  At low energy scales charge transport in the insulating Si:P is dominated by
activated hopping between the localized donor electron states. Thus,
theoretical models for a disordered system with electron-electron interaction
are appropriate to interpret the electric conductivity spectra. With a newly
developed technique we have measured the complex broadband microwave
conductivity of Si:P from 100 MHz to 5 GHz in a broad range of phosphorus
concentration $n/n_c$ from 0.56 to 0.95 relative to the critical value
$n_c=3.5\times 10^{18}$ cm$^{-3}$ corresponding to the metal-insulator
transition driven by doping. At our base temperature of $T =1.1$ K the samples
are in the zero-phonon regime where they show a super-linear frequency
dependence of the conductivity indicating the influence of the Coulomb gap in
the density of the impurity states. At higher doping $n\to n_c$, an abrupt drop
in the conductivity power law $\sig(\omega)\sim\omega^\alpha$ is observed. The
dielectric function $\eps$ increases upon doping following a power law in
($1-n/n_c$). Dynamic response at elevated temperatures has also been
investigated.
