  Studies of different experimental groups that explore the properties of a
two-dimensional electron gas in silicon semiconductor systems ((100) Si
metal-oxide-semiconductor field-effect transistors (MOSFETs) and (100)
SiGe/Si/SiGe quantum wells) in the vicinity of the metal-insulator transition
are described and critically analyzed. Results are identified that are common
to all research: (i) the effective mass of electrons measured at the Fermi
level in the metallic regime increases as the electron density decreases and,
if extrapolated, tends to diverge; (ii) the behavior of the energy-averaged
mass in the metallic region is quite different in the two systems: in
Si-MOSFETs, it also exhibits a tendency to diverge, while in the SiGe/Si/SiGe
quantum wells it saturates in the limit of low electron densities; (iii) there
is a small number (depending on the sample quality) of localized electrons in
the metallic phase; (iv) the properties that the electron system exhibits in
the insulating phase in the vicinity of the metal-insulator transition are
typical of amorphous media with a strong coupling between particles.
