Films of semiconductor
nanocrystals are used as a novel\, low-cost electronic materials for opto
electronic devices. To achieve their full potential a better understanding
of their conductivity as a function of concentration of donors is require
d. So far\, it is not known how many donors will make a nanocrystal film m
etallic. In bulk semiconductors\, the critical concentration of electrons
at the metal-insulator transition is universally described by the famous M
ott criterion. We show theoretically that in a dense NC film\, where NCs t
ouch each other by small facets with radius r <\;<\; d\, the critical
concentration of electrons N at the metal-insulator transition satisfies t
he condition is given by N r^3 = 1.

\nThis critical concentration is
typically 100 times larger than the Mott one. In the accompanying experime
nts\, we investigate the conduction mechanism in films of phosphorus-doped
silicon nano-crystals. At the largest electron concentration achieved in
our samples\, which is half the predicted N\, we find that the localizatio
n length of hopping electrons is close to three times the nano-crystals di
ameter\, indicating that the film approaches the metal-insulator transitio
n.