  We report electrical transport measurements of arrays of PbSe nanocrystals
forming the channels of field effect transistors. We measure the current in
these devices as a function of source-drain voltage, gate voltage and
temperature. Annealing is necessary to observe measurable current after which a
simple model of hopping between intrinsic localized states describes the
transport properties of the nanocrystal solid. We find that the majority
carriers are holes, which are thermally released from acceptor states. At low
source-drain voltages, the activation energy for the conductivity is given by
the energy required to generate holes plus the activation over barriers
resulting from site disorder. At high source-drain voltages the activation
energy is given by the former only. The thermal activation energy of the
zero-bias conductance indicates that the Fermi energy is close to the
highest-occupied valence level, the 1Sh state, and this is confirmed by
field-effect measurements, which give a density of states of approximately
eight per nanocrystal as expected from the degeneracy of the 1Sh state.
