It is difficult to change the intrinsic carrier concentrations of metals and semimetals and therefore 2D gases in such materials was not discussed. Why?
Due to the infinitely large dielectric constant of metals at zero frequency and long wavelengths, it is practically impossible appreciably to manipulate the charge density in these materials by means of an external electric field; static electric field of macroscopic wavelength is very effectively screened inside a metal, physically due to an appreciable density of electronic states at the chemical potential. This is not the case when the host material is insulating (or semiconducting), where the dielectric constant is typically of the order of 10. Historically, one in fact began with metallic thin films; in the autobiography of the late Sir Nevill Mott one reads that his father, Charles Francis Mott, as student of physics under J.J. Thompson did experimental work towards exactly what was achieved decades later with doped semiconductors (the experiments, performed at the Cavendish Laboratory, failed to the disappointment of the young Charles Mott).
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