  The study of the high-energy (MeV-GeV) part of GRBs spectrum can play a
crucial role in investigating the physics of the prompt emission, but it is
often hampered by low statistic and the paucity of GeV observations. In this
work, we analyze the prompt emission spectra of the 22 brightest GRBs which
have been simultaneously observed by Fermi/GBM and Fermi/LAT, spanning 6 orders
of magnitude in energy. The high-energy photon spectra can be modelled with a
power-law $N(E)\propto E^{-\beta}$ possibly featuring an exponential cutoff. We
find that, with the inclusion of the LAT data, the spectral index $\beta$ is
softer than what typically inferred from the analysis of Fermi/GBM data alone.
Under the assumption that the emission is synchrotron, we derive the index
$p\sim2.79$ of the power-law energy distribution of accelerated particles
($N(\gamma)\propto \gamma^{-p}$). In 9 out of 22 GRB spectra, we find a
significant presence of an exponential cut-off at high-energy, ranging between
14 and 298 MeV. By interpreting the observed cut-off as a sign of
pair-production opacity, we estimate the jet bulk Lorentz factor $\Gamma$,
finding values in the range 130-330. These values are consistent with those
inferred from the afterglow light curve onset time. Finally, by combining the
information from the high-energy prompt emission spectrum with the afterglow
lightcurve, we provide a method to derive the distance R from the central
engine where the prompt emission occurs. These results highlight the importance
of including high-energy data, when available, in the study of prompt spectra
and their role in addressing the current challenges of the GRB standard model.
