  Using Floquet dynamical mean-field theory, we study the high-harmonic
generation in the time-periodic steady states of wide-gap Mott insulators under
AC driving. In the strong-field regime, the harmonic intensity exhibits
multiple plateaus, whose cutoff energies $\epsilon_{\rm cut} = U + mE_0$ scale
with the Coulomb interaction $U$ and the maximum field strength $E_0$. In this
regime, the created doublons and holons are localized because of the strong
field and the $m$-th plateau originates from the recombination of $m$-th
nearest-neighbor doublon-holon pairs. In the weak-field regime, there is only a
single plateau in the intensity, which originates from the recombination of
itinerant doublons and holons. Here, $\epsilon_{\rm cut} = \Delta_{\rm gap} +
\alpha E_0$, with $\Delta_{\rm gap}$ the band gap and $\alpha>1$. We
demonstrate that the Mott insulator shows a stronger high-harmonic intensity
than a semiconductor model with the same dispersion as the Mott insulator, even
if the semiconductor bands are broadened by impurity scattering to mimic the
incoherent scattering in the Mott insulator.
