectric surface caused by inhomogeneous polarization switching. Electrons and ions with energies up to several hundreds of eV were been recorded due to a high surface potential of the switched ferroelectric.

A new mechanism of polarization switching and electron emission in ferroelectric cathodes is proposed. Surface flashover plasma of a ferroelectric origin was observed on a polar ferroelectric surface ͓D. Shur, G. Rosenman, and Ya. E. Krasik, Appl. Phys. Lett. 70, 574 ͑1997͔͒. Simultaneous measurements of switched charge and plasma density show that expanding surface plasma represents a dynamic switching electrode. Direct measurements of ion/electron emission currents and surface analysis implemented by different analytic tools indicate that electrons and ions from the surface plasma contribute to spontaneous polarization screening. The high energy of charged particles emitted from the surface plasma is ascribed to a high surface potential during polarization switching.

A pulsed ferroelectric electron cathode with a current density of up to 45 A based on ceramic is presented. The experimental set-up for the cathode triggering allowed us to achieve a perveance of more than P and an energy spread in the generated electron beam as low as 100 eV (FWHM).