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We have studied LO phonon-plasmon coupled modes by means of Raman scattering in n-InP for carrier densities between 6× 10 16 and 1× 10 19 cm− 3. A line-shape theory based on the Lindhard-Mermin dielectric function that takes into account the nonparabolicity of the InP conduction band as well as temperature and finite wave-vector effects is used to fit the Raman spectra and extract accurate values of the electron density. The results obtained from the Lindhard-Mermin model are compared with the charge density determinations based on the Drude and the hydrodynamical models, and the approximations involved in these models are discussed.