This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
Figure 4318 shows the differences of operating electrical field and temperature to emit electrons in various electron guns.
Figure 4318. The differences of operating electrical field and temperature to emit electrons in various electron guns.
Cold field-emission (CFE) guns have high temporal coherence due to ΔE ≈ 0.3 eV and high spatial coherence because of the very small source size. However, CFE guns require a UHV (ultra-high vacuum) environment (10-8 Pa or better) in order to prevent build-up of contamination on the apex and, in general, the beam current falls during the operation of the microscope as gas molecules adhere to the gun tip.
CFE gun emitter is normally made of tungsten with the surface of the (310) plane, working at room temperature without heating. Contamination of residual gases on the surface of the emitter generates emission noise. For the same reason, the cold emission guns are not very stable. For instance, some emission current from a freshly cleaned tip only takes 40 min to decay to 50%.  Therefore, regular maintenance, the so-called flashing process, is often needed. Furthermore, a table on the comparison of various electron sources is presented in page1409.
Compared to the cold field emitter, the Schottky electron source has a much larger emission area. The energy resolution of EELS with a Schottky electron gun is limited to 0.5–1.0 eV due to thermal broadening. The energy resolution is increased by over a factor of two (e.g. ∼0.2–0.4 eV) with cold field emission or even monochromatic electron sources.
 Krivanek OL, Corbin GJ, Dellby N, Elston BF, Keyse RJ, Murfitt MF, Own CS, Szilagyi ZS, Woodruff JW. An electron microscope for the aberration-corrected era, Ultramicroscopy, 108 (2008) 179–195.