We investigated longitudinal optical (LO) phonon-plasmon coupled (LOPC) mode in undoped GaAs/n -type GaAs epitaxial structures using Raman scattering and terahertz time-domain spectroscopic measurements. In the Raman measurement, the LOPC mode, the frequency of which is determined by the doped electron density in the n -type layer, was observed in addition to the LO phonon in the undoped layer. Contrastingly, in terahertz time-domain measurements, the frequency of the observed coherent LOPC mode is dominated by a photogenerated electron density: frequency-tunable LOPC mode. This indicates that the LOPC mode is generated in the undoped layer. Thus, the probing mechanisms for the LOPC mode are different in the Raman and terahertz time-domain measurements. The built-in electric field exists in the undoped layer. This field enlarges the initial polarization of the LO phonon and, as a result, enables to observe the coherent LOPC mode generated in the undoped layer with the use of the terahertz time-domain spectroscopy.