  We develop a theory of far from the equilibrium transport in arrays of tunnel
junctions. We find that if the rate of the electron-electron interactions
exceeds the rate of the electron-phonon energy exchange, the energy relaxation
ensuring the charge transfer may occur sequentially. In particular, cotunneling
transport in arrays of junctions is dominated by the relaxation via the
intermediate bosonic environment, the electron-hole excitations, rather than by
the electron-phonon mechanism. The current-voltage characteristics are highly
sensitive to the spectrum of the environmental modes and to the applied bias,
which sets the lower bound for the effective temperature. We demonstrate that
the energy gap in the electron-hole spectrum which opens below some critical
temperature T* due to long-range Coulomb interactions gives rise to the
suppression of the tunneling current.
