Electronic structure and chemical composition of GaAs-based semiconductors are considered to be well characterized by photoelectron spectroscopy and other surface-sensitive methods. However, effect of ion irradiation on electron binding energies and band structure of semiconductors can drastically distort results of diagnostics. This effect was studied by synchrotron-based XPS applied to n-GaAs wafer after 1250 eV - Ar+ ion exposure with fluence Q ~ 1 × 1015 ions/cm2 which is typical for preparation of atomically clean surfaces. Mechanical action of ions was shown to change the n-GaAs electron binding energies by the value of the bandgap width due to creation of defect states, and conversion of the conductivity type from n to p. The Ga3d and As3d core-level binding energies for p-GaAs and n-GaAs were measured in one experiment on the p-n plane structure formed due to irradiation: EB(p/n) = 19.3/20.4 eV and EB(p/n) = 41.3/42.4 eV. The p-layer nanothickness was determined and the band diagram of the p-n GaAs structure was constructed. The revealed effect may be comparable with core-level chemical shifts and should be taken into account to avoid mistakes in the XPS chemical composition diagnostic of GaAs-based semiconductors.